The Big Bang of Creation occurred about 13.8 billion years ago. Until now, the farthest star whose light was detected by Hubble Space Telescope astronomers existed when the universe was about four billion years old. The discovery was made four years ago and was 30% of its current age, at a time that scientists refer to as “Redshift 1.5.”
Scientists use this term because as the universe expands, light from distant objects is stretched or “shifted” to longer, redder wavelengths as it travels toward us.
In an international collaboration, including researchers from Ben-Gurion University (BGU) of the Negev in Beersheba, Hubble has established an extraordinary new benchmark – detecting the star’s light that existed within the first billion years after the universe’s birth. This is the farthest individual star ever seen to date. The find, called Earendel, is a giant leap further back in time from the previous single-star record holder.
The newly detected star, whose official name is WHL0137-LS, is so far away that its light had taken 12.9 billion years to reach Earth, appearing to us as it did when the universe was only seven percent of its current age, at Redshift 6.2.
Usually, we can observe galaxies containing billions of stars at these distances, and the smallest objects previously seen at such a great distance are clusters of stars embedded inside early galaxies. Until recently, the possibility of observing a single star at such a distance would be considered to be an impossibility. But a cosmic alignment of a massive cluster of galaxies between us and the distant star magnified the star by at least a few thousand times and thus essentially allowed the researchers to observe a single star from the early universe that, without the aid of lensing, there would have been no chance to observe.
“We almost didn’t believe it at first because it was so much farther than the previous most-distant, highest Redshift star,” said astronomer Brian Welch of Johns Hopkins University in Baltimore, the paper’s lead author describing the discovery. It was just published in the prestigious journal Nature. The discovery was made from data collected during Hubble’s RELICS (Reionization Lensing Cluster Survey) program, led by co-author Dan Coe at the Space Telescope Science Institute (STScI), also in Baltimore.
Massive bodies bend spacetime to create a lens in the sky effectively. “The lensing phenomenon opens the door to learn about dark matter and distant galaxies,” noted BGU Prof. Adi Zitrin, one of the study’s lead researchers. “But usually, the magnification from lensing reaches an order of a few. As a regular lens we know from everyday life, a gravitational lens forms high magnification regions called caustics, where light rays focus.”
A similar feature continued Zitrin, “can be seen on the bottom of a pool on a sunny day from ripples in the water. The lensing magnification at the caustic of a gravitational lens can be very high and even reach millions.”
But the higher the magnification, the smaller the region it magnifies. It is thus rare to find a star that precisely aligns on the caustic of a galaxy cluster lens. Only in 2018, with the discovery of the first lensed, distant star, it was properly understood that it was possible to see single stars at these distances by using gravitational lensing.
“The current discovery has another important aspect to it,” explained Zitrin, “as it also opens the door to learn about stars in the early universe, where we have little information on their physical characteristics and their contribution to the early ionization of hydrogen throughout the universe.”
Welch called the highly magnified star Earendel, which means “morning star” in Old English. The research team estimated that Earendel is at least 50 times the mass of our Sun and millions of times brighter, rivaling the most massive stars known. Since detecting the first lensed cosmological star in 2018, a few such high-magnification events have been detected but at somewhat smaller distances.
Various groups try to focus on and monitor galaxies that happen to sit on caustics (a curve to which each of the light rays is tangent, defining a boundary of an envelope of rays as a curve of concentrated light), waiting for one of their stars to approach the curve and light up telescopes on Earth and in space.
Zitrin’s group is a member of various such programs with the Hubble and James Webb Space Telescopes. Astronomers expect that Earendel will remain highly magnified for years to come. It will be observed by NASA’s James Webb Space Telescope, whose high sensitivity to infrared light is needed to learn more about the star because its light is stretched (redshifted) to longer infrared wavelengths due to the universe’s expansion.
“With Webb, we expect to confirm Earendel is indeed a star and measure its brightness and temperature,” Coe said. These details will narrow down its type and stage in the stellar lifecycle. “We also expect to find that the Sunrise Arc galaxy lacks heavy elements that form in subsequent generations of stars. This would suggest Earendel is a rare, massive metal-poor star,” Coe said.
“With Webb, we may see stars even farther than Earendel, which would be incredibly exciting,” Welch said. “We’ll go as far back as we can. I would love to see Webb break Earendel’s distance record.”
Zitrin says that he indeed expects Webb to detect stars even farther away. “Thanks to the spectral shape of stars, which follows what’s called a blackbody, the redshifted spectrum at large distances compensates for the flux loss due to the larger distance, and so we should see lensed stars at even earlier times. We hope that we will be able to see the first stars that formed in the universe with this phenomenon.”