Brown Dwarfs
Astronomers have long conjectured the existence of objects that have some of the characteristics of stars
and some of planets that are called brown dwarfs.
Brown dwarfs never quite made it
as stars, because they did not have a high enough mass to initiate and sustain fusion reactions.
They collapse out of hydrogen clouds, not out of protoplanetary disks (like stars),
but radiate energy only by gravitational contraction, not from fusion
(like planets).
Their masses are expected to range from several times the mass of
Jupiter to a few percent of the Sun's mass. The cooler brown dwarfs
may resemble gas giant planets in chemical
composition, while hotter ones may begin to look chemically more like stars.
They are difficult to detect since they are small and of very low luminosity.
Gliese 229B
The adjacent 1995 Hubble Space Telescope image shows a faint object called
Gleise 229B (abbreviated as GL229B) that is
thought to be a brown dwarf.
Originally discovered at Palomar Mountain,
GL229B orbits the red dwarf
star Gliese 229, which is about 18 light-years away in the
constellation Lepus. It is thought to be about 20-50 times the mass of
Jupiter, but of much higher density so it is about the same diameter as Jupiter.
The primary star is off the image
to the left and suppressed with a coronagraph, but its glare still dominates the image.
The spike in the lower right of the image is an artifact from the optical system.
The separation of the
primary star and the brown dwarf is only 7 arc seconds, corresponding to
a distance about the same as the separation of the Sun and Pluto.
The brown dwarf is
more than
100,000 times fainter than the Sun and
at the time of its discovery was the faintest object ever seen around another star.
It's surface temperature is estimated to be about 1300 K, much hotter than any gas giant
planet but much cooler than any star.
Neither Planet nor Star?
Gliese 229B appears to be too hot and massive to be a planet, but too small and cool to be a star.
One way to identify a brown dwarf is to use its spectrum to learn what elements and compounds
are present. The IR spectrum of GL229B obtained by the Hale Telescope at Palomar looks like
the spectrum of a gas giant planet. Most telling is evidence of methane gas, common in gas
giants but not found in stars. Generally, methane can survive only in atmospheres having
temperatures lower than about 1500 K.
Thus, Gliese 229B is thought to be the first brown dwarf discovered.
