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Hubble Space Telescope Reveals the Farthest Individual Star

Icarus, an enormous blue star located over halfway across the universe, is the farthest individual star to be ever viewed. In general, even when viewed through the world’s largest telescopes, it is considerably faint to view.

However, astronomers used NASA’s Hubble Space Telescope and could pinpoint this distant star and set a new distance record through a natural cosmic quirk that immensely amplified the feeble glow of the star. They even used Icarus to investigate one dark matter theory, and to examine the composition of a foreground galaxy cluster.

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, shown at left, sits between Earth and the galaxy that contains the distant star. The panels at the right show the view in 2011, without Icarus visible, compared with the star’s brightening in 2016. (Image credit: NASA, ESA, and P. Kelly (University of Minnesota))

The star, which is located in a very distant spiral galaxy, is so distant that it has taken nine billion years for the light from the star to reach Earth. It seems as it did when the age of the universe was around 30% of its current age.

The use of gravitational lensing to discover Icarus has introduced a new method for astronomers to investigate individual stars in faraway galaxies. These observations offer a detailed, rare insight into the way stars evolve, specifically the most radiant stars.

This is the first time we’re seeing a magnified, individual star,” explained Patrick Kelly, study leader and former University of California at Berkeley postdoc, who is now a postdoc at the University of Minnesota, Twin Cities. “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.”

Gravity as a Natural Cosmic Lens

The cosmic quirk that enables this star to be seen is a phenomenon known as “gravitational lensing.” Gravity exerted a massive, foreground cluster of galaxies functions as a natural lens in space that amplifies and bends light. At times, light from a single background object looks like multiple images. The light can be largely magnified, rendering distant and extremely faint objects bright enough to be viewed.

Considering Icarus, a natural “magnifying glass” is formed by a galaxy cluster known as MACS J1149+2223. Positioned around five billion light-years from Earth, this massive cluster of galaxies is located between the Earth and the galaxy containing the distant star. Astronomers can combine the strength of this gravitational lens with Hubble’s magnificent sensitivity and resolution to view and investigate Icarus.

The team, which included Jose Diego from the Instituto de Física de Cantabria, Spain, and Steven Rodney from the University of South Carolina, Columbia, named the star “Icarus,” after the Greek mythological character who flew very nearer to the Sun using wings made of feathers and wax that melted. (MACS J1149+2223 Lensed Star 1 is its official name.) Quite similar to Icarus, the background star had only transient glory when viewed from Earth: It briefly shot up to 2000 times its original brightness upon being temporarily magnified.

Models indicate that the immense brightening was possibly due to the gravitational amplification of a star, analogous in mass to the Sun, in the foreground galaxy cluster when the star moved in front of Icarus. The light from the star is generally magnified by nearly 600 times due to the mass of the foreground cluster.

Characterizing Icarus

When the astronomers used Hubble to monitor a supernova in the far-distant spiral galaxy, in 2016, they accidentally zeroed in on a new point of light not very far from the magnified supernova. Studying the position of the new source, they deduced that it has to be considerably more highly magnified when compared to the supernova.

Upon investigating the colors of the light emitted by this object, they found out that it was a blue supergiant star. This kind of star is hotter, more massive, considerably larger, and probably hundreds of thousands of times inherently brighter when compared to Sun. However, at this distance, it would still be very far away to view without the amplification of gravitational lensing, even if Hubble is used.

How did Kelly and his colleagues recognize that Icarus was not another supernova? “The source isn’t getting hotter; it’s not exploding. The light is just being magnified,” stated Kelly. “And that’s what you expect from gravitational lensing.”

Looking for Dark Matter

The nature of dark matter in the cluster was tested by detecting the amplification of a single, pinpoint background star. Majority of the mass of the universe is made of an invisible material called the dark matter.

Investigation of the materials floating around in the foreground cluster enabled the researchers to test one theory which proposes that dark matter could be made mostly of a large number of primordial black holes formed at the time of the birth of the universe and having masses tens of times larger when compared to the Sun. The results of this distinctive test disapprove that theory since light fluctuations from the background star, observed through Hubble for 13 years, would have appeared different in the presence of a swarm of intervening black holes.

Once NASA’s James Webb Space Telescope is instituted, astronomers anticipate that many more stars like Icarus could be discovered. Webb’s exceptional sensitivity will enable measurement of even greater details, including whether these distant stars rotate. Magnified stars such as these might even be discovered to be pretty common.

The Hubble Space Telescope is a project of international collaboration between NASA and European Space Agency (ESA). The telescope is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, performs Hubble science operations. The Association of Universities for Research in Astronomy, in Washington, DC, operates STScI for NASA.

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