Stars align for Hubble Space Telescope discovery

Earendel shone brightly—one of millions of stars observed in data collected from the Hubble Space Telescope's Reionization Lensing Cluster Survey program—lining up perfectly to stand out among the rest. The star is the most distant star yet observed, and paves the way for a new way to study the distant universe.

Brian Welch, a PhD candidate in astronomy and astrophysics at Johns Hopkins, discovered the star. His research focuses on studying distant galaxies using gravitational lensing, where massive foreground objects distort and magnify the light from background objects—a pivotal process in making the Earendel discovery.

RECORD BROKEN: Hubble observed the farthest individual star ever seen!

This extraordinary new benchmark detected light from a star that existed within the first billion years after the universe's birth in the big bang.

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— Hubble (@NASAHubble) March 30, 2022

The find is a huge leap further back in time from the previous single-star record holder; detected by Hubble in 2018. That star existed when the universe was about 4 billion years old, or 30% of its current age, at a time that astronomers refer to as "redshift 1.5." Scientists use the word "redshift" because as the universe expands, light from distant objects is stretched or "shifted" to longer, redder wavelengths as it travels toward us.

The newly detected star is so far away that its light has taken 12.9 billion years to reach Earth, appearing to us as it did when the universe was only 7% of its current age, at redshift 6.2. The smallest objects previously seen at such a great distance are clusters of stars, embedded inside early galaxies.

"We almost didn't believe it at first, it was so much farther than the previous most-distant, highest redshift star," Welch said. He is lead author of the paper describing the discovery, published in the March 30 journal Nature with co-author Dan Coe at the Space Telescope Science Institute in Baltimore.

Earendel is one star among millions, but in the magnified galaxy Welch was studying, the star aligned perfectly to appear magnified by a factor of thousands, highlighting the star among the rest. Welch made a model of the lensing effect of the galaxy cluster in which Earendel was found; in making that model, he discovered the object was too small and too highly magnified to be anything larger than a star. While Welch says entire galaxies look like small smudges at such far distances, gravitational lensing offered the magnification and distortion necessary to make the discovery.

Because Welch made the discovery, he had the chance to choose the star's nickname. He thought Earendel, an Old English word meaning "morning star," was fitting, as he says the star is seen during the era often referred to as the cosmic dawn.

The research team estimates that Earendel is at least 50 times the mass of our sun and millions of times as bright, rivaling the most massive stars known. But even such a brilliant, very high-mass star would be impossible to see at such a great distance without the aid of natural magnification by a huge galaxy cluster, WHL0137-08, sitting between us and Earendel. The mass of the galaxy cluster warps the fabric of space, creating a powerful natural magnifying glass that distorts and greatly amplifies the light from distant objects behind it.

Thanks to the rare alignment with the magnifying galaxy cluster, the star Earendel appears directly on, or extremely close to, a ripple in the fabric of space. This ripple, which is defined in optics as a "caustic," provides maximum magnification and brightening. The effect is analogous to the rippled surface of a swimming pool creating patterns of bright light on the bottom of the pool on a sunny day. The ripples on the surface act as lenses and focus sunlight to maximum brightness on the pool's floor.

This caustic causes the star Earendel to pop out from the general glow of its home galaxy. At this point, astronomers are not able to determine if Earendel is a binary star, though most massive stars have at least one smaller companion star.

Astronomers expect that Earendel will remain highly magnified for years to come. It will be observed by NASA's James Webb Space Telescope, which launched in December. Webb's high sensitivity to infrared light is needed to learn more about Earendel, because its light is stretched (redshifted) to longer infrared wavelengths due to the universe's expansion.

Earendel's composition will be of great interest for astronomers, because it formed before the universe was filled with the heavy elements produced by successive generations of massive stars. If follow-up studies find that Earendel is only made up of primordial hydrogen and helium, it would be the first evidence for the legendary Population III stars, which are hypothesized to be the very first stars born after the big bang. While the probability is small, Welch admits it is enticing all the same.

"Earendel existed so long ago that it may not have had all the same raw materials as the stars around us today," Welch explained. "Studying Earendel will be a window into an era of the universe that we are unfamiliar with, but that led to everything we do know. It's like we've been reading a really interesting book, but we started with the second chapter, and now we will have a chance to see how it all got started," Welch said.