It is rare for a new kind of star to be seen in the sky. Especially when dead stars eat other dead stars and leave behind dead stars that look like they are still alive.
Let's go back up a second.
A star like the Sun can generate energy through fusion of hydrogen and helium in its core. The inert helium gets very hot as it builds up. The outer layers of the star expand to compensate for the extra heat that comes from the core becoming totally made of helium. The star is red.
The gas at the surface of this bloated star feels less gravity holding it on, but a huge amount of energy pushing it upward from below. The core of the star is exposed to space when the outer part of the star is ejected because of this. We call it a white dwarf. The white dwarf is almost entirely made of helium.
The star can become a red giant and a white dwarf at the same time. If there is enough pressure on the core that it can start to blow up. This makes carbon and oxygen, which sink to the center pretty quickly, leaving the helium floating on the surface with just a trace of carbon and oxygen that can be detected. Since they are mostly carbon and oxygen, we call them CO white dwarfs. They can create lovely planetary nebulae by driving fierce winds of gas away.
Things start to get weird here. There is a star where the core is not a white dwarf yet, but is on its way. The outer layers are mostly blown off and just a thin layer of helium is left, which may still be fused into carbon and oxygen. These stars have some characteristics that make them look like normal stars, but they are smaller and fainter, so they are called subdwarfs. They are called sdOs for subdwarf O stars because they look like normal stars.
Things recently got even weirder here. Two unrelated sdOs were found, one of which had surfaces mostly composed of helium. One of the two is about 9000 light years away and the other is 23,000 light years away.
Most hot subdwarfs have a small amount of carbon and oxygen on their surfaces, and even that is rare. But these two? They have more than a trace. Both have surfaces of about 60 percent helium, but they also have 15 and 25% carbon, and 23 and 17% oxygen.
That is a lot. A lot. Where could all that come from?
They were both white dwarfs and then ate their companions. Which were dwarfs from CO.
I know, what? An example of just one pair is needed to avoid more confusion. Two normal Sun-like stars were circling each other long ago. One died, grew into a red giant, shed its layers, and became a CO white dwarf. The other star became a white dwarf after doing the same thing. There are two white dwarfs that are different types.
They spiraled together, possibly by emitting waves, though the exact mechanism is not important here. The CO dwarf was low mass, and that means it was bigger. White dwarfs are unimaginably compressed by gravity and have weird rules. Matter in these conditions shrinks as you add mass, instead of getting bigger. White dwarfs are larger than high-mass ones.
Why is that important? The surface gravity of the smaller, more massive one is stronger, and as the two get closer the larger one can take material off the surface of the other. The CO dwarf is eaten by the helium dwarf. The object has a white dwarf core covered in helium but also has a lot more C and O than you would expect, and it looks like a subdwarf.
This is a very unlikely scenario because you need a lot of conditions to achieve such a weird object. They found two, but it was pretty nifty to find one. There are more that are waiting to be found.
Why is this important? Stars die in a variety of ways, and understanding these pathways tells us about how they live, how they die, and what is left after. Sometimes white dwarfs in systems explode, creating powerful supernovae that are bright enough to be seen for billions of light years. Sometimes they lack the mass to do this, and we end up with weird sdOs that will eventually become white dwarfs. The majority of stars will eventually become white dwarfs, so understanding these compact objects is part of understanding stars.
The stars are the building blocks of the universe. That is a good reason to figure it out. Plus? It is just cool.
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