A brown dwarf is sometimes classed as a failed star and sometimes as a half-way house between a planet
and a star and sometimes as a pseudo-star. They are probably best thought of as the smallest stars. Either
way, a brown dwarf is a star that has insufficient mass to burn hydrogen by nuclear fusion, but is too massive
to be an ordinary planet. The minimum mass for full stardom is about 8% of the Sun's mass (or 80 times
Jupiter's mass), less than this and when a star contracts from a nebula during its birth, it's core will not reach
the critical 2 million Kelvins temperature (about 2 million Celsius) necessary to initiate hydrogen fusion into
heavier elements. In a star the onset of fusion marks its birth as a true star, and the energy released by
fusion opposes gravity and stops the star from collapsing.
What distinguishes a brown dwarf from a planet, is that the brown dwarf does initiate a brief episode of
nuclear fusion, not of hydrogen, but of light elements like deuterium (heavy hydrogen, an uncommon form
of hydrogen) which they burn for up to 100 million years. They burn there small amounts of fuel so slowly that
they are not very bright stars. Objects with less than 1% the Sun's mass never reach the critical core
temperature of one million Kelvins required to burn deuterium, and so are planets rather than brown dwarfs.
There isn't much deuterium in stars, however, and a massive star will consume all of its deuterium very
quickly, but the dim, cool brown dwarfs manage to shine a lot longer with this small amount of fuel.
Brown dwarfs also have lithium in their atmospheres. Young stars also have lithium, but this is rapidly burnt
away, whilst brown dwarfs may never become hot enough to burn lithium, although the more massive brown
dwarfs may burn their lithium in about one billion years. Brown dwarfs, unlike stars, are also cool enough to
have methane in their atmospheres, so looking for lithium and methane may indicate that a star is actually a
Brown dwarfs also shine from the heat they were left with after their formation (by contraction from a nebula
of gas), but once this and their nuclear fuel is depleted, brown dwarfs will very slowly cool and become too
faint to be classed as stars.
Having lost the radiation pressure that keeps more massive stars from collapsing what happens to a brown
dwarf once its fuel runs out? The brown dwarf will not collapse under its own gravity (unlike stars which
collapse into white dwarfs, neutron stars or black holes) because its mass is not high enough. However, it will
slowly contract, releasing more heat, until its core becomes supported by a degenerate gas of electrons.
This is actually similar to what happens in a white dwarf, but white dwarfs are hot and bright. Degenerate
matter is a strange state of matter which occurs at very high pressures and densities. The electrons in the
material are squeezed so tightly together that the only thing that stops them merging and collapsing is
quantum mechanics, which dictates that two electrons cannot occupy the same state (so they cannot merge),
such material is called degenerate. (Photons, the particles of light, on the other hand, are happy to coexist
and can be merged together).
Note: The burning of deuterium is more explosive than the burning of normal hydrogen, and this might give
brown dwarfs especially turbulent cores (?). The artistic impression above, attempts to capture atmospheric
turbulence, perhaps as a result of explosive nuclear reactions inside the brown dwarf, or as a result of
irregular cooling (and perhaps convection).
What would a brown dwarf look like?
Actual data is lacking. Many artists depict brown dwarfs as super-sized gas giants, with banded clouds in
their atmospheres. However, the exact atmosphere will depend on mass and temperature of the brown dwarf
and its life stage. One must remember that brown dwarfs begin as stars and after a brief episode of stardom
they become super-giant planets. The thinking behind the illustration above was that of a brown dwarf in
transition between stardom and planet-dom. The image is dimmer than it would appear to the naked eye, to
show the dusty regions and the hot spots.
As stars, burning deuterium, one might expect their atmosphere to be dominated by convection cells, similar
to those in other stars like the Sun. This mixing would likely prevent cloud-bands from forming. Our young
brown dwarf, depicted above, has a heterogenous atmosphere because it has become enshrouded in a shell
of dust, which has darkened the atmosphere, except where hot spots of activity break through the surface as
'hot islands', hot filaments and hot spots. At this stage the brown dwarf would still be bright, since it is still
radiating much heat, though the image here has been dimmed for clarity. The darker areas represent the
shroud of dust. Dust may occur in the upper atmosphere as it may form there: the relatively low temperature
permit molecules to form and these may coalesce into dust grains. Also, some dust may have accumulated
during accretion in the early stages of formation, though most of this would probably have been in the form of
a small disc. This dust may eventually be blown away, sink into the atmosphere or coalesce into clouds or
rings. Once nuclear fusion switches off, it would take a long time for the last thermonuclear energy to reach
the upper atmosphere and convective turbulence might dominate the atmosphere for quite some time.
Cooling would be further slowed as the large cores of these planetary brown dwarfs would generate much
heat by radioactive decay. Eventually, though the planet's rotation is expected to dominate the weather
systems and the brown dwarf might then be characterised by banded clouds, though it should be
remembered that not all giant planets have banded atmospheres. In our model above, convection cells are
beginning to give way to a more banded atmosphere.
Several new spectral classes have been proposed for brown dwarfs / very cool stars. In our article on main
sequence stars we discussed the spectral classification of stars as, from hottest to coolest: O, B, A, F, G, K
and M; M and some k stars being red dwarfs. Cooler still are the brown dwarfs, with the hottest brown dwarfs
belonging to class L, which are envisaged to be more star-like in their atmospheric structure, perhaps with a
fully convective layer. Cooler than these are the T dwarfs, which possess the signature of water vapour in
their atmospheres. Cooler still is the proposed class of Y-dwarfs which may look more like gas giants with
well-banded atmospheres. Our model above is perhaps most like the notion of a T dwarf coming to the end
of its stellar youth. Personally, I expect that closer observations of brown dwarfs will change our current
concepts of their atmospheres and range of appearances.
Last update: 9/1/2013.
Above: a Pov-Ray model of a brown dwarf in transition from stardom, or perhaps a newly born
brown dwarf. Hot spots erupt through the dusty shell enshrouding the cooling star. [This model
was produced using stochastically generated patterns and produced entirely from code.]
Above: a cool T class brown dwarf. Over billions of years, the
photosphere has cooled below 2200 K, resulting in a break down
pf photospheric convention and the establishment of cloud bands
due to the dwarf's rotation. Methane formation has caused a
blueing of the atmosphere.
Larger brown dwarfs cool more slowly and thus their photosphere
remains convective for longer, possibly for several billion years.