The Sciences

Dim, faint, and small is no way to go through life, son

Bad AstronomyBy Phil PlaitDec 12, 2008 4:00 PM


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Here's an unusual record broken: astronomers have found the dimmest stars ever!

Adam Burgasser (and old friend and colleague, married to an old friend too; hi Gen!) led a team at MIT that found the dim bulbs. They are brown dwarfs, objects with too much mass to be considered planets but too low mass to maintain hydrogen fusion in their cores like normal stars. Some people call them "failed stars", but I don't like that. Maybe they're just overachieving planets. Adam's team observed the brown dwarf 2MASS J09393548-2448279 (which they call 2M0939 for short), which lies about 17 light years away. They used Spitzer Space Telescope to get spectra of the star, which allowed them to get its temperature as well as its chemical composition. They found a temperature of about 600K (330 C or 620 F), making this the coolest brown dwarf ever discovered! However, they had a mystery on their hands. They can measure how bright the brown dwarf is, and found that it was almost exactly twice as bright as its temperature indicates. What gives? The answer was obvious enough: 2M0939 is actually a binary star, two brown dwarfs in a tight orbit around each other. They are so close together they appear as one star. This means that each star has a luminosity -- that is, total energy emitted from the surface -- of only about one-millionth the luminosity of the Sun. So not only are they the coolest self-luminous objects ever seen, they are also the dimmest. They each have a mass of about 0.01-0.05 times that of the Sun, or about 10-50 times that of Jupiter. That means they are too low mass to have sustained fusion of hydrogen into helium in their cores, so they aren't proper stars. Most likely they were able to fuse deuterium when they were young and hotter, but that has long since shut down. Looking at the spectra Adam got makes me smile (the journal paper with the spectra is posted as a PDF on Adam's site). There are clear indications of ammonia, methane, and even water vapor (steam!) in the atmospheres of these two lightweight objects. The Sun, for example, is far too hot to have those molecules, and you usually see that sort of thing in planetary atmospheres! That's why brown dwarfs are so weird. They have characteristics of both planets and stars. You have to be careful when studying them, because their borderline stance between these two classifications can be a little confusing. I studied brown dwarfs for a couple of years, and never got tired of how simply weird they are. The pair making up 2M0939 (taken together) are the fifth closest brown dwarfs known, yet they are so dim you need a big telescope to see them at all. If you put them in place of the Sun in the solar system, from the Earth they would appear far dimmer than the full Moon in the sky! And they would look, oddly enough, magenta to the eye because of their peculiar spectral properties; they don't put out much light at all in the visible part of the spectrum. But you probably wouldn't enjoy the view much, because at that distance from the 2M0939, we'd be frozen solid. Oh well. So my congratulations to Adam and his team. Of course, records were meant to be broken... but then again, I'd put money on Adam being the one to break his own record. Because they are so dim, brown dwarfs like this are extremely tough to find, even though they may be among the most common objects in the Universe. But our telescopes get better, and we have smart, dedicated people like Adam and his team out there looking. Who knows if we'll find one even closer? Or, in this case, find two.

Artist's conception of 2M0939. Credit NASA/JPL-Caltech

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