To understand where we came from—how earth, the solar system, the galaxy became what they
are today—we need to understand how it all began.
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 done so for one lonely star at the end of the universe.
But it won’t be lonely for long.
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.
Now 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 galaxies—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.
Studying the early galaxies 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 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 in the alternate spelling Earendil. A name borrowed by Tolkien for a character in his Silmarillion.
You know, the half-elven Earendil the Mariner,
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 story.
A little more romantic perhaps than a frost-bitten big toe.
But 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 for distance 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.
Now, 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
allowing it to see galaxies in the early universe.
Seeing one of the individual 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, but its 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 lensing.
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'll
have a focal curve called a caustic.
They’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.
The caustic curve represents boundaries in the plane of the source.
When the source crosses those boundaries new images appear or vanish.
As the source approaches a caustic curve, multiple images can come together and overlap as they merge.
Leading to a large increase in the amplification of it's brightness.
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 its cluster.
The rest of Earendel's home galaxy spans extends far beyondthis particular caustic, and is actually being
magnified and smeared by the gravitational field of the entire cluster to a much lesser
degree than Earendel is.
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 one of 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 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 Webb 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, that makes it at least 50
times more massive than 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 star.
This isn’t unusual.
50 solar mass stars 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 proposed explanation is that the main blue star of this binary pair shines
about 600,000 times brighter than the sun, while the companion is a red giant star shining
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.
Meanwhile 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 those simulations against.
But still, those simulations have revealed tantalizing possibilities.
For example, we know that the gas from which these 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 simulations show that this early, pristine gas should've produce more massive stars
than later, polluted gas—something we’ve discussed previously.
In the Milky Way today around 3% of new stars have masses 10 times the sun’s
mass or higher.
The majority will be relatively low mass stars like our sun or even 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 the very first generation of 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 we can learn important
things about this star's parents.
All of this is really important for understanding how our modern very sparkly universe of stars
and galaxies pulled itself together from the
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 being a first generation star.
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
star formation in the early universe.
Now 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 that star.
I do enjoy 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.