As predicted by Einstein's General Theory of Relativity, the path light takes is deflected measurably from a straight line when it passes close to a massive object. As a result, a galaxy can act in much the same way as an irregular glass lens, to produce a distorted or multiple image of anything that happens to lie behind it along the same line of sight. A single quasar may appear as two or more quasars. And a distant galaxy may be distorted almost beyond recognition. The phenomenon is called 'gravitational lensing'.
The four outer bright spots in this gravitationally lensed system, known as the Einstein Cross (or by its catalog number G2237+0305), are all images of a single distant quasar. The bright spot in the center is a direct image of the central bulge of a spiral galaxy located roughly 400 million light years from Earth. The spiral galaxy is acting as the gravitational lens. Its gravity bends the light from the single, more distant quasar into the pattern of four that we see from Earth. The quasar is about 20 times farther from Earth than the lensing galaxy.
The Einstein Cross was discovered by John Huchra of the HarvardSmithsonian Center for Astrophysics at the Whipple Observatory near Amado, Arizona, USA. This HST image revealed it with unprecedented clarity even though it was obtained prior to the First Servicing Mission.
Each of the four gravitationally lensed images is formed by light that took a slightly different path from the quasar to the Earth. While each path is roughly 8 billion light years long, the paths may differ by a light year or so. As a result, when the quasar undergoes a sudden change in brightness, each of the four lensed images will show the same brightness change, but at different times. Precise measurements of these events, and the time lags that separate them, can pin down the distance scale of the gravitational lens system, including both the spiral galaxy and the quasar. That in turn can yield an accurate value for the Hubble Constant, a standard and still uncertain measure of the distance scale of the universe.
Camera: Faint Object Camera
Credit: NASA and ESA
