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Skywatching

A hole in time and space

We have just seen released one of the most important astronomical images in years: an image of a black hole.

It shows a black, circular blob surrounded by a glowing ring of hot material. Calculations by Albert Einstein and others suggested they exist, and we have indirect astronomical evidence they do, but this is the first time we have actually seen one.

This triumph is the result of an international effort involving eight radio telescopes. The black hole in the image lies 55 million light years away, in the core of the galaxy Messier 87, in the constellation of Virgo.

It has a mass about 6.5 billion times that of the sun and is bigger than the solar system. Black holes exist because of two things:

  • a process called runaway gravitational collapse
  • space-time is drastically distorted by strong gravitational fields.

Matter, as we see it around us every day, is made up of atoms. Each one of these consists of a nucleus, a collection of protons and neutrons, with a cloud of electrons orbiting around it.

However, the main ingredient is simply empty space. Here on Earth, atoms are generally not compressible. However, in stars and galaxies this is not true.

When small stars like the sun run out of fuel, they collapse, and the weight of the infalling material compresses their cores so that the atoms shrink as much as they can while still being atoms.

The result is a body about the size of the Earth made of material so heavy a teaspoonful weighs a few tonnes. The star has become a white dwarf.

Bigger stars die more spectacularly. They explode, producing shock waves that blast material into space, and also shock waves going inward. These can compress the cores so that the atoms collapse into neutrons.

The star shrinks to an object a few kilometres across, where a teaspoonful may weigh about a billion tonnes. We have a neutron star.

If the star is really big, the shock waves in its death explosion can push the core material past the trigger point for runaway gravitational collapse. Pulled by its own gravity, the core will then shrink without limit, down to something infinitely small and infinitely dense — a singularity.

However, as this happens, the growing intensity of its gravitational field will distort space-time to that it forms an envelope completely enclosing the singularity. Light and material will go in, but nothing can come out.

The event horizon, containing most of the mass of the star, appears dead black. We have a black hole.

These black holes are too small to image unless we get dangerously close to them. However, black holes can be formed another way, in the cores of galaxies, and can be much larger.

Most galaxies, including ours, have black holes in their centres. They have probably been there since the galaxies formed from collapses of gas and dust clouds in the young universe.

These ones can be really big, millions or more times the mass of the sun. They probably formed when the accumulations of gas and dust got so big that the pressures in their cores due to the weight of overlying material pushed them over the trigger point for gravitational collapse.

It would be easier to try imaging one of these.

The one at the core of our galaxy is an obvious target, but for the first black hole image, it pays to search for a big one. It has long been suspected that the galaxy Messier 87 is host to a particularly big black hole.

That is why it was selected for the first attempt to image one. Fascinatingly, the result looks pretty well what was expected, a dark object with hot material spiralling into it, so although there is a lot of science to do before we understand black holes, it seems we are going in the right direction.

  • Mars lies low and inconspicuous in the west after dark.
  • Jupiter, shining like a searchlight, rises around 1 a.m.
  • Saturn comes up 3 a,m.
  • The moon will reach last quarter on the 26th.

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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]



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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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