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The Long Arm of the Low

Week of April 27, 1998

I am always amazed at what a web astronomy is. Any bit of research, no matter how small, always seems to have some impact on every other aspect of the science. For example, I have recently become interested in very low mass stars; stars with less than 15% or so the mass of the Sun. These stars are interesting for many reasons, but one of the most important is that there appear to be a lot of these tiny stars. When making stars, nature likes the little ones, while making very few big ones. It's like hitting a pane of glass with a hammer; you get one or two big pieces, a few more smaller pieces, and zillions of shards.

This artwork was painted by my old friend Dan Durda, an astronomer who is also a gifted artist. The painting is of an extremely low mass star called Gliese 229B or Gl229B for short. Gl229B is what we call a brown dwarf; a star with such a low mass that it cannot even fuse hydrogen into helium in its core. Gl229B orbits a more massive, but still very low mass star, called (you guessed it!) Gl229A. That's the red star in the background. In this image, Dr. Durda has added a fictional planet circling the brown dwarf. Dr. Durda also has quite a few more space art works on his web page. You can even buy copies for yourself if you like them!

When a gas cloud collapses to form stars, it is very rare to get a powerhouse like Rigel (in the constellation of Orion), which has more than 20 times the Sun's mass and puts out tens of thousands of times the Sun's energy every second. Stars like the Sun are pretty common, but stars with about 15% of the Sun's mass are everywhere! They are so faint, though, that they are hard to see. The problem is that the luminosity of the star (that is, how much energy it gives off) depends on the mass. Rigel is more than 20 times the Sun's mass and can be seen with the naked eye even though it is tremendously far away. The Sun is a modest star compared to Rigel, and would be invisible to the naked eye at a distance of only 100 light years (the Galaxy, to compare, is about 100,000 light years across). The nearest star to the Sun is Proxima Centauri, a feeble red dwarf. It is only 4.3 light years away, yet so faint that it can only be seen through a telescope!

So why are low mass stars the most common? No one knows. Worse, there appears to be a cutoff; at some point, suddenly it appears to become hard to make stars below a certain mass. This is a recent discovery, and no one understands it either.

So what is the impact on the rest of astronomy? These stars are so faint the are almost undetectable even when they are close by, yet so common that the Galaxy might be filled with them. There are reasons to believe that that we can only see about 10% of the mass of the Galaxy. For every ton of luminous matter out there (stars, gas, hot dust, whatever) there are nine more tons of stuff we cannot see. This has profound implications for how the Galaxy behaves (like how fast it rotates, how hard it pulls gravitationally on other galaxies, etc.) and also on the Universe itself. We do not fully understand the shape of the Universe, but we do know it depends on the mass in it. These small, nearly invisible stars have a long reaching arm that may indeed shape the fate of everything we know.

©2008 Phil Plait. All Rights Reserved.

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