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
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
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
(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
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
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.