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A star cluster that is home to a middleweight black hole.

The newly identified black hole, circled, sits within a star cluster.

Image courtesy S. Farrell, U. Sydney/U. Leicester/ESA/NASA

Andrew Fazekas

for National Geographic News

Published July 5, 2012

There's a strange new brute on the celestial block—the middleweight black hole, a new study says.

After nearly three years of spying a superbright object nearly 300 million light-years away, astronomers with NASA's Chandra X-ray Observatory and SWIFT telescope recently announced the discovery of HLX-1, the first representative of a new type of black hole. (See black hole pictures.)

(Related: "New Class of Black Hole Found? [2009].")

Until recently, black holes were thought to come in only two sizes: Small stellar varieties that are several times heavier than our sun, and supermassive black holes that pack the gravitational punch of many million suns—large enough to swallow our entire solar system.

Notorious for ripping apart and swallowing stars, extra-large black holes live exclusively in the hearts of most galaxies, including our own Milky Way.

The new middleweight black hole is between these two types—equal to the matter of about 90,000 suns.

New Black Hole Relics of the Early Universe?

An international team, who discovered HLX-1 "almost by accident" in 2009, noticed the object was pumping out copious amounts of x-rays and radio flares—not from within the core of its host spiral galaxy, but some 12,000 light years beyond.

"Our observations from 2009 and 2010 showed that HLX-1 behaves similarly to the stellar [low] mass black holes, so we worked out when we should be expecting to see radio flares from HLX-1, and when we made more observations in August and September 2011, we did," said study leader Natalie Webb, of the Centre d'Etude Spatiale des Rayonnements in France.

The origin of these intermediate black holes may lie in centers of globular clusters, where hundreds of thousands of stars are densely packed together by gravity.

Alternatively, the middleweights may be true ancient relics of the universe, formed by the very earliest stars, said Webb, whose study appears tomorrow in the journal Science.

(Read more about the origins of the universe.)

"At the dawn of the universe, very massive stars may have existed—maybe as much as ten thousand times the mass of our sun—and these stars would have 
a very short lifetime and end their lives as intermediate mass black 
holes," Webb said.

Middleweights May Explain Black Hole Giants

The very existence of middleweight black holes may also be key in solving how their supermassive cousins formed. (Read about NASA's new "black hole hunter.")

For instance, Webb suspects the middleweights may in fact be the supermassive black holes' progenitors.

These giants may either form when a single intermediate black hole gobbles enough matter to grow into a supermassive black hole with at least a million solar masses.

Or, a number of intermediate black holes "merged in the early universe to form the supermassive black holes we see today," Webb said.

Either way, without further surveys, it's impossible to tell how common middleweight black holes are across the universe.

"It's difficult to assess observationally, as [HLX-1] is the only good candidate," Webb said.

"But some people think that there may be hundreds in each and every galaxy."

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