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In 1981, astronomers announced the discovery of a whopper of a star named R136a.
It was so massive, you’d need over 2000 Suns to match its bulk.
Now, stars can get pretty big, but that sounds downright impossible.
And around a decade later, that’s exactly what it turned out to be.
R136a was really at least 12 different stars in a trenchcoat.
The most massive star in that trenchcoat,
and the most massive star we’ve ever found, is called R136a1.
But over the years, its estimated mass has kept shrinking and shrinking, too.
And pinning down that estimation is super important.
If its mass is too small, it could mean some pretty big consequences
for why we think the universe looks like it does.
If you want to study the stars, you’re going to want to use a telescope.
And a telescope’s resolution refers to its ability to differentiate between
multiple sources of light.
It sounds simple, but visually separating
one giant ball of plasma from a bunch of others right next door
gets a lot harder when all of them are 163,000 light years away.
Over the centuries, astronomers have gotten a lot better at achieving high resolution.
One solution is just to build bigger telescopes.
The larger the light-collecting area, the clearer the image comes out.
But by the 1980s, astronomers were still witnessing impossibly massive stars like R136a.
Because it’s not just telescope size that matters. Technique does, too.
R136a was finally found to be more than one star
through the use of a technique called speckle imaging.
It basically films the star instead of taking a single snapshot.
By compiling the individual frames from that movie,
astronomers can remove the blurring done by the Earth’s atmosphere,
and use those huge, ground-based telescopes to the absolute best of their abilities.
The models used to analyze data have also grown by leaps and bounds.
Researchers use models to make what are essentially incredibly well-educated guesses
about stars that are too far away to see with the same detail as our Sun.
And these models are constantly being fine-tuned
as we learn more and more about how stars are born, grow, and change.
Computers being exponentially more capable
of running incredibly detailed calculations doesn’t hurt either.
So, because of all this, R136a1 has “shrunk” over several studies.
What started as thousands of solar masses became between 170 and 230 solar masses today.
It’s still the most massive star ever discovered,
but if you go by a traditional understanding of stellar physics,
you might be tempted to say that’s still too big.
Because stars are subject to the Eddington limit.
It’s the maximum mass a star can reach where the radiation
pressure pushing out still balances the gravitational pressure pulling in.
All of the nuclear reactions going on inside a star produce a lot of energy,
and the energy creates pressure that pushes out and away from the core.
Below the Eddington limit, a star’s gravity can
counteract this pressure and keep all the star’s stuff held together.
But above that limit, the pressure wins the battle and burps off the
top layers into the void of space until everything balances out again.
For a long time, observations suggested that the
modern universe had a sort of cap for each star: around 150 solar masses.
If that were true, it would suggest that our latest measurements of R136a1 are still wrong,
or that it's breaking the laws of physics.
But now we know it’s a bit more complex.
Each star has its own Eddington limit that depends on a lot of different factors,
including its unique chemical composition.
Stars like our Sun are made with a sprinkling of
elements heavier than helium, which astronomers collectively call metals.
And those stars generally have lower Eddington limits.
But stars that are almost pure hydrogen and helium can really collect some heft.
These are the very first generation of stars.
But they’re known as Population III stars,
because sometimes astronomers like to name things in the most confusing way possible.
Theoretically, these stars made the metals that keep a lot of modern stars small.
And their Eddington limit could have been anywhere between 300 and 1000 solar masses.
Now, R136a1 isn’t a Population III star.
Astronomers haven’t actually managed to find one of those, yet.
But it did form in an area with very low levels of metals,
which gives it a roughly similar composition,
and makes its seemingly impossible mass totally plausible.
The problem is not that the star is too big.
In fact, it’s the exact opposite.
R136a1, while still the title-holder of Most Massive Star, is now small
enough to threaten our understanding of how our universe got so metallic.
See, astronomers think that stars over 300 times the mass of the Sun
can explode in a special kind of supernova called a pair instability supernova.
Many metals are made by supernovas,
but pair instability supernovas are on a whole different level.
A single one of these explosions could seed more metals out into the universe
than all of the regular supernovas combined.
And theoretically, many Population III stars would have been massive enough to explode this way.
It’s how astronomers think most of the atoms
in our universe that aren’t hydrogen or helium came to be.
But we’ve never found definitive proof that one ever happened.
Back when R136a1 was over 300 solar masses,
astronomers were really hopeful that they’d found their ticket.
If they can identify just one modern star in that mass range,
that would pretty much confirm some of the very first stars could have gotten that massive, too.
But now, things aren’t looking so bright.
Because R136a1 has a tiny amount of metals,
and its updated mass is well below that 300 solar mass limit,
Population III stars may not have gotten massive enough to trigger pair instability supernovas.
And no pair instability supernovas would mean that we have a whole lot
of metal atoms in the universe, but no idea where they came from!
Astronomers would need to rewrite the textbooks!
We still have a lot to learn about supermassive stars: how they form, how they work,
and what they have to teach us about the universe.
Losing R136a1 as a promising lead might not prove ideas the way we had hoped,
but it’s still a piece of the puzzle.
And even when they aren’t the coveted corner pieces,
astronomers are always happy to have as many of them as possible.
Thanks for watching this episode of SciShow Space.
And speaking of super special supernovas,
we’ve got an episode breaking down the five
biggest, baddest types that can happen throughout the universe.
Check it out!
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