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The mass of brown dwarfs (center) is in between that of stars and planets.
In 1963, 22-year-old NASA researcher Shiv Kumar was studying very low-mass stars, and he wondered how low they could go. Physical laws dictate that if a ball of gas weighs less than 1.6x1029 kilograms, it won't have enough gravity to begin fusion, the process that makes stars shine. If fusion fails to fire, what remains? Kumar could think of no good reason why a "failed star" couldn't exist.
Three decades after Kumar published his theory to a skeptical scientific community, the first failed stardubbed a "brown dwarf" was discovered in the night sky. In astronomy, observer and observed are separated by billions of kilometers, and technology can't always keep pace with curiosity. That's why theory often precedes discovery. Even today, studying brown dwarfs relies on the affinity between the theorists who dream about the Universe's possibilities and the observers who seek those possibilities.
A New Object
To understand what brown dwarfs are, you have to start with star formation. The process begins when a dense cloud of gas and dust collapses into a compact orb, which slowly heats as the pressure increases at its core. If the orb reaches about 8 percent of the mass of the Sun (equal to about 75 times the mass of Jupiter) the pressure and heat at its core become strong enough that hydrogen atoms fuse into helium, a nuclear reaction that releases intense energy. With the start of fusion, a star is born.
Brown dwarfs are thought to undergo the same formation process as stars, but they fail to accumulate enough mass to start fusion. This may simply be because of a lack of gas and dust nearby. Even without fusion, brown dwarfs glow faintly, mainly in infrared light. In fact, the continued gradual collapse of material under its own gravity generates so much heat that a young brown dwarf may emit the same amount of light as a red dwarf (a very small star). The term "brown dwarf" was coined in 1975 by astronomer Jill Tarter, now the director of research at the SETI (Search for Extraterrestrial Intelligence) Institute, to describe Kumar's predicted objects. Since "red dwarf" was already taken, Tarter reckoned that brown was cooler than red, but not totally burned-out and dark.
In the mid-1980's, astronomers began searching in earnest for brown dwarfs, relying on theoretical models to guide them. Models describe aspects of the Universe using a framework of mathematics and physical laws. Models suggest how new objects may be found by predicting what characteristics to look for. New observations, in turn, confirm the accuracy of a model and reveal its weaknesses. "There's always a process of back and forth between modeling and observations until we're satisfied that we understand what is going on," says Didier Saumon, an astronomer at Los Alamos National Laboratory who models brown dwarf properties. With this continuous feedback, scientists make models that more closely reflect the actual Universe and predict things that cannot currently be observed or measured.
In the first two decades after Kumar's prediction, the key impediment to finding a brown dwarf was the tools available. Spotting these isolated, faint objects in the vastness of space requires knowing where to look, and wide-angle sky surveys simply weren't sensitive enough to pick up their weak light signals. "The technology was not ready," says Saumon. "It turns out that brown dwarfs were much fainter than we had expected. The earliest searches could not possibly have found them."
Instead, early brown dwarf seekers found a number of false alarms, mostly cool red stars and some large planets. On first detection, these objects fit the profile of a brown dwarf, but further measurements revealed that they were either above the critical star mass or within the mass range of planets.
It wasn't until astronomers loosened the constraints of the theoretical predictions that the first bona fide brown dwarf was confirmed. The object, Gliese 299B, was proven to be a brown dwarf by astronomer Rebecca Oppenheimer and collaborators in 1995. Oppenheimer, now an associate curator of astrophysics at AMNH, had obtained a spectrum of a faint companion to a low-mass star, a technique that analyzes light to determine the types of atoms and molecules in a cosmic object. Even though the companion didn't exactly match what theorists predicted a brown dwarf would look like, the spectrum revealed that the object contained methane. Since methane can only form in objects cooler than stars (a star's intense heat prevents the atoms from bonding), its presence proved that the faint companion was cool enough to be a brown dwarf.
Today astronomers know of nearly a thousand brown dwarfs in the Milky Way galaxy and suspect that throughout the Universe there could be just as many brown dwarfs as there are stars.
The Road Ahead
Not all the brown dwarfs that have been discovered can be studied in detail, and scientists still have a long way to go toward understanding brown dwarfs in general. "We've been studying stars for almost a century and we know quite a bit about them," says Saumon. "But when brown dwarfs came on the scene . . . everything about them was new. All the work that we had done to understand stars, we have to go out and do for brown dwarfs."
Telescopes on the summit of Mauna Kea on the Big Island of Hawaii.
The Keck II telescope, located on the chilly, oxygen-deprived peak of Mauna Kea on the Big Island of Hawaii, has been used to hunt for and study brown dwarfs since its construction in 1993. The high-altitude island location offers Keck II and the peak's 13 other observation facilities very low distortion from the atmosphere and almost no city lights, so the telescope gets one of the clearest possible views of the night sky.
About twice a year, Quinn Konopacky, a postdoctoral researcher at Lawrence Livermore National Laboratory, heads to Keck to observe brown dwarfs and measure their masses. Knowing the mass of these objects not only confirms a brown dwarf's status but also determines its age, density, surface composition, how quickly it will cool over its lifetime, and a swath of other information. To measure masses, Konopacky seeks pairs of brown dwarfs in orbit around each other. The Keck II telescope uses a cutting-edge technology called adaptive optics to remove the blurring effect of Earth's atmosphere, which can make two closely orbiting objects appear as one. Without adaptive optics it is often impossible to study brown dwarf pairs from the ground.
Stuck somewhere between stars and planets, brown dwarfs can help improve scientific understanding of both. The atmospheres of cool, faint brown dwarfs and those of gas giant planets may share some characteristics, such as the presence of clouds. And understanding why brown dwarfs fail to become stars can help reveal why true stars succeed. "Studying brown dwarfs is, I think, critical for our understanding of the Universe as a whole," says Konopacky. "Brown dwarfs are probably the most common outcomes of the star formation process in general. So if you don't really understand a lot about brown dwarfs, you could argue that you don't understand a lot about star formation itself."
The Tail End?
A brown dwarf as seen by the Spitzer Space Telescope.
What lies beyond the coolest, faintest brown dwarfs scientists can currently detect? Astronomers now believe that there may be brown dwarfs cool enough to have atmospheres that contain ammonia, a gas present in the atmospheres of some giant gas planets like Jupiter. These super-cool brown dwarfs, identified as Y-type dwarfs, are expected to emit almost no optical light and therefore must be found by telescopes that can detect longer wavelengths of infrared light than can be seen from the ground. The Wide-Field Infrared Survey Explorer (WISE) satellite telescope that was launched in 2009 has the capability to locate Y-type brown dwarfs, so observations may soon catch up with theory.
"Astronomy is special in that sense that, whenever we have new technology that allows us to peer farther in the Universe, or measure things more precisely, we discover new objects that nobody's ever thought of," says Saumon. "And so that's one thing that makes it really exciting, is the sense that there are unknown things out there still waiting to be discovered."