BAN #364: Blue supergiant shell game

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

I have a fondness for shells around stars.

Stars are big balls of gas. Near the ends of their lives their cores heat up, and that extra energy percolates upward into the outer layers. When you heat a gas up it expands, so the outer part of the star swells up, and you get a red giant. Well, for stars like the Sun. If the star is massive, a dozen or more times the Sun’s mass, the energies involved are much larger and you get a red supergiant.

The gravity at the surface of the star drops a lot because it’s so big, but at the same time the star gets a lot more luminous. This means an atom of gas feels a big force outwards from the light but only a small force downward from gravity, so it gets blown off the star. Overall, the star starts blowing a pretty decent wind of particles, like the solar wind but much stronger.

Over time this wind forms a spherical shell around the star, sometimes light years across. We call this a giant outer halo. They’re big and faint and hard to spot. I found one by accident surrounding the nebula NGC 6826 while looking at it for my Master’s Degree, which is one reason I like them. I also went on to do my PhD research on winds blown from a star before it blew up, cementing my relationship with these things.

The winds don’t last long, a few tens of thousands of years, before the star either blows off all its outer layers or the stars, um, blows up. A massive star has a lifespan of millions of years, so the wind stage is really short comparatively, which means finding them is rare. That’s why a team of astronomers went looking for some by searching through archived images.

They looked at data from the infrared mission WISE and found an interesting structure around the star ALS 19653, a blue supergiant about 5,000 light years from Earth. The infrared nebula isn’t spherical but instead has a dumbbell shape — probably due to the star orbiting a second, unseen star. As they swing around the gas expelled forms a disk around the stars, and the wind for the star when it swells up travels more easily up and down relative to the disk, forming an hourglass shape. The nebula is over a light year across.

They looked in visible light images and found not one but two giant outer haloes, the outer one moving faster than the inner one — which is why the outer one is bigger. That means the star had two episodes of mass loss. Given the speed and size of the outer halo, they estimate this mass loss began a mere 10,000 years or so ago. In fact, with all these data in hand, they think the star used to be a binary, but the two stars merged into one. I’m guessing it’s when the massive star swelled up; it would’ve engulfed the other star, and friction/drag would’ve dropped it down deeper into the massive star. I’m a little hazy on the timescales of all this and what happened when, but then it’s not clear how all this works.

But it does happen, and it happens a lot. Massive stars are commonly binary, and many with haloes have a dumbbell-shaped nebula closer in. Supernova 1987A, my PhD thesis subject, has a weird hourglass structure and a dense gas ring around it that we still don’t understand. When we looked at it with Hubble it was the only one of its kind known. Some years later Sher 25 was found in NGC 3603, and now several more like it have been seen.

The thing is, the star that blew up to make Supernova 1987A did so maybe 40,000 years after making its ring, and that strongly implies these other stars are ticking time bombs. They’ll explode too, so it’s important to study them now and understand why they lose mass, how they do it, why some are spherical and others not, and get a grip on how this all works. Creating giant outer haloes are The Beginning Of The End for these stars, and given they detonate into some of the most violent physical events in the Universe, understanding them is a pretty good idea.

Et alia

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