The Hubble Space Telescope has found what appears to be the most distant star
ever detected. It was found in a galaxy a whopping 12.9 billion light-years away. And
that means we're seeing the star as it appeared less than a billion years after the Big Bang.
The discovery was announced in a new paper by Brian Welch et al. in the journal Nature.
They found the star while using Hubble to search for galaxies in the early Universe
that are being magnified by gravitational lenses. Gravitational lenses allow us to
glimpse objects that are otherwise much too far and too faint to be detected. In this case,
the star was found in a galaxy that existed when the Universe was less than a billion years old.
Gravitational lenses have revealed galaxies that are much further back in time. But this is
the earliest epoch of the Universe that we've ever seen a single star. For this reason, the
team dubbed the star "Earendel", which is the Old English word for "Morning Star". OK, there's a lot
to unpack here. There's the gravitational lensing, how they figured out the background, galaxies
distance, and how they know that this tiny blob of light is actually a star within that galaxy.
So let's break this all down. First, there's the whole gravitational lensing thing.
It's a prediction of Einstein's theory of relativity, which states that matter and energy
bend the spacetime around it. Anything traveling through that curved spacetime, including light,
finds its path deflected as well. Now, if the lensing mass and the background object are
both concentrated and the background object lies directly behind the lensing mass, then the light
from the background object will get distorted into a perfect ring of light called an "Einstein Ring".
We sometimes see Einstein rings naturally when a background galaxy lies almost directly behind a
massive compact foreground galaxy. But to gravitationally lens the very weak light
from a very distant galaxy, you need the mass of an entire foreground cluster of galaxies.
But those clusters are lumpy, with blobs of matter here and there.
So instead of a ring, the background galaxy gets distorted into several arcs of light.
Exactly how those arcs are formed depends on the geometry of the alignment and the
distribution of matter in the lensing cluster. So even though the galaxy doesn't look like a galaxy,
you can still measure the redshift of the light in the arcs,
and from there determine from what time period in the early Universe the light is coming from.
And that's because the farther away a galaxy is from Earth, the faster the Universe's expansion
carries it away from us. The faster it moves away from us, the more its light gets stretched
out to longer or redder wavelengths. So by measuring the arc's redshift, we effectively
work out how early in the Universe's history we are looking. Now, by early Universe,
we're talking about a time frame starting around 400 million years after the Big Bang.
Our best understanding is that that was the time when the Universe cooled
enough for matter to clump together to form the first stars. And those first
stars would have been ionizing the surrounding hydrogen and helium gas. And that's why this
box is alternately referred to as the "epoch of realization" or "the cosmic dawn". To that end,
astronomers have been using Hubble as part of the Reionization Lensing Cluster Survey, or RELICS.
The idea here is to obtain infrared imaging of the most massive galaxy clusters to look for
the brightest and highest-redshift gravitational lenses. In 2016. Hubble imaged one of the lensing
clusters called WHL0137–08. It happens to feature a particularly long and bright arc with a redshift
of 6.2. That's a high redshift! It puts the lens galaxy at approximately 12.9
billion light-years away. Or put another way, about 900 million years after the Big Bang.
Since that puts the background galaxy close to the Cosmic Dawn, the RELICS team dubbed it the
"Sunrise Arc". But how do they measure the redshift of the arc? Well, normally that's
done by taking a galaxy's spectrum and measuring how much spectral lines are shifted from their
known rest wavelengths. But that technique only works if there's enough light available to get
a good spectrum. Even though the Sunrise Arc is considered bright for gravitational lens,
it's still much too faint for Hubble's spectrographs.
However, we can take advantage of the fact that the light from the original galaxy
will pass through clouds of hydrogen gas on its way to Earth. As it does so,
hydrogen absorbs light at a particular set of wavelengths. A for simplicity,
I'm just going to show one of those shorter wavelengths here. As the light travels through
the Universe, it redshifts and carries the absorption line to a longer wavelength.
But hydrogen in the next cloud absorbs the light that has now shifted
into this special wavelength and produces another absorption line.
By the time the light reaches another cloud, the first two lines have shifted further into the red
and a new absorption line is created behind them. The process keeps repeating until over time,
the spectrum starts to build up a forest of repeating hydrogen absorption lines.
The higher the galaxy'd redshift, the more of these clouds its light had to pass through,
and the thicker the forest becomes. In fact, you can really see this effect when comparing the
spectra of two quasars with different redshifts. The higher redshift quasar is missing much more
of its shorter wavelength light. Now, the Sunrise Arc is too faint for Hubble to take a spectrum,
but the same phenomenon means that as the redshift increases, the galaxy starts to
fade away at shorter wavelengths and is only visible at longer or redder wavelengths.
So, if you take multiple images of the same galaxy through different filters of Hubble's camera,
you can estimate its redshift based on what wavelength the galaxy remains visible.
This is a technique called photometric redshift. In fairness, it's less precise
than spectroscopy because each filter allows a range of wavelengths to pass through.
That makes it harder to know exactly what wavelength the light was being cut off.
Still, the results converged on a redshift of about 6.2. Now, that's a real whopper
of a redshift! So the team made some follow up observations with Hubble in 2019 to try to get
some more light and glimpse some details within the arcs. Now typically, arcs will show repeated
images of the same feature within the background galaxy, and those images are created in a pattern.
If you recognize the pattern, you can figure out what features in the galaxy they belong to.
For example, these three blobs in cyan are a set of images coming from one star-forming
region in the background galaxy, while these three blobs in magenta are coming from another cluster.
But then there's this tiny bit of light in the arc that turns out to be the star Earendel.
Now, how could they possibly know that this faint poof of light
is actually a star in the background galaxy? Well, this is where it gets
really cool. All of the arcs and features are caused by the various clumps of
matter in the lensing cluster. By analyzing how these features are arranged you can map
out where along the face of the cluster the matter can create the strongest possible magnification.
These trace outlines called "critical curves". And the result looks a little like a contour map.
In theory, a point source of light located directly on this critical curve
will become infinitely magnified. Now, obviously, that's not going to happen because there are no
zero-volume point sources of light anywhere in the Universe. But a small source like a
star can easily get magnified by thousands of times if it happens to fall close to this line.
In fact, we can see this effect in a swimming pool on a sunny day. The ripples in the water
concentrate the sunlight into a network of bright regions at the bottom of the pool. Likewise,
the critical lines of a gravitational lens trace out the rippled spacetime of the lensing cluster.
Earendel appears directly on or extremely close to one of the critical lines.
The team calculated that this rare alignment is magnifying Earendel's light anywhere from 1000
to 40,000 times. That's an insane amount of magnification! Earendel's location on the
critical line explains why there's only one image of the star instead of two or more. The images of
the background star are so close together, they appear as a single point in the Hubble image.
So the region of space that's coming from has to be really small. Now, exactly how small depends
on how you model the lens. And that's because each model makes certain assumptions about the
distribution of matter and dark matter in the cluster, the alignment geometry of the lens and
the background star, and many other factors. Using different lensing models, though, the team found
the radius of the region couldn't be any larger than 0.36 parsecs. That means the diameter of
Earendel is region of space is no larger than 2.3 light-years across. For comparison, the distance
between the Sun and the next closest star, Alpha Centauri, is about 1.3 parsecs or 4.3 light-years.
Obviously, star clusters can fit many, many more stars into a small region of space. But even the
smallest, most compact star cluster we know of has a diameter of about six light-years across.
Earendel's estimate maxes out at just two light-years, or one third the size. Now to
be clear, nobody thinks Earendel is a single star with the diameter of two light-years.
Rather, the volume of space it's located in cannot be any larger than two light-years across.
Now, that's still large enough to accommodate a new kind of ultra-dense cluster of stars. And
that's not completely implausible either, because the Universe was a lot smaller
and had a higher average density back then. However, Earendel appears faint
despite being magnified many, many more times than the surrounding cluster images.
And that means Earendel has to be much less massive than a cluster.
So working backwards, the team estimated that Earendel's mass has to be somewhere
in the 50 to 100 solar mass range. Now, that's insanely massive for a single star!
But again, that's not unheard of either. And besides, high-mass stars
would have been more common in the early Universe. And sure, there's enough mass and
more than enough room to make Earendel a binary or even a triple-star system.
But the thing about those massive binary systems is that typically one of the two
stars is much more massive and luminous than the other. So even then, the light we are seeing is
probably coming from just one of the stars in the Earendel system. But there is another possibility:
what if we're just being fooled? What if Earendel isn't really a star in the source galaxy at all?
Well, the team wondered about this as well. If Earendel really was a - let's say - a
dwarf galaxy in the lensing cluster, then its redshift would be much lower than that of the
rest of the Sunrise Arc. In fact, the team managed to rule out an interloper this way.
And that leaves another possibility that Earendel is actually a faint star or even a brown dwarf in
the Milky Way, and it just happens to have the same red color as the rest of the arc.
However, the chances that a star of any kind, let alone a red dwarf, would happen to line up
with both the Arc and the critical curve by random chance works out to be around 0.01%.
It's not impossible, but it's very, very unlikely.
Still, the only way to know for sure is to take a proper spectrum of the star.
Not only would that tell us what kind of star Earendel really is, but it could also directly
measure its redshift as well as the redshift of the rest of the arc. And that's why the same team
are going to use the James Webb Space Telescope to do just that. If Webb confirms that Earendel
really is a star in that distant galaxy, it could settle the most provocative question of all:
is Earendel one of the very first generation of stars to form in the Universe?
It's an exciting prospect, but I wouldn't be surprised if the answer were "no".
Earendel's redshift puts it at around 900 million years after the Big Bang.
Now, even if it were "only" a 50 solar mass star, it wouldn't have been around for very long,
so it had to form at about the same time it was discovered.
Meanwhile, the very first stars would have formed around 500 million years earlier.
Such stars would have been made of pure hydrogen and helium. They also would have been extremely
massive, weighing in from a hundred to possibly a thousand times the sun's mass.
After just a few million years, these mega-stars would have ended their lives
in titanic supernova explosions, creating the first elements heavier than helium.
We call those elements "metals". Those metals would have been blessed into the surrounding
interstellar medium and triggered the next generation of star formation.
Now, given that Earendel didn't form until 500 million years later,
it wouldn't be surprising if Webb finds metals in its spectrum. But even then,
we'd want to compare Earendel spectra to the spectra of stars that are around today.
All of the second-generation stars that are still around today are low-mass dwarfs. Anything heavier
than eight solar masses would have exploded as supernovae long before the Sun ever formed.
So how exciting would it be to find the kind of star that led to the creation of our own?
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