It’s always amazing when astronomers find a new feature in a familiar astronomical object. Sometimes, though, it actually shocks me.

Like, seeing a bar-shaped cloud of ionized iron going right across the middle of the Ring Nebula, and there’s no real explanation of what it’s doing there [link to journal paper].

This is so cool!

The Ring Nebula is one of the most iconic gas clouds in the sky. Located about 2,600 light-years away in the constellation of Lyra, it’s an absolute observational staple for amateur astronomers. It’s no exaggeration to say it’s literally the first thing I look at when I haul my own ‘scope out in the summer. It looks like a ghostly gray smoke ring in the sky by eye, and photos reveal a series of colors in it due to different gases like hydrogen and oxygen that glow at different wavelengths.

It’s what’s called a planetary nebula, the material cast off by a somewhat Sun-like star when it dies. When the star runs out of hydrogen fuel in its core, a complicated series of events occurs that result in the star expanding its outer layers enormously, which also cools them. The star becomes a red giant. Those outer layers are now tenuously held on by gravity, and the fierce energy coming up from the core (which has contracted and heated up a lot) can blow that material away into space. Once it’s gone, the exposed core, now called a white dwarf, zaps it with ultraviolet light and causes the gas to glow.

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In the simplest case the gas will expand away in a sphere and you get a circular nebula that looks like a soap bubble. The Ring Nebula is, duh, ring-shaped, so astronomers assumed for decades that this was the case for it.

But as we’ve learned in recent decades, most stars have planets orbiting them, and roughly half have stars as stellar companions as well. If the dying red giant gets large enough to consume these other objects, they can orbit inside the giant, running through the material like the blades of a baker’s mixer through batter. The red giant spins up, and that changes things.

The outer layers of the star feel a centrifugal force, and blow out more along the equator, creating a denser disk of material. Matter blown up and down, away from the plane of the disk, travels more freely, and you wind up with an hourglass-shaped nebula. Some of these get extremely elongated, and some are more thick around the middle.

The Ring is one of those; it’s more or less barrel shaped and we happen to be looking down the long axis, from above one of its poles, so it appears as a ring.

So. A team of astronomers tested a new instrument on the William Herschel Telescope (or WHT), part of what’s called a science verification, to make sure things are all working properly. This is common with new cameras, and they’ll usually observe well-studied objects as a way to compare their results with ones already confirmed. For this new instrument they decided to look at the Ring.

The instrument is interesting. It’s called WEAVE, for the WHT Enhanced Area Velocity Explorer. It uses an array of fiber optics to look at an area of the sky, picking out over 500 individual spots (like looking at the sky through a bunch of very narrow soda straws). The light from each fiber is then sent to a spectrograph, which breaks the light up into individual wavelengths (think of them as colors) — a spectrum. What they wind up with is a three-dimension “data cube”: like a 2D image, but at each point in the image they also have information about the colors there as well. Once the data are taken, you can then make images of the object at different wavelengths. Since different elements emit light at different wavelengths, you can easily pick out different elements you want to see and make a map of them.

And this is where things took a hard left. Looking at the light from, say, oxygen and hydrogen, the nebula looked like it always does. But then they noticed that at one particular wavelength, emitted by ionized iron, there was a feature never before seen in the nebula. A ragged bar of gas stuck right across the middle of the nebula, going from one inside edge to the other!

What the what?

Not only has this feature never been detected before — mind you, in one of the best and most-observed objects in the sky — nothing like it has been seen in any planetary nebula. We sometimes see inner rings, for example, where the gas thrown out in the plane of the star’s equator piles up. But to see that inner ring as a line you have to be looking down on the nebula from above its equator, and like I said earlier we’re looking down from above the pole of the Ring Nebula. So that doesn’t make sense.

Sometimes the inner star will blow out narrow beams of material that we call jets, and they can kinda sorta look like this bar. However, the material in the bar isn’t moving anywhere near the speed of typical jets, and why would they only be seen in iron wavelengths and not the more common ones in hydrogen and oxygen?

To be honest, the astronomers who discovered this feature don’t know what it is either. Weirdly, the bar doesn’t go through the central star, like you’d expect if the star were somehow blowing this material out. It’s offset from the star just a bit, so again, that’s pretty weird (the star itself is offset from the center of the ring, though it’s centered in a much larger ring of colder material outside the main ring). It’s not mentioned in the journal paper, but the press release mentions that it could be from a planet that’s getting zapped by the star, or was partially vaporized by the star as it expanded into a red giant. That’s an interesting idea, but I’m not sure how that would manifest as a bar-like object across the middle. A spiral or ring pattern would make more sense.

So it’s a mystery! Astronomers love this sort of thing. Another fun puzzle to figure out! What’s needed now are much deeper images of this structure, either with WEAVE or from a different telescope outfitted with the right kind of filter or spectrograph.

I’d love to see these observation followed up. Finding out the Ring has this bar is pretty shocking, like discovering your best friend of several decades has a secret life you knew nothing about, nor understand.

That would certainly be worth a second look, don’t you think?

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