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The baby days of the universe were weird times. For hundreds of thousands of years after the Big Bang, it was too hot for even atoms to form, so everything in existence was just a soupy mess of particles.

Eventually, things started to cool down, and particles could clump together to form hydrogen and helium — first as charged ions, and then as nice, neutral atoms. So it seemed like things were going to settle down. But then, the first stars showed up.

And they kind of disrupted everything. They turned all the neutral hydrogen floating around back into ions, created a bunch of new elements, and just generally made a mess. But without them, you wouldn’t be here.

Astronomers think the first stars started to form as early as 30 million years after the Big Bang, when the universe was still made of neutral atoms. Like stars today, they formed when clouds of gas collapsed, cramming atoms together so tightly that they fused. These nuclear fusion reactions give off a ton of light and heat, and are what produce heavier elements and release the energy that powers stars.

In the early universe, the gravity from dark matter is what originally pulled in those neutral hydrogen and helium atoms, gathering them into dense clumps and clouds. We still aren’t positive exactly what dark matter is, because we can’t directly detect it, but we do know that its gravitational pull helped make some parts of the early universe denser than others. Eventually, these clouds were so dense that they collapsed, and the atoms inside then began to combine in nuclear fusion reactions, forming stars.

And suddenly, the universe was a whole lot brighter. Those first stars were a lot different from the stars we see around us today. For one, they were originally made only of hydrogen and helium — because those were the only elements around.

All modern stars also contain heavier elements, like carbon, that they’ve fused from lighter elements or collected from older, exploded stars. These heavier elements usually make stars cooler and smaller. That’s because heavy elements tend to cool down clouds of gas, and it takes less mass for a cold cloud to collapse and form a star.

So if your gas cloud is full of heavier, cooler elements, it will form a star much faster than a cloud just made of hot hydrogen. And because it took less stuff to form, the star will be smaller, too. But with only hydrogen and helium around, the first stars were a lot larger and hotter than anything we see today.

They were probably 30 to 1000 times the mass of the Sun — and up to three times as massive as any star we’ve ever found. On their surfaces, they were about 15 times hotter than the Sun’s surface — at least twice as hot as the hottest star we know of. They were actually so hot that they didn’t give off that much visible light — instead, they gave off powerful ultraviolet radiation.

Like some kind of gigantic blacklight from the most extreme tanning bed ever. And this radiation caused kind of a mess in the baby universe. The UV rays knocked some of the electrons off the neutral hydrogen atoms — and maybe the helium ones, too — turning them into charged ions again.

A bubble of re-ionized atoms formed around each star and, as more stars formed, there were more re-ionized bubbles. Eventually, over the next 900 million years, these bubbles all merged. And once again, the gas spread all throughout the universe was made of ions — except with some big, fiery stars this time.

This period in history is called re-ionization, and it changed the universe for good. Today, the gas that’s floating around between galaxies — known as the intergalactic medium, or IGM — is still mostly made of ionized hydrogen. And it’s all thanks to those first stars.

But the stars did a lot more than knock the electrons off the gas in the IGM. As the years went on, they eventually did create heavier elements in their cores, as hydrogen or helium atoms were fused together. And because they were so hot, they probably created them a lot faster than today’s stars.

Then, when the first stars exploded into the first supernovas, they flung those heavier elements — like carbon and oxygen — into the rest of the universe. Those atoms were incorporated into the new, smaller stars we see today — as well as everything else, like comets and planets and moons and the screen you’re using to watch this video. So basically, without those first stars, you wouldn’t be here.

By learning more about exactly when and how re-ionization happened, scientists can learn even more about how it’s influenced the universe. And as we build newer and even more powerful telescopes, they’re hoping to find more answers. But there’s one thing we know for sure: without the first stars, the universe would be completely different.

It would still just be made of hydrogen and helium, and it would be really dark. Which is a lot less exciting. And kind of sad.

So even though the first stars disrupted everything in the baby universe, we should probably be thankful that they did. Thanks for watching this episode of SciShow Space! If you’d like to keep learning more about the universe with us, you can go to youtube.com/scishowspace and subscribe. [ ♪ Outro ♪ ].