Black holes and quasars

Introduction

This chapter offers an overview of black holes and quasars and their connection to Einstein's theory of relativity. Black holes and quasars are distant energy sources known to modern scientists. The term 'quasar' means 'quasi-star', or something that looks like a star but in fact is not. According to astrophysical research data, a quasar is a galactic nucleus (centre) powered by a super-massive black hole. A black hole is a spot in the universe where the gravity is extremely strong.

Formation of the black holes

Gravity can make or destroy stars. It can gather clumps of gas and dust from interstellar clouds, compress them and, if they are sufficiently massive, ignite thermonuclear reactions in their cores. Then, for millions, or billions, of years they produce energy, heat and pressure which can balance the inward pull of gravity. When the star's energy sources are exhausted, however, gravity shrinks the star, turning it into a white dwarf - a cooling down super-dense star that has exhausted its nuclear fuel. A white dwarf collapses further and becomes a neutron star - an extremely dense star with a powerful gravitational pull. The energy released in this collapse turns the neutron star into a supernova - an exploding star. This explosion can be as bright as an entire galaxy of stars. When very large supernovae collapse completely they become black holes.

What is a black hole?

A black hole is a region of space in which the pull of gravity is so strong that nothing can escape. It is called a 'hole' because things can fall into it, but cannot get out of it. This hole is 'black' because even light cannot escape from it. So, a black hole is an object for which the escape velocity (the velocity required to break free from an object) is greater than the speed of light.

The boundary of a black hole is called the event horizon, because any event which takes place within it is forever hidden from anyone watching from outside. A black hole has only three properties: mass, spin and electrical charge, which is probably zero.

Black holes come in three different sizes: small or mini, medium and large or super-massive. There is strong evidence to suggest that medium black holes form as a result of the collapse of massive stars. Apparently, super-massive black holes exist in the core of many galaxies. Black holes are detected by the very strong gravitational force they produce that affects its surroundings. See image 1.

What is a quasar?

The first quasars were discovered with radio telescopes in the late 1950s.Quasars are also called quasi-stellar objects, or QSOs, which means that they may look like stars but they are not. A typical quasar produces more light each second than an entire galaxy of stars. It does so from a region of space the size of our solar system. Many quasars also emit radio waves. Scientists believe that black holes and quasars come in 'one set'. Both quasars and black holes are astronomical sources of energy. A quasar is thought to be a luminous galactic core, powered by a super-massive black hole. Quasars are so far away from us that their light has taken several billion years to reach the Earth.

All quasars have large redshifts, which, according to the Doppler effect, means that they are very distant. The closest known quasar is about 780 million light years from us. A light year is the distance light travels in a vacuum in one Julian year, or in 365.25 days. Some quasars change their luminosity rapidly. In astronomy, luminosity is the amount of energy a body radiates per unit of time. A rapid change in luminosity means that quasars are small because an object cannot change faster than the time it takes light to travel from one end to the other. Today more than    60 000 quasars are known. See image 2.

Black holes and Einstein

Einstein used his theory of relativity to explain the structure and origin of the universe. Einstein's general theory of relativity describes gravity as a curvature of space-time. According to Einstein all objects cause this curve but under normal conditions (on Earth) it will not be detected even by the most sophisticated machines because it is so small. So, when we talk about different types of motion on Earth we use Newton's laws of gravity.

When we talk about different events in the universe, Einstein's theory of relativity applies. Very massive, dense objects like black holes generate much stronger gravity. This type of gravity can affect the intervals of time and space. Different phenomena that occur only on a universal scale, like bending starlight because of gravity or a tiny shift in the orbit of the planet Mercury, have been explained using Einstein's theory of relativity. See image 3.