Armagh Observatory: School Student Essays

Black Holes

Grainne Thie, Presentation Secondary School, Tralee, Co Kerry, 5 April 2001

The mass of a star is proportional to the gravity, so when a star gains a huge amount of mass, the gravity increases to a point where nothing can escape being pulled in once it crosses the event horizon – not even light. (The event horizon is the boundary of a black hole)

A star can increase in mass by amalgamating with other stars. When the supermassive star implodes (collapses under it’s own gravity) a black hole is formed. Many black holes have been found in the centres of galaxies. Astronomers found them in neighbouring galaxies, where they can look into the core. Here they found that millions of stars are moving at an extremely rapid speed. They are orbiting so quickly that they would normally fly out of the centres of their galaxies. Astronomers believe that a black hole is holding the stars in its grip. Examples of the latter are such galaxies as M-87, M-32, M-1 and our own Milky Way.

Black holes emit huge amounts of X-rays because stars falling into them get compressed and heated and release large amounts of energy before they are engulfed by the black hole.

It was thought that black holes don’t give out any energy at all (except when engulfing stars). However, in 1971 a Russian physicist, Yokov Zel’devich thought that black holes may emit protons and other particles. It was proven by Stephen Hawking three years later that black holes emit subatomic particles. These are now called Hawking radiation. From the previous proof, Hawking also concluded that black holes can evaporate.

Quasars are extremely bright galaxies which were first detected by their powerful radio emissions. Red shifts suggest that these galaxies are at least 10 billion light years away. If this is true then they radiate more energy than hundreds of ordinary galaxies and are much smaller then these galaxies. Huge black holes are believed to be at the centre of such galaxies. It is not known why these quasars aren’t as far away as the red shifts indicate. These red shifts may be obscured by huge gravitational fields.

So when do black holes begin to evaporate? The following theory may be a possibility in some cases: as black holes swallow more and more material and grow larger, the quasars get brighter and the wind stronger. Eventually, the wind becomes so strong that it overcomes the gravity of the galaxy and blows all the gas away. With its gas supply cut off the hole will stop growing. It would take a very long time for black holes to evaporate – a hole of 30 solar masses would take roughly 10^61 times the age of the universe! Much smaller black holes, however, would evaporate in less than the age of the universe.

Even though black holes are thought to inevitably evaporate, it is now thought by some scientists that Hawkings evaporation could stop, leaving a tiny long-lived black hole behind. This theory was triggered when scientists had a solitary atom which they believed to be cadmium atom but its frequency shifted by 0.002%. This puzzling occurance lead to the discovery that this atom was a billion billion times heavier than an ordinary atom. Even more surprising , they found that instead of a nucleus, was a hole, a black hole!

This has led some physicists to believe that black holes may remain for billions of years before evaporating or they may stop evaporating completely. Such a theory would require a complex knowledge of quantum mechanics makes this theory highly improbable.

Some believe that black holes helped shape galaxies by acting as a central gravitational pull. Many galaxies are seed shaped the stars travel in an oval shaped orbit. It is thought that as the stars pass near to the centre, they are pulled in by the black hole and forced to remain orbiting around the black hole. Such a galaxy would settle into a more stable, settled sphere – like the Milky Way.

How to find a black hole

To find a black hole you must look among the X-ray stars. Find an X-ray star that is associated with another star. The other star must be overflowing into the X-ray star. Show that the invisible companion star cannot be a neutron star or a white dwarf. This can be done by showing that the X-ray star is 3 solar masses or more (a neutron star or white dwarf cannot by any bigger than two solar masses.

So if the invisible star is larger than 3 solar masses, it must be a black hole.