Earendel: Earendel, the farthest known star in the universe, is rediscovered by the ‘James Webb’ | Science

Although the new space telescope james webb has monopolized covers with his recent images, his predecessor, the venerable Hubble, launched in 1990, is still operational and promoting new discoveries. Like, for example, the observation in March of Earendel, the most distant star.

Few stars have their own names. In this case, it is derived from archaic English and means, more or less, “dawn star”. Fans of Tolkien’s sagas will remember that one of the protagonists of The Silmarillion responds to a very similar name, but it is a pure coincidence.

The surprising thing about Earendel is his remoteness. The light that has caught the Hubble was emitted when the universe was less than a billion years old, meaning that it has been traveling through space for almost 13 billion years before leaving its faint trace on the electro-optical sensors of the Hubble.

It is very unlikely that the circumstances that have allowed us to take this look at such an old star are given. For this reason, from the outset, it was considered a prime objective for the james webb, launched on December 25, 2021. Fortunately, given the time of year, the southern constellation of the Whale where Earendel hides is within telescope range. With its increased resolving power and ability to see in the infrared, the james webb took on July 30 a new, even more detailed image, both of the star itself and of the arc of light that surrounds it, which is responsible for the favorable optical effect that amplifies its brightness. A trace of warped light that has also been given a name: Arc of the Dawn.

Cosmic magnifying glass and redshift

In December, the webb it will repoint its mirror there to attempt a spectral analysis to confirm or rule out the presence of heavy elements. At the moment, and only with the images of the Hubble and the webb, Earendel has already generated more than 4,700 articles in scientific publications (almost 40%, from authors associated with European institutions). Not in vain is it the most remote individual object that —today— we can distinguish in the cosmos. Although there are already reports that three or four more stars have been identified, very old and also favored by another cosmic “magnifying glass”.

The image taken by the 'Hubble', with the galaxy where Earendel is indicated by an arrow.
The image taken by the ‘Hubble’, with the galaxy where Earendel is indicated by an arrow.POT

With stars so remote, astronomers don’t talk about distance so much as “redshift,” a measure of how much their light has been “dilated” as a result of the expansion of the universe. In the case of Earendel, that index is 6.2, which places it 28 billion light-years from us. The one that held the previous record – nicknamed Icarus, in the constellation of Leo – does not reach half that figure.

It seems like a paradox: How is it possible to see an object at that distance when the universe has existed for much less time? His light, whose speed cannot exceed any physical body, should not have had time to reach us yet.

The answer is that space is not static, but is constantly expanding. When Earendel’s light began its journey, the universe was very young and therefore much smaller than it is now. Since then, space has been growing and in its expansion it has been separating more and more the galaxies it contains.

Lonely Star

But what has surprised him most is that Earendel is an isolated star, not a galaxy. The oldest galaxies do not appear in the photos of the Hubble like the beautiful spirals that we know, but like irregular masses of gas with reddish tones in which no structure can be distinguished. In reality, this color is false, the result of the treatment of the images. Most of the light they emit has migrated to the infrared, precisely because the expansion of space has been stretching their wavelengths until they get to that end of the spectrum.

The 'Hubble' image, with Earendel marked with a green star.
The ‘Hubble’ image, with Earendel marked with a green star.POT

Earendel is a huge star. Rather, it was, because its fires have been out for eons. It is likely to be a sample of the legendary Population III, the first stars to appear after the Big Bang. They had to be formed only by primordial hydrogen and helium. No other element was involved in its composition, for the simple reason that atoms of other metals did not yet exist. These heavier atoms would form as a result of the nuclear reactions that occur during the evolution of these stars. When they exploded, after a short and violent life, they would spread all those products throughout the cosmos that would serve as the building blocks of the next stellar generation.

It is estimated that Earendel had a mass between fifty and a hundred times greater than the Sun, with a surface temperature of 20,000 degrees (the Sun is only a third of that figure). That would make it extremely luminous, with a bluish-white glow. But no matter how bright, an isolated star that has not gone supernova should be invisible from such a distance.

That we can see it is due to an unusual chance. Between that star and us stands a small group of galaxies whose gravity acts like a lens that both concentrates and distorts the light from more distant objects.

Furthermore, Earendel is precisely in a narrow area of ​​that giant magnifying glass where the light is strengthened the most. In optics it is known as a caustic and in the telescope image it appears as a thin arc of light. It is the same effect that occurs at the bottom of a swimming pool, when the ripples of the water on the surface form brighter bands, or like light passing through a glass, which creates abstract shapes on the table.

Thanks to this effect, the star’s light could be increased between a thousand and forty thousand times, enough for the Hubble can distinguish it. Of course, after having accumulated nine hours of exposure looking towards the same region of the sky. Literally, the telescope has been accumulating one by one the photons that arrive from Earendel after their long journey through the universe.

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