JWST spies on the galactic downtown

November 27, 2023   Issue #648

Subscribers light up my sky like half a million stars 

Pic o’ the Letter

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

The center of the Milky Way is a busy place.

Our galaxy is huge, and like any grand city the downtown bustles. There’s a supermassive black hole there with over four million times the mass of the Sun, and while it does profoundly affect the environment around it, its total mass is tiny compared to what surrounds it. Millions upon millions of stars are within the heart of the Milky Way, as well as millions of times the Sun’s mass worth of gas. Some of it is forming stars, while some of it is blasting outwards due to exploding stars or newborn massive stars, blowing mighty winds that carve and shape the gas crowding the galactic core.

It’s a mess. One particularly fraught region is called Sagittarius C (or Sgr C for short), a couple of hundred light years out from the true galactic center. It’s a huge complex of thick gas and dust, some of it very cold, some heated by newborn stars embedded within. While it’s been studied by astronomers using various telescopes, JWST took a look at it last year, peering deeply into it in the infrared, getting the highest resolution (highest detailed) image of it ever taken at those wavelengths, and what it saw was gorgeous chaos:

Ye, and may I add, GADS. [Click here for the full-res version, because WOW]

The first thing to note in this image is that essentially every dot in it is a star in our galaxy, and not an entire galaxy very far away. We’re used to seeing galaxies scattered across JWST images, because distant galaxies are bright in infrared. That’s also because infrared light can travel through clouds of cosmic dust pretty easily, allowing us to see past the murky confines of the Milky Way and into the intergalactic space beyond.

But in the center of the Milky Way the dust is so thick that even that light eventually gets absorbed, so all we see are the stars there. And the gas. Oh my, that gas.

Different objects emit light at different wavelengths, and we use filters to select for those colors. Cobalt blue in this image is actually light at 1.62 microns (about twice the longest wavelength our eyes can see), and is emitted by cool stars — red dwarfs and red giants. Teal is 4.04 microns, where warm hydrogen gas glows. Orange is 3.6 microns, emitted by stars mostly. Red is 4.7 microns, emitted by cool molecular hydrogen (two H atoms bound together, called H2).

Now look again at the image. You can see lots of deep blue stars, as well as reddish-oranges ones, so those are cooler stars. I’ll add that there are something like half a million stars in this whole image. And we’re only seeing a small portion of the galactic center here!

But holy wow, look at that teal gas at the lower left! That’s hydrogen heated by newborn stars, or stars struggling to be born (what we call protostars). The structure is very interesting; all those filaments in it are probably due to shock waves compressing the gas. The shock waves are moving outward from newborn massive stars blasting out light; that heats the gas and causes it to expand. These stars can also blow fierce winds of subatomic particles that can compress gas around them, though on a far smaller scale.

There’s an interesting hole — or I should say, “hole” — in that gas though, just above it. Here’s a closer look:

Hmmmm. The notch is almost certainly due to extremely dense gas and dust, so thick that it’s really blocking light from behind it. Note how few stars we can see in it! And that red blob. I’ve seen something like that before: OMC1, a weird cloud of gas with dozens of thin fingers pointing outward from it. OMC1 is thought to be the site where three stars collided and merged, creating a colossal explosion of energy and gas that blasted away dense knots of material, creating the radial fingers. But that’s not what’s going on here; in this case the astronomers who took this image think that’s a protostar cluster, with many stars forming in that small clump. Their combined energy would indeed produce a similar effect as Orion C.

Note that a lot of the hydrogen gas in the teal cloud seems to be aligned with the red blob, and some of it is tangent to it, like it’s the force of the red blob compressing much of the gas. It does seem to be profoundly impacting the less dense gas in that cloud below it.

Here’s another “hole” in that gas, though with less activity going on:

This one is located to the extreme right of the teal gas. It’s not a hole; again, it’s a clump of gas and dust so dense that light can’t penetrate it, so we see…. nothing. Or very little, just a few stars. Remember, seeing black in astronomy doesn’t mean nothing’s there. It might mean there’s so much there that it’s blocking the view. It’s nice to have your perceptions challenged.

And then, off to the right, is that huge and complex cloud of reddish molecular hydrogen gas (remember, it’s not really red, it’s just colored that way in this image so we can see it; in this image everything would be invisible to our eyes because we don’t see infrared):

Molecular gas is cooler than atomic gas, so this is probably denser material forming stars. Most of the stars in it look red to me, but that may be because the cloud is so thick it blocks the shorter wavelengths, leaving only the longest (“reddest”) wavelengths to get through. The shape is a bit complex, no doubt carved by the winds and light of stars forming inside.

By the way, this issue is free, so feel free yourself to share it with someone else who might enjoy it:

I am speculating for the most part here, based on my experience looking at images like this, and I could be hugely off base in my interpretation. But here’s the thing: This region of the galaxy has never been observed in such detail before, so no one really knows what’s going on here yet. It takes time to interpret these images. They were taken to investigate the formation of high- and low-mass stars, but the paper from this work hasn’t been published yet. I look forward to reading it! For one thing, several of the authors are at the University of Virginia, where I got my degree (and now live near once again), but also because star formation is fascinating, and the deep center of the Milky Way is still quite mysterious. Is starbirth there similar to what happens way out here in the galactic suburbs, 26,000 light-years removed? Or are the processes there very different due to how crowded it is? Observations like these from JWST may help us figure that out.

Et alia

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