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  • Rapt in Awe

    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″

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! 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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