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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! 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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