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Skywatching

Black hole eats a star a day

Every few years there seems to be a significant astronomical discovery.

This then becomes a hot topic for research as people try to learn more about it. At the moment the hot topic is probably fast radio bursts (FRBs), millisecond bursts of radio waves coming from sources all over the sky.

In the 1960s the interest was in quasars. These objects are tiny, appear starlike, but lie millions or billions of light years away, and are radiating immense amounts of energy.

The radio emissions from these vary over months or even days, which means they are very small, so small that a new radio astronomical technique, called very long baseline interferometry (VLBI) was developed mainly in an attempt to measure their sizes.

In VLBI carefully timed information from radio telescopes thousands of kilometres apart are brought together and combined in a way that emulates a radio telescope thousands of kilometres across. Canada was the first country to succeed with this technique.

Quasars are small, maybe with diameters not much bigger than the Solar System. The only engine we know of capable of generating such a huge amount of energy in such a small volume is a black hole.

Black holes are formed when matter becomes extremely compressed. This can happen in very massive objects, where the core is compressed by the overlying weight of material. It can also happen in the death throes of giant stars.

When they run out of fuel, they collapse and explode. Huge shock waves travel inward, compressing the core to the point where it may become a black hole.

The gravity at the surface of an object depends on its mass and diameter. If we concentrate that mass in a smaller diameter, the surface gravity increases dramatically.

Eventually, the surface gravity may become large enough to distort the fabric of space-time until it closes off, enclosing the object in a shell known as an event horizon.

The Earth would become a black hole if it were compressed to a diameter of 1.75 cm.

The event horizon allows the gravity of the black hole to pull things in, but nothing gets out, hence the name, black hole. As things fall in, they get extremely hot, and by the time they vanish inside, the amount of energy produced is close to what we would get if we totally annihilated that matter.

This makes black holes the most efficient energy generators we know of. What goes inside, stays inside. The black hole at the centre of our galaxy has swallowed stars, and we can see others that are on their way in, on a one-way trip.

However, luckily, our black hole is too far away, too small and not energetic enough to pose problems for us, at least for now.

Quasars are located in the cores of galaxies that happen to contain particularly large and active black holes, which gorge on anything that comes too close, and radiate prodigious amounts of energy.

The blast of energy can push gas and dust out of the galaxy. Since this is what is used to make stars and planets, this halts their creation, and may render any existing planets uninhabitable.

There is a distant quasar located more than 13 billion light years away, which means we see it as it was more than 13 billion years ago, less than 700 million years after the Big Bang. It has a really massive black hole in the centre.

The blast of particles and radiation from it has blown away all the gas and dust. The formation of stars, planets and living things ended there before it had even got started.

The most massive black hole known so far is on average swallowing a star every day. A video showing a computer simulation of a black hole eating a star can be found at this URL. It was made by the NASA Goddard Space Flight Center.

So far, it looks as though there is little chance of the Sun suffering such a fate, hopefully.

  • Mars is high in the south after dark.
  • You might glimpse Mercury, very low in the sunset glow.
  • The Moon will be Full on the 28th.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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