Key to Astronomical Mystery Discovered : Science: Sighting of a potential ‘brown dwarf’ could confirm existence of celestial bodies that are neither planets nor stars.
After nearly a decade of searching, astronomers have discovered a new member of the stellar family that is certain to shed light on several longstanding mysteries. The sighting marks the most conclusive evidence yet of a “brown dwarf,” a body that exists in a semi-permanent state of adolescence, not quite small or cold enough to be a planet, not quite big or hot enough to be a star.
Despite their somewhat murky status in the stellar pecking order, brown dwarfs occupy a central place in astronomy: They are a favorite candidate for the mysterious “dark matter” that may make up as much as 90% of the universe. In addition, they add an intriguing new piece to one of the juiciest puzzles in astrophysics--how stars are born, mature and ultimately settle on a size.
“We’re very excited about it,” said UC Berkeley astrophysicist Gibor Basri, who found a possible brown dwarf in June. “This should be an even better brown dwarf than ours was. It’s a far superior candidate.”
The finding was announced in today’s issue of the journal Nature by three astronomers at the Astrophysics Institute of the Canary Islands in Tenerife, Spain. It will by no means clinch the identity of the elusive dark matter that pulls gravitationally on everything in the universe yet remains stubbornly unseen. The recipe for dark matter probably contains many different ingredients, theorists say, of which brown dwarfs make up only a small fraction. (Unlike some other forms of dark matter, brown dwarfs should be visible to powerful-enough telescopes.)
But the finding is significant, said physicist Lorne Nelson of Bishop’s University in Quebec, because it heralds “a whole new class of astronomical objects which we believe exist in very large numbers. It would have been somewhat of an embarrassment if we hadn’t found any.”
A brown dwarf hovers in a kind of twilight zone between planet and star, rather like a 17-year-old who is old enough to drive, but not old enough to vote or drink alcohol. Both teen-ager and star face identity problems.
A bona fide star, like our sun, congeals out of a huge swirling cloud of intergalactic gas and dust. The gravitational pressure of its enormous weight ignites a nuclear fire in its belly that fuses hydrogen into helium, releasing energy in the process. “Once it’s found its power source, it can burn happily for a very long time,” Basri said.
Planets, on the other hand, even huge ones like Jupiter, are too puny to generate the kind of pressures needed to sustain sizable nuclear fires. A brown dwarf starts out like a star, but never gets quite massive enough to ignite. Instead, it stokes a small nuclear fire that “poops out at some point,” Basri said.
As real stars burn, they also destroy whatever traces of the element lithium happen to be in the original cloud. Brown dwarfs never get hot enough to burn lithium, so astronomers look for traces of the element to help draw the line between star and dwarf.
Basri’s dwarf passed the lithium test, but some astronomers found it too big for a dwarf, and instead called it a “transitional object.”
The new object reported today, Basri says, “is cooler and fainter,” which makes it a better candidate. “The object we found is just under the star limit.” Like Basri’s dwarf, the new object was found in the Pleiades star cluster, which is part of the Milky Way. Its apparent size and temperature put it squarely in the family of brown dwarfs.
To officially qualify, however, it will have to pass the lithium test. That means waiting for observing time on the only instrument sensitive enough to detect the lithium signature--the Keck telescope in Hawaii.
Drawing the line between dwarf and planet is even more difficult than distinguishing dwarfs from stars, however. Even a planet like Jupiter might produce a minuscule amount of nuclear burning in its core, Basri said. An object five times more massive than Jupiter could qualify as dwarf or planet--depending in part on whether it was found orbiting a star, or on its own.
Yet pinning down the dwarf population is extremely important for understanding how stars are born and mature. For reasons astronomers cannot explain, very large stars are exceedingly rare. At the same time, very small stars are turning out to be much rarer than they should be.
Stars are born in huge clouds of gas and dust that condense into globs of matter under the attraction of gravity. Because gravity is insatiable and pulls on everything in its grasp, most stars should be huge. “Your typical star would be more massive than the sun,” Basri said. Yet most stars are considerably smaller.
On the other hand, astronomers see far fewer small stars and brown dwarfs (and their close cousins, the red dwarfs) than they might have expected. “Nature is not as prolific as we thought” in producing very small stars, Basri said.
