Into a Black Hole

In the sky this week…

  • Jupiter, Mars and Venus form a procession out of the sunrise glow.
  • Saturn is high in the south.
  • The Moon will be New on July 1.

Ken Tapping, June 29, 2011

On March 28 astronomers witnessed a cosmic tragedy — the death of a star. In this case the star did not end its life by becoming a nova or supernova, which is how most stars end. This one was swallowed by a black hole.

The astronomers were looking for objects known as “gamma ray bursters.” Satellites pick up pulses of gamma rays; the trick is to quickly point telescopes in the direction that the gamma rays came from in order to see what sort of object produced them.

Gamma rays are the highest-energy form of electromagnetic waves. They are extremely dangerous to living tissue, so it is very fortunate that our atmosphere blocks any cosmic gamma rays heading in our direction. These rays are given off by some radioactive elements and are one of the reasons we need extensive shielding around nuclear reactors. They are also produced during the explosion of nuclear bombs. However, the biggest reactor or nuclear bomb we have here on Earth would not be even remotely observable at cosmic distances. Something very dramatic must be occurring to produce the strong bursts our satellites are detecting. The collapse of ageing giant stars seems to be the source of most gamma ray bursts. Even with the huge amount of energy available, gamma ray bursts are usually short, typically less than a minute, or often less than a second.

Artist’s rendition of an energy rich black hole

Artist’s rendition of an energy rich black hole

The event observed on March 28 started like a typical gamma ray burst, but instead of ending seconds later, it kept on going for days and eventually a month. This could not be a collapsing star; it had to be something much more energetic and drawn out. Astronomers were observing a dying star, but one experiencing a very different fate; it was being torn apart and swallowed by a Black Hole.

Black holes are masses of material that have become so compressed under their own gravity that not even their light can escape. Our galaxy and most others have massive black holes in their cores. Usually collapsed giant stars, there are also many others scattered around the cosmos. These black holes are a serious threat to anything that comes within close proximity, which is what happened to this unfortunate star.

Initially the star just spiraled closer and closer to the black hole. However, it eventually reached a point where the black hole’s gravitational pull was strong enough to start tearing the star apart — its outer layers going first. The star’s material formed a belt around the black hole and was pulled in, bit by bit, with the whole process taking about a month. Just before vanishing, the material got incredibly hot, radiating gamma rays, and then it was gone.  Black holes are probably the most efficient energy machines in the universe, achieving almost total conversion of the mass it sucks in into energy.

This discovery was a matter of looking in the right direction at the right time. It is sobering to think that there is a high probability that this star had planets. However, if any of them bore life, it would have been snuffed out by the radiation from the black hole long before the system was destroyed.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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