Quasars are galaxies with massive black holes at their cores, around which vast amounts of energy are being radiated. So much light is emitted that the nucleus is much brighter than the rest of the entire galaxy. No one knows for sure how quasars form, or how their stupendous energies are produced, and so they are intrinsically interesting to astronomers studying the nature of black holes as well as the evolution of galaxies. Our own galaxy, for example, has a massive black hole at its center, and perhaps someday it will become a quasar.
Because they are so bright, quasars can be seen even when they are very far away. The quasar APM08279+5255 has several other claims to fame. It is not only far away (its light was emitted when the universe was about 10% of its current age), its light is being magnified by a galaxy fortuitously lying between us. (Einstein predicted that light can be bent by gravity.) Light from APM08279+5255 traveling towards earth happens to pass an intervening galaxy that acts as a "gravitational lens" that bends and magnifies the light. The quasar is thought to be intrinsically very luminous, shining with the equivalent of about 50 trillion stars, but the lens collects and magnifies that light by a further factor of one hundred. As a result, APM08279+5255 appears as the most luminous known object in the universe. Indeed, astronomers originally thought it was a star in our galaxy.
Sensitive optical images find that the gravitational lens refocuses the light into two (perhaps three) adjacent, distorted images of the same quasar. At least, that's what astronomers suspected based on optical studies. SAO astronomers Melanie Krips and David Wilner used the Submillimeter Array (SMA) to study APM08279+5255 with very fine spatial resolution. They report confirming clear evidence of two separate peaks. But their careful modeling of the millimeter images, accounting for Einstein's lensing effect, shows that the images are not distortions of the same bright nucleus, but rather a single image of a double source. The result implies that in addition to the black hole, the nucleus of APM08279+5255 has a bright region of active star formation a few hundred light-years away from the black hole, perhaps triggered by radiation from the black hole. It is the only known example of a quasar with a bright starburst near the black hole. Such extraordinary spatial information about an object whose effective distance (at least when talking about angular sizes) is about three billion light-years is possible only because of the combination of the gravitational lens and the SMA's capabilities.