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Failed Stars

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Ken Tapping, May 20, 2009

In the sky this week...

> Saturn dominates the southern sky during the evening.

> Jupiter rises about 3 a.m., Venus and Mars about 4 a.m.

> The Moon will be New on May 24.

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Stars are formed when cosmic gas clouds, which are composed mainly of hydrogen gas, collapse. During the collapse, denser lumps form in the cloud. Under the impact and pressure of the in-falling material, each growing lump gets hotter and hotter, and increasingly compressed. If the temperature in the core of the lump reaches about 20 million degrees Celsius, and the pressure gets high enough, nuclear fusion starts. Hydrogen is transformed into helium plus energy. The new star starts to shine, and evaporates the rest of its birth cloud so that we get to see it. Thanks to the Hubble Space Telescope and ground-based instruments such as the Canada France Hawaii Telescope, and the two Gemini Telescopes, we have seen stars at all stages in the birth process.

However, when a cloud starts to collapse, we have no guarantee that there is enough material in the cloud to make a star. Sometimes there is plenty, and we get a star that has grabbed ten or more times the amount of material our Sun was born with. Then we get a very bright star that burns for less than a billion years before running out of fuel and blowing itself up. On the other hand, a star starting with maybe 10% or less of the mass of the Sun becomes a dim, red dwarf star, niggardly in its use of fuel and possibly lasting as long as our universe. On the basis that, on average, one gets more small things than big things, we would expect the dim stars with small masses to be more common than "superstars" with maybe a hundred times the mass of the Sun. On that basis, we would expect low-mass stars to be very common.

If that newly-forming star does not collect more than about 75 times the amount of material in Jupiter, the largest planet in our Solar System, the temperature and pressure in the middle will never get high enough for nuclear fusion to start. It will not start shining and has failed to graduate as a star. It will still be a hot ball of gas from the energy released in its formation, but there will be no energy source in the middle. It will shrink a bit and slowly cool off as it radiates its heat into space. Intuitively we would assume there must be a lot of these "Failed Stars" out there; however, they can be hard to observe. They shine only dimly, and are still surrounded by the remains of their birth clouds. With a surface temperature of only about 1000 Celsius, they would shine dull red, and would really be best visible in telescopes sensitive to infrared telescopes. Astronomers refer to these faint, failed stars as "brown dwarfs".

Thanks to modern telescopes, we are finding these strange failed stars. It looks as though there are at least as many of them as there are normal stars. Logic would suggest that if brown dwarfs are this common, even smaller lumps, not even graduating as brown dwarfs, would be even more common. We've known about these smaller lumps for a long time. We live on one of them; they're called planets. Now we know that planets are very common; many stars have them. It's just that, as in the case of brown dwarfs, we only recently developed the means to detect them.


Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca