One of the great challenges for astronomers is to understand when the first stars formed and what they were like. They already have some clues.
First, hydrogen and helium formed about 380,000 years after the Big Bang. This is what the first stars were made of. And second, the oldest galaxies formed about 400 million years after the Big Bang.
An ancient star
So the first stars must have formed at some point in between. The best estimate is that the earliest stars began to shine about 100 million years after the Big Bang. But the truth is that no one knows because these stars have never been observed. Until this year, the oldest observed star formed about 4.4 billion years after the Big Bang. This is significantly later than this first period of star formation.
The James Webb Space Telescope captured a star that began to shine just 900 million years after the Big Bang. The images are better than those taken earlier this year by the Hubble Space Telescope, which discovered this ancient object, and provide astronomers with their first look at a star from this early period of the universe.
Earendel’s host galaxy, the Sunrise Arc, is scattered across the sky by gravitational lensing (Source: arxiv.org/abs/2208.09007)
Astronomers call this ancient star Earendel, a word derived from Old English meaning “rising light.” It began burning about 13 billion years ago, but due to the expansion of the universe, it is now about 28 billion light-years from Earth, making it the most distant star ever observed.
Earendel can only be observed due to an extraordinary cosmic coincidence. As seen from Earth, it lies behind a huge galaxy cluster called WHL0137-08, with a gravitational field that focuses its starlight toward Earth, magnifying it up to 40,000 times.
This gravitational lensing effect makes Earendel’s host galaxy look like a spot of light in the distant universe – astronomers have named it the Aurora Borealis. The rainbow contains bright nodes of light along its length, and one of these nodes is Earendel.
The JWST images improve on Hubble’s and narrow down the potential size of the star. The star appears to be a point source of light, suggesting that it cannot be larger than 4,000 AU in diameter. “These new observations strengthen the conclusion that Earendel is best explained by a single star or a system of multiple stars,” said Brian Welch of Johns Hopkins University in Baltimore and colleagues who analyzed the images.
The team says the photometry suggests Earendel has a surface temperature between 13,000 and 16,000 Kelvin. This in turn suggests that the star is a hydrogen-burning B-type giant star with a mass somewhere between 20 and 200 times that of the Sun.
Binary option
However, the team also says that Earendel could be a binary system and that different combinations of stars could better fit the observed data. Indeed, the best fit is a combination of a luminous cool star and a hot companion.
At the moment, the data does not allow the team to solve this problem. However, JWST should observe Earendel again later this year, when more data may help constrain the nature of this star or star system. It will also provide astronomers with more data about one of the earliest stars in the universe.
It’s an interesting piece of work that shows the extraordinary light-gathering power of JWST and its older relative, Hubble, and how this instrument is already changing the way we see the universe.
Reference: JWST images of Earendel, the extremely magnified star at redshift z=6.2: arxiv.org/abs/2208.09007