Ellen Phiddian
Cosmos science journalist
Astronomers have used a trick of gravity and the James Webb Space Telescope (JWST) to picture 44 individual stars in a galaxy 6.5 billion light-years away.
The galaxy, dubbed the Dragon Arc, hails from a time when the Universe was half its current age.
The observation, which uses a technique called gravitational lensing, marks the largest number of individual stars seen in the distant Universe.
“To us, galaxies that are very far away usually look like a diffuse, fuzzy blob,” says Yoshinobu Fudamoto, an assistant professor at Chiba University in Japan, and lead author on a paper describing the research, published in Nature Astronomy.
“But actually, those blobs consist of many, many individual stars. We just can’t resolve them with our telescopes.”
Instead, the team used the laws of physics. Very massive objects, like galactic clusters, cause a natural magnification with their strong gravitational fields. This is gravitational lensing.
The JWST is sensitive enough to use gravitational lensing to see super-distant objects.
“These findings have typically been limited to just 1 or 2 stars per galaxy,” says Fudamoto.
“To study stellar populations in a statistically meaningful way, we need many more observations of individual stars.”
The Dragon Arc lurks behind a cluster of galaxies known as Abell 370, which both magnifies it and makes it appear stretched.
But while the galaxy cluster performs a lot of the magnification work, it’s stars within the cluster that can help provide fine detail.
“Inside the galaxy cluster, there are many stars floating around that are not bound by any galaxy,” says co-author Eiichi Egami, a research professor at the Steward Observatory in the University of Arizona, USA.
“When one of them happens to pass in front of the background star in the distant galaxy along the line of sight with Earth, it acts as a microlens, in addition to the macrolensing effect of the galaxy cluster as a whole.”
The team used 2 JWST pictures to count 44 individual stars, which would otherwise be too faint to see.
“By observing the same galaxy multiple times, we can spot stars in distant galaxies because they appear to pop in and out of existence,” says Fudamoto.
“This is a result of the varying effective magnifications from the macro- and microlensing effect as the microlensing stars move in and out of the line of sight.”
The team found that many of the stars they identified were red supergiants, at the end of their lives.
“This groundbreaking discovery demonstrates, for the first time, that studying large numbers of individual stars in a distant galaxy is possible,” says co-author Fengwu Sun, now a postdoctoral scholar at the Harvard & Smithsonian Center for Astrophysics.
