- Stars race around a black hole at the center of the Andromeda
galaxy so fast that they could go the distance from Earth to the Moon in
six minutes.
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- The finding, announced today, solves a mystery over the
source of strange blue light coming from Andromeda's center. But it generates
a new puzzle: The stars' phenomenal orbital velocity suggests they should
never have formed in the first place.
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- Astronomers first spotted the blue light near Andromeda's
core in 1995. Three years later, another group determined that the light
emanated from a cluster of hot, young stars. Nobody knew how many were
involved.
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- New data from the Hubble Space Telescope reveal more
than 400 blue stars that formed in a burst of activity roughly 200 million
years ago, astronomers said.
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- The stars are packed into a disk that is just 1 light-year
across.
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- That's amazingly compact by cosmic standards. A light-year
is the distance light travels in a year, about 6 trillion miles (10 trillion
kilometers). The nearest star to our Sun is about 4.3 light-years away.
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- Unlikely Setup
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- "The blue stars in the disk are so short-lived that
it is unlikely in the long 12-billion-year history of Andromeda that such
a short-lived disk would appear now," said Tod Lauer of the National
Optical Astronomy Observatory in Tucson, Arizona. "We think that the
mechanism that formed this disk of stars probably formed other stellar
disks in the past and will trigger them again in the future. We still don't
know, however, how such a disk could form in the first place. It still
remains an enigma."
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- To determine the velocity of the stars, Hubble was used
to calculate the compression and stretching of the light waves coming from
the stars. This Doppler technique is based on the same phenomenon that
changes the sound of an ambulance siren that's moving first toward you
and then away.
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- The stars are traveling at 2.2 million mph (1,000 kilometers
per second). They could circle the Earth in 40 seconds. The fastest among
them orbit the center of Andromeda in 100 years.
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- Control Freak
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- The stellar speed is controlled by the galaxy's central
black hole. Such frenetic activity was thought to prevent star formation.
Stars form when a knot of gas and dust collapses under its own gravity.
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- "Gas that might form stars must spin around the
black hole so quickly -- and so much more quickly near the black hole than
farther out -- that star formation looks almost impossible. But the stars
are there," said team member Ralf Bender of the Max Planck Institute
for Extraterrestrial Physics in Garching, Germany.
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- The observations may provide clues to the activities
in the cores of more distant galaxies that cannot be observed so well.
At about 2.5 million light-years away, Andromeda is the nearest large galaxy
to our own Milky Way.
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- Lauer led an international team of researchers. The results
are published today in the Astrophysical Journal.
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- Common Activity?
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- The new observations also provide clinching evidence
that Andromeda's central dark object is a black hole and not something
else. It packs a mass of 140 million suns, the new study finds.
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- Ultimately, the strange goings-on in Andromeda may turn
out to be commonplace.
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- "The dynamics within the core of this neighboring
galaxy may be more common than we think," Lauer said. "Our own
Milky Way apparently has even younger stars close to its own black hole.
It seems unlikely that only the closest two big galaxies should have this
odd activity. So this behavior may not be the exception but the rule. And
we have found other galaxies that have a double nucleus."
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- Hubble is a cooperative effort between NASA and the European
Space Agency.
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- Copyright © 2005 SPACE.com.
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