Ken Tapping, 23 October, 2013
Years ago there was a popular computer game called Pacman, which involved dealing with, among other things, creatures that wandered around gobbling up anything they encountered. Scattered around the universe are cosmic pacmen that make the ones in the computer games seem benign and friendly by comparison.
A week or two ago astronomers observed a small asteroid being chewed up and absorbed by a distant white dwarf star, designated GD-61. It got too close and was destroyed by tidal forces. This event was of special interest because the asteroid was made largely of ice, supporting the possibility of watery planets like ours orbiting other stars.
Back in the 1700’s Isaac Newton made his landmark proposal that every piece of material in the universe attracts every other piece with a force that increases with the masses involved and falls off with increasing separation. This means that if two objects are close enough to each other, the attractive force between their facing sides will be much stronger than between their other sides.
On Earth the difference between the Moon’s pull at to the side of Earth facing it and the other side of the world gives rise to the tides. The Moon pulls the ocean on the side of the Earth facing it up into a bump and pulls the Earth itself away from the ocean on the other side, leaving a second bump. Because this example is so well known, we refer to these gravity difference effects as tidal forces, wherever we see them at work. Imagine what would happen if the Moon’s gravitational attraction were ten, a hundred or even a thousand times greater. This is what can happen if enough mass is compressed into a small enough lump, so that it is possible to pass really close to it. There are objects in the universe which qualify for this, such as white dwarf stars, neutron stars, and most extreme of all, black holes.
White dwarf stars are the final stages in the lives of stars like the Sun. They have run out of fuel and sneezed off their outer layers, leaving only their white-hot cores. They are about the size of the Earth and so dense that a single teaspoonful of their material would weigh several tonnes.
Neutron stars are the remnants of stars a few times the mass of the Sun. These stars exploded, producing shock waves that propagated downwards into the core of the star, compressing it down to a few kilometres in diameter.
Black holes are the most extreme. In this case their material is compressed so far we are not sure exactly how big the resulting lump is. However, their gravitational fields are so strong it distorts the fabric of space-time, concealing them.
When that unfortunate asteroid approached too closely to GD-61, the tidal effects were so strong that material from the side facing the star was torn away from the rest of the object. This went on until the asteroid had been destroyed.
Imagine a star approaching an invisible black hole. If we observe long enough we will see the star starting its final spiral. Then the star gets progressively pulled out of shape, until its material forms a swirl rather like what we see when we drop some cream into coffee that has already been stirred. Finally the star is completely pulled apart and the black hole is surrounded by a disc of star debris, gradually falling into the hole.
It looks as though we really do live in a “dog eat dog” universe, where everything is busily swallowing everything else. Will the universe last long enough for it to contain just one fat, well-fed black hole? What could happen then? At some point, will that hole open up and suddenly start to expand, starting the whole thing over again?
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