The Infrared Universe


MISSING MASS - BROWN DWARFS?

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 - objects whose mass is between twice that of Jupiter and the lower mass limit for nuclear reactions (0.08 times the mass of our sun). Brown dwarfs are basically failed stars which did not have enough density at their cores to start nuclear fusion. The conversion of hydrogen into helium in a star's core by nuclear fusion is what fuels a star. This fusion process requires an extremely high density at the star's core to compress the hydrogen atoms together and produce helium. Another component of the missing mass may be the burned-out cores of dead stars. When most stars run out of fuel and their fusion reactions stop, they eventually cool off to the point where they no longer radiate enough visible light to be detected by optical telescopes.

Artist's rendition of a brown dwarf

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 right 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.


Philip Lucas (Univ. Hertfordshire) and Patrick Roche (Univ. Oxford), UKIRT

Artist's rendition (Robert Hurt, IPAC)


The discovery of the objects which make up the missing mass will also give astronomers a better idea about the fate of our universe. 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 everytime 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 of the missing mass will likely be a key to answering this question.


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