SPACE TELESCOPE SCIENCE INSTITUTE OBSERVER Volume 1, Number 3, 1991
HST science update for educators
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Science Editor - Ray Villard
Art Director - John Godfrey, Jr.
Astronomy Illustrator - Dana Berry
Distribution Manager - Cheryl Gundy
The Observer is published quarterly by the Educational and Public
Affairs Office of the Space Telescope Institute.
NASA ESA
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In this issue:
o A new solar system
o Monster star
o Close encounters
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Acknowledgements: The editor wishes to gratefully thank,
Albert Boggess, Robert Brown, Jeff Hester, F. Duccio Macchetto,
Georges Meylan, Francesco Paresce, Jim Pringle, Mike Shara, Nolan
Walborn, Harold Weaver, James Westphal.
The Space Telescope Science Institute is operated for the National
Aeronautics and Space Administration by the Associate of
Universities for Research in Astronomy, Inc. The Hubble Space
Telescope is a project of international cooperation between the
National Aeronautics and Space Administration and the European Space
Agency.
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A NEW SOLAR SYSTEM?
NASA's Hubble Space Telescope has taken s close-up look at what just
might be a neighboring solar system. The Space Telescope didn't
uncover planets, but it did observe extremely complex and dynamic
activity in a gas disk encircling the star Beta Pictoris, located 56
light-years away. This billion year-old star might tell us
something about how planetary systems form.
The search for planets beyond our solar system has long tantalized
astronomers. Planets are necessary prerequisites for life as we
know it. Planetary systems may be rare or abundant in the Galaxy.
Either way the implications are profound.
At present we know of only one planetary system, our own.
Astronomers have long sought to understand how our solar system came
into being. It was once thought that a chance close encounter
between the Sun and a bypassing star have birth to planets. The
idea was that a long streamer of gas was pulled from the Sun and
then condensed to form planets, like a string of pearls.
The 18th century philosopher Immanuel Kant saw the solar system as a
normal by-product of the Sun's birth, rather than a cosmic accident.
Kant proposed the nebular hypothesis: that the planets coalesced out
of the same gas cloud that contracted to form our Sun.
There is convincing evidence today that our solar system is a relic
of a vast pancake-shaped disk of dust and gas which accompanied the
newborn Sun. All the planets move about the Sun in the same
direction. Their orbits lie more or less in the same plane. Disks
are commonly seen around newborn stars.
Astronomers would like to understand what conditions might
precipitate planet formation before an embryonic disk dissipates.
Beta Pictoris first drew attention of astronomers in 1983.
Observations with NASA's Infrared Astronomical Satellite (IRAS)
showed an unusually large amount of infrared radiation in the star's
vicinity. Researchers concluded that this radiation came from cool
dust encircling the star.
Follow-up ground based images revealed an intriguing disk-like
structure which appears to be tilted almost edge-on to Earth, and is
nearly ten times the diameter of our own solar system (which is
loosely defined as the diameter of Pluto's orbit, about 12 billion
kilometers).
Detailed analysis showed that the Beta Pictoris disk is made up of a
swarm of orbiting particles which appear to have agglomerated from
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much smaller interstellar grains. Because Beta Pictoris is estimated
to be nearly one billion years old, these dust particles should have
already coalesced to form planet-sized bodies, according to current
theories of planet formation. It's also possible that, for reasons
not understood, planets may have never formed. However, if planets
do exist, astronomers suspect there could be a huge "doughnut hole"
gap in the middle of the disk where newborn planets have swept up
dust.
The problem is that the glare of Beta Pictoris drowns out any images
of that portion of the disk which is close to the tar. By splitting
starlight into its component colors through spectroscopy, astronomers
have probed the inner region of the disk which is too close to the
star to be imaged directly. Spectrograms made with the
International Ultraviolet Explorer (IUE) satellite, reveals that
there is also a cloud of gas around the star. This gas cloud is
much smaller than the dust disk, and is approximately the size of
Mars's orbit, which defines the periphery of our inner solar system.
It's not clear where this gas comes from. The gas may be produced
by slow decomposition of the solid particles in the disk, it may
have been shed by Beta Pictoris, or it may have evaporated from cool
objects perhaps resembling comets that may have formed.
Using the Goddard High Resolution spectrograph (GHRS) aboard Hubble
Space Telescope, astronomers have studied the gaseous disk in
unprecedented detail. The GHRS was used to study the distribution
and motions of singly ionized iron atoms. Besides being important
scientifically, iron is a useful tracer of the distribution and
behavior of other less readily detected gases swirling about Beta
Pictoris. To identify changes in the turbulent gas cloud, HST
observations were made on two separate occasions 23 days apart. The
GHRS detected gas clumps passing in front of the star as evident in
rapid changes in the ultraviolet spectrum. This revealed that the
gas cloud's structure changed dramatically over the period between
the two observations.
Space Telescope observations confirm earlier ground-based data that
suggested gas clumps are falling into the star at more than 200,000
kilometers per hour. These clumps may be icy comets, which have
formed in the disk, only to be swallowed up by their parent star.
These complex dynamics don't offer direct evidence for the presence
of planets. However, if the clumps really are icy comets, then
bodies at least the size of our Moon could be perturbing the comets
such that they fall into the star.
At present, astronomers are far from certain whether planets really
exist around Beta Pictoris. Future observations will seek
additional direct and indirect evidence for the presence of planets
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in the Beta Pictoris system. A long-term, systematic survey of
other stars with disks might eventually offer compelling evidence
that planets are abundant throughout the galaxy. Where there are
planets, there may be life as well.
CAPTIONS:
A ground-based telescope image of a broad flattened disk of dust
encircling the star Beta Pictoris. The glare of the star is blocked
out by an "occulting finger" which cut across the center of the
image. This also obstructs the inner ten billion kilometers of the
disk, where planet-building may have already taken place. The
diameter of Neptune's orbit is added for scale. (Courtesy F.
Paresce, C. Burrows, & ESO)
Though never imaged directly, presumably the dust disk extends close
in to the star Beta Pictoris (unless planets have formed and swept
up material). HST spectroscopic observations centered on an inner
gas disk, which is about the diameter of Mars' orbit. The gas disk
may be fed by dust grains which vaporize at temperatures of 10,000
degrees Kelvin.
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HOW TO BUILD A SOLAR SYSTEM
This is the most widely accepted scenario for out solar system's
formation. It's not clear, as yet, whether this process is rare
or common among other stars.
1. Nearly five billion years ago a huge, amorphous cloud of cold gas
and dust begins to contract, perhaps from a shockwave from a nearby
supernova. Most of the cloud collapses under gravity to form a
rapidly spinning protostar.
2. Residual dust and gas form a broad, flattened disk. Though this
circumstellar disk is supported by centrifugal force, dynamic
processes quickly change the disk. Some of the dust grains spiral
into the protostar and gas is ejected along the star's spin axis as
bipolar jets.
3. Planet-building begins in the denser, cooler portions of the
disk where tiny grains of dust stick together through chance
collisions. This snowballing process continues until bodies form
which are several kilometers across.
4. These clumps continue to grow into protoplanets by collisions
with other bodies, and by gravitationally sweeping up all material
in their vicinity. The larger protoplanets quickly gain mass and
clear out noticeable gaps in the disk, like wide grooves in a
phonograph record. The disk grows clumpier and thinner as the
planets gobbled up or ejected smaller bodies through a "demolition
derby."
5. A strong stellar wind from the newborn Sun sweeps out remaining
dust and gas to leave behind a family of surviving bodies -- the
nine planets.
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ETA CARINAE: A MONSTER STAR AT THE BRINK OF DESTRUCTION
HST has provided a detailed view of a star on the brink of
catastrophe. This blue supergiant, called Eta Carinae, is 100 times
more massive that our Sun, and intrinsically several million times
brighter.
The monster star is embedded in the giant Carina Nebula, one of the
most prominent objects in the southern Milky Way, which contains
clusters of massive hot young stars.
Space Telescope observations reveal complex new structures in a
dusty nebula surrounding the star, and unusual wave-like features in
a jet of material ejected from the star.
Few known stars are as massive as Eta Carinae. Such supergiant
stars burn their nuclear fuel rapidly and are short-lived. They
eventually self-detonate in supernova explosions.
Eta Carinae is a unique "Rosetta Stone" for understanding the
evolution and physics of the massive stars. These stars play a
fundamental role in the evolution of the galaxy. They enrich
interstellar space with heavier elements which are then recycled
into the formation of new stars and, presumably, planets. Most of
the elements that we are made of, including oxygen, carbon and
calcium, were synthesized inside massive stars, and then blown into
space.
For the past century, astronomers have known that Eta Carinae is an
extremely variable star. During an outburst in 1843, Eta Carinae
reached a visual magnitude of -1, making it the second brightest
star in the night sky, behind only Sirius (it has since faded to
sixth magnitude, the limit of naked-eye visibility). During that
outburst, Eta Carinae also blasted a large amount of gas into space.
Until recently, astronomers thought the star itself had exploded in
1843. However, infrared light measurements taken in 1969 shown that
Eta Carinae is the brightest infrared object in the sky behind only
the Sun and the Moon. This means the star is still there, but it is
now hidden inside a dense dusty cloak that resulted from the 1843
outburst.
This material can be seen from the ground today as a small oblong
nebula which is expanding away from the star. This bright inner
nebula has been dubbed the Homunculus or "little man" because it has
appendages that vaguely resemble a head, arms, and feet.
The small scale clumpiness and well defined edges of the Homunculus,
as revealed in the HST image, suggests that it is a very thin shell
of material, more like a soap bubble rather than a filled volume,
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like a baseball.
This dusty shell could have been ejected in a single burst from the
star. Another possibility is that the material was compressed by
fast speed material ejected from the star later in its lifetime. In
either case, the new image shows that the shell has become
fragmented and is clumpy down to the limit of HST's resolution.
The HST image clearly shows the small knots and filaments which
trace the locations of the shock fronts. A well collimated jet of
material can be seen flowing away from Eta Carinae. Two parallel
lines which mark the edges of the jet suggest that it has a narrow
tube-like structure.
The new image also reveals the presence of a fascinating "ladder
like" structure associated with the jet. The "rungs" of the ladder
may represent some kind of wave phenomenon in the flow of material
away from the star. One interpretation is that these are standing
waves, much like sound waves inside an organ pipe. Another
possibility is that they are ripples in the flow of material along
the jet's bow shock, much like the ripples seen in a pond.
Previous studies of the spectrum of the light emitted from the
Homunculus show that this gas is enriched in the elements helium and
nitrogen which are formed in the interiors of massive stars by the
nuclear burning of hydrogen. This means that Eta Carinae is
ejecting processed material from deep within the star. A massive
star reveals such material late in its life.
Eta Carinae will probably explode as a supernova within the next
10,000 years or so. In fact, since Eta Carinae is 9,000 light-years
away, it has probably ALREADY self-detonated. It's likely that a
blast wave of radiation and neutrinos from the Eta Carinae supernova
is heading toward us, right now.
An even more bizarre possibility is that Eta Carinae is so massive
it has instead IMPLODED into a black hole, WITHOUT blasting off its
outer layers.
The answer to Eta Carinae's ultimate fate awaits a future generation
of astronomers. If we are very lucky, we may just witness the
supernova ourselves.
CAPTIONS:
Artist's concept of the variable blue supergiant star Eta Carinae,
one of the brightest and most massive stars in our Galaxy. During
its outburst in 1843, Eta Carinae shed a tremendous amount of gas
and dust into space. Such are, super-massive stars like Eta Carinae
are inherently unstable and explode as supernovas after only a few
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million years. (D. Berry)
Space Telescope's view of Eta Carina --
A. Clumps in the fragmented gas shell are ten times the size of
our Solar System.
B. Ejecta from the star slams into slower moving gas, creating
a ridge of emission.
C. Two lines mark the edges of a tube-like jet of material
flowing outward.
D. This ladder-like structure may represent some kind of wave
phenomenon.
Computer Model -- This is one interpretation of the complex features
seen around the star Eta Carinae. In this model, the Homunculus is
an egg-shaped shell around the star. Lobes of hot dust (red) lie
near the star. A narrow northern jet and broader southern jet
(violet) of outflowing material escape through a hole in the top and
bottom of the shell. The jets collide with slower moving material
and create bowl-shaped shock fronts (blue). (D. Berry)
A negative image of Eta Carinae, as photographed in 1975 with the
four-meter telescope at Cerro Tolo inter-American Observatory, in
Cerro Tolo, Chili. Note the double knot (at the 11:00 o'clock
position). Ejecta from the star slams into slower moving gas to
create a ridge of emission to the lower right. Resolution is one
arc second in this one-minute photographic exposure (Courtesy Nolan
R. Walborn & CTIO)
A Hubble Space Telescope observation of the star Eta Carinae in 1990
with the Wide Field and Planetary Camera (WF/PC), reveals far more
detail. When compared to the foreground, it's clear that the know
is moving away from the star at 2.8 million kilometers per hour.
(Courtesy WF/PC Team)
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CLOSE ENCOUNTERS OF THE STELLAR KIND
Space Telescope has uncovered stars which behave as if they've found
the legendary fountain of youth. Called "blue straggler" stars,
they appear as if they had actually rejuvenated themselves and
changed from old age back to young stars.
Like people, all stars are born, age and die. Our Sun is a typical
middle-aged star. In about 5 billion years it will swell to a red
giant star. The Sun will then lose much of its hydrogen fuel into
space, and its nuclear furnace will burn out. The Sun will then
collapse to a white dwarf -- a glowing cinder, no larger than Earth.
In 1953, astronomer Allan Sandage found a puzzling new population of
stars which seemed to go against these rules of stellar evolution.
He detected hot young blue stars in a globular star cluster.
Sandage was puzzled because globular clusters are veritable stellar
old folks homes. They are full of ancient red giant stars, which
are as old as the universe.
Sandage dubbed the new stars "blue stragglers" because they looked
like they were left behind by other blue stars which long ago
evolved on to cool red giant stars.
Astronomers have been puzzled by blue stragglers ever since then.
The simplest explanation is also the least likely -- that blue
stragglers formed much later in a cluster's life. When globular
clusters formed many billions of years ago, they were stripped of
the residual dust needed for new star formation. So all of a
cluster's stars should have formed at about the same time.
Another idea is that blue stragglers are somehow far more efficient
at mixing their internal hydrogen supply so that they can shine much
longer than stars normally do.
Stars are inefficient nuclear furnaces. A typical star consumes
only about 10 percent of its hydrogen fuel through nuclear fusion.
Stars like our Sun "puff off" much of the rest of the remaining
hydrogen late in their lives. Blue stragglers would need to
approach an extraordinary efficiency of 100 percent to essentially
"stretch out" their youth, and burn brightly for billions of years
longer than normal.
A star could more efficiently use its fuel supply if it were
captured by another star. In a binary star system, the less massive
star would siphon fresh hydrogen from its more massive and hence
faster evolving companion star. With the new fuel supply, the
smaller star would heat, growing bluer and hotter.
In stellar encounters which are more nearly head-on collisions, the
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stars might actually merge, mixing their nuclear fuel and "re
stoking" the fires of nuclear fusion.
The Hubble Space Telescope observations seem to support this idea of
stellar capture and merger. HST's high resolution and UV
sensitivity to ultraviolet light have been utilized to uncover many
blue stragglers concentrated in the core of a nearby globular
cluster called 47 Tucanae. HST resolves about 600 stars in the core
where ground-based images yield only a few dozen stars. When
astronomers compared HST images with visible light images of 47
Tucanae, they discovered 21 blue straggler stars that are
exceptionally bright in ultraviolet light.
Though the blue stragglers represent only a tiny fraction of the
cluster's population, they could play an important role in the
cluster's dynamic evolution. If blue stragglers are binaries, they
whirl about each other, like a pair of ice skaters. As they do,
they become tremendous batteries of kinetic energy. Other stars in
the cluster can tap this energy and get a boost in speed. Just a
few blue stragglers can stir up the motion of thousands of other
stars in the cluster, like a pair of egg beaters. So besides
rejuvenating each other, blue stragglers may keep the cluster active
and dynamic.
Space Telescope will be used to probe other globular clusters to
look for concentrations of blue stragglers. If they typically form,
this may explain why the cores of globular clusters aren't even more
densely packed with stars. The cores may in fact collapse but then
rebound due to the presence of blue straggler stars, like a rubber
ball that has been squeezed and then relaxed.
CAPTIONS:
Space Telescope's Faint Object Camera resolves at least 600 stars at
the core of the globular cluster 47 Tucanae. Some of the brightest
stars in this ultraviolet picture are blue stragglers which have
congregated at the core. The field of view is less than one light
year across (Courtesy F. Paresce & FOC Team)
One of the most magnificent globular clusters in the heavens, 47
Tucanae contains several million stars, which are concentrated
together in a "beehive swarm" only a few hundred light-years across.
One of the closest globular clusters (15,000 light years) 47 Tucanae
is a favorite target for studying stellar dynamics. (Courtesy
European Southern Observatory)
Stars are so crammed together at the core of 47 Tucanae that even
under the best observing conditions, ground-based telescopes can
resolve only a few dozen stars within the same small region that
Space Telescope has probed (box). Blue straggler stars are hidden
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among the cluster's red giant stars in this visible light picture.
Image is 4 light-years across. (Courtesy Georges Meylan & European
Southern Observatory)
Astronomers estimate the age of a star cluster by taking an
inventory of the stars it contains. Stars are classified according
to color temperature (blue stars are hot; red stars, cool) and
intrinsic brightness, or luminosity. This graph, called the
Hertzprung-Russell diagram, is of a typical stellar population in a
young cluster. Most of the stars are in the stable portion of their
lives and hence lie along a track called the Main Sequence.
This H-R diagram shows the stellar population in 47 Tucanae. The
arm of the main sequence containing the bright blue stars has almost
disappeared. These stars have evolved to ancient red giants.
However, there is a small population of blue stars that look like
they are trailing or left behind by other blue stars which long ago
evolved to the red giant stage.
The sky is ablaze with stars in this imaginary view from the surface
of a planet located at the heart of globular star cluster 47
Tucanae. The average distance between stars here is a fraction of a
light-year. Many of the red giant stars appear brighter than Venus
does from Earth. Countless, fainter red dwarf stars blanket the
heavens. Blue straggler stars stand out like sparkling diamonds.
In the foreground, a blue straggler forms as a result of a chance
close encounter between two Sun-like stars (D. Berry)
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