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à=Ð/Ð8šThe Nearby Stars: A Tour of Our Neighborhood
When we look up into the night sky, it is easy to imagine, as did the ancient Greeks, that all the stars are the same distance, stuck to some sort of Celestial Sphere that rotates through the heavens as the Earth stands still. This conclusion would be very wrong, however. It has taken astronomers thousands of years to realize that stars are different distances from us and that the scale of our universe is truly enormous.
Stars that lie in the same constellation only appear to be near each other from Earth’s perspective. If we were to move even a few light years away from Earth, we would see a change in the allignment of the constellations as nearby stars moved to the side faster than the farther background stars. A carefull analysis of the stars shows that there are 28 stars within 12.5 light years of Earth, scattered more or less randomly through the sky. Most of these (17) are small, dim red dwarfs which are only visible through powerful telescopes even though they are nearby on the cosmic scale. The larger, hotter stars are increasingly rare. Our sun is one of the larger stars in the immediate neighborhood, and there is good evidence that this holds true for other areas of our galaxy as well. It seems that dim stars in the universe are like rabbits and bureaucrats: They may not be very bright, but there are a lot of them.
The following table shows the names, distances, stellar class, right ascension (longitude), and declination (latitude) of the 28 stars within 12.5 light years. We will look at the stars within ten light years in turn.
Alpha Centauri
Our nearest neighbor is actually a trinary (triple) star system consisting of two medium size stars orbiting closely around each other and a third, distant red dwarf orbiting the middle two. This red dwarf, called Proxima Centauri, is actually our nearest neighbor currently. It has a distance of 4.27 light years from us and is over 300 times as far from the central pair as Pluto is from our sun. It is also called Alpha Centauri C.
Alpha Centauri A is very similar to our sun, a yellow G2 class star. Some astronomers feel its similarity could be due to both it and our sun having formed at the same time from the same nebula of dust and gas. This is something that may never be proven, however. Alpha Centauri B is an orange K2 star, smaller and dimmer than A. They orbit about each other at roughly the distance of Uranus from our sun with a period of 80 years. This close orbit of two massive objects would make stable orbits for planets unlikely. The Alpha Centauri system would therefore be a poor candidate for a life-bearing planet.
Barnard’s Star
This is a very modest little red dwarf in the constellation of Ophiucus. In 1916, Edward Barnard discovered that it had the largest proper motion of any star in our neighborhood. In about 10,000 years it will actually pass us closer than Alpha Centauri is now. It appears to move about 0.5 degrees (the diameter of the Moon as seen from Earth) every 175 years.
Careful observations of this movement over many years reveal a slight wobble in Barnard’s Star’s motion, which can only be explained as the gravitational attraction of an unseen companion, probably a gas giant planet several times larger than Jupiter. We cannot see this planet directly because the star is too bright to make out the much dimmer planet, but the tale-tell tug on the star proves it must be there. Peter van de Kamp has found evidence that there may actually be two planets orbiting around Barnard’s Star. They would both be too far from the star to be in a habitable zone. This does not mean, however, that smaller planets the size of Earth don’t exist in the same system which are two small to make a detectable additional wobble in the star’s motion.
Wolf 359
This red dwarf in Leo is the dimmest of all the stars in our neighborhood. If the sun were replaced with Wolf 359, it would only be ten times brighter than a full moon on Earth and it would be difficult to make out its spherical shape without a telescope.
For Star Trek officianadoes, this is the site where the Federation lost 40 ships in an attack by the Borg.
BD +36 degrees 2147
Another unremarkable red dwarf in the constellation of the Big Dipper. It is suspected to have at least one planet because of the wobble effect (see Barnard’s Star). It is also known in some charts as Lalande 21185. The BD in its name comes from “Bonner Durchmusterung”, the name of a 19th century star catalog which first listed it.
Luyten 726-8
This binary red dwarf system in Cetus is also known as UV Ceti because one member is an ultraviolet flare star. Many stars make solar flares, including our sun, but they are usually lost in the glare of the main star. UV Ceti is so dim that the flare stands out brighter than the red star it comes from, which creates a temporary increase in its apparent magnitude.
The two stars have an orbital period of 25 years and are about 6 AU apart. They have about equal mass, totaling only 30 % of our sun.
Sirius
On clear winter nights as you look up toward the constellation of Orion the Hunter you will notice a brilliant white star below and to the left. This star is the brightest star in our sky, called Sirius. It is also the “Dog Star” because it forms the eye of the Great Dog, Canis Majoris. Sirius is bright for two reasons: first, because it is has a spectral class of A1, it is intrinsically bright. Its surface temperature is about 10,000 K. If Sirius were to somehow trade places with our sun, our days would be over 20 times as bright as they are now. Sirius is also bright because it is relatively close to us; only 8.6 light years away.
Carefull photographic analysis of Sirius detected a slight wobble in its proper motion (path through the sky). In 1834, F. W. Bessel noticed that it appeared to be swinging back and forth over about two arc seconds with a period of 50.09 years. Bessell became convinced that Sirius must actually be a double star system, with the wobble being caused by the gravitational attraction of an unseen companion. In 1862, this companion was seen with a new 18-inch telescope by Clark. It is known as Sirius B, or “the Pup.” The two stars of this binary system orbit around each other at a distance of about 20 AU.
Sirius A is about 2.2 times as big as our sun and is the largest star in our neighborhood. Sirius B has almost the same mass as our sun (.94 times) but is 10,000 times fainter than Sirius A. This could only be explained if Sirius B were about the size of the earth. A quart bottle full of this star would weigh more than a jumbo jet airliner. This incredible density was debated by astronomers for many years until it was finally verified by Eddington in 1924 by measuring a predicted red shift of the light coming off the star. Called a white dwarf, it was finally explained in 1940 by Chandrasekhar as the leftover corpse of an old, medium size star after it has thrown most of its material into space as a planetary nebula.
Ross 154
Another red dwarf, this star lies in Sagittarius on a line toward the center of our galaxy. Because that region of the sky is so packed with stars, it is difficult to identify this star even with powerful telescopes.
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