A new
discovery of a middleweight black hole suggests black holes come in all sizes.
Black holes
can't be seen, but they're detected by noting their effects on stars or gas
around them. They're so dense that nothing, including light, escapes them. Only
two classes of black holes are firmly established to exist: Stellar black holes
typically weigh a few times the mass of the sun; supermassive black holes are
loaded with millions or billions of solar masses.
Astronomers
have long
debated the existence of a class of middleweight black holes, which could
be a missing link in the evolution of the universe's first stellar black holes
to supermassive black
holes that anchor most major galaxies.
Data
gathered by instruments on the Hubble Space Telescope and the Gemini South
telescope in Chile show that the sky's largest star cluster, Omega Centauri,
might harbor an elusive intermediate-mass black hole in its center.
"This
result shows that there is a continuous range of masses for black holes, from
supermassive, to intermediate-mass, to small stellar mass types," said astronomer
Eva Noyola of the Max-Planck Institute for Extraterrestrial Physics in Germany,
and leader of the team that made the discovery, which was announced today.
Located
17,000 light-years away, Omega Centauri has a long history of perplexing
astronomers. Nearly 2,000 years ago, scientists listed it as a single star. In
1677, Edmond Halley revised Omega's identity as a nebula, and it wasn't until
the 1830s that English astronomer John Herschel recognized it as a globular
cluster.
The entire
cluster contains about 10 million stars tightly bound by gravity, making Omega
Centauri among the biggest and most massive of some 200 globular clusters
orbiting the Milky Way. These clusters tend to house mostly ancient stars and
are thought to be remnants of the early universe.
By
measuring the velocities of stars in the cluster's center, Noyola and her
colleagues calculated Omega's total mass to be far higher than expected based
on the number and type of stars seen.
The
researchers suspect the missing mass comes from a black hole weighing 40,000
solar masses at the center of the cluster.
Other
possible explanations for the missing mass include a collection of unseen
burnt-out stars, such as white dwarfs or neutron stars, or a group of stars
with elongated orbits that would make the stars closest to the center appear to
speed up and result in an overestimation of mass. Noyola says these
alternatives are unlikely, partially due to the fact that both scenarios would be
very short-lived.
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