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MOST black holes come in one of two sizes—regular and extra-super-huge.
Astronomers have long searched for an intermediate-sized black hole and now
they’ve found one. Studying this middleweight should help them understand how
the supermassive black holes at the centre of all galaxies and quasars are
formed.

Most black holes are born from a single star, but those at the hubs of
galaxies can be millions or billions of times more massive than the Sun. The new
middleweight black hole weighs in at roughly 500 solar masses.

At a press conference at NASA headquarters in Washington DC last week, a team
described how they found it when they trained the satellite-borne Chandra X-ray
Observatory on the nearby galaxy M82. Other satellites had seen intense X-rays
coming from this oddball galaxy, but Chandra pinpointed a handful of bright
spots. The brightest of these spots was 100 times more intense than the most
powerful star-sized black hole in our own Galaxy.

The intensity of the X-rays rose and fell erratically over several months,
showing that they did not come from a single fading explosion. The intensity
also oscillated with a period of 10 minutes, the telltale sign of a black hole
slurping up hot gas from the middle of a rotating and undulating disc.

“This is the first object that we have in the middle [mass range] and the
fact that we know this is due to this 10-minute variability. That’s important,”
says Richard Griffiths of Carnegie Mellon University in Pittsburgh, who with
Andrew Ptak found evidence of M82’s black hole last year.

The new black hole provides a solid lead to a long-standing mystery, says
Stuart Shapiro, an astrophysicist at the University of Illinois at
Urbana-Champaign. “Every galaxy and quasar has a big black hole in the centre of
it,” he says. “How did it get there? How did it form?”

One curious aspect of the new black hole may help answer these questions.
It’s 600 light years from the centre of M82 and this slightly off-centre
position points to the galaxy’s violent past—perhaps even to the
conception of the black hole. Three hundred million years ago M82 collided with
its neighbour, M81. The collision seems to have created a dense region of gas in
which stars are born 10 times more often than in our own Galaxy. The black hole
is in the middle of this stellar nursery.

Ordinary stars in such a crowded environment can collide, merge and
ultimately collapse into a black hole, says Shapiro. Indeed, 10 years ago he
used computer simulations to show that this process could produce a black hole
of several hundred solar masses. Big black holes might grow into giants by
gobbling up other stars or by merging with one another. However, Martin Ward, an
astronomer at the University of Leicester and leader of the observing team, says
there are other theories of how jumbo black holes form. “I predict,” Ward says,
“that theorists will dust off their theories and adjust them a bit, because we
now have some hard numbers to work with.”