Paid subscribers give me great feedback

Sometimes there are questions in astronomy that seem like they have easy answers, but then when you look into them a bit more carefully you find it gets confusing fast.

For example, we know lots of stars are born in stellar clusters, collections of as few as dozens or as many as hundreds of thousands of stars. These clusters are themselves formed from clouds of gas and dust, some relatively small and some huge.

We know the clouds start to collapse — perhaps they collide with another cloud, a nearby supernova goes off and compresses it, or just over time the cloud cools and loses the internal heat needed to support it (like a hot air balloon) — and stars form as clumps of the cloud shrink down. Some stars accumulate a lot of mass (like a dozen or more times the sun’s mass), and get incredibly luminous, blasting out intense amounts of light, including ultraviolet light. This is a decently high-energy form of radiation, which pushes against and dissolves away the cloud around it in a process called photodissociation. 

This overall process is generically called feedback: clouds form stars that then affect the cloud itself. In this case, the luminous stars are keeping the cloud from making more stars near them! It also curtails already forming stars, so they tend not to get as massive.

Massive stars are rare and take a lot of material to make, so they’re more likely to be found in massive or dense clouds, where lots and lots of stars are being born all at once, forming a stellar cluster. If this happens near the edge of a giant cloud, the combined might of all the massive stars’ destructive light can puncture through the cloud to interstellar space, causing a blowout. We see this a lot; in fact the famous and iconic Orion Nebula is just such a structure, blown out of the side of one part of the immense Orion Molecular Cloud Complex.

This can then go on to affect star formation in more clouds, because now the light from those massive stars can travel farther. When you get enough clusters forming in a galaxy, the star formation rate can be affected globally. It’s a big deal.

So. Getting back to the thesis of all this, which is something that seems like it should be obvious but is difficult to prove: how long does all this take? If you start with a collapsing cloud, what’s the timescale before the stars blow through the cloud and emerge so that they’re visible from the outside? And most importantly, does this depend on the mass of the cluster?

In other words, do big clusters (ones that tend to make a larger number of massive stars) tend to form more quickly and blow out their gas, or does that happen faster with smaller clusters, which have less gas to blow away when they form? It seems like a simple question, but computer models that simulate the physics of star formation struggle with this. There are tons of complicating issues. For one example, massive stars explode after only a few million years, which helps clear away the gas. But what astronomers want to know is what happens before stars start to explode. How good are massive stars at clearing away their cocoons?

Some observations have been useful here, of course, but it’s hard. We can look at clouds in our own galaxy, which are close to us and therefore we can see in more detail… but, ironically, the presence of opaque dust inside our galaxy blocks the view. Looking at other galaxies is easier, but most observations lack the resolution needed to see smaller details due to the larger distances to the targets.

So a team of astronomers used JWST to look at four nearby galaxies (M51, M83, NGC 628 and NGC 4449). JWST has very keen eyesight in infrared, which is helpful because infrared light can get through the dust more easily than visible light. They looked at pretty big areas of each galaxy, mapping thousands — thousands! — of star clusters to measure their ages and find out how long it takes for them to disperse their natal material. Oddly, studies done before have done this, but haven’t looked at the difference between more and less massive clusters.

What they found isn’t surprising, but it is important: sure enough, more massive clusters clear away material faster [link to journal paper]. The most massive clusters swept away their cocoons in about five million years, and less massive ones took more like 7-8 million. Getting these numbers means astronomers can fine-tune their models more, make them match what nature is really doing. That will help us understand better how stars form.

When you go out on a sunny day, think about that star shining down on you: it formed some 4.6 billion years ago, but we aren’t really sure exactly how. There’s good evidence it formed in a big cluster, which means there were likely lots of massive stars nearby. How did that affect the proto-sun’s growth? How might it have affected our stellar siblings that formed alongside the sun? And how did those luminous, beefy stars contribute to how other stars formed even farther away in the galaxy?

We do have a good grasp on how gas and dust clouds turn into stars, but the details are still elusive. Studies like this will help us nail them down.

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!