Tuesday, January 15, 2008

Taking a census of the neighborhood

Image Credit: NASA/CXC/M. Weiss

Today, I continue to talk about science presented at the American Astronomical Society meeting, held last week here in Austin, TX. Yesterday I talked about goings-on in our own Solar System. Today, we step back a bit and look at our corner of the Milky Way Galaxy. Several different people presented results of ongoing attempts to identify our Sun's closest neighbors.

It shocks a lot of people to realize that we don't know if we've found all of the sun's closest neighbors yet. But the problem is that most of the stars in our galaxy are not bright, but very faint. Our sun's closest neighbor, Proxima Centauri, is so faint, we can't see it without a telescope. Proxima Centauri has a companion, Alpha Centauri A, that is nearly a twin of our sun; Alpha Centauri A is the fifth brightest star in our sky (after the Sun, Sirius, Canopus, and Arcturus). Sirius is relatively nearby, and is also very bright. But most of the Sun's neighbors are like Proxima Centauri -- feebly glowing red dwarf stars. So, it is not surprising that many of these may have escaped our notice even to this day.

Fainter than the faintest stars, brown dwarfs are often considered "failed stars" (an artist's rendition comparing brown dwarfs to the sun and Jupiter is above). They are too small to sustain nuclear fusion, which powers all true stars, and so although they start off quite hot, they cool off and slowly fade away, like the embers in a dying fire. Because of this, brown dwarfs close to the sun may be too cool to see at all in visible light; many brown dwarfs only glow in infrared light. We are just starting to explore the infrared sky, and so new brown dwarfs are being discovered all the time, and most are within just a few dozen light-years of the Earth. But there could be very cool brown dwarfs closer than Proxima Centauri that we just haven't found yet.

In short, the results of these surveys is that they are finding new neighbors, but not tons of them. These studies are important, because the relative numbers of different types of stars in our neighborhood, where we think we've counted most or all of the stars, is used in studying clusters of stars and other galaxies, where we have no hope of seeing the faintest stars and so have to guess how many stars are there.

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