Brown dwarfs within 10 parsecs

By July 2006, astronomers had found at least 15 brown dwarfs within 10 parsecs (32.6 light-years) of Sol, although these objects are extremely dim compared to OBAFGK stars. Some astronomers believe that brown dwarfs may be as numerous as stars in the Milky Way. Unfortunately, none are bright enough to observe with the unaided Human eye in Earth's night sky.

Spectral Populations of Brown Dwarfs

According to some theorists, a celestial object with a mass of less than about 75 Jupiter-masses -- around seven percent of Sol's mass -- cannot sustain significant nuclear fusion reactions in its core and so will not destroy the lithium in its atmosphere. Hence, this mass threshold divides brown dwarfs from the smallest, dimmest red dwarf stars. Establishing a lower mass limit for brown dwarfs has proven to be more difficult. Some astronomers would like to set the minimum mass limit at 13 Jupiter-masses, because less massive objects cannot even fuse deuterium.

© Anglo-Australian Telescope Board
(Image by Chris Tinney)




Wide field, "true-color" image
with satellite trail (more).




Brown dwarfs are actually
reddish, like the binary
DENIS 1228-1547, or
magenta in color for the
cooler and dimmer T-dwarfs
that are rich in methane.

Although brown dwarfs are similar in size to Jupiter, they are much more massive and dense enough in their cores to produce their own light (mostly infrared wavelengths), whereas Jupiter shines with reflected light from the Sun. When brown dwarfs are very young, they are relatively luminous because they do generate some radiative energy from the fusion of deuterium ("heavy hydrogen") into helium nuclei, which is used up in a few tens of millions of years. Subsequently, brown dwarfs glow much more feebly from the heat generated by the release of gravitational energy as they slowly contract. By definition, the object's core temperature must be less than three million degrees, as that is the critical temperature required for substantial nuclear reactions to take place. However, surface temperature is dependent on its mass, which will be lower for lower mass objects. Hence, brown dwarfs are expected to have a surface temperature around 1,000 K and cool down as they get older, as initial nuclear fusion of deuterium at the beginning of its life cannot be sustained very long. Because of their low surface temperature, brown dwarfs are not very luminous (more).

Robert Hurt, IPAC, NASA

Larger and jumbo illustrations.

In near-infrared, M and L dwarfs are slightly
orange or red compared to Sol, while
methane-rich T dwarfs are bluish from
methane absorption of green and red light,
similar to Jupiter (more).

The smaller red dwarf stars, brown dwarfs, and gas giant planets like Jupiter all have approximately the same size, less than a tenth of Sol's diameter. At around one billion years in age, red dwarf stars and L-type brown dwarfs are red, while the less massive T dwarf is dimly magenta, due to the absorption of green wavelengths by sodium and potassium atoms. In near-infrared light, red dwarfs and L dwarfs are slightly orange or red compared to the Sun, but methane-rich T dwarfs are distinctly blue due to a lack of light in the green and red portions of the spectrum caused by absorption from methane. Methane is also abundant in the atmosphere of Jupiter and this gas, along with clouds and bands of other complex molecules, gives rise to alternating patches of pink and blue on Jupiter and possibly the cooler brown dwarfs as well (Kirkpatrick et al's L&T Dwarfs; M,L, and T dwarf classification; and Adam J. Burgasser's T-Dwarfs page.)

Jeffrey L. Linsky, JILA,
STScI, NASA


Larger image.


Brown dwarfs, like Van
Biesbroeck's Star (Gliese
752 B) have less than 20
percent of Sol's mass and
so can transport core heat
through convection only,
unlike its larger and brighter
companion Gl 752 A (more).

Because a brown dwarf does not have a strong central source of nuclear energy, its interior should be a rapid "boiling," convective motion. When combined with the rapid rotation that most brown dwarfs exhibit, convection sets up conditions for the development of a strong, tangled magnetic field near the surface. Astronomers believe that this magnetic field can create strong flares. As turbulent magnetized hot material beneath a brown dwarf's surface conducts heat to its atmosphere, it would allows electric currents to flow and produce an X-ray flare, as has been detected from LP 944-20. The absence of X-rays from LP 944-20 during non-flaring periods suggest that the million-degree Celsius upper atmosphere, or corona, created by flares disappears as its surface temperature cools below around 2,773 degrees K and becomes electrically neutral.