The beginners' guide to black holes - not even light can escape

Monday, 16 May 2016

A supermassive black hole with millions to billions times the mass of our sun is seen in an undated NASA artist's concept illustration. In this illustration, the supermassive black hole at the centre is surrounded by matter flowing onto the black hole in what is termed an accretion disk. This disk forms as the dust and gas in the galaxy falls on to the hole, attracted by its gravity. Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole's spin, according to a NASA news release.

The recent proof of the existence of gravitational waves resulting from the collision of two black holes has brought black holes once again to the public's attention. Most people only have a vague idea of what a black hole is, so here's an introductory guide.

If you throw a ball into the air it will fall back to Earth. This is because you don't have the strength to give the ball enough energy of motion (kinetic energy) for it to overcome the pull of the Earth's gravity.

The speed that the ball must acquire to be launched free from the Earth's gravity is called its escape velocity. The force or pull of gravity decreases with distance from the Earth (more correctly, as the inverse square of the distance), so the escape velocity decreases with height above the Earth's surface.

The value of the escape velocity at the surface is a useful number to compare astronomical objects. The escape velocity of anything, such as a ball or spacecraft, at the Earth's surface is 11.2 kilometres per second. It can be shown that the escape velocity at the Earth's surface depends on the mass of the Earth (kilograms) divided by the Earth's radius (for spherically symmetrical astronomical objects, the escape velocity is actually proportional to the square root of the mass of the object divided by its radius).

The point is, the more matter that is packed into a small region, the greater the mass to radius ratio and hence the more powerful the gravitational field will be and the higher the velocity required to escape.

The mass divided by the radius of the Moon is only 5 per cent that of the Earth, so the Moon has a weak gravity, hence a small escape velocity, 2.5km per second at its surface. The Sun's mass divided by its radius is 3000 times greater than that of the Earth. This translates to an escape velocity of 618km per second – more than any rocket could deliver, even if it could withstand the heat.

Low- to medium-mass stars are between about 0.5 to 8 times the mass of our Sun. When such a star has consumed all of its nuclear fuel, the powerful gravity of its mass causes the star to collapse and form a white dwarf star.

Typically a white dwarf has the mass of the Sun crushed into an object the size of the Earth. Its mass divided by its radius is more than 330,000 times that of the Earth. The colossal power of the gravitational field of a white dwarf means its escape velocity is a staggering 6500km per second.

When a star of more than eight times the mass of the Sun runs out of fuel it can undergo gravitational collapse to produce an object that is about 1.5 times the mass of the Sun crushed into a radius of only 10km. The result is a neutron star - an object with a mass to radius ratio that is literally astronomic – its surface gravity is about 1 billion times that on Earth. The escape velocity of a neutron star is a mind boggling 200,000km per second.

When a massive star, at least 10 to 20 times the mass of the Sun, runs out of fuel, the subsequent gravitational collapse usually produces a supernova explosion. The aftermath of this is an object with a mass to radius ratio of more thn 700 million times that of the Earth.

Its gravity is so powerful that its escape velocity is 300,000km per second – the speed of light. Nothing can escape, not even light, so it is black – a black hole. (Research suggests that quantum effects enable black holes to leak mass but there is no classical method to escape the vice-like grip of a black hole).

It is possible for very small and very large black holes to exist, just as long as enough matter can be crammed into a sufficiently small region. About 75,000 billion billion tonnes crammed into a soccer ball would give it an escape velocity of the speed of light – a black hole. Alternatively, an object with a huge radius can be a black hole provided it has enough mass. The black hole at the centre of our galaxy has a radius of 12 million kilometres, its mass is 4.1 million times that of the Sun.

We have much more to learn about black holes and hopefully by analysing gravitational waves from cosmic events involving black holes we can unveil some of the secrets of these fascinating objects.

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Dr Roger Hanson is a New Plymouth-based chemical engineer with a PhD from the University of Cambridge.