Webb Spots Gravitationally Magnified Stars 6.5 Billion Light-Years Away

Using James Webb Space Telescope observations, astronomers have identified more than 40 microlensed stars in a single galaxy behind the galaxy cluster Abell 370 at redshift of 0.725 (dubbed the Dragon arc) when the Universe was half of its current age.

“This groundbreaking discovery demonstrates, for the first time, that studying large numbers of individual stars in a distant galaxy is possible,” said Dr. Fengwu Sun, a postdoctoral researcher at the Harvard & Smithsonian’s Center for Astrophysics.

“While previous studies with the NASA/ESA Hubble Space Telescope found around seven stars, we now have the capability to resolve stars that were previously outside of our capability.”

“Importantly, observing more individual stars will also help us better understand dark matter in the lensing plane of these galaxies and stars, which we couldn’t do with only the handful of individual stars observed previously.”

In the study, Dr. Sun and colleagues analyzed the Webb images of a galaxy known as the Dragon arc, located along the line of sight from Earth behind a massive cluster of galaxies called Abell 370.

Due to its gravitational lensing effect, Abell 370 stretches the Dragon arc’s signature spiral into an elongated shape – like a hall of mirrors of cosmic proportions.

The astronomers carefully analyzed colors of each of the stars inside the Dragon arc and found that many are red supergiants. This contrasts with earlier discoveries, which predominantly identified blue supergiants.

According to the researchers, this difference in stellar types also highlights the unique power of Webb observations at infrared wavelengths that could reveal stars at lower temperatures.

“When we discovered these individual stars, we were actually looking for a background galaxy that is magnified by the galaxies in this massive cluster,” Dr. Sun said.

“But when we processed the data, we realized that there were what appeared to be a lot of individual star points.”

“It was an exciting find because it was the first time we were able to see so many individual stars so far away.”

“We know more about red supergiants in our local galactic neighborhood because they are closer and we can take better images and spectra, and sometimes even resolve the stars.”

“We can use the knowledge we’ve gained from studying red supergiants in the local Universe to interpret what happens next for them at such an early epoch of galaxy formation in future studies.”

Most galaxies, including the Milky Way, contain tens of billions of stars. In nearby galaxies such as the Andromeda galaxy, astronomers can observe stars one by one.

However, in galaxies billions of light-years away, stars appear blended together as their light needs to travel for billions of light-years before it reaches us, presenting a long-standing challenge to scientists studying how galaxies form and evolve.

“To us, galaxies that are very far away usually look like a diffuse, fuzzy blob,” said Dr. Yoshinobu Fudamoto, an astronomer at Chiba University.

“But actually, those blobs consist of many, many individual stars. We just can’t resolve them with our telescopes.”

The findings were published in the journal Nature Astronomy.

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Y. Fudamoto et al. Identification of more than 40 gravitationally magnified stars in a galaxy at redshift 0.725. Nat Astron, published online Jaunary 6, 2025; doi: 10.1038/s41550-024-02432-3