Earendel: A star seen by the Hubble Space Telescope

The big news from the Hubble space telescope this week was that it had spotted the most distant star ever found at 12.9 billion light-years away and nicknamed irendel now you'll probably see this in the news already but now it's time to dive into all of the nitty-gritty astrophysics details like how did they find this thing how do they know how far away it is and how do they even know that it's a star and is there a chance that it could be something else entirely so let's dive into this this study has been led by welch cohen carburetors who have been targeting big clusters of galaxies with the Hubble space telescope.

                           

Since about 2016 in the hope of finding things like this known as arcs these are galaxies in the background behind the ones in the big galaxy cluster in the foreground that you can see and the cluster is just one big like lump of matter right you know like hot hydrogen gas and dark matter just all pot marks then with galaxies of stars kind of like a chocolate chip cookie and all of this matter curves space right this is how einstein described gravity and his theory of general relativity which means that light from those background galaxies travels across that curved space and gets bent into these arcs this is what's known as gravitational lensing because that cluster of galaxies in the foreground acts like a lens to change the path of light just like the bottom of a stem wine glass does when it passes in front of a candle in the background and just like a lens that cluster of galaxies can also magnify things in the distance as well taking those you know very faint distant galaxies that we wouldn't usually be able to spot i'm making them much brighter so that we can study them and because light has this finite speed limit that it can't go fast and light takes time to travel to us.

So we're seeing those very distant galaxies as they were billions of years ago when the universe was much younger so by studying them what we can do is compare what did the universe look like then compared to what it looks like now when we look at nearby galaxies and can we piece together what's happened in terms of how the whole universe has evolved to give us what we see today so this is what welch and cohen collaborators were looking for at first with what's known as the relics survey and in that data they spotted the longest arc they've ever seen at a distance of 12.9 billion light years away they know that this arc is that far away because of the different wavelengths of light that they can and can't detect from it so when the light from this galaxy has been traveling through the universe to as the universe has been expanding so the light is stretched to longer wavelengths it is red-shifted but on its way it also passes through random clouds and clumps of hydrogen gas that just permeate the universe and the hydrogen gas actually steals away a very specific color or wavelength of light as it passes through as the light travels further through the universe.

More light gets red shifted to become that specific wavelength that hydrogen likes to absorb and steal away and so what happens is you start to lose all the light beyond a certain wavelength if you take a spectrum of the light from the galaxy where you split the light into all of its different component wavelengths and get a trace of how much of each wavelength you detect then it's really easy to spot right like in the example i'm showing here but the giant art they detected here was actually too faint for hubble spectrograph to even be able to detect so instead what you can do is you can take images through filters that only let in like certain regions certain wavelengths or colors of light and from where you do and don't detect light anymore you can pinpoint well how much has the light been redshifted and therefore how long the light has been traveling for to know how far away the object was when the light left it and in the case of this arc that they spotted that was when the universe was less than a billion years old practically a baby in comparison to the 13.8 billion years old that we think it is now so what they did after spotting this arc was they actually applied to observe it again with the hubble space telescope.

This time for longer in the hope of detecting collecting more light to see more detail they got that image back in image that the art was originally spotted in now along any arc you also get clumps as well which could either be light from the center of a galaxy where it's brightest or from a star cluster in that galaxy that's really quite bright as well and usually those are seen in multiple places along the arc due to how the lensing works like the things highlighted in cyan here this is a star cluster scene three times but then there's also this bright point which is only seen once in both the images taken in 2016 and in identified as a single star in this galaxy that's 12.9 billion light years away we'll get to how they identified it as a single star in a minute but i just wanted to talk about how it was even possible for them to detect a single star at that kind of distance because when you look up to the night sky every single star that you see is from our own galaxy the milky way a few tens of thousands of light years away.

The hubble space telescope has just enough resolving power to make out individual stars in the andromeda galaxy which is our largest nearest neighbor galaxy but anything further than that and the light from the stars have just merged to give us just this glow of a galaxy it's only when they die and go supernova do we have a hope of picking out individual stars but even that has its limits in terms of distance out to about 10 ish billion light years away that isn't what we've got here this isn't a ridiculously bright supernova what we've got is a star that's just in the right place at the right time the star just happens to be perfectly aligned with the clump of mata that's doing the lensing so much so that it gets magnified to over a thousand times brighter than it normally is so that we can spot it the best analogy that i've heard for this is that the matter in the cluster is acting kind of like the ripples on the surface of a swimming pool you know it transforms the equal amounts of sunlight that's incident on the surface of the pool into these patterns on the bottom of the pool which are analogous to the arcs you know in these patterns.

You can see there's always that odd bit that is that much brighter than the rest this is what's happened to this star whl0137-ls but nicknamed by the authors as a irendell from the old english word meaning morningstar or dawnstone this is why it's been nicknamed this dawn star you know the first star at the dawn of the universe you get it right but there's been a lot of comparisons also to tolkien's lord of the rings because there's a character and a star in lord of the rings that's called erendill i give you the light of neal our most beloved star and for good reason too right tolkien was an english literature expert he was the professor of anglo-saxon at pembroke college oxford when he wrote the hobbit and he got the name from an old english poem that mentioned the anglo-saxon name irendelle meaning morningstar that is a massive tangent though so let's get back to the astrophysics how do we actually know that irendel is a single individual star well one of the main reasons is because of its size alone because from the shape of the art you can actually construct a map of how the matter in the galaxy cluster that's doing the lensing is distributed to give you the lensing effect that you see from that you can then work out.

Okay well how big must the object be to be making this clump of light that i've seen in my arc and there's a lot of different assumptions that go into this and those are the different algorithms that you can use to pull this out as well so you get a few different answers but as you can see from the nature article where this study was published all of these models say that erindel is at least less than 0.3 parsecs in radius or about two light years away so to put that number into context think about how the fact that the sun's nearest star to it proxima centauri is about four light years away and the thing that's been lensed here that's making this bright spot is smaller than two light years across now admittedly the region of the milky way where the sun is is pretty sparse right it's pretty under dense compared to a lot of other regions in the milky way especially star clusters where you can get a huge number of stars in a very small space but the smallest densest star cluster that we know is six light years across and they've estimated their end deal is a maximum of two light years across is probably smaller than that so that's like a third of the size of the densest star cluster we know.

So while it could be that you know maybe star clusters in the early universe were much smaller and denser than we see now more likely hypothesis that we have is that it is a single star or maybe even a binary star system of two stars orbiting each other which was quite common for massive stars and could have been more common in the early universe too or even the other option for a rendel is that it is a milky way star that just happens to be in the foreground getting in the way and is right in the firing line that places it directly on that arc it's not very likely because again arendelle has that same pattern of light in the different filters where you see that big drop off whereas it's very unlikely that a star in the milky way would ever have that again taking the light from arundel and splitting it into its component wavelengths to get a spectrum would definitely tell you which one of those options it would be because each type of star has a very specific signature.

A very specific trace of light shape of light when you do this and get a spectrum so you could be able to see if it was just one specific star if it was a mix of two or a mix of many of them in a cluster again this is far too faint to do with hubble the spectrograph that hubble has on board needs far more light to be able to do that and split it to get into the component wavelengths this is a job for the jamestown space telescope and in fact welch and cohen collaborators have already applied to observer rendel with the jameson space telescope to confirm this and have been awarded time in the very first round of observations in webb's first year it just depends on the visibility from its position at lagrange.2 when it actually gets observed it's really going to be something to look forward to in james webb's first year of operations you know confirming whether this is a single star and then if it is what type it is as well because currently from its brightness in the different filters they've estimated that it's either an ob or a star round about 50 times the mass of the sun with a temperature about 20 hotter than the sun now the reason that me and my colleagues are so excited about this discovery is that it could turn out that this could be what's known as a population three star now this is a hypothetical type of style we've never observed these before but the idea is that it would be made of purely hydrogen and helium the stuff that was knocking around just after the big bang and wouldn't be polluted by any of the heavier elements like carbon or nitrogen because they'd be some of the very first stars ever formed in the universe that would actually kick-start that production of those heavier elements when they finally died and went supernova throwing all that material back into space and actually dispersing the elements like carbon oxygen nitrogen that go into making rocky planets and you know the ingredients for life as well we've never observed a start like that though because you know it's so distant and they just all merge together into these fuzzy blobs of very very distant galaxies but it is something that the james webb space telescope has been designed to do and the fact that we now have a candidate for one of those stars so that we know where to point the james webb space telescope first is amazing you know because even if you know jdbst finds that it's actually you know not an individual star that it's a binary system of stars or if it's a cluster of stars we'll still have observations of some of the very first stars to have ever formed in the entire universe