Record broken: Hubble Space Telescope spots farthest star ever seen

Editor’s note: This story is featured in the 2022 year in review.

NASA's Hubble Space Telescope has established an extraordinary new benchmark: detecting the light of a star that existed within the first billion years after the universe's birth in the Big Bang — the farthest individual star ever seen to date.

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 star, nicknamed Earendel by astronomers from the Old English word meaning “morning light,” 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.

The paper describing the discovery of the newly detected star, which has been published in the journal Nature, is led by Brian Welch of Johns Hopkins University and Dan Coe of the Space Telescope Science Institute, and includes Arizona State University co-authors Rogier Windhorst and Francis Timmes.

“This is a monumental discovery with Hubble,” said Windhorst, who is a Regents Professor and astronomer at ASU’s School of Earth and Space Exploration and an interdisciplinary scientist for the James Webb Space Telescope. “Hubble revealed a star in the first billion years directly through a very lucky alignment in Einstein's Cosmic House of Mirrors — a gravitational lensing galaxy cluster. When we predicted in 2018 that this could be observed by Webb, we weren't sure how often we would see such highly magnified early stars.”

Hubble was able to detect Earendel by looking through space warped by the mass of the huge galaxy cluster WHL0137-08, an effect called gravitational lensing. Earendel was aligned on or very near a ripple in the fabric of space created by the cluster's mass, which magnified its light enough to be detected by Hubble.

"Normally at these distances, entire galaxies look like small smudges, with the light from millions of stars blending together," Welch said. "The galaxy hosting this star has been magnified and distorted by gravitational lensing into a long crescent that we named the Sunrise Arc."

When stars align

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, located between us and Earendel.

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. Its brightness is magnified a thousandfold or more. At this point, astronomers are not able to determine if Earendel is a binary star, though most massive stars today have at least one smaller companion star.

This detailed view highlights the star Earendel's position along a ripple in spacetime (dotted line) that magnifies it and makes it possible for the star to be detected over such a great distance — nearly 13 billion light years. Also indicated is a cluster of stars that is mirrored on either side of the line of magnification. Credit: NASA, ESA, Welch (JHU), Coe (STScI), Pagan (STScI)

Confirmation with James Webb Space Telescope

Astronomers expect that Earendel will remain highly magnified for years to come. It will be observed by NASA's James Webb Space Telescope. 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.

“Webb is uniquely built and suited to find and study these early stars as they become visible, for a period of time, as highly magnified into Webb's infrared images,” Windhorst said. “And now we have an early-bird preview of one such object with Hubble. Webb will measure the star's light power, temperature and its fraction of atomic elements heavier than helium.”

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.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

This release was written by the Space Telescope Science Institute press team with contributions from Karin Valentine of ASU’s School of Earth and Space Exploration.