It's a mystery. I don't like mysteries! They give me a bellyache, and I've got a beauty right now...
---Captain James T. Kirk
Planetary nebula are beautiful clouds of gas that form when stars like the Sun die. The star blows off a series of super-solar winds, ejecting gigatons of gas into space. As this happens, the star itself exposes its hot, dense core, which emits scads of ultraviolet light. This high-energy light ionizes the gas, creating what is essentially a cosmic neon sign a few trillion kilometers across.
These nebulae take on weird shapes depending on lots of factors. Some have binary stars in them, for example. As the stars orbit each other, they tend to emit gas preferentially in a flattened sphere, like a basketball someone has sat on. The nebula takes on an ellipsoidal shape, and sometimes even an hourglass shape. If it's the latter, you can see a bright ring surrounding the central stars, which is the waist of the hourglass.
Behold the planetary nebula SuWt2, located 6500 light years away. That's it in the image above, taken in 1995 from a 1.5 meter telescope in Chile. You can see the ring which is the hourglass waist, and just make out a little bit of the lobes of the hourglass; at least near the central ring (deep exposures show more).
But there's a problem. The gas is lit up, so one presumes the bright star in the center is what's doing the illuminating. But if you study that star, you run into a major problem.
For one thing, the star isn't a star. It's two stars. That's OK, and even good: that would explain the shape of the nebula. But the major monkey in the wrench is that neither star is bright enough or hot enough to do the job. They're A stars, meaning they're hotter and brighter than the Sun, but not enough by a long stretch to power the nebula. They simply don't have what it takes to make the gas so bright.
Usually, the star in center of a nebula like that is a white dwarf, a compact, dense, hot object leftover when a Sunlike star sheds its mortal coil. So seeing two normal stars there is weird. It's like hearing a dog barking behind a door, and opening it to see a squid*.
So where's the white dwarf? I'll cut to the chase: we don't know.
1) It faded away. White dwarfs don't generate energy any more, so over time they fade. However, that usually takes a long time, like millions or billions of years. The nebula can't possibly be that old; such objects are a few thousand years old. So that seems unlikely to me.
2) It might be hidden. Before the star died, it would have shed lots of dust, which can enshroud it. That seems unlikely as well; it would take a lot of dust to do that.
3) Or maybe, along those lines, the binary is hiding it. Maybe the binary is not associated with the nebula; the pair of stars just happens to be sitting in the way. If that's the case, their light might be overwhelming the white dwarf.
Except... a telescope that sees UV light saw no sign at all of any white dwarf, or any star that might be zapping the gas. The binary wouldn't be able to hide that.
This is really weird.
As it happens, the hourglass shape is something I'm familiar with: Supernova 1987A had one around it. The star that blew up had previously created the giant structure, then lit it up when it exploded. Interestingly, the explosion seems to have left nothing behind; even after more than 20 years there's no sign of the usual neutron star or black hole.
It's difficult to see how that ties in here, though. Had the star that formed the SuWt 2 nebula exploded, it would have been spectacularly bright at its distance of only 6500 light years. And if it happened too long ago (long enough that no one happened to notice it, like thousands of years ago), the nebula would have been torn apart by the debris by now.
Since I have some experience with planetary nebulae, I talked to the two astronomers who released this news, Howard Bond and Katrina Exter. They're baffled by this object, and we had a jolly good time tossing around ideas. I have to wonder... the binary stars do appear to be in the center of the ring, or close enough. And they're weird too: they rotate too slowly, as it happens, and they appear to have almost exactly the same mass, and they both appear to be at the same stage in the life, and that happens to be where hydrogen fusion in their core has stopped, and they're contracting and heating up.
So it makes me think that if you have a weird object, and the stars in the center happen to be weird, then maybe that's correlated. But none of us could figure out how. So in the end -- or at least, wherever we are now with this bizzare thing -- we just don't know what's generating the energy needed to light it up. Unlike the movies, not all scientific puzzles get wrapped up neatly at the end, with the scientists laughing and clinking their glasses together as the credits roll.
But still, unlike our dear Captain Kirk, we do love a mystery. So we'll keep at this one, and I just bet one day I'll be posting about SuWt 2 again, and I'll have an answer for you.