At first glance, the universe can seem pretty straightforward, like when it comes to dying stars.

According to the standard story, which matches most of what we see around us,

small stars die relatively quietly, while big ones explode as supernovas.

But while that sounds neat and tidy and all, the reality is, big stars are complicated.

In the last two weeks, a pair of papers in the journal Nature

has helped us address two outstanding mysteries about these stellar giants:

how they really die, and how big they can actually get.

Both papers remind us that, for as nice as it would be to have one clean set of rules

to explain space, things aren't always that easy. But they are way more interesting.

The first paper was on hypernovas.

Kind of like some regular supernovas, hypernovas can form when a star's core collapses really quickly.

But while supernovas form from stars a few times more massive than the Sun,

hypernovas form from stars tens of times as massive.

That makes them much brighter,

and they've produced some of the biggest explosions since the Big Bang.

But there's also a lot we don't understand about them.

For example, according to models, the gas near the star's poles

should produce huge jets of radiation known as gamma-ray bursts, or GRBs.

Sometimes, we do see this happen.

But we've also seen hypernovas without these bursts, and it's not super clear what's going on.

Previous work has suggested this could be thanks to what's called a cocoon:

a cloud of gas near a star's pole that's heated by a jet of GRB radiation.

Scientists hypothesize that some jets can punch through these cocoons,

but others get absorbed and never make it out, which explains the missing gamma-rays.

The problem is, cocoons are notoriously hard to study, either because they're outshined

by the GRB itself, or because we don't notice them in time.

So it's hard to figure out how true this hypothesis is, or to investigate it further.

That is where this new paper comes in.

By studying a special hypernova observed in 2017,

authors were able to look at this process in more detail than ever before.

Their hypernova was special because it involved a pretty dim GRB.

It was bright enough to detect, but it wasn't bright enough to outshine the cocoon,

and that allowed the team to study the explosion within a day of it happening.

That turned out to be really helpful.

For one thing, the results back up what we used to think about cocoons.

In a model of their hypernova, the team saw the jet lost some of its energy to the gas, but not all of it.

If it had been stronger, it probably would have punched all the way through.

And if it had been weaker, it would likely have been absorbed,

just like previous studies said would happen.

Also, by analyzing the light from around the explosion, the team was able to tell us more

about what these cocoons are actually like.

They discovered that the gas was moving, like, almost ridiculously fast: Some materials were

going up to a third the speed of light, which is faster than anything we've seen in similar explosions.

The gas also contained different elements during the first day of explosion than later on,

things like iron, cobalt, and nickel.

One researcher suggested these elements were probably produced in the star's core as it collapsed.

That means that, not only can studying cocoons tell us how these explosions work,

but they can also tell us what it's like inside of them.

And the best part?

The researchers collected a ton of data, so there might be even more to learn.

But either way, this is a big step in the process of understanding how big stars die,

and it has some researchers pretty excited.

Of course, hypernovas, and the stars that cause them, can only get so big.

That's because if too much gas falls together as a star is forming,

the gas will heat up and push more gas away.

That generally stops stars from getting much heavier than a couple hundred times the Sun's mass.

But that also poses a problem.

See, early galaxies had supermassive black holes at the center, just like today's galaxies do,

that could have been millions or billions of times the Sun's mass.

But black holes generally form from dying stars,

and then they grow when gas or stars fall into them.

So if early supermassive black holes were formed from stars only a couple hundred times

the Sun's mass, there wouldn't have been enough time for them to get so big.

Fortunately, a paper published this Wednesday is challenging some of those assumptions,

by saying that the early universe totally could have produced stars that weren't just

a measly hundred or so solar masses.

They could have been ten thousand solar masses.

To figure this out, the paper's authors simulated giant gas clouds starting about

200 million years after the Big Bang.

As the clouds crashed into each other, they condensed into lots of different stars.

But the biggest stars didn't form in the center of the action with the others.

They were out in, like, the countryside, or, at least the suburbs.

There, they could keep growing as gas clouds merged together,

but their heat didn't get added to the heat from the other stars.

The more heat there is, the more pressure there is to push the extra gas away,

so by staying out of that crowded environment, these stars could get huge.

Admittedly, these enormous stars were pretty rare, only two formed out of hundreds of cases,

but that's still common enough to make supermassive black holes much less mysterious.

And thanks to research from the last few years, the team can even explain why we haven't

seen enormous hypernovas from these giants.

Some extremely massive stars can skip the supernova entirely and collapse straight into

black holes at the end of their lives.

Other research has even suggested that big collections of gas could fall together

in just the right way to skip the star stage, too.

They'd go straight from gas cloud to black hole.

The new simulations didn't get into whether either of these scenarios happened to the first monster stars.

But they do give us a pathway going from hydrogen to enormous black holes,

which is exactly what we need if we're going to understand the earliest galaxies.

Thanks for watching this episode of SciShow Space, which is officially

our 500th episode on this channel!

We started SciShow Space back in 2014, and we have loved getting to explore all of the

mystery, discovery, and straight-up ridiculousness of the universe with you over the last five years.

If you want to help us make our next 500 videos, you can check out patreon.com/scishow.

And if you want to keep learning about space with us, and your place in it,

just go to youtube.com/scishowspace to subscribe.

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