Hubble spots most distant single star ever seen, 12.9 billion light-years away

On March 30, 2022, astronomers announced they had spotted a single star farther away than any star humanity had ever seen before. The Hubble Space Telescope had captured light from a star designated WHL0137-LS, which scientists nicknamed Earendel—an Old English word for morning star or rising light. That light had traveled 12.9 billion years to reach Earth, meaning we were seeing the star as it existed when the universe was only about 900 million years old, just 7 percent of its current age.

Earendel is a monster of a star. It weighs at least 50 times what our sun weighs and shines millions of times brighter. Yet under normal circumstances, it would be invisible from Earth. The distance alone would render it undetectable. The previous record holder, discovered by Hubble in 2018, dated to when the universe was about 4 billion years old—nearly five times more recent than Earendel. Before that, the farthest objects astronomers could resolve were clusters of stars embedded in distant galaxies, their individual light merged into a collective glow.

What made Earendel visible was an accident of cosmic geometry and a phenomenon Einstein predicted more than a century ago. Between Earth and Earendel sits an enormous galaxy cluster called WHL0137-08. Its gravitational pull is so immense that it warps the fabric of space and time itself, bending light the way a lens bends it. Astronomers call this gravitational lensing. The light from Earendel, traveling through space for nearly 13 billion years, passed through this warped region and was magnified—amplified enough that Hubble could detect it. The effect is similar to how ripples on the surface of a swimming pool focus sunlight into bright patterns on the pool's floor. By extraordinary chance, Earendel aligned almost perfectly with a curve in this warped spacetime that provided maximum brightening, causing the star to stand out sharply from the general glow of its host galaxy, which astronomers named the Sunrise Arc because the lensed light had been stretched into a long crescent.

Brian Welch, an astrophysicist at Johns Hopkins University and the lead author of the study published in Nature, was careful to clarify what this discovery did and did not mean. Hubble has observed galaxies at far greater distances, he noted, but those observations show the blended light of millions of stars. Earendel is the most distant object from which astronomers have ever isolated and studied the light of a single star. "This finding gives us an opportunity to study a star in detail in the early universe," Welch told Space.com.

Yet Earendel itself is young—probably only a few million years old. The 12.9 billion years of travel time does not mean the star has existed for 12.9 billion years; it means we are seeing it as it was 12.9 billion years ago. Given its enormous mass, Earendel almost certainly did not survive to the present day. Massive stars burn through their fuel rapidly and end their lives in violent explosions or collapse into black holes. The oldest stars known today formed around the same era as Earendel but are far less massive, which is why they have continued burning until now.

Many details about Earendel remain unknown. Astronomers are uncertain of its exact mass, brightness, temperature, and spectral type. They cannot even confirm whether Earendel is a single star or a binary system—most stars of its mass typically have smaller, dimmer companions, and it is possible that Earendel is simply outshining a partner star too faint to detect separately. NASA's James Webb Space Telescope, recently launched, will conduct follow-up observations to analyze Earendel's infrared light and resolve many of these mysteries. That information could illuminate how the universe's first stars formed and behaved, before successive generations of massive stars had seeded the cosmos with heavy elements.

Welch emphasized the broader significance. "I think one of the most exciting things about this result is that it opens a new window into the early universe," he said. Normally, at such vast distances, astronomers see only entire galaxies as small, fuzzy objects and must infer details about their stars from the combined light of millions. With Earendel, magnified by gravitational lensing, scientists can study a single star's light independently and compare it directly to stars in the Milky Way, looking for differences that will deepen understanding of how stars formed in the universe's infancy.