A globular gift from JWST

February 27, 2023 Issue #531

The edge-on disk galaxy NGC 5866. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); W. Keel (University of Alabama, Tuscaloosa)

Subscribers delight me as much as globulars do

Pic o’ the Letter

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

If you’ve been following me for a minute then you know I love me some globular clusters — tightly packed roughly spherical collections of hundreds of thousands of stars, all held together by their mutual gravity. They’re fascinating scientifically but also a delight at the eyepiece, and I’ve seen dozens through my own telescope.

They’re incredible when viewed by bigger ‘scopes, too, like Hubble. I’ve written about them many times on The Old Blog.

I’ve been wondering when we’d see the first globular images from JWST, and hooray: One was just released!

The globular cluster M92, seen by JWST. Credit: NASA, ESA, CSA, A. Pagan (STScI)

Wow! But what are you seeing here, and why? Let me ‘splain.

M92 is big as globulars go, roughly 100 light-years wide. The diameter of these beasts is a bit difficult to nail down because they aren’t solid; the number of stars drops with distance from the center so there’s no real edge (like our atmosphere, which just gets lower in density the higher up you go).

In cases like these astronomers have different ways of characterizing the size. For example, they fade with distance from the center until they sortof blend into the sky of background stars around them. You can draw increasingly large circles around the center and add up all the light you see in them; eventually you reach stable amount once the globular ends and you’re just seeing background stars. A good measure of size in that case is the half-light radius; the distance where a circle contains half the total light. For M92 that’s about 8.5 light years. Another measure is what’s called the tidal radius: How big the cluster can get before the gravity of the Milky Way galaxy starts stripping stars away from the cluster’s suburbs. For M92 that’s more like 110 or so light-years. That’s an upper limit to how big it can be.

Astronomers are very interested in the stellar population of globulars — that is, the kinds of stars in them, and how many of each there is. For one thing this can tell you the age of the cluster. Also, the distribution of stars (like, how many low-mass red dwarfs are there compared to heftier stars like the Sun) tells us about how stars formed in the cluster billions of years ago.

This can be hard to measure from the ground because globulars tend to be far away — M92 is over 26,000 light-years from us — so seeing the faintest stars is hard, plus they’re so crowded you need super-high-resolution images to prevent them from overlapping.

This is where JWST comes in. On top of its sharp eyesight it also sees in the infrared, where these lowest mass stars put out most of their light, so they are easier to spot .

Detail of M92 in the JWST image, from the section on the right of the image above. Credit: NASA, ESA, CSA, A. Pagan (STScI)

That’s one reason these images were taken (you can read the observation proposal here). Another was because, with JWST, we need new ways to process and analyze the images. The shapes of the stars seen are due to the unique optics of the observatory, and require new methods to deal with. So the scientists who took the image wanted a crowded field with lots of faint stars so they could test various ways of dealing with the images. M92 is perfect for that.

Hubble’s view of M92 taken in 2014. Credit: ESA/Hubble & NASA Acknowledgement: Gilles Chapdelaine

They also wanted — and I love this so much — to be able to see if and by how much stars have physically moved over the years in the cluster. Hubble took images of M92 in 2014, and over the time since the stars will have moved a little bit in their orbits around the cluster center. For many, that change will be visible in these images, so as a bonus we get to measure those orbits from these JWST images. The sizes and shapes of the orbits can be used to find the mass of the cluster (currently estimated to be about 300,000 times that of the Sun).

And of course an obvious question you have (thanks for your patience) is: what’s the deal with that big gap in the image? M92 is a bright cluster — it’s actually visible to the naked eye from a very dark site! — and JWST has a very large mirror which collects a lot of light. So even short exposures can saturate the detectors, overexposing them. The glare from all those stars in the center (and their diffraction spikes) will spill over into the outer parts of the cluster in the JWST images, too, messing up the images. How to prevent that from happening?

Well, NIRCAM — the camera used for these observations — has two separate detectors in it, spaced a small distance apart. The astronomers placed the center of the cluster in that gap between the detectors, which blocked all that bright light from the cluster core. Each detector therefore only saw stars closer to the edges. Clever. It does mean we don’t see the core, but the bright crowded stars there would be a mess anyway.

The dwarf galaxy WLM, observed by the Very Large Telescope Survey Telescope. Credit: ESO

This observation project also looked at two nearby dwarf galaxies that also have resolved stars in them, much for the same reasons they looked at M92. One of them, I was delighted to see, is the galaxy WLM — for Wolf–Lundmark–Melotte, an elongated irregular galaxy about 3 million light-years away. I wrote about it (again on The Old Blog) if you want details, but it’s an isolated weird little thing, possibly undisturbed by nearby bigger galaxies since its birth billions of years ago, making it a great laboratory to study the evolution of small galaxies and their stellar population.

I hope they release those images! They should be quite lovely.

And, of course, JWST will be observing more globulars as time goes on. Hubble images were a huge boon to astronomers, since it can see fainter stars, but JWST is literally designed to see even fainter ones, so I’m really hoping we’re about to experience another big leap in our understanding of these gorgeous and intriguing objects. Stay Tuned.

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P.S. If you’re wondering what the sky would look like from a planet orbiting a star in a globular cluster, do I have something for you! My upcoming book Under Alien Skies has a whole chapter on that exact topic. And yes, the sky would be brain-fryingly incredible. You can find out for yourself in Chapter 8!

Et alia

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