Astronomers now know the reason for this. The lines in the spectrum of these star-like objects are shifted to the red by an enormous amount. In other words the sources must be very far away. 3C 273 proves to be more than 2,000 million light-years away. No star can be seen from this distance. Therefore it cannot be an ordinary star, even though it looks like one. And for it to be visible from such a distance, it must be hundreds of times brighter than an ordinary galaxy.

Astronomers have since discovered more than 1,500 other quasars, or quasi-stellar radio sources.

Quasars do not shine steadily like ordinary galaxies. They vary in brightness over periods of days or years. For this reason they cannot possibly be as big as an ordinary galaxy. For if a quasar changes in brightness in a year, say, it cannot be more than one light-year across. And if it changes in brightness in a day, it cannot be more than a light-day across. From its variation in brightness, 3C 273 works out to be less than one-hundredth of a light-year across, which makes it only one ten-millionth the size of a typical galaxy!

However, it seems that quasars are not separate bodies. They appear to be eruptions at the centre of massive galaxies are too faint to be visible at the distances involved. Astronomers believe that a quasar is a black hole at the centre of a distant galaxy, wich consumes all the matter around it. The whirling matter being sucked into the hole creates an amazing source of energy and powerful "jets" of material (top), which are projected out of the galaxy's glowing core.

A black hole is created when aging stars collapse. It is a region of space with super-high gravity, which swallows matter like a cosmic vacuum cleaner. It is surrounded by a rapidly ratating disc of hot gas. Matter attracted by the hole's enormous gravity acquires great amounts of energy. This is released as radiation when the matter ploughs into the disc.