Astronomy Picture of the Day

Explanation: Here is a rather typical quasar. But since quasars are so unusual it is quite atypical of most familiar objects. Of the two bright objects in the centre of this photo, the quasar is on the left. The bright image to quasar's right is a star, the faint object just above the quasar is an elliptical galaxy, with an apparently interacting pair of spiral galaxies near the top. Quasars appear as unresolved points of light, as do stars, and hence quasars were thought to be a type of star until the 1960s. We now know that the brightest quasars lie far across the visible universe from us, and include the most distant objects known. Quasars may occupy the centres of galaxies and may even be much brighter than their host galaxies. In fact, the centres of many nearby galaxies have similarities to quasars - including the centre of our own Milky Way Galaxy. The exact mechanism responsible for a quasar's extreme brightness is unknown, but thought to involve supermassive black holes. This picture represents a milestone for the six-year-old Hubble Space Telescope as it was picture number 100,000, taken on June 22, 1996.