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R136a1 is the most massive star that astronomers have ever discovered. It's so massive you might think the laws of physics wouldn't allow it. But it turns out that its current mass estimate is actually so low that it threatens our understanding of how the universe got to be where it is, today!

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[♪ INTRO] 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 trench coat. The most massive star in that trench coat,   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! [♪ OUTRO]