Observations of record-breakingly distant galaxies by the James Webb Telescope (JWST) have been making the headlines lately. Just fuzzy red dots to us, those early galaxies are so bright and massive, that they’re challenging our understanding of galaxy formation and the physics of the early Universe.
JWST’s latest surprise may not sound as revolutionary, but it could be the first direct sign of the Tyrannosaurus Rex of stars. The spectrum of one of those distant galaxies shows hints of a huge amount of carbon – much more than could have been formed through familiar stellar processes.
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The first stars ever to form in the cosmos are thought to have been massive, unfamiliar beasts. Carbon, it's believed, might be a spectral fossil of their bodily remains.
What makes those early stars so exotic? The first generation of stars formed in a more innocent time, before the Universe became littered with dust and dirt and adulterated with heavy elements. It was a time when the cosmos was simple and clean, and, as a result, star formation was much, much more difficult.
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In principle, the recipe for a star has a very short ingredient list. Hydrogen gas. That’s it.
The most simplistic description of a star is a giant ball of hydrogen that, via nuclear fusion sustained by the heat and pressure of its own gravity, converts vast amounts of hydrogen into helium, from the lightest element to the next.
Stars on the more massive side can also burn other elements in their cores, but they all start the same way. The star is officially born when hydrogen fusion begins in its core.
But getting that gas to collect and compress enough to ignite is surprisingly difficult. The problem is, to get enough gas together to ignite and form a star, you have to first, paradoxically, cool it down.
The temperature of a gas is really just a measure of how quickly its component particles (atoms or molecules) are moving around. Cold things move slowly, hot things move fast. Imagine you’re a sports team coach trying to gather together a group of excited small children into a huddle. As long as they’re running around at top speed, they’re never going to be able to collect in a group. You need to get them all to slow down first.
In star formation, we have a similar problem. Nuclear fusion won’t start until you have a lot of gas compressed into very high pressures and temperatures in the core of a dense star, but to get that gas together, first, you need to get it relatively cold.
For the stars we see today, this is accomplished mostly with dust. Clouds of gas and dust collect in the denser parts of the galaxy and start gravitating together. As the molecules bounce around and collide, instead of just ricocheting off like billiard balls, some of them can temporarily store that collision energy internally.
In some cases that means the collision causes a molecule to vibrate, or flex, like a bouncing spring. In others, that might mean an atom or molecule’s electrons get excited to higher energy states.
Either way, two atoms or molecules can end up going slower than they were before they collided, and they can later release the stored-up energy out into the cosmos as radiation. The end result is that the cloud is just a little bit cooler, and the bouncing gas particles are just a little more chill. That process continues until the gas gets dense enough to form a star.
The problem with this story is that the most important molecules for cooling – the ones that can store and re-release energy the best – didn’t yet exist when the first stars formed. At that time, the Universe was all hydrogen and helium and just a tiny smattering of lithium, all of which formed during the first few minutes of the cosmos.
All the heavy elements, carbon and nitrogen and oxygen and the components of that squishy dust can only be formed inside stars or in the supernova explosions at their deaths. So how did the first stars manage to form at all?
It is actually possible for gas to cool just with hydrogen, but it’s much less efficient, and takes a lot longer – pure hydrogen isn’t very good at storing and re-radiating energy. The general expectation among astronomers is that the first stars were much more massive than stars tend to be today because it takes more gravity to force gas together when it can’t cool efficiently.
When those massive, pristine stars reached the ends of their lives, they went supernova, exploded in dramatic fashion and littered their surroundings with heavy elements for the first time. That began the process of filling in the periodic table with heavier elements and made things much easier for subsequent generations of stars, like our Sun. It also allowed for the existence of rocky planets, and, ultimately, us.
As JWST continues to give us glimpses into the most distant galaxies in the cosmos, we’re learning more and more about not just physics in unfamiliar environments, but about our own origins in the cosmos.
We may not yet be seeing the cosmic dinosaurs themselves, but we’re picking apart their bones, studying their footprints, and learning more every day about how their deaths made our lives possible.
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