Occasional paper: The Light of an Older Heaven

And then the light of an older heaven was in my eyes
and when my vision cleared, I saw Titans.

— Alan Moore

Today’s Occasional Paper comes to us from the James Webb Space Telescope.

So let’s start with some basics:  nothing can travel faster than the speed of light.  So when a telescope looks out into space, it’s also looking back into time.  Look at the moon?  You’re seeing it as it was when the light left it’s surface about 1.5 seconds ago.  Look at the Sun?  You’re seeing it as it was 8 minutes ago.  The Sun could have exploded 5 minutes ago, and there’s no way you could possibly know about it until 3 minutes from now.

Okay, so keep going.  Look at the nearest star?  You’re seeing it as it was about four years ago.  Look at the center of our galaxy?  30,000 years.  The light from there left around the high point of the last Ice Age.  Look out of our galaxy, at our neighbor galaxy Andromeda?  About 3 million years.

Now it starts to get weird and interesting.  Because as we start to look at things that are billions-with-a-b light years away — very distant galaxies — things start to change.  That’s because we’re looking back into the distant past of the Universe.  And the Universe is only 13.5 billion years old, so… yeah.  In theory, if you had a strong enough telescope, you could see back to the Big Bang and the beginning of everything.

Of course it’s not that simple.  The Universe is expanding.  Distant galaxies are receding from us.  More distant galaxies are receding faster, often at significant fractions of the speed of light (from our perspective).  This means that the distance to them is greater than you might expect.  It also means that their light is “red shifted” by the Doppler effect.  Also, while the Big Bang was very bright, once it cooled down the Universe was just a hot dark cloud of gas, mostly hydrogen with a bit of helium mixed in.  In that earliest pre-dawn epoch, there was not much to see, and no light to see with… until the first stars switched on.

And now for a brief historical digression.

You’ve probably heard of the James Webb Space Telescope, JWST or “the Webb ” to the cool kids.  For three years now it’s been the most powerful telescope in the world.  (Okay, not exactly “in the world”, because JWST is about a million miles out in space.  Really.  It’s just about one million miles.)


[come on, doesn’t that just look cool as all heck]

Well, a hundred years before there was the Webb, there was the Hooker:  the 100 inch Hooker telescope at Mount Wilson Observatory, located in the mountains near Los Angeles, California.


[also cool.  aw yeah girders!]

“100 inch” means the size of the reflecting mirror at the telescope’s heart, 2.5 meters to the rest of the world.  When the Hooker went live in 1919, it was the biggest, most powerful telescope in the world, just as the Webb is now.  And it kept that record for almost 30 years, until a slightly bigger telescope was brought online in 1947.

— You might perhaps wonder: was it really a good idea to build the world’s largest telescope just outside of Los Angeles?  And the answer is, at the time that decision was made (around 1910) Los Angeles had about 1/20th of its current population and basically no automobiles.  The Hooker is still around and, yup, it struggles with light pollution and smog.  But it’s too big to move, so astronomers have come up with various workarounds.

Now, really big telescopes are always in demand.  Astronomers are constantly clamoring for time on the scope to look at whatever their particular obsession is.  The Webb has a committee that makes these decisions.  Let’s just say it sometimes gets a little intense.  And a thing that never happens — ever — is one person getting hundreds of hours in a single year on one of these giants.

Except for this one time in 1944, when one guy got exactly that.

That was the middle of World War Two, yes?  So pretty much all of the nation’s scientists were being called off to work on war-related stuff.  We all know about the Manhattan Project, but that’s just the most famous.  (It wasn’t even the biggest.  The B-29 was the biggest.)  Scientists were being dragooned to work on all kinds of stuff.  Artillery, bomb sights, torpedoes, radar,  you name it.

But there was this one astronomer named Walter Baade who got passed over for all that.  See, Baade had been born in Germany in 1893.  He came to the US on a Rockefeller fellowship shortly before Hitler came to power.  And Baade looked back at what was happening in his home country, said “hard nope”, and decided to just stay in the US.  

(Yes, mid-20th century scientists were required to smoke a pipe.  It was a whole thing.)

Anyway, in 1942 Baade was still German.  He was a permanent resident but he never took US citizenship.  So after the war broke out, he was a resident enemy alien.  He was required to register, observe a curfew, check in regularly with a minder, and not travel outside Los Angeles County.  

But here’s the thing:  Baade didn’t want to travel outside Los Angeles county.  He wanted to spend all his time up at the Mount Wilson Observatory.  So a friendly colleague went to the Army guy in charge of LA’s enemy aliens and said, look, let’s make this easy — he’ll basically never leave the observatory.  Just bend the curfew so that he can spend his nights up there.  And the Army guy — busy, presumably, with rounding up dangerous Japanese-Americans — said  whatever, fine, just make sure he reports in regularly.

So Walter Baade basically got to spend the next couple of years up on a mountain top with the world’s most powerful telescope.  And since most of his colleagues were busy doing War Stuff, Baade had the giant telescope almost entirely to himself.  And as an extra-bonus layer of raspberry frosting, Los Angeles was blacked out at night — because war, right?  So Baade had incredibly perfect conditions for observing.  No astronomer had ever been given such an opportunity, and no astronomer has had one like it since.



(You might be thinking, wait a minute.  We were in the middle of the Second World War.  Walter Baade was an enemy alien.  But instead of locking him up or sending him to a camp or deporting him,  the US government just… sent him to Astronomer Heaven?  And the answer is:  yes, that is exactly what happened.  What can I say?  It was a different time, strange and backwards.)

So what did Baade do?  Well, he looked at a lot of stars.  And he made a couple of big discoveries, including the first pretty good estimate of the actual size of the universe.  But the one that interests us today is this:  Baade discovered that all stars seemed to be divided into two groups.  He named these two groups Population I and Population II.  (Yes, those are terrible names.  Astronomers are terrible at naming things.)

So there are a bunch of differences between these two groups.  Here’s the big one:  Population I stars have (relatively) large amounts of heavier elements in them.  Population II stars have (relatively) much less.  And Baade didn’t just observe this.  He guessed the reason:  Population II stars have fewer heavy elements because they are older.

See, when large stars die, they explode as supernovas.  And in that process, they create a lot of heavy elements.  “Heavy” is relative here.  Astronomers refer to all the chemical elements heavier than hydrogen and helium as “metals”.  Seriously, they do.  If you’re an astronomer?  Carbon is a metal, oxygen is a metal.  There’s hydrogen, there’s helium, and then… metals.  Astronomers are terrible at naming things, remember?  It’s like they can only count one, two, many.


[the periodic table you learned in school]



[the periodic table according to astronomers]

But anyway: all the metals in the universe (other than a tiny wisp of lithium, never mind that) were created by dying stars.  In the earliest days of the Universe, before there were any stars, there were no elements heavier than helium — no metals.

Population I stars, like the Sun, are stars that have a lot of metals.  The sun is about 75% hydrogen, 23% helium, and 2% other stuff — metals.  Two percent may not sound like a lot but it’s actually a huge amount as these things go.  The Sun is a fairly metallic star.  Population I stars generally are between 1% and 2% metals.  Population II stars, on the other hand, are metal-poor.  They usually have a lot less than 1%, often less than 0.1%.  

And the reason for this, as Walter Baade correctly guessed, is that Population II stars are old.  They formed when the universe was much younger.  Metals come from dying stars, right?  Well, Population II stars formed before many stars had died yet.  Population I stars, on the other hand, were born later, after a lot of dying stars had enriched the interstellar medium with heavier elements.  Our Sun is something like a fourth or fifth generation star.  The metals in the Sun?  and most of the stuff that makes up the Earth, your body, and the device that you’re reading this on?  Forged by dying stars long before the Sun began to condense.



Okay, all good.  Except… a piece was missing.

See, Population II stars have few metals.  But they don’t have no metals.  So they can’t be the very first generation of stars, formed from the primordial gas clouds soon after the Big Bang.  Those primordial gas clouds were nothing but hydrogen and a bit of helium.  Stars formed from them would have, not few metals, but no metals at all. 

So there had to have been at least one generation of stars before Population II.  Astronomers tentatively named these hypothetical first generation stars Population III, because of course they did.  (Astronomers are terrible at naming things.)  And they spent the next 70 years looking for them.

Without success.  Nobody has ever seen a Population III star.

Why not?

I will now skip over 70 years of argument and debate to give the current consensus: nobody has seen Population III stars because they were only around very briefly, for a short period of time when the Universe was very young.

Why?  Well… see, there’s a limit to how big a star can be.  If a star tries to form that’s bigger than that limit, it basically blows itself apart.  However, in the conditions of the very early Universe, this limit may have been set much higher.  It may have been possible to form ridiculously large stars, hundreds or even thousands of times more massive than the sun (and millions of times brighter).  In fact, current thinking is those may have been the only stars that could form back then.  Smaller stars, like our Sun, might not have been able to form until the Universe had cooled down a bit.

So (current thinking is) those first-generation, OG stars were huge.  And because they were huge, they burned hot and bright, radiating immense amounts of energy, including lots of ultraviolet light and X-rays.   And then after a very short period of time (for a star) they died in violent explosions.

There was an age of Titans — gigantic, brilliant superstars, blue-hot and far brighter than any star that can exist in the Universe today.  But it was brief, over in the cosmic blink of an eye.  And then the Universe took the exploded remnants of those giants and used them to make metal-poor Population II stars, and then it used /their/ exploded remnants to make metal-rich Population I stars, and here we are.

(I pause here to imagine the way the Universe looked back then.  If you had a time machine and could go back to the age of Population III stars, you’d see space filled with incredibly bright stars — far more of them and far brighter than our sky today.  And mixed in, among and between them, you’d see a bunch of little fuzzy smudges and spirals:  galaxies.  Today the naked eye can only see three galaxies in the sky, the two Clouds of Magellan and a faint little blur that is Andromeda.  But in those days, before the expansion of the Universe had pushed everything apart, you could have seen dozens or hundreds.  This was the youth of creation, fiery and crowded.)

So this is why we have never seen a Population III star.  Yes they were very big and bright, but they only existed very briefly, a very very long time ago, and then they all exploded and died.  Their ashes live on in us, which is both poetic and also literally true.  But that’s not very helpful to frustrated astronomers who want to actually see a Population III star.  

Enter the James Webb Space Telescope.  Looks back in time, right?  95% of the way back to the Big Bang, give or take.  The Webb can literally see a younger Universe, when things were rather different than they are today.  Can it see Population III stars?

Maybe.

Here’s the tricky bit: Population III stars would have been very, very short-lived.  So looking far back in time is necessary but not sufficient.  The Webb would have to observe a galaxy at the exact, brief moment when it had Population III stars.  Look just a tiny bit closer/more recent,  and they’re all gone.  Look just a tiny bit further / back in time, and… well, no stars exist yet, so nothing to see.  The Webb is looking back over 13 billion years into the past, trying to catch a sliver of time that lasted maybe just one million years.  Not easy.

But it gets worse.  At these cosmic distances, even the Webb couldn’t easily see a single Population III star.  It would be much easier to spot a galaxy full of them.  But that far back in the youth of the Universe, individual galaxies were mostly small.  Over time, the ones that didn’t get pulled apart by the expansion of the universe tended to combine like drops of water, merging into the bigger galaxies that we see today.  Our own Milky Way galaxy is a bloated cosmic cannibal:  it appears to have devoured at least a dozen smaller galaxies over the eons. 

So back in the day, most galaxies were small, meaning even harder to see.  But astronomers are clever little creatures.  So some of them came up with a possible workaround:  don’t look for the Population III stars.  Look for the effects of Population III stars.  If you can’t see the animal, look for its tracks; if you can’t see the fire, look for smoke.

And here is where the paper comes in: a bunch of astronomers using the Webb have spotted what they’re pretty sure is such an effect.  Specifically, they’ve observed a small, distant galaxy which they have given the euphonious name of RXJ2129-z8HeII, because — I want to emphasize this — astronomers are terrible at naming things.

The interesting thing about this small, distant, ancient galaxy is that it has clouds of helium.  But not normal helium.  The helium in these clouds has been massively ionized, meaning its electrons stripped away. 

If you remember high school chemistry, you might recall that helium doesn’t like to be ionized.  It’s a very stable noble gas that holds its electrons tightly.  In order to ionize it, you need a massive blast of high-energy radiation: ultraviolet light and X-rays.  The only thing that could do that?  A lot of Population III stars.  Astronomers think they’re seeing a “hybrid” galaxy right caught in the act of moving from Population III to Population II.  Its Population III stars are mostly gone, which is why we don’t see them.  But the helium that they ionized is still hanging around: it hasn’t had time to de-ionize and calm down yet.  The fire may have gone out, but there’s still a cloud of smoke to show it was there.

This isn’t quite the same as actually seeing one, of course.  But we’re getting very close.  We don’t have a good photograph of the beast, but we can see its tracks and droppings.  Another year or two, and the Webb may finally pull this off.

Or not.  Maybe we’ll have years or decades of indirect and suggestive evidence, but that definitive picture will elude us until the Webb goes dark.  We don’t know.  We think we’re close, but the beast may elude us yet.

But still.  We’re rummaging around in the bright youth of the universe, trying to catch the first starlight in a net and bring it home.  That’s a thing.

— And what of the time before the Population III stars?  The Dark Age, when the cooling Universe was just clouds of gas, slowly condensing to form galaxies?  That gets beyond the scope of the paper or this post.  So I’ll close with a bit of science-fictional speculation from nearly a hundred years ago.

“But now there appeared a new trouble. Some of the eldest of the nebulae complained of a strange sickness which greatly hampered their meditations. The outer fringes of their tenuous flesh began to concentrate into little knots. These became in time grains of intense, congested fire. In the void between, there was nothing left but a few stray atoms. At first the complaint was no more serious than some trivial rash on a man’s skin; but later it spread into the deeper tissues of the nebula, and was accompanied by grave mental troubles. In vain the doomed creatures resolved to turn the plague to an advantage by treating it as a heaven-sent test of the spirit. Though for a while they might master the plague simply by heroic contempt of it, its ravages eventually broke down their will. It now seemed clear to them that the cosmos was a place of futility and horror.

“Presently the younger nebulae observed that their seniors, one by one, were falling into a state of sluggishness and confusion which ended invariably in the sleep that men call death. Soon it became evident even to the most buoyant spirit that this disease was no casual accident but a fate inherent in the nebular nature.

“One by one the celestial megatheria were annihilated, giving place to stars.”

— Star Maker, Olaf Stapledon, 1937

[Andromeda galaxy, 2017, Wikimedia commons, copyright David Dayag]