Quasar: the most distant observable single object near the limits of the visible universe.
The first quasars were discovered in the 1950s with the help of radio telescopes. At that time, strong radio sources were found in space that did not seem to belong to any visible star or galaxy. Their nature remained mysterious until the discovery of their extreme ►redshift in 1963. This discovery told us two things. On one hand, the quasars had to be very far away from us; on the other, they had to have an extremely strong radiance level in order to be discernible at such distance. Indeed, the quasars' radiance in the region of radio waves can be as much as 100 times stronger than that of the entire Milky Way and some billion times stronger than that of our sun.
It is only since the discovery of ►black holes that we have known of a mechanism that causes such luminosity. Quasars are supermassive black holes at the centers of young galaxies that attract matter from their environments by their gravity. This matter does not immediately fall into the hole but orbits it and creates a disk, the so-called accretion disk. This disk heats up to a temperature of several million degrees due to friction. It is the heat radiation of the heated accretion disk that causes the characteristic luminosity of the quasars. Only due to this radiation does the rotating matter decelerate and eventually plunge into the black hole; in this manner the brightest quasars swallow up roughly a thousand times the mass of our sun per year.
Quasars' radiation can fluctuate. Sometimes it changes over weeks or days, enabling us to determine their distance if ►gravitational lenses happen to lie in their route. Sometimes we can observe radiation bursts that last only for a few seconds, indicating that the radiation comes from a relatively small region.
The most distant quasar to be discovered so far is SDSS J1148+5251, which has a redshift of 6.43. It was first observed in April 2003. As you can see from the diagram in the article on ►distance, its light required 12.8 billion years to travel to us; thus, it was emitted a mere 900 million years after the ►Big Bang. Consequently, we don't see the quasars in their current state, in which they are much further — up to 40 giga-lightyears — away from us and their accretion disks have long since disappeared after swallowing up all matter in their vicinity. Instead, we see them as they were long ago during the universe's infancy.