Quasar Quest

What is a Quasar?

The Lovell Telescope at Jodrell Bank

In the early 1960s, Radio Telescopes were beginning to show astronomers the Universe in an entirely different way. Because radio waves are usually produced in very different ways than ordinary, visible, light, by finding objects that produce a lot of radio "light" it is possible to look at different kinds of physics and chemistry.

One mystery was that some of the objects in the first catalogues of radio sources seemed to have no visible counterpart - these seemed to be objects that produced lots of radio waves but hardly any normal light at all. After much work it was realised that the radio waves were in fact coming from objects that looked like faint, blue stars.

Quasar 3C273

The quasar is the blue-ish dot in the centre!

That was surprising enough, since stars do not normally produce much radio emission, but when astronomers took spectra of the "stars", they were very strange - apparently made of elements that had never been seen before! These strange objects were called Quasars (for Quasi-Stellar Radio Sources) and were soon studied by many astronomers hoping to crack the problem.

It wasn't until 1963 that the mystery was solved when Maarten Schmidt realised that the spectrum of one quasar called 3C273 could be explained if the elements were normal ones like Hydrogen, but if the whole spectrum had been shifted a long way towards the red.

Redshifts had been seen for many years in normal galaxies and were known to be related to distance - the larger the shift towards the red, the further away the galaxy. However, the shift here was much larger than any ever seen before - which must mean that the quasar was a very long way away.

Energy to Burn

Of course, something that is a long way away should also appear very faint. So, one mystery was replaced with another. Since the Quasars are a very long way away, they must be producing an enormous amount of energy for us to be able to see them at all! In fact, they would have to be the most energetic objects in the universe.

Eventually it was realised that there was only one kind of object powerful enough to produce the light we could see - Supermassive Black Holes!

Getting Light from Black Holes

Near a Black Hole

An image from the Hubble Space Telescope showing the disc of dust and gas surrounding a black hole.

Cygnus A

A radio observation of the jets from a quasar-like galaxy. The colours are false.

Most people know that black holes swallow everything, even light, so how do we "see" them? In fact we cannot, but we can see the effect that the black hole has on the stars, dust and gas around it.

The black hole in a quasar is actually in the centre of a galaxy, surrounded by stars and gas. The gravity of the black hole can make some of ths stars and gas spiral in. Eventually it will fall into the black hole, but as it falls in, it gets very fast and bits of star and gas crash into each other.

These collisions produces huge flashes of light and, because they are happening outside the black hole, that light can escape. Lots of collisions happening all the time produce the light that we see - not just the normal visible light, but light all across the electro-magnetic spectrum.

In some quasars, the collisions also throw some of the gas out of the quasar at very high speeds (close to the speed of light). This can form a giant jet of high-speed, glowing gas flying away from the quasar and reaching lengths of more than 1,000,000 light years!

Lots of questions

There are now many hundreds of known quasars and we know that they are formed when a supermassive black hole in the centre of a galaxy is pulling in and destroying stars and gas around it, but there are still lots of important things we do not know about them.

The aim of this experiment is to help to answer one or more of these questions. You will be doing this by studying the way in which the brightness of a quasar varies over a period of a few weeks.