Inside a 300-light-year-long cosmic sea serpent, stars are helping make new stars

July 31, 2023   Issue #597

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Astronomy News

It’s a big Universe. Here’s a thing about it.

Astronomers have seen unambiguous evidence of triggered star formation in Nessie!

OK, yeah, reading that again I see I have to back up a bit.

There are a couple of different places where stars can be born in our galaxy. One is huge, cold, dense clouds of gas called molecular clouds — I just wrote about these in BAN 576 in fact.

The other is a bit surprising: long, sinuous filaments of cold gas that look like dark serpents slithering across the stars. They’re sometimes called the “bones of the Milky Way” [link to paper].

The first one seen in our own Milky Way galaxy was discovered in infrared images taken by the now defunct Spitzer Space Telescope, and is a monster: over 300 light-years long and only a few wide, it looks like a sea serpent, and was quickly nicknamed Nessie. I wrote about it when it was discovered back on The Old Blog™.

Nessie is cold — about 10K (-263°C) or just barely above absolute zero— and dense, averaging about 100,000 atoms per cubic centimeter. Most gas clouds are more like a few atoms per cc, so to an astronomer it’s pea soup. Also, space is big, so this stuff adds up; there’s enough material to make about 200 stars like the Sun along every light-year of its length!

And we know it’s making stars; on the right side of that image you can see a teardrop-shaped bubble. That’s a star-forming region, where stars are actively being born. Massive stars, several times more hefty than the Sun, are extremely bright and emit powerful winds of subatomic particles, and these push against the gas, heating it and inflating the bubble. You can see how the gas is thicker at the edge of the bubble where it’s piled up due to this snowplowing action.

But here’s a question: When these massive stars form, do they help or hinder further star formation? If they can plow up the gas then that force could cause material to collapse and form stars. But if they’re too vigorous they can disperse the gas, suppressing star birth. Which one dominates?

New observations just announced pretty conclusively show that in at least this case it’s the former: The massive stars are triggering new stellar birth.

The lead astronomer on these observations, James Jackson, gave a press conference about it which you can watch, though it gets a little technical in places:

A close-up of the teardrop shows where it’s happening. Right at the edge of the teardrop are a bunch of massive stars being born, creating a little bump in the side, a bubble on the bubble, or a sub-bubble (I just made this up and should probably pronounce it subbable). Just underneath that bump is an extremely bright newborn star, called (deep breath) AGAL 337.916-00.47. It’s one of the most luminous protostars known, just blasting out infrared light as material from around it falls onto the star and heats up tremendously in the process.

Not only that, but the protostars is sitting right where the hot gas from the bubble is hitting the cold gas from the filament. That’s pretty provocative, and provides a compelling case that the massive stars are helping make new stars. But, is that a coincidence? It’s not conclusive, and we need direct evidence.

That’s where the new observations come in. Using the (now defunct) flying infrared observatory SOFIA and the Australia Telescope Compact Array radio observatory, astronomers looked at ammonia gas in this region of the sky. Ammonia is convenient because it emits one kind of light when it’s embedded in hot gas, and a different kind of light when it’s in cold gas (hence the use of two different observatories that see different kinds of light). They also looked at carbon that emits light when it’s in warm gas (mind you, all this gas is cold in human terms, so think of cold, warm, and hot as relative terms).

What they found is a smoking gun. The massive stars are surrounded by hot gas, as expected, which is cooler farther out from them. But the protostar is surrounded by warm gas precisely where the hot bubble gas and the cold filament gas interact. Not only that, they found an ammonia maser, which is like a laser but with microwave light (radiation with wavelengths longer than infrared light but shorter than radio waves). Masers can occur when warm dense gas is hit by a shock wave, like when expanding hot gas slams into it… exactly what you’d expect if the massive stars are pushing out vast waves of hot gas that are slamming into the cooler gas and compressing it.

If you were expecting an actual post about an actual sea monster then sorry about that. But c’mon, you know this is WAY cooler. So why not share it with a friend?

And that is the recipe for star formation. So it really looks like the massive star are triggering the birth of protostars, or at least AGAL 337. All the pieces fit together pretty tightly.

There’s another idea that this process is self-sustaining in a way. Some of the stars that are born this way will themselves be massive, and could create hot bubbles that trigger more births a little further down along the filament. Some of those are massive, so you get another bubble, and eventually you get a string of these bubbles like a pearl necklace. The astronomers liken it to a domino effect, each bubble causing the formation of the next along the line. This can be seen in the Spitzer image of the cat’s Paw Nebula, which is loaded with bubbles that are all thought to have formed in this way.

I’ll add this isn’t always the case. If the massive stars really are super vigorous they can indeed suppress star formation, but that’s not the case here. How cool would it be to have a time-lapse video of Nessie lasting, oh, say a hundred million years, as new stars form down its length like a chain of firecrackers going off?

Et alia

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