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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 North: May is the month Polaris ( the North Star) gets two bright flankers!

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

Click here to download a printer-friendly version of this chart.

Is the North Star – Polaris – our brightest star? No! And it certainly won’t look that way this month as it shares the northern sky with two very bright stars. But, read on. Polaris is not nearly as dim as it looks!

If you have been learning your guidepost stars as they rise in the East, you won’t be surprised by the two bright stars which flank – and outshine – our pole star in May. To the northwest is Capella, a star we first met when it rose in the northeast in November. In May the northeast is dominated by a star that is almost Capella’s twin in brightness, Vega, a guidepost star we introduce in May. (See “Look East!” for more about Vega.) As a bonus we also have the twin guidepost stars, Castor and Pollux, making their way into the northern sky high above Capella. But let’s focus on Capella and Vega.

New star watchers frequently assume the North Star, Polaris, will be the brightest star in the sky. It isn’t even close! It is bright, but its fame comes because it’s very, very close to where the axis of the Earth points to the north celestial pole. So it serves anyone trying to find true north as a very good guide. But when it comes to brightness, it’s in the same league as the stars in the Big Dipper. Quite bright, but it can’t hold a candle to Capella and Vega. When you look at a list of the brightest stars, Vega is number 5 and Capella number 6. Polaris, our North Star, is number 48!

As simple as one, two, three!

That doesn’t mean Polaris is a slouch, though. First, in the eastern sky in May you meet Spica. (That’s on our chart for the east.) One distinction of Spica is that it’s as close to being magnitude 1 as any star gets. A distinction of Polaris is, as Spica defines magnitude 1, Polaris defines magnitude 2. (To be precise it’s magnitude 2.02.) Vega and Capella are extremely close to magnitude 0. Vega is 0.03 and Capella 0.08. Good luck on telling the difference! This month, if you look north 90 minutes after sunset, you may think Capella is a bit brighter actually – but if it appears that way it will be because it’s a bit higher in the sky and thus is not dimmed by having to fight its way through as much of our atmosphere as Vega is doing at the moment. So don’t try to split hairs. And yes, you’re right – they are NOT really as “simple as one, two, three” – on the magnitude scale they are as simple as zero, one, two – but that doesn’t sound as good! (Vega and Capella are zero; Spica is magnitude one, and Polaris, magnitude two.)

So which is really the brightest star of these four? Are you ready for this? Polaris! That’s right – if you put all four stars at the same distance, Polaris would appear to be the brightest. Remember, that the lower the magnitude number, the brighter the star. In absolute magnitude – the brightness we give to a star if they are all shining fromt he same distance  -these four stars line up this way:

  • Polaris -3.4
  • Spica -3.2
  • Capella 0.1
  • Vega 0.3

And those absolute magnitudes also reflect their order in distance from us.

  • Polaris 433 light years
  • Spica 250 light years
  • Capella 45 light years
  • Vega 25 light years

So sometimes a star is very bright because it’s – well, very bright. But sometimes it only appears to be very bright because it is very close to us. If you put our closest star into this group, our Sun – remember, it is just 8 light minutes from us – in absolute magnitude it would be by far the dimmest of this group – absolute magnitude 4.9! So while Polaris doesn’t look all that bright, it really is a very bright star! Another way to think about this is if you move our Sun out to where Polaris is, it would be about magnitude 10! You would need binoculars or a telescope to see it!

Click image for larger view of this chart. Yellow circle represents typical field of view for low power binoculars, such as 10X50.

To get an idea of the difference between Polaris and our Sun, point your binoculars towards Polaris.  You should be able to make out the “Engagement Ring” asterism – granted, a crude ring with Polaris as the diamond.  This asterism points you towards the true north celestial pole  – just avery short distance to the other side of Polaris –  and also gives you a good idea of about how far Polaris is from that pole.  Small binoculars will not show you the companion of Polaris, but to get an idea of how bright our Sun would be at the same distance, look for the star labelled 9.8 – and if you can’t see it, see if you can see the star that’s a bit brighter labelled “9.”  Don’t expect to see these instantly. Sit calmly, relax, and keep looking for at least a minute.

And here’s one more cool secret about Polaris. It has a companion that just happens to be quite dim – magnitude 9. It’s fun to see the two of them if you have a small telescope, though it’s not all that easy because Polaris is so much brighter than its companion. But if you get a chance to see Polaris and its companion in a telescope, remind yourself that the very faint companion is still a bit brighter than our Sun would look at this distance. This companion, known as Polaris B, was discovered in 1780 by William Herschel, and for many years Polaris was thought to be a binary star – that is, a system of two stars orbiting about a common center of gravity. But Polaris was holding one more surprise – it’s really a triple star.

The top image shows Polaris and its faint companion that can be seen in any decent backyard telescope. The bottom image shows the second companion, Polaris Ab, which has only been seen by using the Hubble Space Telescope.

This has been known for some time, but no one could see the third star until they turned the Hubble Space telescope on it in 2006. That’s when NASA released the first image of this third companion. The accompanying press release explained it this way:

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings  in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arc second from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light years, that translates into a separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

So as I said, Polaris is no slouch. It not only is a very bright star, but it also has two companions, and scientists are still studying it because it is unusual in other respects. We’ll talk about those other differences another month.

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″
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