SF State professor discovers dwarf star

SF State astronomy Professor Geoff Marcy and UC Berkeley astronomer Gibor Basri announced this summer that they had verified the existence of a brown dwarf, a theorized object bigger than a planet but too small to be a star.

Announcing their results in July at a meeting of the American Astronomical Society, the confirmation made front page news across the country and stirred up the astronomy community, because brown dwarfs are thought to be the missing link in several cosmic mysteries.

"Nature tends to be continuous. It is very logical to wonder whether there could be something in between planets and stars," Marcy said. "Jupiter, the largest known planet, is 100 times less massive than the lowest mass star known, so there has been a gap between the least massive stars and most massive planets. This discovery begins to fill that gap."

A brown dwarf is essentially a star that does not burn hydrogen and therefore does not shine brightly, making it almost impossible to detect. Although there have been many suspected brown dwarf candidates over the past 15 years, observational confirmation has eluded astronomers.

Marcy used the world's largest telescope, the 10-meter Keck Telescope on Mauna Kea in Hawaii, to gather the data needed to confirm the existence of a rumored brown dwarf, known as PPL 15, in the Pleiades Star Cluster.

Marcy and Basri focused the Keck telescope on PPL 15 for over five hours in order to collect enough radiation to get a spectrum-a stellar fingerprint that tells what atomic elements are present in a star. The key spectral test for a brown dwarf is the presence of lithium.

The lithium test is the most reliable way to determine whether or not a candidate is a bona fide brown dwarf or just a small dim star..

Lithium will burn at a much lower temperature than hydrogen. When a star is formed, it has a certain amount of lithium in it. By the time a star hits middle age (about a hundred million years old), it will have burned all of its lithium in its nuclear furnace. A middle aged brown dwarf, however, will have retained most of its primordial lithium.

"If you can find a very low mass cool star that has retained its lithium, then it is clearly a unique bird from a totally different flock," Marcy said

Brown dwarfs are thought to make up a large portion of the "dark matter" in the universe that is central to several cosmological "missing matter" problems. According to astronomers, distant galaxies rotate in such a way that can only be explained by the presence of a large amount of mass in their galactic halos. But no such matter has ever been detected.

One theory is that this missing matter is composed of dark matter-large planetoid or brown dwarf like objects that don't radiate, called Massive Compact Halo Objects (MACHOS). The method used to confirm PPL 15 will provide a means for determining the total mass contribution of MACHOS in a galaxy and how it should affect these rotational motions.

To cosmologists, the missing matter problem concerns the very fate of the universe. According to Albert Einstein's General Theory of Relativity, matter warps the structure space-time. That is, it produces a geometry or shape that the universe moves in as it expands in time. If we know how much matter there is in the universe, then we can predict this geometry.

With too much matter, the universe will have a "closed" geometry-one that will collapse back in on itself in the distant future in a "Big Crunch." Not enough matter and the universe will expand forever in an "open" geometry. But just the right amount of matter and the expansion of the universe will come to a halt in what is called a "flat" geometry.

Current observational estimates on the amount of matter in the universe seem to indicate that the universe has an open geometry. However, cosmological theories of the early universe predict a flat geometry. This is a serious discrepancy between theory and observation, which has resulted in a missing matter problem. Being able to determine the amount of matter that brown dwarfs contribute to the total mass of the universe will help in the resolution of this discrepancy.

"On the cosmological level, it's conceivable that the brown dwarfs make up a sizeable portion if not all of the dark matter," Marcy said. "So indeed it could be that 90 percent of the universe is brown dwarfs, and we spend all our time studying atoms and molecules and computers and so on when in fact most of the universe is in the form of brown dwarfs. So we astronomers may have actually been missing the iceberg and studying just the tip."

Marcy predicts that an explosion of brown dwarf confirmations will follow their discovery.

"It's fine that we have found one brown dwarf, but what is really exciting is that this one brown dwarf will serve as a bench mark. So now it's easy. You don't need to go and take a spectrum anymore. We now know what the brightness and color of a brown dwarf is, so everything fainter and redder is also a brown dwarf."

But before the astronomical community accepts PPL 15 as the definitive brown dwarf benchmark, additional confirmations of fainter candidates that pass the lithium test must be made.

"This is a tremendously exciting result," said Dr. James Liebert, an astronomer at the University of Arizona and an authority on brown dwarfs, to the New York Times. Dr. Liebert said it was "absolutely crucial to find other fainter ones" and determine that they also display a clear lithium signature.

Marcy and Basri are returning to the Keck 10-meter telescope in November to look at a brown dwarf candidate fainter than PPL 15 by a factor of 2. Marcy said that if it passes the lithium test then it will make their current results "rock solid." If the fainter candidate doesn't have lithium, then the entire theory of brown dwarfs will be thrown out and everything will be cast in doubt. "And that," adds Marcy, "is what makes science so wonderful."

[ back to Golden Gater Online August 31, 1995 ]

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