Washington, April 3: NASA's Hubble Space Telescope has discovered the farthest individual star ever seen - an enormous blue stellar body nicknamed Icarus located over halfway across the universe. The star, harboured in a very distant spiral galaxy, is so far away that its light has taken nine billion years to reach Earth. It appears to us as it did when the universe was about 30 percent of its current age.
Normally, the star would be too faint to view, even with the world's largest telescopes. However, through a phenomenon called gravitational lensing that tremendously amplifies the star's feeble glow, astronomers were able to pinpoint this faraway star and set a new distance record.
"This is the first time we're seeing a magnified, individual star," said Patrick Kelly, who led the study as a postdoctoral fellow University of California at Berkeley. "You can see individual galaxies out there, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions," said Kelly, who is no at the University of Minnesota in the US.
Gravitational lensing occurs when forces of gravity from massive cluster of galaxies acts as a natural lens in space, bending and amplifying light. Sometimes light from a single background object appears as multiple images. The light can be highly magnified, making extremely faint and distant objects bright enough to see. In the case of Icarus, a natural 'magnifying glass' is created by a galaxy cluster called MACS J1149+2223.
Located about 5 billion light-years from Earth, this massive cluster of galaxies sits between the Earth and the galaxy that contains the distant star. By combining the strength of this gravitational lens with Hubble's exquisite resolution and sensitivity, astronomers can see and study Icarus.
The team - including Jose Diego of the Instituto de Fisica de Cantabria in Spain, and Steven Rodney of the University of South Carolina in the US - dubbed the star Icarus, after the Greek mythological character who flew too near the Sun on wings of feathers and wax that melted. Its official name is MACS J1149+2223 Lensed Star 1. Much like Icarus, the background star had only fleeting glory as seen from Earth. It momentarily skyrocketed to 2,000 times its true brightness when temporarily magnified.
Models suggest that the tremendous brightening was probably from the gravitational amplification of a star, similar in mass to the Sun, in the foreground galaxy cluster when the star moved in front of Icarus. The star's light is usually magnified by about 600 times due to the foreground cluster's mass. Detecting the amplification of a single, pinpoint background star provided a unique opportunity to test the nature of dark matter in the cluster.
Dark matter is an invisible material that makes up most of the universe's mass. By probing what is floating around in the foreground cluster, scientists were able to test one theory that dark matter might be made up mostly of a huge number of primordial black holes formed in the birth of the universe with masses tens of times larger than the Sun. The results of the test disprove that hypothesis, because light fluctuations from the background star, monitored with Hubble for 13 years, would have looked different if there were a swarm of intervening black holes.