Hubble Finds Mysterious Disk of Blue Stars Around Black Hole
Astronomers using NASA's Hubble Space Telescope have identified the source
of a mysterious blue light surrounding a super-massive black hole in our
neighboring Andromeda Galaxy (M31). The strange light has puzzled astronomers for
more than a decade.
Image above: Andromeda and its complex core can be seen in the illustration and two images [above]. The illustration [lower, right] shows the disk of blue stars nested inside a larger ring of red stars. The Hubble photo [upper, right] reveals Andromeda’s bright core. The image at left shows the entire galaxy. Credits: (Image left) copyright 2002 R. Gendler, Photo by Robert Gendler/ (Image top right) NASA/ESA/T. Lauer (NOAO/AURA/NSF)/(Image bottom right) NASA/ESA/A. Feild (STScI)
The blue light is coming from a disk of hot, young stars whipping around the
black hole in much the same way as planets in our solar system are revolving
around the sun. Astronomers are perplexed about how the pancake-shaped disk of
stars could form so close to a giant black hole. In such a hostile environment,
the black hole's tidal forces should tear matter apart, making it difficult for
gas and dust to collapse and form stars. These observations, astronomers say, may
provide clues to the activities in the cores of more distant galaxies.
By finding the disk of stars astronomers have collected what they say is ironclad
evidence for the existence of the super-massive black hole. The evidence has
helped astronomers rule out all alternative theories for the dark mass in
Andromeda's core, which scientists have long suspected was a black hole.
"Now that we have proven that the black hole is at the center of the disk of blue
stars, the formation of these stars becomes hard to understand," said Ralph
Bender of the Max Planck Institute for Extraterrestrial Physics in Garching,
Germany. "Gas that might form stars must spin around the black hole so quickly
that star formation looks almost impossible. But the stars are there," he added.
"Seeing these stars is like watching a magician pulling a rabbit out of a hat.
You know it happened but you don't know how it happened," said Todd Lauer of the
National Optical Astronomy Observatory in Tucson, Ariz. He and a team of
astronomers, led by Bender and John Kormendy of the University of Texas in
Austin, made the Hubble observations. The results will be published in the
September 20, 2005, issue of the Astrophysical Journal.
As far back as 1995, Hubble revealed a strange blue light in Andromeda's core
that astronomers said might have come from a single, bright blue star or perhaps
from a more exotic energetic process. Follow-up observations with Hubble in 1998
suggested that the light is a cluster of blue stars.
New observations by the Hubble's Imaging Spectrograph (STIS) reveal that the blue
light consists of more than 400 stars that formed in a burst of activity about
200 million years ago. The stars are tightly packed in a disk only a light-year
across. The disk is nested inside a ring of older, redder stars, seen in previous
The astronomers also used STIS to measure the velocities of those stars. They
obtained the stars' speeds by calculating how much their light waves are
stretched and compressed as they travel around the black hole. Under the black
hole's gravitational grip, the stars are traveling very fast: 2.2 million miles
an hour. They are moving so fast it would take the stars 40 seconds to circle the
earth and only six minutes to arrive at the moon.
While astronomers are surprised to find a disk of stars swirling around a super-
massive black hole, they believe the puzzling architecture may not be that
unusual. "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," he added.
For imagery and additional information about the research on the Internet, visit:
Goddard Space Flight Center