Earendel, Our Most Distant Star

This past week brought another new discovery from the James Webb Space Telescope. I mean, I realize I could say that basically any day of any week and it would be true. But part of the promise of Webb is not just showing just completely new things but allowing us to deepen our understanding of things we already knew something about. That’s what happened this week, and that’s why I like it.

So let’s follow Webb as it stands on the shoulders (mirrors?) of its predecessor and compatriot, the Hubble Space Telescope, and peers back into the depths of time to the most distant star we’ve ever seen.

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The Sunrise Arc

These days it feels like Webb gets most of the hype, but Hubble is still awesome. Over several observations spread throughout 2016 and 2019, the old observatory peered at the galaxy cluster WHL0137-08 and discovered a smeared out streak of light near the cluster’s edge. 

 This is the hallmark of a gravitationally-lensed galaxy. Basically, the galaxy that this smear of light originally came from is sitting somewhere behind WHL0137-08 from our point of view. We shouldn’t be able to see it at all—the cluster is in our way. 

But gravity is a strange and funny thing and it messes with light the same way it messes with everything else. WHL0137-08, being a clump of galaxies, has a huge amount of mass in it and therefore a whole big lot of gravity. The gravity of the cluster bends the light coming from the distant background galaxy, the same way a lens will bend light coming through a telescope (hence the term “gravitational lensing”). This means light from the background galaxy actually moves around the cluster and becomes visible to us (and Hubble) as these stretched out streaks. 

These smears of light don’t look like a galaxy—they look like somebody gave a toddler a paintbrush and let them loose on the sky—but they can tell astronomers a lot. We know, for instance, that we’re seeing this particular gravitationally-lensed galaxy as it was about 900 million years after the Big Bang. Astronomers gave the galaxy the designation WHL0137-xD1. But since it looks like a red arc of light they also gave it a lovely nickname: the Sunrise Arc. And it turned out to be offering us something fascinating.

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The Morning Star

Sometimes the Universe lets things line up juuuuuuust right. When things are in just the right places, sometimes gravitational lensing will not only bend light but will magnify it, allowing us to see details that would normally be impossible to make out. Such was the case with the Sunrise Arc. Part of it lies perfectly in a sort of ripple produced by the lensing. And sitting on that ripple, magnified by a factor of at least 4,000 by the lensing effect, is a star. 

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While we’ve been able to make out galaxies so distant that we see them as they were only a few hundred thousand years after the Big Bang, individual stars are a different story. They are so tiny (relatively speaking) that it’s impossible to make them out at huge distances. The fact that we can see this star is a huge pile of cosmic coincidences, but we can see it, and that makes it the most distant star we’ve ever seen. 

It has another annoying muddle of letters and numbers as its official designation, but when astronomers announced its discovery to the world in March 2022, they called it Earendel, the Morning Star. Who says scientists don’t have a sense of poetry?

(This is the part where I, as a diehard fan of all the works of J.R.R. Tolkien, am required to point out that this Old English word is also the inspiration for the name of Tolkien’s hero Eärendil, who sails the heavens of Middle-earth as its brightest star. This symmetry pleases me to no end. Okay, thank you for the indulgence, moving on now.)

From Hubble to Webb

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It was in March of 2022 that Hubble showed the world that Earendel was there. It was only a few months later, in July, that Webb released its first images and announced itself ready to begin full science operations. One of the first things it observed was Earendel and now, a year later, the results of that observation have been released. 

Webb didn’t just look at Earendel with a camera, it also looked with a spectrometer. This splits the light from the star into its components and allows us to learn a lot more about what’s going on with the star. For one thing, that’s how we divide up stars into different classifications—we’ll classify a star with a letter based on what its spectrum look like. The letter classes are O, B, A, F, G, K, and M. In that order. Don’t try to make it make sense. There’s a whole big long history behind it that I might go into sometime.

The Webb data suggests that Earendel is a main sequence B-type star. “Main sequence” means it’s in the prime of life, neither very young nor approaching its end. B-type stars are big and hot, with the smallest of them over twice the size of our Sun and running many thousands of degrees hotter. They tend to look blue to the human eye. If you look at the constellation Orion in the winter sky, his shoulder Bellatrix and his knee Rigel are B-type stars. 

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Stars like these will frequently be found with a companion star as part of a binary system. A smaller, dimmer companion would be almost impossible to see at Earendel’s distance (we have trouble seeing companion stars in our own Milky Way sometimes. Those suckers can be good at hiding). But not being able to see something doesn’t mean we can’t guess it’s there.

The spectrum of Earendel taken by Webb shows hints that it’s not just the light of Earendel we’re seeing, but also the light of a cooler, dimmer, redder companion. What’s especially nifty is that the wavelengths that this light comes in are outside of Hubble’s range of view. There is no way Hubble would ever have been able to detect the companion at all. It’s just not designed that way. Webb is, and that’s why when they team up we can discover things that would otherwise remain hidden. And I love that.

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A Haunting Note

One final thought. As a B-type star, Earendel’s life span would be, on average, around a billion years. We’re seeing it as it was only 900 million years after the Big Bang. Even if we assume we’re seeing it somewhere near the beginning of its lifetime, Earendel has been dead for around 12 billion years. And yet it’s still sharing its life and light with us, like a ghost telling its story. And that’s cool.

But there’s also that potential companion star. The smaller, cooler, dimmer, redder types of stars live a very long time—trillions of years for the smallest ones. So let’s imagine that the companion is, in fact, there, and is a small red star, and it survived Earendel’s death (most B-type stars are too small to die in an explosive supernova, but not all of them). If that is the case, then the companion is likely still around today, drifting through whatever the Sunrise Arc has evolved into over the eons, many billions of light years away from us. And that is also very cool.

I mean, if that doesn’t get ya, I can’t do anything for you.