Hubble gets head count of elusive brown dwarf stars
NASA NEWS RELEASE
Posted: August 24, 2000
Astronomers using NASA's Hubble Space Telescope have taken attendance in a class of brown dwarfs and found indications that these odd and elusive objects also tend to be loners.
The Hubble census -- the most complete to date -- provides new and compelling evidence that stars and planets form in different ways.
Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light that they emit is so faint it's hard to tell how many of them are scattered throughout the galaxy, and how they're formed.
The Hubble census finds that, like stars, there are more low-mass brown dwarfs than high-mass ones, and this trend continues down to low, nearly "planetary" masses. "In this respect, the isolated, or free-floating, brown dwarfs found by Hubble appear to represent the low-mass counterparts of the more massive stars," added Najita. "This suggests that stars and free-floating brown dwarfs form in the same way."
However, the Hubble finding also offers the strongest evidence so far that free-floating brown dwarfs are far different than the recently discovered planets that orbit nearby stars. Najita's team found brown dwarfs more often alone than in orbit around other stars. "This suggests that the extra-solar planets and, by extension, the planets in our own solar system, formed very differently from how the Sun and other stars formed," Najita noted.
Only a few years ago, it was commonly believed that brown dwarfs are rare, perhaps because the process that makes stars "stops working" at lower masses. "Nature does not discriminate between stars that can shine by fusion and lower-mass objects that are unable to do so," said Najita. "In fact, the universe easily makes brown dwarfs of all masses, from the most massive to the least."
The study also found that brown dwarfs are unlikely to contribute significantly to the mysterious, unseen "dark matter" that dominates the mass of our galaxy and the universe. Although Hubble found that brown dwarfs are abundant, it turns out that they are not common enough to explain the dark matter. Najita and her colleagues conclude that brown dwarfs probably contribute less than 0.1 percent of the mass of our Milky Way's halo.
"The ability to measure the temperature of each star solved several problems simultaneously," Najita said. "In addition to helping us distinguish the cluster brown dwarfs from background stars, we were also able to measure the masses of the brown dwarfs without having to assume their age. This greatly improved our mass estimates."
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy (AURA), Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NOAO is operated for the National Science Foundation by AURA, Inc.