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Hubble Space Telescope uncovers the farthest star ever seen

An international team has collaborated in the observation of Icarus, an enormous blue star 9 billion light years from the Earth detected by Hubble Space Telescope. José M. Diego, a researcher from the Institute of Physics of Cantabria (IFCA) - a joint Institute of the University of Cantabria and the Spanish National Research Council (CSIC), has participated in this discovery.

Icarus, whose official name is MACS J1149+2223 Lensed Star 1, is the farthest individual star ever seen. It is only visible because it is being magnified by the gravity of a massive galaxy cluster located about 5 billion light-years from Earth. Called MACS J1149+2223, this cluster sits between the Earth and the galaxy that contains the distant star. The cosmic quirk that makes this star visible is a phenomenon called “gravitational lensing.” Gravity from a foreground, 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. Combined with Hubble's resolution and sensitivity, this distant star has been detected and analyzed.

The team of astronomers dubbed the star “Icarus,” after the Greek mythological character who flew too near the Sun on wings of feathers and wax that melted. Icarus came as close to this "Sun" that reached glory as his Greek counterpart. "We were able to establish that Icarus is a blue supergiant star. A star type that is much larger, massive, hotter and possibly thousands of times brighter than the Sun but which, at that distance, is impossible to observe individually even for Hubble unless we have the gravitational lens phenomenon", says Ismael Pérez Fournon of the IAC. The models they have made to explain this magnificent astronomical event indicate that Icarus was amplified by a star similar to the Sun present in the intergalactic medium of the star cluster. The alignment was perfect and there was an amplification of the Icarus light by a factor of, at least, 10000.

Normally, it would be impossible to view. In fact, it is only possible to see individual stars of our own galaxy, the Milky Way, and galaxies in our vicinity - even with the world’s largest telescopes! But through a quirk of nature that tremendously amplifies the star’s glow, astronomers using NASA’s Hubble Space Telescope were able to pinpoint this faraway star and set a new distance record. They also used Icarus to test one theory of dark matter, and to probe the make-up of a galaxy cluster. These results have just been published in Nature Astronomy.

The star, harbored in a spiral galaxy, is so distant that its light has taken 9 billion years to reach us, which is equivalent to 70% of the age of the Universe. “This is the first time we’re seeing a magnified, individual star,” explained the co-author of the study Patrick Kelly of the University of Minnesota. “We can see very distant individual galaxies, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions”, he added. "Until Galileo observed the sky through his telescope, one could not see the hundreds of thousands of individual stars that make up what is known as the 'Camino de Santiago', a bright diffuse area of the sky", explains Pablo Pérez González, researcher in the Earth Physics and Astrophysics Department at the UCM. "Until 2016 it was only possible to observe individual stars in the Milky Way or in a few galaxies very close to us, a few million light years away. Today, it is already possible to observe an individual star on the other side of the Universe that, in fact, no longer exists. But we have not only been able to see it using a human invention, but also thanks to the magnificence of Nature itself and to the laws of Physics, among which is the disturbance exerted by a mass in the photon path. It's really fabulous.", praises Pérez González.

In addition to the UCM and the University of Minnesota, the Institute of Physics of Cantabria (IFCA), the University of the Basque Country (UPV), the Institute of Astrophysics of the Canary Islands (IAC), the University of La Laguna (ULL) and the University of South Carolina have participated in this work.


Kelly and his colleagues saw several sudden changes in the star's brightness caused by the microlens effect dued to the gravitational effect of stars belonging to the cluster. "There are individual stars and dead stars, white dwarfs or neutron stars, for example, floating in the middle of the cluster. They're actually so weak we can't see them. But we know they're there, because every time one of them passes just in front of the faraway star in perfect alignment, we see Icarus get brighter", Kelly explains. "So we have both a macro-lens effect produced by the entire mass of the cluster, and a micro-lens effect produced by individual objects floating in the intergalactic medium."

Detecting Icarus with Hubble was so extraordinary that all the telescopes of the world began observing it. "In Spain we have the largest optical-infrared telescope in the world, the GTC, so Spanish astronomers involved in the project from UCM, IFCA, UPV and IAC contacted the GTC's Director and he granted 4 hours of observation that same night", said Pablo Pérez González. "GTC was the only telescope to detect this star so far from the ground, given that Icarus is very weak", explained UCM researcher.

Icarus' discovery is not only exceptional because we have seen such a distant star for the first time. Detecting the brightness amplification of an individual star allows to study the nature of the dark matter in the cluster in a unique way. By exploring what floats in it, the team of astronomers has been able to test a theory about the nature of dark matter in which most of it are primordial black holes, that they would have a mass equal to several dozen suns, and that they would have formed at the birth of the Universe.


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