The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/93_13.txt

Text File | 1996-01-12 | 7KB | 146 lines

CONTACTS:  Ray Villard, STScI
          410-338-4514

          Guido De Marchi, STScI
          410-516-8604

          Francesco Paresce, STScI
          410-338-4823

PRESS RELEASE NO.:  STScI-PR93-13


HUBBLE FINDS EVIDENCE OF STELLAR CLOSE  ENCOUNTERS:
BRIGHT BLUE STARS AND NAKED CORES

NASA's Hubble Space Telescope (HST) has discovered a new
population of stars  isolated deep in the core of M15, one of the densest
globular star clusters.

The stars are among the hottest stars observed in the core of a globular
cluster.  The most likely explanation for their existence is that they are the
"naked cores" of stars that have been stripped of their outer envelope of
gas, according to astronomers.

"This is amazing; these objects represent a totally new population of
very blue stars,"  says Guido De Marchi, of the Space Telescope Science
Institute (STScI), Baltimore, Maryland, and the University of Florence,
Italy. "When we started wondering what  they could be, we realized that
they may be among the first observed stars to have been stripped."

De Marchi and Dr. Francesco Paresce (STScI and European Space
Agency) explain that this could only have happened if stars are so
crowded together in the core they can be stripped of much of their
gaseous envelopes by the gravitational pull of  bypassing stars.

This stellar cannibalism could only take place where stars are so
crowded together that chances for close encounters are exceptionally
high, they said.  De Marchi and  Paresce interpret the existence of this
new class of stars as possible evidence that the center of the globular
cluster has contracted to an extremely dense condition called
"core collapse."

This research, by De Marchi and Paresce, is being announced at a press
conference  today at the meeting of the American Astronomical Society
in Berkeley, California.

 The astronomers were surprised to discover about 15 hot blue stars
segregated at the  very core.  Their surface temperatures are above
60,000 degrees Fahrenheit (our   Sun's surface is 11,000 degrees
Fahrenheit).   This discovery was possible only with the Hubble
Space Telescope because it can resolve stars at the dense core
of M15 that are only a blur from the ground.  The observations
also required Hubble's sensitivity to ultraviolet light to distinguish
the hot stars from the surrounding cooler stars.

Such very hot blue stars can be made in several ways besides
stellar stripping:   magnetically stirred-up super massive stars, white
dwarfs, or planetary nebulae.  However, the researchers are quick to
point out that none of these scenarios explain why the stars are so
concentrated and so numerous only at  M15's core.

"This rules out a number of other hypotheses," says De Marchi.  He
explains that all   the blue stars lie within a one light-year radius at the
very core of the cluster.  What's   more, 90 percent  of them are
concentrated at the very center of this volume, within a  four-tenth
light-year radius.


CLOSE ENCOUNTERS OF THE STELLAR KIND

According to this scenario, the new population of blue stars was once
the cores of red  giant stars.  Such stars expand to enormous sizes late
in their lives, due to changes in   the nuclear "burning" at their cores.
(If our sun were the size of a red giant it would engulf the inner solar
system, out to the diameter of Mars' orbit.)

Red giant stars are so distended that they have a weak gravitational
hold on their   outer envelope of cool gas.  If a normal, main sequence
star passes within a few   stellar radii it can rob gas from the red giant.
This stripping process can, in theory,  expose a star's core -- the nuclear
fusion "engine" that powers stars.

However, conditions where stars are so crammed together are unusual.
For example,   in our own stellar neighborhood, the stars are typically a
million times farther apart   than the distance between the sun and Earth.
Conversely, due to the relentless pull of   gravity, the stars at the core
of M15 have converged so that they are at about 500  times the distance
between the Earth and the sun.  If our planet were there, we would
see a hundred thousand stars closer than Proxima Centauri, the
closest star to the  Earth; the night sky would look simply fantastic!

The astronomers used Hubble Space Telescope's Faint Object Camera
(FOC) to probe   the core of M15 (15th object in the Messier Catalog)
which is located 30,000  light-years away in the constellation Pegasus.
M15 is visible to the naked eye as a   hazy spot 1/3rd the diameter of
the full moon.


CORE COLLAPSE

Globular clusters are compact "beehive swarms" of several hundred
thousand stars   loosely held together under the mutual pull of gravity.
The stars are deflected by   gravity if they pass near each other. During
such close encounters a smaller, less   massive star steals momentum
from the larger star.

Because of these near-collisions, the massive stars lose momentum
and "fall" toward   the center of the cluster, like marbles rolling to the
bottom of a funnel.  Given   enough time, massive stars should accumulate
at the cluster's center.  Theoretically,   this could become a runaway
collapse where stars quickly crowd together.

Previous Hubble observations suggest that the cluster probably contains
powerful  energy "storage batteries" in the form of double star systems,
which prevent the core  from imploding all the way down to a black hole.
The rapid orbits of two stars about   each other in tight binary systems
create a powerful reservoir of kinetic energy.   A  few double stars can
stir up the motion of in-falling stars. This would cause the core   to
rebound, like squeezing and relaxing a rubber ball.

Astronomers have long sought evidence for core collapse at the heart of
very dense clusters like M15.  To estimate the true stellar density from
ground-based visible light  photographs,  however, has been difficult.
The Hubble observation doesn't tell   whether the core is still collapsing
or rebounding.

Previous research by a team led by Paresce found that another class of
unusual blue  star, dubbed blue stragglers, also dwells at the cores of
some clusters.  However,  even the "stragglers" are not as hot nor as blue
as the new population of blue stars in  M15.  Most of the blue stragglers
are probably double stars that gravitationally  capture each other.  The
capture stirs-up the stragglers' nuclear fuel.  The star "resets
its clock" to relive a bright and hot youth.

The researchers plan to use Hubble to peer into the cores of other
globular star  clusters to see if this new class of star dwells elsewhere
as well.

                         * * * * * *

The Space Telescope Science Institute is operated by AURA
(the Association of  Universities for  Research in Astronomy, Inc.)
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 ESA (the European Space Agency).