JWST Discovered The Farthest Star Ever Seen! (Script) (Patreon)

To understand where we came from—how earth, the solar system, the galaxy became what they are today—we need to understand the beginning of time. For example, how did the first galaxies pull themselves together from the dark universe-filling ocean of gas that followed the Big Bang? With the James Webb Space Telescope we’re starting to be able to find those first galaxies. It’s hard work because at those crazy distances all we see is tiny, faint and fuzzy blobs. If only we could see the individual stars in those galaxies we could learn so much more. Well, now using this one weird trick we can do exactly that. Or at least we have one lonely star at the end of the universe. But it won’t be lonely for long.

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A top contender for most distant star we can see with the naked eye is a huge variable star in the constellation Cassiopeiae. It’s 16,000 light years away, so just a fraction of the distance across the Milky Way. Stars more distant blend together into the unresolved Milky blur that gives our more galaxy its name. We can pick out those more distant stars with telescopes. But it was only a bit over a century ago that our telescopes got good enough to pick out the brightest individual stars from our nearest sizable neighboring galaxy, Andromeda. As our telescopes improved we were able to resolve those individual pinpricks of light to greater and greater distances. But it feels like there’s a fundamental limit. Our telescopes can only get so big, they can only stare for so long. The most distant galaxies seem destined to remain faint fuzzy blobs for a very long time to come.

Unless, but some happy chance, an individual star at the end of the universe happens to be directly behind an entire cluster of galaxies, resulting in an enormous amplification of its light by gravitational lensing. And that happy chance has happened. Meet Earendel - the farthest star ever seen - shining out from 28 billion light years distant. That’s double the distance of the previous record holder, and it may even be our very first direct glimpse of the first generation of stars to form in our universe.

Earendel was first spotted by the Hubble Space Telescope in March 2022. It was a lucky find, showing up as an inexplicably bright spot in the Reionization Lensing Cluster Survey, or “RELICS.” The RELICS was all about finding extremely distant ga laxies—galaxies whose light has been traveling to us for most of the age of the universe. By peering back in time this way, we can learn how galaxies first began to grow out of the ocean of dark gas that filled the universe after the Big Bang.

This is one of the main goals of the James Webb Space Telescope, but with some extra cleverness the older Hubble can do some of this work also. It needed a bit of help to see that far, in this case in the form of an additional lens—a gravitational lens. RELICS looked towards and through clusters of galaxies, searching for vastly more distant galaxies behind clusters whose light is warped and magnified by the cluster’s gravitational field. With this one weird trick, the RELICS team managed to discover hundreds of galaxies from the first billion years of the universe. And in one of those galaxies a single star shone out. It seemed impossibly bright, even under the magnification that enhanced the rest of that galaxy’s light.

They named the star ‘Earendel’. It’s … a good name, and one with a deep and nerdy pedigree. It’s the old English for “morning star,” likely another old name for Venus or maybe even sunrises. It probably comes from Old Norse, Aurvandill, a mythological character who’s toe froze off and Thor tossed into the sky to make a “star” - again, probably Venus. That was all new knowledge to me. I knew the name with the alternate spelling EarenDIL as a Tolkien character. You know, the half-elven Earendil the Mariner from the Silmarillion? Father of Elrond and Elros, who wears one of the silmarils on his brow as he sails the sky, and that’s Tolkien’s Venus creation myth. A little more romantic than a frost-bitten big toe. Either way, Earendel-dil is a good name for a very special star.

And a distant one. Earendel is 28 billion light years away. Now astronomers usually don’t talk about light years when the numbers get so large. For such distant objects we just refer to the redshift. Because the universe is expanding, a photon traveling between galaxies gets stretched—its wavelength increases. The more time it spends traveling, the more it’s stretched. So if we measure this redshift of light we know how long ago it must have left its starting point. That also tells us the distance to that starting point.

For comparison, light that’s been traveling to us for 10% of the age of the universe is stretched by about the same factor, giving it a redshift of 0.1. Earendel’s redshift is 6.2—it’s light is stretched by a factor of more than 7. That gigantic redshift means its light must have been traveling for nearly 13 billion years—most of the age of the universe.

The star is NOW 28 billion light years away rather than 13 billion because while its light was traveling to us, it was racing away from us with the expanding universe.

At this crazy redshift, all of Earendel’s visible light and a lot of its ultraviolet reaches us as infrared light. Hubble is optimized for visible light, so it can only pick up a small fraction of Earendel’s light—just its shortest wavelength ultraviolet that ends up still being in the range of Hubble’s sensitivity.

It sure would be nice if we had a space telescope even bigger than Hubble that was sensitive to infrared light. Well what do you know? We do. The James Webb Space Telescope has a collecting area more than five times larger than hubble, and was designed especially for work deep into the infrared spectrum. in the early universe. Seeing one of the stars inside one of these galaxies is a huge bonus.

Here’s the Hubble image of Earendel and here’s the Webb image. This faint streak on either side is the star’s home galaxy, fittingly named the ‘Sunrise Arc.’ The galaxy doesn’t really look like this. It would be a much more compact bundle of billions of stars whose light has been smeared across the sky by the vast gravitational field of this cluster of galaxies that you can see in the foreground. Smeared and amplified—and the amplification is why the RELICS program looked at this thing in the first place.

Gravitational lensing is really an incredible tool. It lets us see much further than we otherwise could. Let’s do a quick review of how this works.In gravitational lensing, rays of light from a distant object that would normally not reach our telescope are deflected toawards us by an intervening gravitational field. In the case of so-called weak lensing, this just leads to a distortion of the image, like looking through rippled water. But if the alignment of the lens and the source is just right then many light rays that would otherwise have traveled in very different directions are deflected towards us. This is called strong lensin g. In the case of a lens with perfect circular symmetry and a perfect alignment of us and lens and source, rays traveling in all different directions reach us. Now the warped image looks like a perfect ring around the lens—what we call an Einstein ring.

We do see Einstein rings, but they’re rare. Normally the alignment isn’t quite perfect and the lens isn’t symmetric enough. Break either of those and the ring fragments. For example, into multiple images for very compact sources, or multiple arcs for extended sources.

Individual lensed images are also magnified. Just like a glass lens can bend diverging rays of light back towards each other to a focus, thereby increasing brightness, a gravitational lens can do the same. For a glass lens the magnification is the greatest if the light source is at the focal point relative to your eye—then all diverging rays are sent straight to your pupil.

Gravitational lenses are messier, and instead of a focal point you have a focal curve called a caustic. It’ll be this star-like structure for a typical lensing configuration with a relatively simple gravitational field. You see similar shapes when you shine light through the stem of a wine class, or through rippled water.

Points near one of these caustic curves will be seen as multiple images. The closer to the curve, the closer together those images become. If an object is directly on the curve, all multiple images come together, resulting in massive amplification. Technically this would be infinite amplification for a point-like object sitting on the infinitesimally-thin curve.

In practice, it’s much more likely that a compact object like a star will sit just off the curve, and so technically will produce multiple images. However we only see those multiple images if they’re far enough apart to be distinguished by the resolution of our telescope. If the object is very close to the curve then the multiple images still blend together and have the effect of extreme magnification.

And that’s exactly what happened with Earendel. In this case it’s very near the caustic curve produced by one of the galaxies in this cluster. The rest of its galaxy spans extends far beyond this particular caustic, and is actually being magnified and smeared by the gravitational field of the entire cluster to a much lesser degree than Earendel. But this star, with its happy position, experiences magnification of at least a factor of 1,000, and could be up to 40,000. At the lower limit, we’re gaining the light collecting power of a telescope 30 times larger in diameter.

On either side of Earendel, you can also see the lensed image of a nearby cluster of stars in the same galaxy. That cluster is close to the caustic curve, but not as close as Earendel. As a result its multiple images are sufficiently separated to be visible.

OK, so now that we’ve found this distant star, what can we learn about it? Like I said, Hubble was only able to capture a fraction of the star’s light due to most of that light being redshifted outside Hubble’s range. But JWST is sensitive to much of Earendel’s now-infrared light.  Here’s what we know so far. “De-redshifting” the light we can compare the spectrum—essentially the raw colour of the star—to known stellar types. Earendel appears to be a blue-ish white type B main sequence star, making it at least 50 times more massive tha n the Sun with a surface of more than 20,000 Kelvin.

On top of that, there also seems to be an excess of red light coming from Earendel, which astronomers think could be due to a companion. This isn’t unusual. 50 solar mass stars very often have binary partners, but the mixing of the light from the two stars combined with the uncertain magnification makes it difficult to say exactly how massive and luminous each star is. One explanation they give is that Earendel is a binary of the main blue star that shines about 600,000 brighter than the sun, while the companion is a red giant star shining that’s about 200,000 times brighter than the sun.

Earendel is important for a bunch of reasons. For example, both it and its newly discovered companion are the first direct data points we have for the earliest stars. We know a lot about star formation in the universe today because we can actually see it happening. We have countless snapshots of star formation in action in star-forming clouds, both in the Milky Way and in relatively nearby galaxies. Our surveys of individual stars across the Milky Way and beyond tell us the final results of the star formation process. We can check our calculations and computer simulations against these observations to figure out how modern star formation works.

While we can still do computer simulations of star formation in the early universe, there’s not much to check them against. Still, those simulations have revealed tantalizing possibilities. For example, we know that the gas from which stars formed back then must have been more “pure” —-almost entirely hydrogen and helium. These days that gas is polluted by heavier elements produced in the deaths of multiple generations of stars. Computer s imulations show that this early, pristine gas should produce more massive stars than later, polluted gas—something we’ve discussed previously.

In the Milky Way today around 3% of nearly formed stars have masses 10 times the sun’s mass or higher. The majority will be relatively low mass stars like our sun or smaller. On the other hand, simulations of early galaxies tell us as many as 20% of stars might have more than 10 solar masses. So really hefty stars like Earendel may have been more of a norm than an exception in the early universe.

Being around when the universe is 900 million years old makes it possible, but unlikely, that Earendel is one of these first generation stars. That would be truly incredible. We’ve never seen one of the founding batch before. But even being from an early generation of stars means it can still tell us important things about its parents. All of this is really important for understanding how our modern very sparkly universe of stars and galaxies pulled itself together from the dark ocean of gas that existed after the big bang.

Earendel is not a totally solved star. We don’t know how much magnification it’s experiencing, so can’t know its true brightness until we model the lens a lot better. And its fainter red companion is even more mysterious. But this is a seriously exciting new line of investigation. While we can’t say a ton yet from the current observations, further observations of this star and the discovery of new objects like it could be revelatory.

For example, if we can get better information about the spectrum of the star we may be able to tell whether it really does have the low abundance of heavy elements expected of an early star. Perhaps we even confirm that it’s mostly raw hydrogen and helium, increasing the chance that it’s of the first generation.

If we can find more lensed stars like this then we can start to build up population statistics. That can tell us if massive stars really were produced in larger quantities back then. The specifics of the resulting population could tell us a ton about how star formation happened back then. Upcoming giant surveys of the sky are definitely going to find, like, millions of lensed galaxies. Hopefully some of them will have a star conveniently located near a caustic curve, enabling us to see it.

I like the severed-digit themed origin stories of the Norse, and especially like the elves-sailing-sky-ships origin story of Tolkien. But as our new and next generations of incredible observatories peer deeper into the cosmic dawn, we no longer have to invent our own origin story. We can know it. Will the help of Earendel, the star at the beginning of the universe and at the edge of observable spacetime.