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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Look East in February 2014: Two dogs – plus Jupiter – rising in a star-spangled spectacular – the Winter Hexagon

We have two “dog stars” on the southeastern horizon early on February evenings  – Sirius and Procyon – and  both are part of what is certainly the brightest, star-spangled  section of our northern night sky – the Winter Hexagon.  Adding to this annual dazzle in 2014 – and brighter than any star – is the “wandering star” (i.e., planet) Jupiter, smack in the middle of the Gemini twins -about halfway between their heads and feet.

Here’s how the Winter Hexagon looked to the camera of Jimmy Westlake who took this gorgeous shot as it loomed over Stagecoach, Colorado, USA.  You may not see the faint band of the Milky Way shown here if you live in a light polluted region, but you certainly should be able to pick out the bright stars that outline the Hexagon, as well as the Pleiades star cluster visible near the top and just right of center.

Click on image for much larger view! (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Look carefully at that photo, then compare it with this star chart, which is what we see from mid-northern latitudes as we look southeast early on a February evening.

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you'll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you’ll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

For a printer-friendly version of this chart, click here.

People in the north tend to think that the stars are brighter in winter because the air is so cool and crisp. That certainly could be a factor. But the simple fact is our winter sky is dominated by a whole lot of very bright stars. In fact, visible from earth are 22 stars of first magnitude. Sixteen  of these are visible from the Northern Hemisphere, and half of these are visible in the area of the Winter Hexagon on a February evening. That means nearly all these bright stars are jammed into a space taking up less than one-quarter of the February night sky – which is  just one-eighth of the total night sky we can see through the year! In other words, if bright stars like these were scattered throughout the night sky evenly there would be 64 first magnitude stars instead of just 22. Add to that the seven bright stars of the Big Dipper being dragged up the northeastern sky by the Great Bear on a February evening, and it is no wonder that in the dead of a northern winter our skies offer a lively, colorful, star-spangled spectacular.

The Hexagon alone contains seven of the first magnitude stars in our sky and an eighth that is the brightest second magnitude star we see. This one – Castor – just misses being first magnitude by a hair.  And nearby is Adhara, a star that sits right on the border between second and first magnitude; plus Regulus, another first magnitude star, is rising low in the east. Whew! That’s a lot. Let’s review.  Going  counterclockwise and starting at the bottom, the Hexagon’s corners are marked by:

  • Sirius, the brightest, and at about eight light years one of the closest, stars in our sky – except the Sun, of course.
  • Rigel, the blue giant that marks one of Orion’s feet.
  • Aldebaran, the brilliant orange star that is the eye of Taurus the Bull and dominates the nearest open star cluster, the Hyades.
  • Capella, now high overhead, is really a complex of four stars that we see as one.
  • Castor and Pollux, the twins, one of which (Pollux) is first magnitude, while Castor is the brightest second magnitude star we see.
  • Procyon, the “Little Dog” star, which is dim only in comparison to Sirius, the “Big Dog.”

And . . .

  • Inside the Hexagon is another first magnitude star, Betelgeuse, the red giant that marks Orion’s shoulder, not to mention the three bright stars of Orion’s Belt – all second magnitude.
  • Regulus, the “Little King,” is a first magnitude star that is rising in the east and bringing us the familiar sickle of bright stars that mark the head of the lion. We’ll study it closely next month.
  • Adhara is the western-most star of the distinctive small triangle of stars beneath Sirius. At magnitude 1.5 I call it a first magnitude star, but others consider this second magnitude. So depending on how you count Adhara there are either 21 or 22 first magnitude stars.

Before leaving the Winter Hexagon, I must stress that  this is not simply a northern hemisphere show – if you live  in Sydney, Australia, you could just rename this the “Summer Hexagon.” I see these stars in the southeast – my friends in Sydney see them in the northeast of their sky – and, of course, since they’re “standing on their heads,” they see them a bit differently – something like this!

The “Winter Hexagon” becomes the “Summer Hexagon” in the Southern Hemisphere, but contains all the same bright stars. (Chart prepared from Starry Nights Pro screen shot.)

February Guidepost Stars

Of the stars mentioned so far, the two dog stars, Sirius and Procyon, plus the fence sitter, Adhara, are the guidepost stars to learn this month. They are the ones you can spot near the southeastern horizon, coming into view about 45 minutes to an hour after sunset. (We’ll have more to say about Regulus next month, and the other stars mentioned we’ve met in previous months.) To see the February guidepost stars – and the asterism of the Virgins –  look low in the southeast about 45 minutes to an hour after sunset.  Here’s what you should see.

Click image for larger version. This chart shows the three guidepost stars for February as they appear about an hour after sunset in the southeast. Sirius is the brightest star we see and Procyon is not far behind, but Adhara is not much brighter than its companions, which form a distinctive, small triangle the ancient Arabs knew simply as “the Virgins.” (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

Procyon, the seventh brightest star we see, is first up in our sky, and thus the highest, of these three. To the southeast and a tad lower is brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world. Adhara is the brightest star in the “Virgins,” a simple,  distinctive  triangle asterism. But, of course, Sirius is dominant – far brighter than any other star we see in our night sky. I always think of Sirius as the eye of the great dog and as he sits, the triangle seems to be his rear haunches. From our perspective Adhara may be just another bright star, but of these three it is really the brightest by far – it’s just much farther away than the other two.  If we compared them side by side we would find that Procyon shines with the light of seven Suns, Sirius 23, and Adhara has a luminosity to the eye of 3,700 Suns! Now that’s bright.  And in another way, Adhara reveals our human bias, for if we had ultraviolet vision Adhara would be the brightest star in our sky, not Sirius. But again – that’s not the way we see it. From our perspective, Sirius and Procyon are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in, or near, the Southern Hemisphere.) But Adhara? Adhara is 405  light years away – about the same distance as the North Star, Polaris. Sirius will frequently seem to be changing colors, but that’s just the effect of our atmosphere. Just as our atmosphere makes our Sun look red when it is rising or setting, it makes any bright star near the horizon appear to dance and change colors rapidly.

The Big Dog as Johannes Bayer depicted him in 1603. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)
The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is known as a “dog star” because it is the brightest star of the classic constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, it’s hard to see how connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers.

Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups”  – the one belonging to Procyon – is impossible to see with a backyard telescope and the other an extreme challenge.  The reason is they are quite dim and being very close to the bright stars, get lost in the glare.

But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs, and white dwarfs are a class of stars far denser than anything we encounter on Earth. Now I always find talk of the density of stars counter-intuitive because it gets drilled into our heads that stars are gas and the gas we encounter in our daily lives is anything but dense!  In fact, it’s quite – well – gaseous!  To appreciate this, let’s take a close look at our own Sun.

Click image for larger view.Sirius – with Sirius B at lower left.  Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas which reaches densities that near the center are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times denser than the stuff in the Sun.

Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. (Think of what that would do to your pool table, not to mention your foot if it fell on it!)

We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is, we end up with a mass roughly that of the Sun but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun. (See why a white dwarf is one million times as dense as the Sun?)

How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups,” the embers are being seen while still white hot. But they will eventually cool.

The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose.

This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy, it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyra.  And at the core of that nebula will be the dying ember we know as a white dwarf.

I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than at other times – and we happen to be in a period of several years when that distance will be growing, and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.) I also plan to take a close look at Adhara with a telescope, for it has a 7.5 magnitude companion just 7 arcseconds away. This should be a challenge – because of the difference in brightness of the two –  but not nearly the challenge that seeing the companion of Sirius is. For those with binoculars and small telescopes, some of the most fascinating objects are in this general area of sky, near, or inside the Winter Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular telescopic open clusters in Gemini and Auriga, M35, M36, M37, and M38. All that star light certainly can make for bright nights during the dark  of a northern winter.

Vital Stats for the Guidepost Stars

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.5,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)

For Adhara:

  • Brilliance: Magnitude 1.5, it has a luminosity to the eye of 3500 times that of the Sun! (In other words, much brighter, really, than Procyon or Sirius.)
  • Distance: 405 light years
  • Spectral Type: B2
  • Position: 06h:59m, -28°:59′:18″

September 2013 – pursuing the not-so-false dawn – plus planets

There’s nothing false about the false dawn – in fact, it’s quite intriguing and somewhat puzzling, but very real. Here’s a cool picture of it.

Yes, I’m talking about the zodiacal light – known for hundreds, if not thousands of years as the “false dawn” because it precedes the usual predawn light. Only it isn’t always so obvious – September and October are the best time to see it in the northern hemisphere early morning sky.  (It is best seen in the early evening sky in February and March.)

Oh  – do keep in mind that the picture above was taken through the thin air and superbly dark skies above the European Southern Observatory in Chile and  benefits from the camera’s ability to do a better job of capturing faint light  than our eyes.  We won’t see it that way. But,  the picture is very useful because it gives us a good idea of the shape and size of what we are looking for when we seek this elusive glow in our skies.

If you want to catch it you have to:

  • Be out two hours before sunrise – and  give your eyes time to dark adapt. It is best seen about 80 minutes  before sunrise.
  • Be in a place relatively free of light pollution – you especially don’t want to be looking at a light dome from a city to your east. If you can see the Milky Way your skies are dark enough – if not, you need to go somewhere where you can see it.
  • Look at a time when the Moon isn’t in the morning sky – in 2013 that means the first two weeks of either September or October. 

Is it worth it – I certainly think so – but then I think September mornings are great anyways because you get to see all the bright stars of the Winter Hexagon without freezing your tail off as you do when they are in the evening sky in January. In addition we have Mars rising low in the East and Jupiter is already pretty high up and should appear near the peak of the zodiacal light – and be brighter than any star.  (Mars will be about as bright as   Castor and Pollux,  two of the bright stars  of the Winter Hexagon. Here’s a chart.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click here for a printable, black and white version of this chart.

For you insomniacs – or folks who just love to get up early when the world is still and most of the neighbors have turned off their lights so the sky is darker, pursuing the zodiacal light is special.

What is it? It is sun light reflecting off a  huge cloud of very fine dust between the Earth and Sun on the plane of the solar system.  That’s been agreed upon for some time.  How much dust?  Well, wrap your mind around this.  Assuming that the dust particles have the same reflectivity as the surface of the moon, it would take one dust particle every five miles to reflect that much light! We’re still looking at an awful lot of empty space. Hmmm. . . there 93 million mile between the Earth and Sun – so if we had a single straight line of dust particles, we’d still have more than 18 million of them – and of course this is much more than one single line.  Now that’s awesome.

But where did all that dust come from?    J. Kelly Beatty goes over the science history in an excellent article in September’s Sky and Telescope  and notes that the current opinion is the dust cloud is a result of short period comets.  Think of a comet as a dirty snowball that melts as it nears the Sun, leaving a trail of dust. That dust stays in orbit. Short period comets are ones whose orbit takes 200 years or less because they have been captured by the gravity of the  planets. (Other comets take much longer to orbit, or simply make a single trip around the Sun.)

Why is it obvious in the morning sky in the fall and the  evening sky in the spring? Because it follows the path of the ecliptic and the ecliptic is more or less straight up and down in the morning at this time of year – and in the evening in February and March. At other times it slants at quite an angle keeping the zodiacal light lower in the sky where it gets lost in the routine dawn light.

There’s a nice little planetary show in the west this month as well, as Venus and Saturn get cozy and on September 8 Venus has a close encounter with the crescent moon right after sunset.   It’s Saturn’s turn the next night.  About a week later  Saturn and Venus should fit comfortably in the same binocular field of view for several days.

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

Update- March 5 – Outlook brightens for PanSTARRS!Events – March 2013: Get set for a real nice – BINOCULAR – Comet and more

NASA guide to PanSTARRS position and tail direction on different dates this month. This is NOT a prediction of tail length or comet brightness. It is likely tobe much shrter and fainter - but comets are full of surprises and so this still has the potential to be really nice.

NASA guide to PanSTARRS position and tail direction on different dates this month. This is NOT a prediction of tail length or comet brightness. It is likely to be much shorter and fainter – but comets are full of surprises and so this still has the potential to be really nice. (Click image for larger version.)

The latest indicators are that Comet PanSTARRS will put on a better show than anticipated just a week ago – as noted, comets are just not that predictable! – here’s a recent news item:

Observers in the Southern Hemisphere have been watching Comet PanSTARRS for weeks, but the Northern Hemisphere is due to get its first looks at one of the year’s most eagerly anticipated sky extravaganzas this week. And there’s good news for northerners: The up-and-down expectations for the cometary show are trending upward again.

March Observing Highlights  –

Comet PANSTARRS and its distant kin, the Zodiacal Light

First, let me stress Comet PanSTARRS is not likely to be nearly as bright as originally predicted – but it still should be a nice comet, especially when viewed with binoculars.  And remember – we have another due in November/December that should be much better. However, with comets we can only make educated guesses – they can – and have – surprised the experts over the years, sometimes under performing, sometimes over performing.

I’m linking this comet with the Zodiacal Light because both might be seen at their best on March 12 after sunset in the west. What’s more,  they are  both essentially dust reflecting sunlight,  presenting a related observing challenge, though they are radically different in size. March 12 may be the earliest time for a good look at Comet PanSTARRS in the early twilight – and it will be the last night in early March for the  Zodiacal Light which can be seen about 80 minutes after sunset for the first 12 days of March – after that the Moon will tend to wash out the Zodiacal Light until the last couple days of the month.

Quick Observing Guide:

  • to observe both comet and Zodiacal Light  at their best, hope for clear skies on March 12 – and some special comet luck 
  • to observe the Zodiacal Light  alone go out any evening during the first 12 days of March 2013 and look for it about 80 minutes after sunset.
  • to observe Comet PanSTARRS it may be visible – especially from low northern latitudes such as the southern US, as early as March 7 or 8th, but the week beginning March 12 will probably give the best opportunity for observers in mid-northern latitudes.

A comet is a “dirty snowball” that “melts” when it gets near the sun, giving off what can be a spectacular trail (tail) of tiny dust particles that reflect sunlight. When we think of a comet we are usually thinking of seeing one with such a tail.  And the Zodiacal Light? It’s tons of inter-planetary dust, much of it having accumulated over the years from many comets that eventually disintegrated as they made several trips around the Sun. And while your best views of Comet PanSTARRS will be when it’s near the Sun – but getting dark – your best view of the Zodiacal Light will be just as full darkness is arriving – about 80 minutes after local sunset.

The Zodiacal Light will be in a fixed position night after night – a huge, but very faint, light cone reaching from the western horizon and slanting up in the general direction of the Pleiades star cluster in Taurus  – Comet PanSTARRS will change position slightly each night, drawing away from the Sun. The Zodiacal Light is most certainly a naked eye phenomenon requiring a good view to the west and  skies that are largely free of light pollution in that direction.

The same basic requirements fit Comet PanSTARRS – you need a good view to the west with an unobstructed horizon, at least for the early – and probably best – views. While it may be visible to the naked eye, the best guess is this will be bets seen in binoculars. So by all means, break out the binoculars! You don’t need any thing special – ordinary, low-power ones will do, though if you have large astronomical binoculars, all the better.  And while you will be searching for the comet in the early twilight, do be careful. Wait until about 15 minutes after sunset before scanning the western horizon for it. At all cost, avoid looking with your binoculars at the sun, as that will seriously damage your eyes.

Yes, you are likely to hear that PANSTARRS is visible to the naked eye. Don’t get too excited, though, it’s visibility is a lot like that regular March visitor, the  Zodiacal Light – the numbers in reality don’t really add up. Thus the binoculars are highly recommended – even if its brighter than expected.

Great video guide to the comet from NASA

I read in Sky and Telescope this month that the Zodiacal Light is actually the second brightest “thing” in the Solar system.  Wow! Never prove that from my experience. I  have always found it elusive. I count myself lucky if I can see it at all!  But, of course, Sky and Telescope is right.  Here again there’s an important lesson relating to both the Zodiacal Light and a comet – the brightness they’re talking about is for a point object, but in our view of it, this light is spread out.

So when you hear the Zodiacal Light is beaten only by the Sun in brightness, you have to understand that this is determined by pretending all the light reflected from it was concentrated in a single spot – and it isn’t. It is spread out over a huge area of sky – widest near the horizon and getting narrower as it rises towards the Pleiades. For me it looks much like the Milky Way, only a bit fainter.

The same thing is true of a comet – but to a much lesser degree. That is, Comet PanSTARRS is fairly likely to reach magnitude 2 and if it does, well that’s as bright as the North Star, or most of the stars in the Big Dipper. But – and here’s the catch – that light will be spread out with much of it concentrated in the fuzzy head, but  some also appearing in the tail.

What’s more, as the comet draws away from the Sun it will almost certainly get fainter – and therein lies the crucial problem of seeing a comet at its best. What we are dealing with is a constantly changing set of variables. Generally speaking, the closer a comet is to the Sun, the brighter it is.  However, the closer it is to the Sun, the more it is competing with the lingering sun light. As the twilight deepens, the comet should stand out more – BUT, as the twilight deepens the comet is also getting lower in the sky and that means you’re looking at it through more atmosphere and that makes it appear dimmer.

So the joy – and frustration – of comet hunting is that how the comet looks to you will depend on your local weather, of course, but also exactly when you see it – how bright it is, how high it is, and how dark the sky is around it. That’s what makes viewing – and photographing – comets both fun and challenging.

So what’s the best bet for Comet PanSTARRS – for those in mid-northern latitude somewhere between March 7 and 20 probably about halfway in between. I plan to watch the weather closely from March 10th to 17th and take advantage of any clear evening to look for it. The farther south you are, the sooner it should appear at its best for you – the farther north,  the later in the month it will be at its best.

But remember – on a  clear night early in the month that you go comet hunting – hang around even if the comet is too low to see well – the Zodiacal Light should be best about 80 minutes after sunset when there is no – or little – interference from the Moon. (That means from March 1 to about March 12, 3013.)  If you see the Zodiacal Light – how well you see it depends largely on timing, local weather conditions, and the lack of light pollution.  In other words, it is not quite to finicky as the comet, but still a challenge.

Jupiter – King of the Winter Hexagon!

Wow! What a view to the south!

As the sky darkens on these March evening, don't hesitate to look due south for a wonderful view of Jupiter dominating the Winter Hexagon - thata rea of sky with more birght stars in it than any other! Click the image for a larger version. (Prepared from Starry Nights Pro screen shot.)

As the sky darkens on these March evening, don’t hesitate to look due south for a wonderful view of Jupiter dominating the Winter Hexagon – that area of sky with more bright stars in it than any other! Click the image for a larger version suitable for printing. (Prepared from Starry Nights Pro screen shot.)

The Winter Hexagon is one of my favorite asterisms encompassing a very rich area of sky contains eight very bright stars and that most recognizable of constellations, Orion.  But bright as these stars are, Jupiter will dominate them, outshining even Sirius, the brightest star for norther hemisphere observers. Take a look in that direction about an hour after sunset – in fact, you can’t hope but notice this brilliant area as you scan in the darkening even sky for the Zodiacal Light which shine faintly in a widening cone reaching from near the Pleiades to the western horizon.

And what a fabulous binocular sight!

Use your binoculars to:

  • Look for the fuzzy area in Orion’s sword  which hangs below his belt – the Great Orion Nebulae.
  • Look for the Hyades – the fabulous star cluster that makes up the “V” of Taurus and is just 150 light years away.
  • Look for the Pleiades – my favorite binocular target, a cluster of brilliant gem stone roughly 400 light years away.
  • And, of course, if you can hold them steady enough – brace against a pole, or the corner of a house – try to pick up one or more of the four bright moons of Jupiter.
The "V" of Taurus marks the Hyades cluster and the Pleiades are bit to the right as seen when looking south about 90 minutes after sunset this month.  Watch carefully over the course of the month and you will see Jupiter slowly change position moving towards Aldebaran, the bright star that marks the bull's eye. (Click for larger version. Prepared from Starry Nights Pro screen shot.)

The “V” of Taurus marks the Hyades cluster and the Pleiades are a bit to the right as seen when looking south about 90 minutes after sunset this month. Watch carefully over the course of the month and you will see Jupiter slowly change position moving towards Aldebaran, the bright star that marks the bull’s eye. (Click for larger version. Prepared from Starry Nights Pro screen shot.)

Oh – and I should add that Jupiter will have a real close call with a “young” crescent Moon on March 17, 2013. Exactly how close will depend on where you are, but for me on the East Coast of the US, the Moon will pass within two degrees of the bright planet and add to the fun of binocular observing on that night. They both will fit easily into the same binocular field of view!

Saturn now dominates the morning sky

Think of it as “coming attractions” if you’re not a morning person. Saturn crosses over into our late evening sky and by next month it will be quite easy t see at a reasonable hour.  For March 2013, however, it is primarily the dominant planet in the morning sky.

In fact, this is a rare month for planets – well, I should say planets are rare this month. Jupiter and Saturn are, for all practical purposes, the whole show – the other major planets being too near the Sun for easy viewing.

Our chart shows Saturn at  mid-month and midnight due southeast and about 23 degrees above the horizon. Spica – which is about half a magnitude dimmer than Saturn, will be about 18 degrees away. Be sure to look for the color difference. Saturn should appear creamy – maybe a tad yellow, while Spica is an icy blue.

Saturn and Spica at midnight in March 2013. (Prepared from Starry Nights Pro screen shot. Click image for larger version.)

Saturn and Spica at midnight in March 2013. (Prepared from Starry Nights Pro screen shot. Click image for larger version.)

Look East in February 2013: Two dogs – plus Jupiter – rising in a star-spangled spectacular – the Winter Hexagon

We have two “dog stars” on the southeastern horizon early on February evenings  – Sirius and Procyon – and  both are part of what is certainly the brightest, star-spangled  section of our northern night sky – the Winter Hexagon.  Adding to this annual dazzle in 2013 – and brighter than any star – is the “wandering star (ie.planet) Jupiter, just above Aldebaran.  (Jupiter doesn’t show in our pictures and charts, but you can’t miss it when you go out to look at this section of the sky this year. )

Here’s how the Winter Hexagon looked to the camera of Jimmy Westlake who took this gorgeous shot as it loomed over Stagecoach, Colorado, USA.  You may not see the faint band of the Milky Way shown here if you live in a light polluted region, but you certainly should be able to pick out the bright stars that outline the Hexagon, as well as the Pleiades star cluster visible near the top and just right of center.

Click on image for much larger view! (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Look carefully at that photo, then compare it with this star chart which is what we see from mid-northern latitudes as as we look southeast early on a February evening. (And as noted, in 2013 there will be a “star” brighter than all the others in the chart – the planet Jupiter.)

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you’ll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

For a printer-friendly version of this chart, click here.

People in the north tend to think that the stars are brighter in winter because the air is so cool and crisp. That certainly could be a factor. But the simple fact is our winter sky is dominated by a whole lot of very bright stars. In fact, visible from earth are 22 stars of first magnitude. Sixteen  of these are visible from the northern hemisphere and half of these are visible in the area of the Winter Hexagon on a February evening. That means nearly all these bright stars are jammed into a space taking up less than one-quarter of the February night sky – which is  just one-eighth of the total night sky we can see through the year! In other words, if bright stars like these were scattered throughout the night sky evenly there would be 64 first magnitude stars instead of just 22. Add to that the seven bright stars of the Big Dipper being dragged up the northeastern sky by the Great Bear on a February evening, and it is no wonder that in the dead of a northern winter our skies offer a lively, colorful, star-spangled spectacular.

The Hexagon alone contains seven of the first magnitude stars in our sky and an eighth that is the brightest second magnitude stars we see. This one – Castor – just misses being first magnitude by a hair.  And nearby is Adhara, a star that sits right on the border between second and first magnitude; plus Regulus, another first magnitude star, is rising low in the east. Whew! That’s a lot. Let’s review.  Going  counterclockwise and starting at the bottom, the Hexagon’s corners are marked by:

  • Sirius, the brightest, and at about eight light years one of the closest, stars in our sky – except the Sun, of course.
  • Rigel, the blue giant that marks one of Orion’s feet.
  • Aldebaran, the brilliant orange star that is the eye of Taurus the Bull and dominates the nearest open star cluster, the Hyades.
  • Capella, now high overhead, is really a complex of four stars that we see as one.
  • Castor and Pollux, the twins, one of which (Pollux) is first magnitude, while Castor is the brightest second magnitude star we see.
  • Procyon, the “Little Dog” star, which is dim only in comparison to Sirius, the “Big Dog.”

And . . .

  • Inside the Hexagon is another first magnitude star, Betelgeuse, the red giant that marks Orion’s shoulder, not to mention the three bright stars of Orion’s Belt – all second magnitude.
  • Regulus, the “Little King,” is a first magnitude star that is rising in the east and bringing us the familiar sickle of bright stars that mark the head of the lion. We’ll study it closely next month.
  • Adhara is the western-most star of the distinctive small triangle of stars beneath Sirius. At magnitude 1.5 I call it a first magnitude star, but others consider this second magnitude. So depending on how you count Adhara there are either 21 or 22 first magnitude stars.

Before leaving the Winter Hexagon, I must stress that  this is not simply a northern hemisphere show – if you live  in Sydney, Australia, you could just rename this the “Summer Hexagon.” I see these stars in the southeast – my friends in Sydney see them in the northeast of their sky – and, of course, since they’re “standing on their heads,” they see them a bit differently – something like this!

The “Winter Hexagon” becomes the “Summer Hexagon” in the Southern Hemisphere, but contains all the same bright stars. (Chart prepared from Starry Nights Pro screen shot.)

February Guidepost Stars

Of the stars mentioned so far, the two dog stars, Sirius and Procyon, plus the fence sitter, Adhara, are the guidepost stars to learn this month. They are the ones you can spot near the southeastern horizon, coming into view about 45 minutes to an hour after sunset. (We’ll have more to say about Regulus next month, and the other stars mentioned we’ve met in previous months.) To see the February guidepost stars – and the asterism of the Virgins –  look low in the southeast about 45 minutes to an hour after sunset.  Here’s what you should see.

Click image for larger version. This chart shows the three guidepost stars for February as they appear about an hour after sunset in the southeast. Sirius is the brightest star we see and Procyon is not far behind, but Adhara is not much brighter than its companions, which form a distinctive, small triangle the ancient Arabs knew simply as “the Virgins.” (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

Procyon, the seventh brightest star we see, is first up in our sky, and thus the highest, of these three. To the southeast and a tad lower is brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world. Adhara is the brightest star in the  “Virgins,” a simple,  distinctive  triangle asterism. But, of course, Sirius is dominant – far brighter than any other star we see in our night sky. I always think of Sirius as the eye of the great dog and as he sits, the triangle seems to be his rear haunches. From our perspective Adhara may be just another bright star, but of these three it is really the brightest by far – it’s just much farther away than the other two.  If we compared them side by side we would find that Procyon shines with the light of seven Suns, Sirius 23, and Adhara has a luminosity to the eye of 3,700 Suns! Now that’s bright.  And in another way, Adhara reveals our human bias, for if we had ultraviolet vision Adhara would be the brightest star in our sky, not Sirius. But again – that not the way we see it. From our perspective Sirius and Procyon are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in, or near, the Southern Hemisphere.) But Adhara? Adhara is 405  light years away – about the same distance as the North Star, Polaris. Sirius will frequently seem to be changing colors, but that’s just the effect of our atmosphere. Just as our atmosphere makes our Sun look red when it is rising or setting, it makes any bright star near the horizon appear to dance and change colors rapidly.

The Big Dog as Johannes Bayer depicted him in 1603. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)
The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is known as a “dog star” because it is the brightest star of the classic constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, it’s hard to see how connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers.

Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups”  – the one belonging to Procyon – is impossible to see with a backyard telescope and the other an extreme challenge.  The reason is they are quite dim and being very close to the bright stars, get lost in the glare.

But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs, and white dwarfs are a class of stars far denser than anything we encounter on Earth. Now I always find talk of the density of stars counter-intuitive because it gets drilled into our heads that stars are gas and the gas we encounter in our daily lives is anything but dense!  In fact, it’s quite – well – gaseous!  To appreciate this, let’s take a close look at our own Sun.

Click image for larger view.Sirius – with Sirius B at lower left.  Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas which reaches densities that near the center are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times denser than the stuff in the Sun.

Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. (Think of what that would do to your pool table, not to mention your foot if it fell on it!)

We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is, we end up with a mass roughly that of the Sun but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun. (See why a white dwarf is one million times as dense as the Sun?)

How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups,” the embers are being seen while still white hot. But they will eventually cool.

The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose.

This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy, it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyra.  And at the core of that nebula will be the dying ember we know as a white dwarf.

I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than at other times – and we happen to be in a period of several years when that distance will be growing, and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.) I also plan to take a close look at Adhara with a telescope, for it has a 7.5 magnitude companion just 7 arcseconds away. This should be a challenge – because of the difference in brightness of the two –  but not nearly the challenge that seeing the companion of Sirius. For those with binoculars and small telescopes, some of the most fascinating objects are in this general area of sky, near, or inside the Winter Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular telescopic open clusters in Gemini and Auriga, M35, M36, M37, and M38. All that star light certainly can make for bright nights during the dark  of a northern winter.

Vital Stats for the Guidepost Stars

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.5,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)

For Adhara:

  • Brilliance: Magnitude 1.5, it has a luminosity to the eye of 3500 times that of the Sun! (In other words, much brighter, really, than Procyon or Sirius.)
  • Distance: 405 light years
  • Spectral Type: B2
  • Position: 06h:59m, -28°:59′:18″

Look East in February 2012: Two dogs rising in a star-spangled spectacular – the Winter Hexagon

We have two “dog stars” on the southeastern horizon early on February evenings  – Sirius and Procyon – and  both are part of what is certainly the brightest, star-spangled  section of our northern night sky – the Winter Hexagon.  Here’s how it looked to the camera of Jimmy Westlake who took this gorgeous shot of the Winter Hexagon over Stagecoach, Colorado, USA.  You may not see the faint band of the Milky Way shown here if you live in a light polluted region, but you certainly should be able to pick out the bright stars that outline the Hexagon, as well as the Pleiades star cluster visible near the top and just right of center.

Click on image for much larger view! (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Look carefully at that photo, then compare it with this star chart which is what we see from mid-northern latitudes as as we look southeast early on a February evening.

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you’ll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

For a printer-friendly version of this chart, click here.

People in the north tend to think that the stars are brighter in winter because the air is so cool and crisp. That certainly could be a factor. But the simple fact is our winter sky is dominated by a whole lot of very bright stars. In fact, there are 22 stars of first magnitude – 16 of which are visible from the northern hemisphere and more than half of these are visible in “prime time” on a February evening. And nearly all these bright stars are jammed into a space taking up less than one-quarter of the February night sky – that’s just one-eighth of the total night sky we can see through the year! Add to that the seven bright stars of the Big Dipper being dragged up the northeastern sky by the Great Bear on a February evening, and it is no wonder that in the dead of a northern winter our skies offer a lively, colorful, star-spangled spectacular.

The Hexagon alone contains seven of the first magnitude stars in our sky and an eighth that is the brightest second magnitude star we see – in other words, that one just misses being first magnitude by a hair. And nearby is Adhara, a star that sits right on the border between second and first magnitude; plus Regulus, another first magnitude star, is rising in the east. Whew! That’s a lot. Let’s review.  Going  counterclockwise and starting at the bottom, the Hexagon’s corners are marked by:

  • Sirius, the brightest, and at about eight light years one of the closest, stars in our sky – except the Sun, of course.
  • Rigel, the blue giant that marks one of Orion’s feet.
  • Aldebaran, the brilliant orange star that is the eye of Taurus the Bull and dominates the nearest open star cluster, the Hyades.
  • Capella, now high overhead, is really a complex of four stars that we see as one.
  • Castor and Pollux, the twins, one of which (Pollux) is first magnitude, while Castor is the brightest second magnitude star we see.
  • Procyon, the “Little Dog” star, which is dim only in comparison to Sirius, the “Big Dog.”

And . . .

  • Inside the Hexagon is another first magnitude star, Betelgeuse, the red giant that marks Orion’s shoulder, not to mention the three bright stars of Orion’s Belt – all second magnitude.
  • Regulus, the “Little King,” is a first magnitude star that is rising in the east and bringing us the familiar sickle of bright stars that mark the head of the lion. We’ll study it closely next month.
  • Adhara is the western-most star of the distinctive small triangle of stars beneath Sirius. At magnitude 1.5 I call it a first magnitude star, but others consider this second magnitude. So depending on how you count Adhara there are either 21 or 22 first magnitude stars.

Before leaving the Winter Hexagon, I must stress that  this is not simply a northern hemisphere show – if you live  in Sydney, Australia, you could just rename this the “Summer Hexagon.” I see these stars in the southeast – my friends in Sydney see them in the northeast of their sky – and, of course, since they’re “standing on their heads,” they see them a bit differently – something like this!

The “Winter Hexagon” becomes the “Summer Hexagon” in the Southern Hemisphere, but contains all the same bright stars. (Chart prepared from Starry Nights Pro screen shot.)

February Guidepost Stars

Of the stars mentioned so far, the two dog stars, Sirius and Procyon, plus the fence sitter, Adhara, are the guidepost stars to learn this month. They are the ones you can spot near the southeastern horizon, coming into view about 45 minutes to an hour after sunset. (We’ll have more to say about Regulus next month, and the other stars mentioned we’ve met in previous months.) To see the February guidepost stars – and the asterism of the Virgins –  look low in the southeast about 45 minutes to an hour after sunset.  Here’s what you should see.

Click image for larger version. This chart shows the three guidepost stars for February as they appear about an hour after sunset in the southeast. Sirius is the brightest star we see and Procyon is not far behind, but Adhara is not much brighter than its companions, which form a distinctive, small triangle the ancient Arabs knew simply as “the Virgins.” (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here. Procyon, the seventh brightest star we see, is first up in our sky, and thus the highest, of these three. To the southeast and a tad lower is brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world. Adhara is the brightest star in the  “Virgins,” a simple,  distinctive  triangle asterism. But, of course, Sirius is dominant – far brighter than any other star we see in our night sky. I always think of Sirius as the eye of the great dog and as he sits, the triangle seems to be his rear haunches. From our perspective Adhara may be just another bright star, but of these three it is really the brightest by far – it’s just much farther away than the other two.  If we compared them side by side we would find that Procyon shines with the light of seven Suns, Sirius 23, and Adhara has a luminosity to the eye of 3,700 Suns! Now that’s bright.  And in another way, Adhara reveals our human bias, for if we had ultraviolet vision Adhara would be the brightest star in our sky, not Sirius. But again – that not the way we see it. From our perspective Sirius and Procyon are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in, or near, the Southern Hemisphere.) But Adhara? Adhara is 405  light years away – about the same distance as the North Star, Polaris. Sirius will frequently seem to be changing colors, but that’s just the effect of our atmosphere. Just as our atmosphere makes our Sun look red when it is rising or setting, it makes any bright star near the horizon appear to dance and change colors rapidly.

The Big Dog as Johannes Bayer depicted him in 1603. ((Image courtesy of Linda Hall library of Science, Engineering and Technology.)
The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is known as a “dog star” because it is the brightest star of the classic constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, it’s hard to see how connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers. Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups”  – the one belong to Procyon – is impossible to see with a backyard telescope and the other an extreme challenge.  The reason is they are quite dim and being very close to the bright stars, get lost in the glare. But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs, and white dwarfs are a class of stars far denser than anything we encounter on Earth. Now I always find talk of the density of stars counter-intuitive because it gets drilled into our heads that stars are gas and the gas we encounter in our daily lives is anything but dense!  In fact, it’s quite – well – gaseous!  To appreciate this, let’s take a close look at our own Sun.

Click image for larger view.Sirius – with Sirius B at lower left.  Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas which reaches densities that near the center are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times denser than the stuff in the Sun. Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. (Think of what that would do to your pool table, not to mention your foot if it fell on it!)  We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is, we end up with a mass roughly that of the Sun but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun. (See why a white dwarf is one million times as dense as the Sun?) How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups,” the embers are being seen while still white hot. But they will eventually cool.  The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose. This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy, it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyra.  And at the core of that nebula will be the dying ember we know as a white dwarf. I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than at other times – and we happen to be in a period of several years when that distance will be growing, and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.) I also plan to take a close look at Adhara with a telescope, for it has a 7.5 magnitude companion just 7 arcseconds away. This should be a challenge – because of the difference in brightness of the two –  but not nearly the challenge that seeing the companion of Sirius. For those with binoculars and small telescopes, some of the most fascinating objects are in this general area of sky, near, or inside the Winter Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular telescopic open clusters in Gemini and Auriga, M35, M36, M37, and M38. All that star light certainly can make for bright nights during the dark  of a northern winter.

Vital Stats for the Guidepost Stars

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.5,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)

For Adhara:

  • Brilliance: Magnitude 1.5, it has a luminosity to the eye of 3500 times that of the Sun! (In other words, much brighter, really, than Procyon or Sirius.)
  • Distance: 405 light years
  • Spectral Type: B2
  • Position: 06h:59m, -28°:59′:18″

Coloring the stars – an exercise for all seasons

Trying to identify the true colors of stars, as we see them, is fun, challenging, and instructive.

Your assignment, should you choose to accept it, is to develop accurate color swatches that represent the colors of the bright stars, including our Sun, as they actually are seen and in so doing learn:

  • How to see color in the stars
  • What the color tells us about each star

The chart shows the Winter Hexagon because many of the brightest stars can be seen there all at one time, but it also includes swatches for several bright stars that are prominent in the spring, summer, and fall.

This image of the Winter Hexagon was taken by Jimmy Westlake looking at the skies over Stagecoach, Colorado. Look carefully and you can see color in some of the stars - especially if you click on the image to see the larger version. (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Star colors are real. They relate to a star’s temperature and from them we can surmise much more about a star. But they also are very subtle. I think of them not as colors, but as tints. I see stars essentially as white lights to which a little color has been added to tint it one way or another. I believe most people don’t see the colors at all when they first look at the stars and this can be frustrating, especially if you’ve read that Betelgeuse, for example, is an “orange” star.

With the naked eye you only will see color on the brightest stars because our eye simply needs a lot of light to detect color. In fact, point your binoculars at those bright stars, and you should find it easier to detect the colors because the binoculars gather more light. You can train yourself to see star colors, though people do differ in this ability. But for most, the colors are really quite obvious on some of the brighter stars, once you know what you can expect to see. And that’s what this little exercise is for – learning what you can expect to see.

Your main tools will be the chart and the color table provided here. You’ll have to provide the primary colors (red, yellow, and blue), plus white in some easily blendable medium, such as common water, tempera, or poster paints. Nothing fancy needed and no special painting skills required.

First, here’s the chart you will be coloring.

You can download a version for printing by clicking here.

And here’s the color table you will use as your guide

Your task is simple.

Next to each star on the star chart is its spectral classification. This consists of a letter and number. The letters go from blue to red stars in this order: OBAFGKM. Each letter gets divided into a numerical sub-classification from 0-9. So a “B0” star would be just at the beginning of the “B” category. A “B9” star would be at the end of that category and almost into the next one. The star chart shows the spectral classification for each star. Match that with what you see in the color table. Then determine its color.

You will notice that there are two different color scales in the table. That’s because the way we see color depends in part upon the environment in which we see it. The “conventional color” is what would be seen if the star were put under high magnification and projected onto a white sheet of paper in the daylight. The “apparent color” is what is seen by the naked eye in a dark sky. That is the color you want. That’s what you’ll try to duplicate by mixing your water colors and painting the swatch next to each star so it matches its classification – and thus what you are likely to see in the night sky.

The result will be a chart that will help you know what color to expect to see when you look at the stars in the sky. I should add that if you see a photograph of these stars, the colors will be similar, but different. That’s because the colors in a photograph depend on the color sensitivity of the film or computer chip used to record them – which is not the same as your eye. So you cannot use a photograph as an absolute guide to what you will see. The chart you make, if done well, will be a much better guide.

When you look for star color, make sure the star is high in your sky – hopefully at least 30 degrees or more above the horizon. All bright stars near the horizon will appear to flash many brilliant colors. Those colors – like the colors in our sunrises and sunsets – are caused by the Earth’s atmosphere. When you are looking at an object near the horizon, you are looking through much more air than when you are looking at a star high overhead.

Of course, you are going to have to use your judgment in making the color swatches, and you might experiment a bit on another piece of paper. That’s why I recommend using some sort of water color for this activity – so it’s easy to blend and thin your colors to get the color you want – the one that is closest to what you actually see. Of course to get orange you mix the yellow and red – and white will come in handy to lighten any of the colors.

Get the idea? Will your colors be perfect? I doubt it. But experimenting this way will give you a much better feel for how subtle star colors are and exactly what you are looking for when you go out at night. Too often people are confused and disappointed because they read that Betelgeuse or Aldebaran is a red or orange star – and when they read that, they are thinking of the conventional red or orange – quite naturally – but look at the chart and look at the difference between conventional colors and apparent colors.

More about a star’s spectral class

OBAFGKM is certainly a crazy order, I know. It started out to be an alphabetical list more than a century ago. But as they learned more about the stars, the letters got scrambled. Here’s an easy way to remember the order:

Oh Be A Fine Girl/Guy Kiss Me

At first they thought letters would be enough, but the more they learned about stars, the more they saw there were many subtle variations that were important. So for each letter there is a sub-classification system that goes from 0-9. Thus an O9.5 star, such as Mintaka, is in the “O” spectral class (blue) but about as close as one can get to being a “B” (blue white) star. Don’t be too concerned about these numbers, however. You’ll find it difficult enough just to get colors that accurately match the letter classifications. Besides, I’ve found that different sources sometimes give different numbers for the spectral classification of a specific star, so I see them as a good rough guide as to how solidly a star is into a specific class but not something to take overly seriously in terms of what we can detect with our eyes.

Mintaka, incidentally, is included here because bright “O” stars are hard to find. Mintaka was one of the easiest “O” star to identify, being the western-most star in Orion’s Belt. But coincidentally, Ainitak, the star at the other end of the belt, is also an”O” and  a bit brighter. But ay O9.7 it, too, just makes it into the “O” class by the skin of its teeth. In fact, it’s a bit closer to being a “B” star than Mintaka – but I guarantee you won’t see any difference.

“M” stars are even more difficult to find. True, Betelgeuse is one in the Winter Hexagon, and  in the summer we have another brilliant “M” star – Antares, the brightest star in the Scorpion.  But these are special. They both are Supergiants – stars that are going through their death throes and have expanded tremendously.  The vast majority of “M” stars are of average size, and in fact, these average-sized “M” stars are the most common stars in the universe – yet there is not a single “normal” class “M” star visible to our naked eye, let alone as bright as the stars that form the Winter Hexagon.

I also added our Sun to the chart. DO NOT LOOK AT THE SUN TO TRY TO DETERMINE COLOR. YOU WILL DAMAGE YOUR EYES. We were all taught as children to color our Sun yellow – and this is correct if you are talking about conventional color. But the Sun is a class G2 star, and I suggest you color its swatch the “apparent” color it would appear to our naked eye were we seeing it as just another bright star in our night sky. This means it would appear the same as Capella.

Binocular and telescope users can see many double stars, and some of these provide striking color contrast, such as the blue and gold of Albireo. Seeing two stars close together that are of different colors makes it even easier to see star colors but also presents a whole new set of challenges, and experienced observers frequently differ on what the colors of the double stars are. John Nanson has explored this in an excellent post to the “Star Splitters” blog that we co-author. To learn more about these stars and the special challenges of determining their colors, read John’s post here.

What we can surmise from the colors

As you can see from the temperature scale, blue stars are hot – red stars are “cool.” Cool, that is, as far as star temperatures go. They are still very, very hot: 3700 Kelvin is about 6,200 degrees Fahrenheit! (Steel melts at about half that temperature.)

So once you notice a star’s color, what more can it tell you about the star? A detailed answer is beyond this exercise, but it means you can make a very good guess about some other important characteristics of the star.

Here’s a summary in table form of what the spectral classification tells about the size and life expectancy of a star, and even hints at how it will probably die.

  • Ninety-five percent of all stars are on what is called the “main sequence.” Most of the stars that are not on the main sequence are white dwarfs. But a few others are giants or supergiants. Roughly one percent of the stars fall into one of the giant categories, such as Antares.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter.
  • Age – “O” stars have short lives and thus die first, then “B,” etc. No “dwarf” “K” or “M” star has died yet – the universe isn’t old enough.

Look East in February 2011: Two dogs rising in a star-spangled spectacular known as the Winter Hexagon

We have two “dog stars” on the southeastern horizon early on February evenings  – Sirius and Procyon – and  both are part of what is certainly the brightest, star-spangled  section of our northern night sky – the Winter Hexagon. Here’s how it looked to the camera of Jimmy Westlake who took this gorgeous shot of the Winter Hexagon over Stagecoach, Colorado, USA.  You may not see the faint band of the Milky Way shown here if you live in a light polluted region, but you certainly should be able to pick out the bright stars that outline the Hexagon, as well as the Pleiades star cluster visible in the upper right.

Click on image for much larger view! (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Look carefully at that photo, then compare it with this star chart which is what we see from mid-northern latitudes as as we look southeast early on a February evening.

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you'll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

For a printer-friendly version of this chart, click here.

People in the north tend to think that the stars are brighter in winter because the air is so cool and crisp. That certainly could be a factor. But the simple fact is our winter sky is dominated by a whole lot of very bright stars. In fact, there are 22 stars of first magnitude – 16 of which are visible from the northern hemisphere and more than half of these are visible in “prime time” on a February evening. And nearly all these bright stars are jammed into a space taking up less than one-quarter of the February night sky – that’s just one-eighth of the total night sky we can see through the year! Add to that the seven bright stars of the Big Dipper being dragged up the northeastern sky by the Great Bear on a February evening, and it is no wonder that in the dead of a northern winter our skies offer a lively, colorful, star-spangled spectacular.

The Hexagon alone contains seven of the first magnitude stars in our sky and an eighth that is the brightest second magnitude star we see – in other words, that one just misses being first magnitude by a hair. And nearby is Adhara, a star that sits right on the border between second and first magnitude; plus Regulus, another first magnitude star, is rising in the east.

Whew! That’s a lot. Let’s review.  Going  counterclockwise and starting at the bottom, the Hexagon’s corners are marked by:

  • Sirius, the brightest, and at about eight light years one of the closest, stars in our sky – except the Sun, of course.
  • Rigel, the blue giant that marks one of Orion’s feet.
  • Aldebaran, the brilliant orange star that is the eye of Taurus the Bull and dominates the nearest open star cluster, the Hyades.
  • Capella, now high overhead, is really a complex of four stars that we see as one.
  • Castor and Pollux, the twins, one of which (Pollux) is first magnitude, while Castor is the brightest second magnitude star we see.
  • Procyon, the “Little Dog” star, which is dim only in comparison to Sirius, the “Big Dog.”

And . . .

  • Inside the Hexagon is another first magnitude star, Betelgeuse, the red giant that marks Orion’s shoulder, not to mention the three bright stars of Orion’s Belt – all second magnitude.
  • Regulus, the “Little King,” is a first magnitude star that is rising in the east and bringing us the familiar sickle of bright stars that mark the head of the lion. We’ll study it closely next month.
  • Adhara is the western-most star of the distinctive small triangle of stars beneath Sirius. At magnitude 1.5 I call it a first magnitude star, but others consider this second magnitude. So depending on how you count Adhara there are either 21 or 22 first magnitude stars.

Before leaving the Winter Hexagon, I must stress that  this is not simply a northern hemisphere show – if you live  in Sydney, Australia, you could just rename this the “Summer Hexagon.” I see these stars in the southeast – my friends in Sydney see them in the northeast of their sky – and, of course, since they’re “standing on their heads,” they see them a bit differently – something like this!

The "Winter Hexagon" becomes the "Summer Hexagon" in the Southern Hemisphere, but contains all the same bright stars. (Chart prepared from Starry Nights Pro screen shot.)

February Guidepost Stars

Of the stars mentioned so far, the two dog stars, Sirius and Procyon, plus the fence sitter, Adhara, are the guidepost stars to learn this month. They are the ones you can spot near the southeastern horizon, coming into view about 45 minutes to an hour after sunset. (We’ll have more to say about Regulus next month, and the other stars mentioned we’ve met in previous months.) To see the February guidepost stars – and the asterism of the Virgins –  look low in the southeast about 45 minutes to an hour after sunset.  Here’s what you should see.

Click image for larger version. This chart shows the three guidepost stars for February as they appear about an hour after sunset in the southeast. Sirius is the brightest star we see and Procyon is not far behind, but Adhara is not much brighter than its companions, which form a distinctive, small triangle the ancient Arabs knew simply as "the Virgins." (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

Procyon, the seventh brightest star we see, is first up in our sky, and thus the highest, of these three. To the southeast and a tad lower is brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world. Adhara is the brightest star in the  “Virgins,” a simple,  distinctive  triangle asterism.

But, of course, Sirius is dominant – far brighter than any other star we see in our night sky. I always think of Sirius as the eye of the great dog and as he sits, the triangle seems to be his rear haunches. From our perspective Adhara may be just another bright star, but of these three it is really the brightest by far – it’s just much farther away than the other two.  If we compared them side by side we would find that Procyon shines with the light of seven Suns, Sirius 23, and Adhara has a luminosity to the eye of 3,700 Suns! Now that’s bright.  And in another way, Adhara reveals our human bias, for if we had ultraviolet vision Adhara would be the brightest star in our sky, not Sirius.

But again – that not the way we see it. From our perspective Sirius and Procyon are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in, or near, the Southern Hemisphere.) But Adhara? Adhara is 405  light years away – about the same distance as the North Star, Polaris.

Sirius will frequently seem to be changing colors, but that’s just the effect of our atmosphere. Just as our atmosphere makes our Sun look red when it is rising or setting, it makes any bright star near the horizon appear to dance and change colors rapidly.

The Big Dog as Johannes Bayer depicted him in 1603. ((Image courtesy of Linda Hall library of Science, Engineering and Technology.)

The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is known as a “dog star” because it is the brightest star of the classic constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, it’s hard to see how connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers.

Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups”  – the one belong to Procyon – is impossible to see with a backyard telescope and the other an extreme challenge.  The reason is they are quite dim and being very close to the bright stars, get lost in the glare.

But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs, and white dwarfs are a class of stars far denser than anything we encounter on Earth. Now I always find talk of the density of stars counter-intuitive because it gets drilled into our heads that stars are gas and the gas we encounter in our daily lives is anything but dense!  In fact, it’s quite – well – gaseous!  To appreciate this, let’s take a close look at our own Sun.

Click image for larger view.Sirius – with Sirius B at lower left. Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas which reaches densities that near the center are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times denser than the stuff in the Sun. Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. (Think of what that would do to your pool table, not to mention your foot if it fell on it!)  We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is, we end up with a mass roughly that of the Sun but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun. (See why a white dwarf is one million times as dense as the Sun?)

How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups,” the embers are being seen while still white hot. But they will eventually cool.  The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose.

This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy, it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyra.  And at the core of that nebula will be the dying ember we know as a white dwarf.

I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than at other times – and we happen to be in a period of several years when that distance will be growing, and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.) I also plan to take a close look at Adhara with a telescope, for it has a 7.5 magnitude companion just 7 arcseconds away. This should be a challenge – because of the difference in brightness of the two –  but not nearly the challenge that seeing the companion of Sirius.

For those with binoculars and small telescopes, some of the most fascinating objects are in this general area of sky, near, or inside the Winter Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular telescopic open clusters in Gemini and Auriga, M35, M36, M37, and M38. All that star light certainly can make for bright nights during the dark  of a northern winter.

Vital Stats for the Guidepost Stars

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.5,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)

For Adhara:

  • Brilliance: Magnitude 1.5, it has a luminosity to the eye of 3500 times that of the Sun! (In other words, much brighter, really, than Procyon or Sirius.)
  • Distance: 405 light years
  • Spectral Type: B2
  • Position: 06h:59m, -28°:59′:18″

Look East! February 2010 brings two dogs and an impostor!

We have two “dog stars” on the eastern horizon early on February evenings  – and in 2010 an imposter that nearly outshines them. To see all three, look low in the east about 45 minutes to an hour after sunset – they will be the first  objects visible in the twilight. Together the three make a nice line of bright “stars” from due east to southeast. What’s more, each of the “dog stars” has a “pup” we can’t see with our naked eye – a faint companion star orbiting it, which in many ways is more interesting than the stars we do see. The dog stars also complete two handy winter asterisms.  More on all that later.

Click image for larger chart. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart.

The bright impostor that is nearly due east is the planet Mars, which at the start of the month is about as close to Earth as it will get in two years, and so about as bright as it will get in that time.  The middle star of the three is Procyon, seventh brightest star in our night sky. And to the southeast and a tad lower than the other two  is  brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world.

Not only are these two stars very bright, they are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in the southern hemisphere, or the southern part of the northern hemisphere.)

Later – when it is darker and all three (Mars, Procyon, and Sirius) are higher – look for the color contrast between Mars and these two stars. Early in the evening the colors will be confusing because the two stars will twinkle and Sirius, especially, is noted for flashing all sorts of colors. This is simply because it is so bright and it is so low. Any bright star near the horizon is shining through a lot of air, and it is the air that makes it appear to dance and change colors rapidly. Stars are so distant they are point sources of light. Planets are closer and their light comes from a disc, too small to detect with the naked eye, but still making them tend to shine more steadily.

The Big Dog as Johannes Bayer depicted him in 1603. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is the best known of the two “dog stars,” but it actually rises a little later than Procyon, for those in northern latitudes.  Sirius is known as a “dog star” because it is the brightest star of the constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, you can see that no amount of connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers.

Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups” is impossible to see with a backyard telescope and the other an extreme challenge. The reason is they are quite dim and being very close to the bright stars, get lost in the glare.

But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs and white dwarfs, a class of stars far denser than anything we encounter on Earth.  In fact, to appreciate this, let’s take a close look at our own Sun.

Sirius - with Sirius B at lower left. Click image for larger view. Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas, but even that idea is hard to grasp because we think of gas as something light and wispy, yet gas in the Sun reaches densities that are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times as dense as the stuff in the Sun. Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is we end up with a mass roughly that  of the Sun, but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun.

How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups” the embers are being seen while still white hot. But they will eventually cool.  The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose.

This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyre.  And at the core of that nebula will be the dying ember we know as a white dwarf.

I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than others – and we happen to be in a period of several years when that distance will be growing and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.)

Greater Asterisms

Sirius and Procyon join with Betelgeuse to form the “Winter Triangle,” an asterism of three bright stars that appears in the southeast just as the Summer Triangle stars, Vega, Deneb, and Altair, are bowing off stage to the northwest. I have to admit, though, I’ve never paid any attention to this. If you find it useful, great. If not. . . . well, consider the Winter Hexagon.

The Winter Hexagon is an asterism I love, but to see it requires that you have been learning the guidepost stars for the past few months. If this is your first month on the job, wait until next year. But if you are familiar with these stars from past months, note what a wonderful, huge Hexagon they create, encompassing  a part of the sky that is just afire with bright stars. The Hexagon stars are: Sirius, Rigel, Aldebaron, Capella, Castor/Pollux, and Procyon. Yes, it takes seven stars to make up this six-sided figure because I choose to fudge it a bit and count Castor and Pollux as one point.  (Others just use Pollux, but I have trouble separating these twins.)  The star chart for the Winter Hexagon and Winter Triangle looks like this.

Click image for larger view.

Click here to download a black-on-white (printer-friendly) version of this chart.

Notice that we not only have seven bright stars anchoring this asterism, but there are at least half a dozen bright stars inside it.  I think this large concentration of bright stars is one of the reasons why we think of winter nights as clearer than those of summer.  Truth is, summer nights can be just as clear, but they don’t contain such a dominant concentration of bright stars. For those with binoculars and small telescopes, some of the most fascinating objects are near, or inside this Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular open clusters in Gemini and Auriga.

Vital stats

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.46,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)
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