Black holes are hungry. They grow bigger as they gobble up cosmic debris from nearby stars. Anything too close can get sucked in.
They are the remains of massive stars, scrunched up on themselves until they become infinitely dense. Anything that gets too close gets trapped by their powerful gravity. Even light cannot escape once it passes a critical point, the "event horizon".
That makes them hard to spot. Often the best way is to monitor the movements of nearby stars.
However, one black hole has recently made itself thoroughly conspicuous. After lying dormant for 26 years, it has begun emitting a series of bright cosmic burps.
The first of these "X-ray novas" was observed two weeks ago. Astronomers monitoring the Swift telescope noticed that a strange new bright object had appeared in the sky.
At first the team didn't know what these bright flashes were. They alerted their colleagues, and several other telescopes began monitoring the flares. Some lasted several minutes, others went on for hours.
The astronomers have now found that the flares are coming from an intensely hot disk around the black hole.
This is happening because the black hole is consuming gas and dust from a nearby star, but not all of this stuff is going in. Instead, some of the material forms a ring around the black hole, called an accretion disk. You can see a simulation of this below.
This ring builds up over time, says Swift's director of mission operations John Nousek of Penn State University in Philadelphia, US.
"When it builds up enough material, you get a condition that looks very similar to when a hydrogen bomb has exploded," says Nousek. "A lot of hydrogen under intense pressure and heat makes an explosion that looks like a new star."
This blast is so powerful, it blows away all of the material that had been resting near the black hole.
These types of eruptions are rarely observed.
"Some kind of hiccup happened a couple of weeks ago and suddenly the star has started spewing gas onto the black hole," says Neil Gehrels of NASA's Goddard Space Flight Center in Maryland, US.
Observing these explosions helps researchers learn more about how black holes change over time.
They don't have long. As quickly as the eruptions began, they have now stopped. The black hole has gone back to sleep and we don't know when it will wake up again.