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One colloquial description is that to analyse a body’s chemical make-
brown dwarfs are stars that “didn’t up by looking for these “absorption
quite make it,” and while that’s not lines”. If a brown dwarf has a con-
very scientific it is quite accurate. siderable amount of methane in its
Imagine a gas giant eighty times the atmosphere, the spectrometer will
mass of Jupiter that’s capable of reveal this by showing a black line at
maintaining its own quintillion-ton the appropriate wavelength.
fusion reactor over billions of years,
and you have a picture. However, it’s One of the earliest methods for
still not considered a proper star. It’s identifying a brown dwarf was the
still a dim, dwarf protostar incapa- so-called lithium test. Lithium is
ble of maintaining the thermostatic initially abundant in both stars and
equilibrium (a long-term balance of brown dwarfs. Its fusion tempera-
heat generation and heat output) ture lies just below that of hydro-
that would put it on on the main se- gen, so all evolved stars will quickly
quence of stars. burn through their lithium deposits,
whereas brown dwarfs will retain
Are brown dwarfs really brown in them. This made distinguishing a
colour? Not really, as most pilots will star from a brown dwarf possible for
know. The reason is actually that early astronomers, although there
brown does not truly exist in the op- are other variables like metallicity or
tical spectrum. Compared to a star, magnetic fields that can ‘taint’ this
a brown dwarf emits very little ra- test. It still was of great use to iden-
diation, including visible light. Even tify the first generations of brown
compared to a dim red M-class star dwarfs back in the 20th century and
it’s still comparatively very faint, and laid the groundwork for most scien-
many brown dwarfs emit mostly in tific works on them.
infrared. The result is a very dark red
colour, which made them exceed- So, if brown dwarfs are not real stars
ingly difficult to identify from afar, as that blow up, collapse or inflate at
was the case for Gliese 229B back the end of their lives, what will their
in 1994. inevitable fate be when their fuel
runs out?
Due to their low radiation output,
more complex molecules like meth- Deuterium fusion generates energy,
ane, carbon monoxide or even mo- although only tiny amounts com-
lecular hydrogen can form in their pared to hydrogen fusion. It’s still
atmospheres. These molecules enough to overcome the dwarf’s in-
absorb specific portions of the op- ward gravitational pressure, so the
tical spectrum, namely the more dwarf will radiate away heat (and
energetic — and hence ‘brighter’ — some light) until its fuel is used up.
ones. The result is that many brown In most cases, the reservoirs are
dwarfs actually look magenta in col- thought to last several billion years,
our. The term ‘magenta dwarf’ would depending on the mass. In contrast
probably not have caught on. to main sequence stars, when the
fuel is used up there is no follow-up
You’re peering into a computer-aid- fusion process. The dwarf simply
ed refractor telescope. How would has no fuel left and fusion ceases.
you know you have just discovered What if the resulting ball With all fusion gone and no other el-
a brown dwarf? ements to burn, a brown dwarf will
of gas, called a ‘protostar’, then cool over another couple of mil-
As is often the case in optical as- lacks the ‘final punch’? lion years and then become an inert
tronomy, the answer is spectros- body.
copy. With specific prisms, light can
be split into its spectral components That’s a fate they will most likely
which can be measured with a spec- share in many ways with their ‘cous-
trometer. Most chemical elements ins’, the black dwarfs. Black dwarfs in
absorb specific wavelengths in the turn are hypothesized white dwarfs
optical spectrum, making it possible that have cooled sufficiently to not

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