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Do black holes suck?

Apparently there is a fearsome force that lurks in deep space ready to drag all matter to a crushing, violent end. But Dr Karl isn't so sure and will boldly go where no one has gone before.

Black hole

Artist's impression of a black hole (Source: Reuters)

Black hole! Even the name has a certain menace about it, not to mention their reputation.

Most of us think they are some kind of cosmic vacuum cleaner, sucking anything around them into their voracious maw.

But if our sun were to be replaced by a black hole weighing the same as our sun, the Earth would still travel in the same orbit. The only real difference is that it would be cold and dark, because the black hole would not emit any heat or light.

To understand a black hole, think about the concept of "escape velocity".

If you throw a rock upwards, pretty soon it falls back down again. If you throw it upwards 10-times faster, the rock takes longer to fall down — but it will still fall back to earth.

But if you propel the rock up with a speed of 11.2 kilometres per second, it will never fall back down to earth. So the escape velocity of planet Earth is 11.2 kilometres per second.

A black hole has such a massive density compared to its size, that the escape velocity is equal to the speed of light, or even greater. This means that light itself can never escape, and so we can never see it, hence the name black hole.

The size of our sun is about 1.4 million kilometres across. There are two forces keeping it balanced at that size — the outward pressure of the nuclear fires, and the inward suck of gravity.

These nuclear fires consume about 600 million tonnes of hydrogen each second. At some stage in the distant future (several billions of years away) the nuclear fires will run out of fuel. So gravity will be the dominant force, and the sun will shrink.

As part of the natural evolution of stars, at some late stage in the nuclear burning, most stars toss much of their mass out into space.

If, at the end of nuclear burning, the amount of mass remaining in a star is less than one solar mass (the current mass of our sun), then gravity shrinks that star down into an object called a white dwarf.

Most white dwarfs are roughly the size of planet Earth — about 12,000 kilometres across — but with a mass of about 60% of the sun.

This gives them a density about one million times greater than water.

The astronomers have found about 9000 white dwarfs — in fact, they make up about 6% of the stars in our immediate neighbourhood.

A white dwarf is the fate of all stars that are left with a mass less than 1.4 solar masses, after the nuclear fires fade. In other words, about 97% of all the stars in our galaxy will end up as white dwarfs.

But suppose that you start with a heavier star. If, after the nuclear fires run down, the mass of the star is 1.4–2 solar masses, its gravity will be stronger, and it will shrink itself even smaller down to a neutron star, about 10–20 kilometres across.

A neutron star is about 100 million–1 billion-times denser than a white dwarf. This gives them a typical escape velocity of about half of the speed of light.

So far, the astronomers have found about 2000 neutron stars in our Milky Way galaxy, and in its neighbouring galaxies, the Magellanic Clouds.

But things are very different if, after all the nuclear burning, the star weighs more than about two solar masses.

In this case, it will shrink so far that it will collapse into a point with infinite density and zero volume — let me just emphasise that, not just a colossally high density, but infinite.

The technical name for this is a "singularity" — the famous black hole. This point is hidden inside the so-called "event horizon".

Black holes can cover a range of sizes. If our Sun were to be turned into a black hole, the event horizon would be about six kilometres across.

If an object such as a comet, or even a planet, were on a collision course with the event horizon, yes, it would vanish inside the black hole.

But if it started off in a stable orbit around the sun, then it would continue in the same stable orbit after the sun had been magically turned into a black hole.

So black holes are very weird, but they do not suck everything into themselves.

But, as with all the forces of darkness, it's better to stay out of their way.

Tags: astronomy-space, blackholes, planets-and-asteroids, the-universe

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Published 23 July 2008

© 2014 Karl S. Kruszelnicki Pty Ltd