The universe is a pretty cool place. Literally.
If it hadn’t cooled down from the searing temperature after the Big Bang,
atoms wouldn’t have been able to settle down and create stars or galaxies—or us.
Instead, the universe would just be full of hot, swirling gas.
But luckily, it’s not!
And all the complexity in the universe ultimately owes its existence
to one of the simplest materials possible: molecular hydrogen.
Not only did this molecule play a huge role in building the universe as we know it;
today, it also helps us explore it.
As far as chemical elements go,
regular hydrogen is as simple as it gets: just one proton and one electron.
And just a few hundred-thousand years after the Big Bang,
as the universe expanded and things cooled down a little, these atoms started to form,
and the universe filled with hydrogen gas.
Then, about half a billion years in, this hydrogen started reacting with other particles,
and in the process formed molecular hydrogen.
Molecular hydrogen is just two plain hydrogens stuck together,
which is why it’s also called H2, for short.
And while the difference between one atom and two might seem small,
the introduction of this molecule totally transformed the early universe.
See, at the time, the universe was still extremely hot,
thousands of degrees Celsius, so gas particles were moving around extremely fast.
And because of that, they created an outward pressure
that counteracted gravity and kept the gas from collapsing into things like stars.
And it wasn’t about to cool down all by itself.
Because ordinary hydrogen atoms are like billiard balls,
they bounce off each other and transfer energy, but they don’t lose much overall.
That’s what changed once H2 came into the picture.
When an H2 molecule picks up some kinetic energy from a collision,
it doesn’t use all that energy to bounce off at a faster speed like a billiard ball.
The atoms within that molecule can use some of that energy to vibrate,
spin around their axis, or bump an electron into a higher energy state.
Then, at some later point, they can release that energy as a particle of light.
So as H2 molecules collided with atoms,
they started absorbing energy from them and releasing it as light,
which gradually lowered the temperature of the gas.
As that outward pressure dropped, it gave gravity enough of an edge
to begin collapsing the gas clouds, which is the first step of star formation.
Molecular hydrogen can only cool the gas down to a point,
and scientists still aren’t sure exactly how it remained cool as it contracted.
But eventually, the gas condensed to form stars, planets, and galaxies.
So, ultimately, it was all thanks to molecular hydrogen
that the universe began evolving all the stunning features it has today.
But molecular hydrogen wasn’t just done after that.
It’s still around, and it's still helping gas clouds cool down enough
to collapse into new stars all the time.
In fact, because it’s so easy to make, H2 is still the most abundant molecule in the universe,
and it’s especially common in galaxies.
But even with so much of it around, we only detected it for the first time in 1970.
It wasn’t like scientists didn’t know it was out there.
For a few decades already, astronomers had suspected that
there should be a lot of H2 in space, since it’s so easy to make in chemical reactions.
They just didn’t have a good way to look for it.
Like all molecules, H2 has a chemical signature
that you can detect in light that has passed through it.
But that signature only shows up in UV light,
and most UV light is absorbed by the atmosphere, so we can’t observe it.
It wasn’t until 1969 that physicist and engineer
George Carruthers found a workaround for that problem.
He developed a special camera sensitive to UV light, and NASA sent it up
above the atmosphere on a small rocket designed for brief science experiments.
The camera returned with UV snapshots of starlight passing through interstellar gas,
and it had exactly what they were looking for: the telltale signature of H2.
That was the first proof that this molecule existed in space.
And since then, scientists have gotten a much better handle on
where it tends to show up and what role it continues to play in stars and galaxies.
It’s still tricky to observe directly, so astronomers tend to look for carbon monoxide instead,
which is also abundant in the molecular gas in space,
and happens to make a decent proxy for H2.
And now that we have a way of mapping H2 in galaxies around the universe,
it gives us a special look at the way the universe is evolving now.
For example, it still plays a really important role in star formation.
So based on where H2 is concentrated,
astronomers can get an idea of where new stars are being born, and how quickly.
Plus, since it's an important ingredient in those stars,
the amount of H2 influences their mass, what kind of stars they can become,
and what their planetary systems look like.
So in the end, it not only helped create all the structure in the universe,
but it helps us understand how new stars are forming
and how that structure is continuing to change.
Which is a pretty remarkable feat for a simple little molecule.
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