HST News & Updates Archive
October 8, 1997:
HUBBLE IDENTIFIES WHAT MAY BE THE MOST LUMINOUS STAR
Hubble Identifies What May Be The Most Luminous
October 8, 1997
Space Telescope Science Institute
Astronomers using NASA's Hubble Space Telescope
have identified what may be the most luminous star known -- a celestial
mammoth which releases up to 10 million times the power of the Sun and
is big enough to fill the diameter of Earth's orbit. The star unleashes
as much energy in six seconds as our Sun does in one year.
The image, taken by a University of California,
Los Angeles (UCLA)-led team with the recently installed Near-Infrared Camera
and Multi-Object Spectrometer (NICMOS) aboard Hubble, also reveals a bright
nebula, created by extremely massive stellar eruptions. The nebula is so
big (four light-years in diameter) that it would nearly span the distance
from the Sun to Alpha Centauri, the nearest star to Earth's solar system.
The astronomers estimate that when the titanic
star was formed one to three million years ago, it may have weighed up
to 200 times the mass of the Sun before shedding much of its mass in violent
"This star may have been more massive than
any other star, and now it is without question still among the most massive
-- even at the low end of our estimates," says
Don F. Figer of UCLA. "Its formation and
life stages will provide important tests for new theories about star birth
Violent Eruptions Produce Nebula
The UCLA astronomers estimate that the star,
called the "Pistol Star"
(for the pistol shaped nebula surrounding it), is approximately 25,000
light-years from Earth near the center of our Milky Way galaxy. The Pistol
Star is not visible to the eye, but is located in the direction of the
constellation Sagittarius, hidden behind the great dust clouds along the
The Pistol Star was first noted in the early 1990s
by astronomers in South Africa and Japan, but its luminosity and relationship
to the nebula was not realized until 1995, when Figer proposed in his Ph.D.
thesis that the "past eruptive stages of the star" might have
created the nebula. The Hubble spectrometer results confirm this conclusion.
The astronomers believe that the Pistol nebula
was created by eruptions in the outer layers of the star which ejected
up to 10 solar masses of material in giant outbursts about 4,000 and 6,000
years ago. The star will continue to lose more material, eventually revealing
its bare hot core, sizzling at 100,000 degrees.
Burning at such a dramatic rate, the Pistol Star
is destined for certain death in a brilliant supernova in 1-3 million years.
"Massive stars are burning their candles at both ends; they are so
luminous that they consume their fuel at an outrageous rate, burning out
quickly and often creating dramatic events, such as exploding as supernovae,"
said Mark Morris, a UCLA professor of astronomy and co-investigator. "As
these stars evolve, they can eject substantial portions of their atmospheres
-- in the case of the Pistol Star, producing the nebula and an extreme
stellar wind (outflow of charged particles) that is 10 billion times stronger
than our Sun's."
Why Has it Taken So Long to Find?
The Pistol Star would be visible to the naked eye as a fourth magnitude
star in the sky (which is quite impressive given its distance of 25,000
light-years) if it were not for interstellar dust clouds of tiny particles
between the Earth and the center of the Milky Way that absorb the star's
light. The most powerful telescopes cannot see the Pistol Star in visible
wavelengths. However, ten percent of the infrared light leaving the Pistol
Star reaches Earth, putting it within reach of infrared telescopes, which
have seen rapid technological advances in recent years -- spurred by projects
such as NICMOS.
What Are the Implications?
The Pistol Star was so massive when it was born that it brings into
question current thinking about how stars are formed, say the UCLA astronomers.
In the current view, stars form within large dust clouds which contract
under their own gravity, eventually forming hot clumps that ignite the
hydrogen fusion process.
The star may radiate enough energy to halt the
inward fall of material, thus limiting its maximum mass. The initial mass
of the Pistol Star may have exceeded this theoretical upper limit. "It
is perhaps no accident that this extreme-mass star is found near the center
of the Galaxy," says Morris. "Current evidence leads us to believe
that the star formation process there may favor stars much more massive
than our modest Sun."
Over the coming year, the team will be using the
new near-infrared spectrometer that Ian S. McLean's team is building at
UCLA for the giant 10-meter Keck II telescope in Hawaii. The new instrument
will be used to measure the velocities of the expanding gas shells.
In addition to Figer, Morris, and McLean, the
team also includes Caltech physicist Gene Serabyn and Columbia University
astronomer R. Michael Rich.
The Space Telescope Science Institute is operated
by the Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center, Greenbelt,
MD. The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency.