More than halfway across the universe, an enormous blue star nicknamed Icarus is the farthest individual star ever seen.
Normally, it would be much too faint to view, 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 pinpointed the faraway star and set a new distance record. University of Arizona astronomer Brenda Frye provided foundational research so that the team could use Icarus to test one theory of dark matter, and to probe the makeup of a galaxy cluster.
The team dubbed the star "Icarus," after the Greek mythological character whose wings of feathers and wax melted when he flew too near the sun. Its official name is MACS J1149+2223 Lensed Star 1.
Harbored in a spiral galaxy, the star is so distant that its light has taken 9 billion years to reach Earth, so it appears as it did when the universe was about 30 percent of its current age. Observations of Icarus provide a rare, detailed look at how luminous stars evolve.
"This is the first time we're seeing a magnified, individual star," explained study leader Patrick Kelly of 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."
A Natural Magnifying Lens
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, and sometimes making light from a single background object appear 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 galaxy cluster called MACS J1149+2223 creates a natural "magnifying glass." Located about 5 billion light-years from Earth, this massive cluster of galaxies is situated 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. Using powerful Earth-based telescopes such as the Large Binocular Telescope in Arizona, astronomers can define the shape and boundaries of the gravitational lens.
Frequently, entire galaxies are gravitationally lensed to appear as bright arcs in the sky, instead of the elliptical or spiral shapes that they have when not viewed through a gravitational lens. It is astonishingly rare for a single star in a distant galaxy to be magnified.
"The only way this can happen is if that star passes through a special boundary in the image of this cluster," Frye said. Using observations from the Large Binocular Telescope, Frye helped define this special boundary, called a critical curve, where Icarus' light is magnified.
Besides tremendous large-scale amplification of background light by the galaxy cluster, the team saw additional short and sudden boosts in brightness from the gravity of small clumps of mass within the cluster passing in front of Icarus. This effect is called "microlensing."
"There are individual stars or stellar remnants such as white dwarfs or neutron stars floating in the middle of a galaxy cluster, and every time one of them moves in the right place, it would cause Icarus to brighten," Kelly explained. "So we have both lensing by the entire mass of the cluster and lensing by these individual objects floating around."
After Icarus was discovered, it continued to brighten for a couple of weeks, reaching a peak magnification of at least a factor of 2,000. Models suggest that the tremendous magnification probably was from microlensing by a sun-size star in the galaxy cluster traveling in front of Icarus. As the foreground star moved past, Icarus' light dimmed once more. The light from Icarus is usually magnified by a factor of 600 because of the foreground cluster's mass. By studying the fluctuations in its light from microlensing, astronomers are able to use Icarus to learn about how the foreground galaxy cluster's mass is distributed on a small scale. Scientists may even be able to detect planet-size objects in the galaxy cluster by using this technique.
Discovering the Star
The team had been using Hubble to monitor a supernova in the far-distant spiral galaxy when, in 2016, it spotted a new point of light not far from the supernova. As it was observed, this light source became more than three times brighter in one month.
"We thought we had found another supernova, because we had found one before in this same field," Frye said. "But this time the spectrum was not characteristic of a supernova."
When the team analyzed the colors of the light coming from this object, it discovered it was a blue supergiant star. This type of star is much larger, more massive, hotter and possibly hundreds of thousands of times intrinsically brighter than our sun. But at 9 billion light-years, the star still would be too far away for any telescope to see without the magnficiation from the gravitational lens.
According to all models of the galaxy cluster lens, Icarus appears to be located very close to the line of sight through the foreground cluster, where the strength of the gravitational lens provides maximum magnification. As it passes through this point of maximum magnification, the star may appear to brighten by as much as 10,000 times its true brightness. Thereafter, the light is expected to fade as the star moves past this point, much like the fleeting glory the mythological Icarus experienced as he flew too close to the sun.
Mysterious Dark Matter
Detecting the amplification of a single background star provided a unique opportunity to test the nature of dark matter in the cluster. Dark matter is an invisible, mysterious material that makes up most of the universe's mass.
"The bright galaxies are the tip of the iceberg," Frye said. "Dark matter — like the iceberg that is below the water's surface — will be 10 times the mass of the bright stuff we can see."
Like the stars and planets floating around in the foreground galaxy cluster, the dark matter will change the shape of the critical curve. By probing what can be seen floating around in the foreground cluster, astronomers can determine how much mass is made of dark matter.
"It can place constraints on the composition and structure of the dark matter," Frye said.
For example, the team studying Icarus was able to test one theory that dark matter might be made largely of huge numbers of primordial black holes that formed in the birth of the universe. The results of this unique test disfavor that hypothesis, because light fluctuations from Icarus — monitored with Hubble for 13 years — would have appeared differently if there were a swarm of black holes in front of the star.
In May 2016, astronomers saw a second bright peak near Icarus. This may mean that Icarus is not one single star but a pair of stars that orbit each other.
"The shape of the critical curve changes on tiny, tiny scales constantly," Frye said. "Objects are magnified only in very, very small regions that take complex shapes similar to Norwegian fjords." Moving dark matter or stars in the foreground galaxy could have caused the critical curve to flex, including Icarus' possible companion in its field of magnification.
"It's not completely sorted out whether there is one star multiply-imaged by gravitational lensing or a binary system," Frye said.
If it is two stars, then Icarus would represent not only the farthest star ever seen, but also the most distant binary system ever discovered.
When NASA's James Webb Space Telescope is launched, astronomers expect to find many more stars like Icarus. Webb's extraordinary sensitivity will allow measurement of even more details, including whether these distant stars are rotating. Such magnified stars may even be found to be fairly common.
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute in Baltimore conducts Hubble science operations.
The Large Binocular Telescope Observatory is an international collaboration among the University of Arizona, the LBT Beteiligungsgesellschaft of Germany, the Italian National Institute for Astrophysics, the Research Corporation for Science Advancement and Ohio State University. The University of Arizona hosts the LBT headquarters and is responsible for maintaining the observatory. The mirrors for the LBT were constructed at the Richard F. Caris Mirror Lab at the UA.