The Hubble Telescope Detected the Farthest Star Ever Observed

When the renowned British writer John Ronald Reuel Tolkien, better known as J.R.R. Tolkien, wrote in the summer of 1914 The voyage of Eärendel, the evening star, on his aunt's farm in England, he never thought that more than 100 years later, that created name would be designated to the farthest star ever discovered by mankind.

NASA's Hubble Space Telescope has set an extraordinary new benchmark: detecting the light of a star that existed in the first billion years after the birth of the universe in the Big Bang, making it the most distant single star ever seen until the date.

The find is a big leap further back in time than with the previous single star record; this was spotted by Hubble in 2018. That star existed when the universe was about 4 billion years old, or 30 percent of its current age, at a time astronomers refer to as a “1.5 redshift”. Scientists use the term “redshift” because as the universe expands, light from distant objects stretches or “shifts” to longer, redder wavelengths as it travels towards us.

The newly detected star is so far away that its light has taken 12.9 billion years to reach Earth, and it appears to us as when the universe was only 7 percent of its current age, with a redshift of 6.2. The smallest objects seen earlier at such a great distance are clusters of stars within primitive galaxies.

“At first we hardly believed it: it was much further away than the previous most distant redshift star,” said astronomer Brian Welch of Johns Hopkins University in Baltimore, lead author of the article describing the discovery, published on March 30 in the journal Nature. The discovery was made from data collected during the Hubble Gravitational Lens Reionization Study (RELICS) program, led by co-author Dan Coe at the Space Telescope Science Institute, also in Baltimore.

“Normally, at these distances, entire galaxies see small spots, where the light of millions of stars mixes,” Welch said. “The galaxy that houses this star has been enlarged and distorted by gravitational lenses into a long half-moon that we call the Arc of Dawn.” After studying the galaxy in detail, Welch prolongs that one of the elements is an extremely magnified star that he called Earendel, which means “morning star” in ancient English. The discovery promises to open an unexplored era of very early star formation.

“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 with which we are not known, but that led to everything we know. It's like we've been reading a very interesting book, but we start with the second chapter, and now we'll have a chance to see how it all started,” Welch said.

When the stars align

The research team estimates that Earendel is at least 50 times the mass of our Sun and is millions of times brighter, rivaling the most massive stars known. But even such a bright and massive star would be impossible to see such a great distance without the help of the natural increase produced by a huge cluster of galaxies, WHL0137-08, that lies between us and Earendel. The mass of the galaxy cluster deforms the fabric of space, creating a powerful natural magnifying glass that hardly distorts and amplifies light from distant objects behind it.

Thanks to the rare alignment with the cluster of galaxies that serve as a magnifying lens, the star Earrendel appears directly on a ripple in the fabric of space, or very close to it. This ripple, which is defined in optics as a “caustic”, provides maximum magnification and brightness. The effect is analogous to the undulating surface of a pool that creates bright light patterns at the bottom of the pool on a sunny day. The ripples on the surface act like lenses and focus sunlight to maximum brightness at the bottom of the pool.

This caustic makes the Earendel star stand out from the general glow of its home galaxy. Its brightness is magnified a thousand times or more. At this point, astronomers cannot determine whether Earendel is a binary star, although most massive stars have at least one smaller companion star.

Confirmation with the Webb

Astronomers expect Earendel to remain greatly magnified for years to come. It will be observed by NASA's James Webb Space Telescope. The Webb's high sensitivity to infrared light is necessary to learn more about Earendel, because its light stretches (shifts to red) at longer infrared wavelengths due to the expansion of the universe.

“With Webb we hope to confirm that Earendel is indeed a star, as well as measuring its brightness and temperature,” Coe said. These will narrow down the research on its type and its stage in the stellar life cycle. “We also hope to find that the Arc of Dawn lacks the heavy elements that form in later generations of stars. This suggests that Earendel is a rare, massive, metal-poor star,” Coe said.

The composition of Earendel will be of great interest to astronomers, because it was 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 composed solely of primordial hydrogen and helium, it would be the first evidence of the legendary stars of Population III, which are supposed to be the first stars born after the Big Bang. While the probability is small, Welch admits that it is tempting anyway.

“With Webb, we can see stars even farther away 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.”

This historic discovery was made by an international team of researchers led by Brian Welch, a scientist at Johns Hopkins University in the United States, and in which José María Diego also participated.

KEEP READING: