Brown
Dwarfs
A
Some
parents tell their children that if a celestial body in the night sky seems to
be twinkling, then it must be a star. Actually, it is difficult to differentiate
a star from a planet with the naked eye—a powerful telescope or
spectroscope is needed to establish whether the light from an object in the sky
is being reflected or if it is being produced internally. The former would
indicate that the object is a planet, and the latter, a star.
In
the early processes of star formation, temperature and mass are crucial. A
clump of gas and dust with the potential to produce a star must be massive
enough—at least 8 percent of the mass of the Solar System’s Sun—to be able to
attain a core temperature capable of initiating the process of nuclear fusion.
When the temperature reaches 10 million Kelvin, hydrogen fusion occurs and
energy is released, giving the object luminosity.
B
In the 1960s, astronomers theorized the existence of a
celestial object that defied classification—not quite planet and not quite
star. The American astronomer Jill Tarter classified them as dark sub-stellar
objects that did not have enough mass to sustain hydrogen fusion. This raised
the question: once the star formation process begins,
does a star always result? Astronomers
have discovered that the answer is “not always”.
For
some reason, some gas and dust
clumps that commence star formation do not achieve
the requisite mass and temperature needed to support nuclear fusion. Yet,
because some energy remains, heat is retained—a heat that is greater than what
planets possess. Thus, the object continues to emit radiation. Dr. Tarter originally called them black dwarfs, but
they were renamed brown dwarfs to differentiate them from stellar remnants that no
longer emit a significant amount of light.
In terms of brightness, brown
dwarfs are like rooms with a night light, glowing
only faintly. This is the reason these
objects are also known as failed stars.
C
It
isn’t easy to collect information about brown dwarfs because they are hard to
detect. Their glow is within the infrared range and can only be detected with
sensitive equipment and when the circumstances permit their identification.
In fact, it was only in 1994 that the first confirmed brown dwarf, known as Teide 1, was
discovered by a team of Spanish astrophysicists. It was located in the Pleiades
open cluster through a series of collected images of the constellation as seen
through an 80 cm telescope. About 55 times the mass of Jupiter, its size fell
within the mass range
of brown dwarfs discovered thus far—15 to 75 times the mass of Jupiter.
Astronomers
have since learned that brown dwarfs are actually a bright, deep red in color,
form part of a binary system—with the distance between the dwarf and its
companion star comparable to that of Pluto and the Sun—and has a luminosity
about a tenth of the faintest stellar objects. Its mass spectrum contains water
vapour and methane, which suggests that its surface temperature does not exceed
1,500 Kelvin. About four-hundred dwarfs have been discovered since 1994, and
these have been classified according to size and temperature.
D
Because
brown dwarfs are nondescript, scientists were inclined to believe that they
merely coexist
with their partner star
and are insignificant. Some preferred to
classify them as planets because of their cool atmosphere, but given that their
formation began as stars and that no accretion of spinning dust appeared to be
involved in their making, the classification “failed star” endured. Then, on
July 11, 2000, a nearby brown dwarf called LP 944-20 emitted what appeared to
be a solar flare for two hours. Jupiter emits X-ray flares as well, but the
dwarf’s flare was about a billion times greater than those discharged by
Jupiter.
In
January of 2002, the image of a brown dwarf orbiting a star called 155ge was captured. Its distance from
its companion star was only 14 astronomical units, similar to that between
Saturn and Uranus. Previously, it was believed that distances between brown
dwarfs and their binary companions could only be equivalent to the Sun and
outer planets of the Solar System. Moreover, the sheer size of the brown dwarf
seemed to confirm that dwarfs do not form as planets do, for it would have been
impossible for a planet of that size to be assembled given the
abbreviated distance between the two objects.
Three
years later, an Irish team of researchers caught a young brown dwarf in a
stellar nursery known as Rho Ophiuchi emitting a jet approximately 1.5 billion
kilometers long. Since it is known that young massive stars form similar jets,
the phenomena seemed to cement the notion that brown dwarfs are not ejected by
a companion star,
but rather, they form like any other star.
Just
as unusual was the discovery of proto-planetary disks around several brown
dwarfs in the Chamaeleon constellation. The disks contained dust particles that
had crystallized to the point of
sticking together,
just as planets in the early phases of formation do. The disks also seemed to be flattening,
an indication that they were forming into planets. Scientists surmise
that this suggests
the potential for planets to form around a brown dwarf system. These
discoveries have caused excitement as the potential for further research has increased
now that the
status of the brown dwarf has
risen to
something closer to objects within the stellar class.