Holes in space

Black holes are distortions in the fabric of space-time produced by intense gravitational fields.

This distortions are "closed,” meaning nothing, not even light, can get out. Basically, any body can be turned into a black hole by compressing it enough.

For example, if we compressed the Earth down to a diameter of around nine millimetres, it would become a black hole. Can bodies be compressed that much? Surprisingly, the answer is yes.

We can do it in two ways. One is by simply piling on more and more material, and the other compression by a converging shock wave. It can also be a combination of the two.

Any body in space, including the Earth, Moon, Sun and other planets and stars, have gravity pulling their material inwards and holding them together. Their cores are compressed by the weight of the overlying material.

However, in these instances the cores are easily able to support the weight. Let’s conduct a thought experiment where we just keep piling on more and more material. The core pressure rises and the compression increases.

Matter is composed of atoms: each consisting of a nucleus surrounded by shells of orbiting electrons. However, atoms are mostly just empty space.

If the pressure gets great enough, the electrons move inward to the closest stable paths around the nucleus, shrinking the atom and relieving some of the pressure.  Matter in this condition is known as "degenerate.”

Dwarf stars, like the Sun, will turn into this when they run out of fuels and collapse. The Sun will shrink to a white dwarf star, about the size of the Earth, and so compressed that a teaspoonful will weigh a few tonnes.

If we continue adding material, we can reach a point where the pressure collapses the atoms completely. The neutrons in the atoms remain unchanged, but each proton combines with an electron and becomes a neutron, so we end up with a ball of neutrons.

If the Sun became a neutron star, it would be just a few kilometres in diameter and a teaspoonful of its material would weigh in at roughly 2e15 grams — a two followed by 15 zeros.

If we keep piling on more material, we reach a point where the material is totally unable to resist the compression, and as far as our physics tells us, will shrink forever. It is almost certainly not "forever,” but it is definitely a lot.

Eventually, as the body shrinks, its gravity becomes intense enough to close it off from the rest of the universe. We have a black hole.

It is possible that the giant black holes at the cores of galaxies, including ours, grew to their masses of millions of times the mass of the Sun by grabbing a lot of material as their host galaxies formed in the young universe.

However, in ageing stars, black holes are formed when their cores are compressed by inward-moving shock waves due to huge explosions in their outer layers.

This means we should see two classes of black hole:

• Small ones of a few solar masses, formed during the deaths of giant stars
• Millions of solar mass black holes at the centre of galaxies.

It makes sense, but that does not explain the recent detection of the collision of two black holes of 66 and 85 solar masses. Where did they come from?

We might think that black holes are the essence of modern astronomy. However, their existence was first postulated by John Mitchell, an English clergyman, in November 1784. 

However, his science was a bit off, and it took Albert Einstein to give us give us the physics describing what must be among the most exotic objects in the universe.

• Sept. 22, the Sun crosses the equator, heading south, marking the autumn equinox. It will continue moving south until Dec. 21, the winter solstice.
• After dark, Saturn and brilliant Jupiter lie low in the south
• Mars will be rising in the east.
• Venus, which is even brighter, rises in the early hours.
• The Moon will be New on the 17th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.