Medium-size black holes actually do exist, according to
the latest findings from NASA’s Hubble Space Telescope, but
scientists had to look in some unexpected places to find

The previously undiscovered black holes provide an important
link that sheds light on the way black holes grow. Even more
odd, these new black holes were found in the cores of
glittering, “beehive” swarms of stars — called globular star
clusters — that orbit our Milky Way and other galaxies.

The new findings promise a better understanding of how
galaxies and globular clusters first formed billions of years
ago. Globular star clusters contain the oldest stars in the
universe. If globulars have black holes now, then globulars
most likely had black holes when they originally formed. The
new results indicate that the very sedate, elderly
environments of globular clusters house these exotic objects,
quite unlike the violent cores of some galaxies.

“These findings may be telling us something very deep about
the formation of star clusters and black holes in the early
universe,” said Roeland Van Der Marel of the Space Telescope
Science Institute in Baltimore. “Black holes are even more
common in the universe than previously thought.”

“Not only will we learn about the formation of the black
holes, but these new data from Hubble help us connect
globular clusters to galaxies, providing information on one
of the most important unsolved problems in astronomy today:
how galaxy structure forms in the universe,” added Michael
Rich of the University of California, Los Angeles.

This is reinforced by the uncanny fact found by these
investigations that a black hole’s mass is proportional to
the mass of the stellar environment it inhabits. Supermassive
black holes found by Hubble in the centers of galaxies
represent about 0.5 percent of the galaxies’ mass.
Amazingly, the black holes now found in star clusters, which
are 10,000 times less massive than a galaxy, also obey this
trend. It appears that there is some yet-to-be-discovered
underlying process that ties a black hole to its host in a
fundamental way. Nature is providing a big clue as to how
these systems and their black holes form.

“The intermediate-mass black holes that have now been found
with Hubble may be the building blocks of the supermassive
black holes that dwell in the centers of most galaxies,” said
Karl Gebhardt of the University of Texas at Austin.

Van Der Marel led a team that uncovered a black hole in the
center of the globular star cluster M15, 32,000 light-years
away in the constellation Pegasus. His collaborator Joris
Gerssen, also of the Space Telescope Science Institute,
pinned down the black hole’s mass at 4,000 times that of our

In a separate observing program, a team led by Rich, and
including Gebhardt and Luis Ho of the Carnegie Institution of
Washington, found a 20,000-solar-mass black hole in the giant
globular cluster G1, located 70 times farther — 2.2 million
light-years away — in the neighboring Andromeda galaxy. By
contrast, stellar-mass black holes are only a few times the
mass of our Sun, and galactic-center black holes can be
millions or billions of times more massive than our Sun.

Previously, X-ray observations from the ROSAT Observatory and
NASA’s Chandra Observatory have identified ultra-bright X-ray
sources that could also be interpreted as intermediate-mass
black holes in star-forming galaxies. However, alternative
interpretations for these X-ray sources continue to exist. By
contrast, Hubble’s measurements are based on the velocities
of stars whirling around in the dense cores of globular
clusters, which yield a direct measurement of the black hole

The M15 globular star cluster is close enough that individual
star speeds can be measured. By contrast, the G1 observations
rely on measurements of the collective properties of many
stars. In either case, a black hole can be identified by
using a common Hubble black-hole-hunting technique. Stars
close to the black-hole “whirlpool” orbit at a faster rate.

Astronomers have searched for black holes in globular
clusters for nearly 30 years. The roadblock has been the fact
that ground-based telescopes cannot easily resolve the stars
closest to the suspected black hole. As far back as the
1970s, hunting for globular-cluster black holes was
recognized as a task suited for Hubble Space Telescope’s
exquisite resolution, which is needed for looking close to a
black hole.

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