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SYFY WIRE Bad Astronomy

So what the heck is StDr 56?

By Phil Plait
StDr 56, a possible planetary nebula in the constellation of Triangulum. It’s about the same size as the full Moon on the sky. Credit: Robert Pölz, Marcel Drechsler, Xavier Strottner

Over the years, I've seen a lot of things in the night sky, oh my, yes I have. Galaxies, planets, moons, satellites, balloons, lanterns, rockets, and so much more. Some have baffled me for a moment, but then a deeper look usually solved the case.

It is very rare for me to see something and actually not be sure at all what it is. It is even more rare for such an object to make me literally gasp out loud when I first see it.

But StDr 56 is precisely such an object. It is profoundly beautiful, every bit as much as it is profoundly bizarre.

StDr 56, a possible planetary nebula in the constellation of Triangulum. It’s about the same size as the full Moon on the sky. Credit: Robert Pölz, Marcel Drechsler, Xavier Strottner

See? I told you. Absolutely breathtaking.

But... what is it?

The quick version is, I don't know. The slightly more lengthy version is, it's a nebula, and probably a planetary nebula, but I have never seen one like this, and there are some baffling aspects of it I cannot explain.

StDr 56 was discovered by amateur astronomers Marcel Drechsler and Xavier Strottner, who comb through surveys of the sky looking for planetary nebulae (or PNe) — winds of gas that flow from stars like the Sun when they die, blown when the star turns into a red giant. Eventually the outer layers blow away entirely, revealing the core of the star: a hot dense white dwarf. Ultraviolet light from the white dwarf excites the gas, causing it to glow.

Hundreds of such objects can be found in catalogs. In general they're a few light years across at most. After that, the expanding gas gets too thin for it to efficiently catch the light of the central white dwarf, and the nebula dims. Eventually, generally after a few thousand years, the gas mingles and merges with the gas in interstellar space.

They can take on all manners of shapes; some mundane, like Abell 33, which is a near-perfect soap bubble in space, and some fantastic, like M 2-9, which looks like a pair of squid kissing.

But this? Strottner and Dreschler have found quite a few previously unknown PNe and recorded them in their catalog (called StDr after their names, with the one in question here being number 56), and many of them are odd, but nothing like this.

Drechsler and Strottner named it the Goblet of Fire Nebula. Fair enough.

First of all those long thin filaments are very unusual for a PN. In general, such striping can occur when the gas flows along magnetic field lines. A white dwarf can indeed have a strong magnetic field, but I don't think it could shape the gas structure over the size of a nebula like this. Sometimes gas moves along the Milky Way galaxy's magnetic field lines, so that's a contender. But it's not clear.

In the image, red gas is hydrogen and blue oxygen. Both glow strongly when hit by UV light, so those are pretty common to see in PNe. Also, the oxygen seems to be smaller and inside the structure of the hydrogen. That too is somewhat common in this kind of nebula.

Close-up of StDr 56 showing two stars (highlighted), either of which might be the nebula’s central star.

Drechsler and Strottner identified two possible white dwarfs, either of which could be the nebula's central star. The brighter of the two is called Gaia DR2 300394067131824768, and is about 1,130 light years from Earth. The other is Gaia DR2 300394964780348288 and is 3,800 light years away.

That info in turn tells us how big the nebula might be. Its apparent size on the sky is about a half a degree, the same size as the full Moon. If the first white dwarf is the central star, then at 1,130 light years away the nebula is about 10 light years across. If it's the other star, the nebula is about 33 light years across.

The latter size is huge, so much so that I think it rules out the nebula being that far away. But even 10 light years is extremely large for such an object... but maybe not too large. If the dying star was a massive one (say, 5 times the Sun's mass) then it would blow a large wind into space.

As it happens, StDr 56 is in the constellation of Triangulum, which is well off the plane of the Milky Way. The flat disk of the galaxy contains a lot of gas and dust, and a nebula trying to expand into that material would slow rapidly, limiting its size. StDr 56 being so far off (30°) means it can expand more freely. So that fits, but the large size has me scratching my head. That's into supernova remnant territory, where it takes an exploding star to get gas out that far. It's weird.

I'll note this object is faint. Astrophotographer Robert Pölz took it in Austria using a 25-centimeter telescope and it's a total of 60 hours of exposure time. I mention this because one way to determine what this object is would be to take spectra of, very carefully measuring the wavelengths of light it emits. It's possible to measure the expansion speed doing that, which could immediately tell us if it were a supernova remnant (which expand rapidly, hundreds or thousands of kilometers per second) or a planetary nebula (which expand in the dozens of km/sec range). The problem is, getting a spectrum takes much longer exposure times than an image, and would take a big telescope.

So. I know professional astronomers read this blog, including many who study objects like this. I am putting this out there if anyone wants to follow up. I would love to see even deeper images with big ‘scopes, and love spectra even more.

Just what is StDr 56? Besides jaw-droppingly gorgeous, I mean. Maybe we can find out.

My thanks to Marcel Drechsler for helping me with info on this amazing object.