© 1986, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.
by Sherwood Harrington, Astronomical Society of the Pacific
Sirius looks so bright in part because it is a relatively powerful light producer; if our Sun were suddenly replaced by Sirius, our daylight on Earth would be more than 20 times as bright as it is now!
But the other reason Sirius is so brilliant in our nighttime sky is that it is so close; Sirius is the nearest neighbor star to the Sun that can be seen with the unaided eye from the Northern Hemisphere.
"Close'' in the interstellar realm, though, is a very relative term. If you were to model the Sun as a basketball, then our planet Earth would be about the size of an apple seed 30 yards away from it — and even the nearest other star (alpha Centauri, visible from the Southern Hemisphere) would be 6,000 miles away.
Distances among the stars are so large that it is helpful to express them using the light-year — the distance light travels in one year — as a measuring unit. In this way of expressing distances, alpha Centauri is about four light-years away, and Sirius is about eight and a half light- years distant. (Thus if your students look at Sirius this evening, then they will be seeing light that left the star in 1977!)
The table shows the distances and brightnesses of the dozen stars (distributed among eight star systems) that are known to be within 10 light-years of us. But let's first get to know each of these stars a little better. (An explanation of some of the unusual names follows the table.)
By the way, don't reproach yourself if many of our stellar neighbors are not familiar to you. Only Sirius and two others are bright enough to be seen without a telescope. Our Sun isn't really a very "average, garden-variety'' star at all! Of the 12 nearest stars, only two are brighter, and most of the others are much dimmer. This preponderance of dim stars isn't restricted to our immediate neighborhood. It seems that dim stars in the universe at large are like rabbits and bureaucrats: They may not be very bright, but there are a lot of them.
The two central stars take 80 years to orbit around each other, and they are about 20 times as far from each other as Earth is from the Sun. (This is about the same as the distance from the Sun to the planet Uranus.)
The third star, alpha Centauri C, is sometimes called Proxima Centauri because it is the closest to us of the three. It is a dim, red (cool), small star, and is very far from the central pair; at least 300 times as far as the most distant planet in our solar system is from the Sun! This is still a very small fraction of the average distance between star systems, though, and Proxima is certainly caught in a long and slow orbit around its brighter two siblings. If our Sun had a companion just like alpha Centauri C that far away, it would look like a very ordinary starlike point of light in the night sky, visible to the unaided eye, but dimmer than hundreds of other stars.
While the constellations appear fixed and unchanging over a human lifetime or two, the stars do slowly change their positions over the centuries as the Sun and the other stars move through space at various speeds and in different directions. For most stars, this change in position on the sky is very slow, indeed — the constellations of 10,000 years ago were only slightly different from what they look like now. But stars that are quite nearby can change their positions relatively rapidly, just as an automobile on a street right next to you zips past you quickly, but cars on a distant highway seem to crawl along.
Barnard's Star moves across the sky at a rate of about half a degree (the size of the Moon's diameter) every 175 years. As it moves across the sky, it also is getting closer to us. Calculations indicate that it will pass by us at a distance of 3 3/4 light-years (closer than alpha Centauri) — in about A.D. 11,800!
As the star moves, it doesn't seem to follow a perfectly straight line. Careful observations over several decades by Peter van de Kamp and his colleagues at Sproul Observatory in Pennsylvania indicate that it may be "wobbling'' slightly around a straight-line path. It is possible that this wobble comes from the star's being tugged this way and that by the gravity of one or more large planets orbiting around it. (Currently, planets around other stars can't be seen through telescopes from Earth if those planets are like the ones in our solar system. Not producing light by themselves, they would be very dim and so close to their stars that their feeble, reflected light would be swamped in glare.) Astronomers have found very faint companion stars using the "wobble'' method, but no planets have been confirmed as yet. The work is continuing at several observatories.
Luyten 726-8 A and B are about six times as far apart as Earth is from the Sun, and they take about 25 years to orbit around one another. Each of these little stars is among the lightest weight known: Together their total mass, which they probably share about equally, is only about 30 percent of the Sun's. (Luyten 726-8 is in the constellation of Cetus, the whale.)
Sirius A, electric blue-white and roughly twice as wide in diameter as our Sun. is by far the brightest. hottest (nearly 10,000 degrees Celsius at the surface), and heaviest (about 2.2 times the Sun's mass) star close to the Sun. (The closest star which is even hotter and more luminous than Sirius is Vega, about 25 light-years away from us.)
Sirius B is our nearest example of a white dwarf star: an ultra dense, collapsed core of a star which long ago ran out of fuel to keep its energy-producing nuclear reactions going. While it is only about the size of the Earth — about a million times smaller than the volume the Sun takes up — it weighs fully 94 percent as much as our star! Its material is so compressed that a quart bottle full of its material would have about as much mass as a jumbo-jet airliner. The force exerted by its gravity would literally be crushing; if we could somehow stand on its surface, a 100-pound student would weigh something like 10,000 tons. (The next-nearest white dwarf star orbits around the bright star Procyon, about 11 1/2 light-years away from us.)
The two stars are both blue-white, but radically different in all other respects. They orbit around one another in about 50 years with an average distance between them of roughly 20 times the Earth-Sun distance.
|Name||Distance (light-years)||Apparent Brightness1||Luminosity2|
|Alpha Centauri A||4.3||0.26||1.56|
|Alpha Centauri B||4.3||0.077||0.45|
|Alpha Centauri C||4.2||0.00001||0.00006|
|BD +36 degrees 2147||8.2||0.0003||0.006|
|Luyten 726-8 A||8.4||0.000003||0.00006|
|Luyten 726-8 B||8.4||0.000002||0.00004|
Table adapted from one compiled by Alan H. Batten in The Observer's Handbook 1986, edited by Roy L. Bishop, Royal Astronomical Society of Canada.)
A designation which starts with a Greek letter or a number is based on the star's brightness rank within its constellation. For example, alpha Centauri is the brightest star in the constellation Centaurus. (Alpha is the first letter in the Greek alphabet.) "BD'' stands for "Bonner Durchmusterung,'' a 19th-century star catalog. The other names refer to stars found in individual surveys of the sky.
If a star is found to be a multiple star — that is, two or more stars orbiting around one another — then 'A', 'B', and so on are added to its name to identify its individual members. It's interesting to note that multiple stars are not at all rare; in fact, more than half of the individual stars in our table are members of three multiple-star systems!
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