You’d think we’d have found basically every kind of star by now.

But the universe is a really big place, and there are still entirely new kinds of stars that astronomers think should exist, based on physics, but they’ve never found. And some of them are pretty extreme.

Like quasi stars, otherwise known as black hole stars. If they sound intense, that’s because they are: a quasi star would be fueled by a giant black hole at its core. Astronomers think a quasi star could form when the middle of a huge protostar — that is, a star that’s still gathering material — gets too dense and collapses into a black hole, which then gives off a huge wave of energy.

When that kind of collapse happens today, it produces so much energy that it blows away the rest of the star in a supernova. But early stars were almost entirely made up of hydrogen and helium, which meant they could get a lot bigger than today’s stars. If a star was large enough, it’s possible that the outer layers could’ve absorbed the blast, leading to what was basically a giant star with a black hole in the middle.

Black holes tend to suck in everything around them, which means a quasi star wouldn’t have lasted long. But black holes also produce a lot of radiation, which leads to what’s known as radiation pressure — an outward force that counteracts some of its gravitational pull. The radiation pressure would’ve slowed the star’s death a little bit, but after about 7 million years, a quasi star would’ve been swallowed up for good.

Since modern stars don’t get big enough to become quasi stars, it’s pretty unlikely that we’ll ever find one. We’d have to be able to look inside ancient galaxies billions of light-years away. But if quasi stars did exist, that’s one way the universe might have gotten some of its black holes.

Then there are electroweak stars, which are thought to form after a supernova explosion. After a star goes supernova, it starts to collapse in on itself. The particles inside the star get pressed together so tightly that protons and electrons start combining into neutrons, forming what’s called a neutron star.

And in some stars, the compression doesn’t stop there. The star becomes so dense that the neutrons break down into quarks, the even-smaller building blocks of matter. That’s a quark star, a type of star that’s also been elusive — some astronomers think that a supernova detected in 1987 might have turned into a quark star, but we’ve never found one for sure.

But electroweak stars don’t stop there. In an electroweak star, the temperature is really high — over 8 billion degrees Celsius (14.4 billion FËš), which was the temperature around one ten-billionth of a second after the Big Bang. At such high temperatures, the star keeps collapsing, and the pressure forces quarks to change into leptons, a totally different class of particles that includes electrons.

Electroweak stars would be so hot and dense that they should collapse really quickly. But instead, astronomers think the stars would be held up by radiation pressure, just like in quasi stars. In this case, the pressure would come from the energy produced by quarks turning into leptons, in what’s called electroweak burning.

An electroweak star could survive this way for more than 10 million years. And that whole reaction happens in a core about the size of an apple -- an apple that’s twice as dense as the entire Earth! And electroweak star would be only a few kilometers in diameter, but the whole thing would be supported by electroweak burning from that tiny core.

Based on what we know about them so far, an electroweak star would look pretty much the same as a regular neutron star from here on Earth. But astronomers hope that we’ll eventually be able to predict what the fingerprint of an electroweak star’s light should look like. So maybe we’ll spot these freakishly dense stars someday.

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