Brown Dwarfs

Brown dwarfs are objects which are too large to be called planets and too small to be stars. They have masses that range between twice the mass of Jupiter and the lower mass limit for nuclear reactions (0.08 times the mass of our sun). Brown dwarfs are thought to form in the same way that stars do - from a collapsing cloud of gas and dust. However, as the cloud collapses, it does not form an object which is dense enough at its core to trigger nuclear fusion. The conversion of hydrogen into helium by nuclear fusion is what fuels a star and causes it to shine. Brown dwarfs were only a theoretical concept until they were first discovered in 1995. It is now thought that there might be as many brown dwarfs as there are stars.

Artist's rendition of a brown dwarf

Philip Lucas (Univ. Hertfordshire) and Patrick Roche (Univ. Oxford), UKIRT
Brown dwarfs are very dim and cool compared with stars. The best hope for finding brown dwarfs is in using infrared telescopes, which can detect the heat from these objects even though they are too cool to radiate visible light. Many brown dwarfs have also been discovered embedded in large clouds of gas and dust. Since infrared radiation can penetrate through the dusty regions of space, brown dwarfs can be discovered by infrared telescopes, even deep within thick clouds. Recently, 2MASS (Two Micron All Sky Survey) data revealed the coolest known brown dwarf. To the left is an infrared image of the Trapezium star cluster in the Orion Nebula. This image was part of a survey done at the United Kingdom Infrared Telescope ( UKIRT) in which over 100 brown dwarf candidates were identified in the infrared.

The discovery of objects like brown dwarfs will also give astronomers a better idea about the fate of our universe. The motion of the stars and galaxies are influenced by material which has not yet been detected. Much of this invisible dark matter, which astronomers call "missing mass", could be made up of brown-dwarfs. Our universe is currently expanding, due to the Big Bang. If there is enough mass, it is thought that the expansion of the universe will eventually slow down and then the universe will start collapsing. This scenario could mean that the universe goes through an endless cycle of expansions and contractions, with a new Big Bang occurring every time the universe ends its collapse. If there is not enough mass for the universe to collapse, then it will expand forever. We will only know the fate of the universe when we can accurately estimate how much mass the universe has in it. The detection missing mass objects, such as brown dwarfs will likely be a key to answering this question.

Artist's rendition by Robert Hurt, IPAC

Brown Dwarfs were only a theoretical concept when the Spitzer Space Telescope was first proposed. Since the mid-1990s, various infrared telescopes and surveys have identified a few hundred of these objects. Spitzer will devote much of its time to the discovery and characterization of brown dwarfs. It is expected that Spitzer will study thousands of these objects, including those only slightly larger than Jupiter. This will provide astronomers with enough data on brown dwarfs for good quality statisical studies.