It’s a big universe out there, and even though astronomers have been studying it for hundreds of years, there’s still a lot we don’t know.

We’ve already talked about planets and galaxies that shouldn’t be possible, based on our understanding of the universe. But with over 100 billion stars in just our galaxy alone, there are also plenty of stars out there that are so mind-bogglingly weird, astronomers aren’t sure how they exist.

Here are three of them. There’s a faint star about 4000 light years from Earth, in the constellation Leo, named SDSS J102915+172927. We’ll call it J1029 for short, since that’s kind of a mouthful.

When J1029 was discovered, astronomers found that it had the lowest concentration of heavy elements that we’d seen in any star. Ever. In astronomy, all the elements heavier than hydrogen and helium are considered heavy elements.

And all these heavy elements are called metals, I guess because astronomers like to make things confusing. After the Big Bang happened around 13.8 billion years ago, the first elements that formed were hydrogen and helium and small amounts of lithium. And a couple hundred million years later, the first stars began to form out of those elements.

Since these stars only contained hydrogen and helium and a little bit of lithium, astronomers would classify them as “metal-poor”. Heavier elements didn’t even exist yet — they were made in the cores of stars later on, then spread through the universe by exploding supernovas. We know that J1029 is more than 13 billion years old, and astronomers would expect a star that old to have concentrations of hydrogen, helium, and lithium similar to what existed in the universe right after the Big Bang, plus maybe a few heavier elements.

But J1029 has just a fiftieth the amount of lithium you’d expect, making it so metal-poor that stellar formation models say it shouldn’t even exist. The second star on this list is the most massive star we’ve ever seen. It’s called, R136a1, and it’s 265 times the mass of the Sun — almost twice what astronomers thought was even possible.

They used to think that any star heavier than 150 times the mass of the Sun would radiate so much energy that it would blow itself to smithereens. But somehow, R136a1 managed to get a lot more massive than that. Researchers calculated that at one point, it would have had 320 times the mass of the Sun, and it’s lost some of that mass over time.

Extra-massive stars like this are rare, probably because they don’t live very long. More massive stars have shorter lifetimes because they tend to generate more heat and blow up faster. R136a1 has been around for about a million years, so it’s already halfway through its estimated 2-million-year lifetime.

So we still don’t know how this star is possible, but we do know that we’re lucky we caught it while it was alive. A third unexpected star was born when another star died. SN 1987A gets its name because it’s a supernova that was first detected in 1987.

It’s what’s called a type II supernova, a kind of supernova that’s supposed to end up with either a neutron star or a black hole at its core. Based on the size of the exploding star, 1987A should have formed a neutron star. But it didn’t.

When we look at the supernova, we don’t see anything. And that has astronomers stumped. One possibility is that instead of forming a neutron star, 1987A formed a quark star, a type of star that hypothetically could exist, but we’ve never seen before.

A quark star starts out kind of like a neutron star: the collapsing stellar core condenses to the point where the protons and electrons combine to form neutrons. But then, the neutrons break down into their components, aka quarks. A quark star would be tinier and denser than a neutron star, so if 1987A really does have a quark star at its center, it makes sense that we wouldn’t be able to see it.

But that’s just one possible explanation, and we haven’t been able to confirm it. All we know for sure is that 1987A is really strange. Like all these stars, it’s another reminder that we have a lot left to learn about the universe.

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