What is larger than planets, but smaller than stars? If you guessed brown dwarfs, you are correct. A brown dwarf is a failed star. They exist in a cold, dark world all on their own, wandering through space alone. Some of them make their homes in the shadow of a great star. Many astronomers thought brown dwarfs were very rare, but new sky surveys show that brown dwarfs might be as common as stars even though only a few brown dwarfs have been discovered since the first one was found in 1995.
Difference between Stars and Planets
Stars shine by their own light, while planets shine by reflecting the light from stars. A star forms when a cloud of gas shrinks under the force of gravity and heats up. When the temperature rises high enough for nuclear fusion to take place in the center, this produces energy, and the star begins to shine.
A different process forms planets. The small particles of dust left over by the formation of stars collide and stick together forming larger clumps. The clumps collide to form even larger clumps, and the process continues until most of the dust is used up. This produces a relatively cold body that is neither hot nor heavy enough to produce its own light.
What are brown dwarfs?
Brown dwarfs are thought to be failed stars that did not have enough gas available when they were formed. Since they did not get enough gas, they were not able to increase their mass and temperature enough to produce hydrogen fusion. When this happens, a brown dwarf is formed.
How were brown dwarfs named since they are not even brown? Brown dwarfs are actually red, but the name red dwarfs was already taken. Red dwarfs describe stars that are less than half of the Sunís mass. Before the name brown dwarfs was selected in 1975, these objects were called black stars or infrared red stars.
Looking For Brown Dwarfs
The search for brown dwarfs is long and difficult because they are very dim in the night sky. Also it is difficult to recognize a brown dwarf by just looking at it. We need to know its history and how it formed. This takes time to research so the discoveries of brown dwarfs have been extremely slow. Astronomers have recently found that a good place to look for brown dwarfs is close to known stars. More than half of the stars in our galaxy are found in pairs, 2 stars orbiting their common center of gravity. Researchers therefore felt that many stars that seem alone might actually have a brown dwarf as a companion. Using this fact, astronomers do not have to look at large sections of the sky for brown dwarfs. They can focus their telescopes on small areas near known stars. Even though astronomers began searching for brown dwarfs in the mid 1980ís, it was not until 1995 that they were discovered.
Why are brown dwarfs important?
One of the most important problems in astrophysics is the so called "Missing Mass". Scientists have proved that we have only been able to identify 10 percent of the mass of the universe. So where is the other 90 percent? One theory is that it is in brown dwarfs. Scientists estimate there may be about 100 billion brown dwarfs. Finding out more about these brown dwarfs and where these objects come from will help us to better understand our universe.
Unless otherwise noted, all images courtesy of NASA. Permission for use at http://www.nasa.gov/gallery/photo/guideline.html.
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