this is Earendel, the most distant star ever observed

A gravitational lens allowed Hubble to go back in time to the cosmic Dawn to discover this exceptional star in all respects.

When our elders start having trouble seeing properly, the first instinct is often to put on a pair of glasses. And that also applies to good old Hubble, even if the binoculars in question are very different. Despite his advanced age, he has just made an exceptional observation; using a gravitational magnifying glass, he set his lens on the most distant solitary star ever spotted.

The star was baptized Earendel, from an old Anglo-Saxon term meaning “morning star”. This nickname owes nothing to chance, since it is an object almost as old as the universe itself. She was formed at the very beginning of her life; the signals spotted by Hubble were emitted just 900 million years after the Big Bang – a record for this type of object.

A record star

This means that it is barely younger than our galaxy, whose age was updated following a recent study (see our article). It is therefore a star older than the oldest supernovae ever spotted, and 3.4 billion years older than the star Icarus which held the record until now.

And this is not the only striking element. Earendel is also the most distant solitary star ever documented in the scientific literature; because of the expansion of the universe that has taken its course since that date, it is now located about 28 billion light-years from Earth!

That’s a phenomenal distance, so even a state-of-the-art telescope like Hubble would have a hard time getting anything out of it on its own. At this distance, even extremely bright objects, such as qasars, become exceedingly difficult to identify; as much to say that it was necessary to trick to be able to draw the portrait with an isolated star like Earendel.

A galactic lens of Dantesque proportions

The phenomenon of gravitational lensing schematized by ESA. The grid represents the curvature of spacetime. © NASA, ESA & L. Calçada

To achieve their goals, astronomers have used a new, rather special lens. And they didn’t even need to send a ship to Hubble to install it there, since the latter was already present in space long before the telescope. Indeed, the lens in question is not an optical instrument produced by engineers: it is gravity itself!

This concept derives directly from general relativity conceptualized by Albert Einstein in 1915. One of the key points of this theory, whose immense solidity has again been recently demonstrated (see our article), states that the most massive objects, such as black holes, greatly distort the space-time around them. This has many consequences, but there is one that particularly interests us in this case; the light path precisely follows this curvature.

This is a very important phenomenon for astronomers, because it is possible to exploit it directly. Indeed, this curvature of space-time can be likened to a cosmic lens of absolutely enormous proportions; with a bit of luck and on the condition of being sufficiently meticulous in the preparations and the alignment, it is therefore possible touse a massive celestial body to “zoom in” to an object directly behind it.

It is this technique, called the “gravitational lens”, which allowed astronomers to spot Earlendel completely by chance. In this case, they did not use a single object, but a entire galaxy cluster which was their original object of study. On a large scale, it can be likened to a single object of phenomenal mass; it therefore generates a significant curvature of space-time, and therefore behaves like a cosmic lens of absurd proportions.

© Science: NASA, ESA, Brian Welch (JHU), Dan Coe (STScI); Image processing: NASA, ESA, Alyssa Pagan (STScI)

Many outstanding questions

This phenomenon allowed Earlendel to appear discreetly at the periphery of Hubble’s field of view; aware of having got their hands on a potentially interesting element, they therefore concentrated on this small luminous task, with the result that we now know.

Astronomers have notably managed to determine its distance thanks to the redshift phenomenon. To summarize the concept very briefly, we know that the expansion of the universe affects electromagnetic waves like light; the farther a ray of light travels, the more its wavelength is shifted towards the infrared, hence the term redshift. We can therefore use this offset to estimate the distance of an object.

Unfortunately, this technique, as elegant as it is interesting, also has some limitations. Unlike the lenses of telescopes like Hubble, which are carefully calibrated over years, these gravitational lenses are far from perfect. Researchers must therefore provide a lot of analytical work to interpret the images thus produced, which are often distorted and full of visual artifacts.

So many questions remain unanswered. It starts with the very nature of Earlendel. What is its composition? Was it really a solitary star, or were we dealing with a couple belonging to the same binary system? So many questions that do not yet have an answer at the moment.

©: Hubble / NASA / ESA.

An exceptional witness to the Cosmic Dawn

But that does not mean that this discovery is anecdotal. Indeed, her age implies that she was born in a time dubbed “Cosmic Dawn.” It is a moment that is by definition extremely difficult to observe; it was preceded by what is now called the “dark age”, during which light could not yet travel freely in the universe.

This is a big problem for researchers; for as one might expect, this light plays an eminently important part in the observations. The fact of having found a star born at the extreme limit of the transition between the dark age and the cosmic dawn is therefore a superb discovery.

It will certainly provide decisive elements for our understanding of these times, and consequently, of the origins of the universe as we know it today. From now on, all that is expected is the James Webb Space Telescope, which was specifically designed to look into the origins of the universe.

The text of the study is available here.

Source: Journal du Geek by

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