Probing deep within a neighborhood stellar nursery, NASA’s
Hubble Space Telescope uncovered a swarm of newborn brown dwarfs. The
orbiting observatory’s near-infrared camera revealed about 50 of these
objects throughout the Orion Nebula’s Trapezium cluster [image at right],
about 1,500 light-years from Earth. Appearing like glistening precious
stones surrounding a setting of sparkling diamonds, more than 300 fledgling
stars and brown dwarfs surround the brightest, most massive stars [center
of picture] in Hubble’s view of the Trapezium cluster’s central region.
The brown dwarfs are too dim to be seen in an image taken by the Hubble
telescope’s visible-light camera [picture at left].

Credits for near-infrared image: NASA;
K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.);
and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R.
Thompson (Steward Observatory, University of Arizona, Tucson, Ariz.)

Credits for visible-light picture: NASA,
C.R. O’Dell and S.K. Wong (Rice University)

Don Savage

Headquarters, Washington, DC

(Phone: 202/358-1727)

Ray Villard

Space Telescope Science Institute, Baltimore, MD

(Phone: 410/338-4514)

Suzanne Jacoby

National Optical Astronomy Observatory, Tucson, AZ

(Phone: 520/318-8364)

RELEASE: 00-130

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.

“Because the brown dwarfs bridge the gap between stars
and planets, their properties reveal new and unique
insights into how stars and planets form,” said Joan Najita
of the National Optical Astronomy Observatory (NOAO) in
Tucson, AZ. Her study with fellow NOAO researcher Glenn
Tiede and John Carr of the Naval Research Laboratory,
Washington, DC, will appear in the October issue of the
Astrophysical Journal.

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 inventory was carried out using Hubble’s infrared
vision to measure the brightness and temperature of stars
in the cluster IC 348, located in the constellation
Perseus. Because the cluster is young, the brown dwarfs in
the cluster are intrinsically brighter, which made it easy
to detect about 30 brown dwarfs. A critical step in the
observation was picking out the brown dwarfs from the
clutter of background stars. To tackle this problem, Najita
and colleagues developed a new technique using Hubble’s
NICMOS camera. The procedure measures the strength of an
infrared water-absorption band in the atmospheres of the
stars. The strength of the band is a sensitive measure of
each star’s temperature.

“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.

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