What is Coronal Gas?


Stars are hot. Space is cold. We’re all familiar with that, right?

Ok, good.

Technically, it’s more complicated than that. Space isn’t completely frigid—absolute zero, the temperature at which there is no heat whatsoever, is purely theoretical and not thought to exist in the universe. But it is pretty darn cold.

And stars aren’t always very hot—there is one newly discovered star that’s only as hot as fresh coffee. (It’s a brown dwarf, and if you go by the definition of a star as an object that’s ignited hydrogen fusion in its core, then it doesn’t actually count.)

In general, though, stars are pretty darn hot. Some special types of stars reach up to 200,000 K—that’s 359,540.33℉. Our own sun is about 5,778 K, which much cooler, but still almost ten thousand degrees Fahrenheit.

As a rule, we can think of stars as being much hotter than the space in between…except in the case of coronal gas.


Not too many posts ago, I showed you this image of the galaxy.

Now what the heck is going on here?

If you’re now feeling as if you’ve missed out…don’t worry. This isn’t a sight you’d see with your own eyes. Well, it is, but only if you have x-ray vision, which would make you not quite human.

This is an image taken with special telescopes designed to collect x-rays emitted by objects in the universe—rather than the visible light that the optical telescopes you might be familiar with collect.

All that glowing stuff? That’s coronal gas.

Now, here’s the million-dollar question. How the heck is the space between the stars producing x-rays? X-rays are some of the highest-energy forms of radiation in the universe, so it would take something pretty darn hot to produce them.

Well, stars can emit x-rays. They burn hot because of hydrogen fusion in their cores. But there’s certainly no hydrogen fusion happening inside coronal gas. It needs extremely high pressures, but coronal gas has a very low density and thus a very low pressure.

Then what the heck is making it so hot?

The answer does, interestingly enough, come in the form of stars. Coronal gas isn’t producing its energy the same way stars do—it got its energy from stars.

Most detailed image of the Crab Nebula

Meet the Crab Nebula, one such culprit of coronal gas formation.

The Crab Nebula is a supernova remnant—basically what’s left over when a massive star reaches the end of its life cycle and explodes. I’ll go into more detail about how and why this happens later on when I talk about star life cycles.

Anyway, what’s important is that a supernova is a very powerful explosion. Large amounts of very hot gas are blasted outward in expanding bubbles, and these bubbles become coronal gas.

These coronal gas bubbles have no way of maintaining their own heat—they simply remain the temperature of their parent supernova.

This, of course, begs the question…will coronal gas cool off eventually? As stars continue to go supernova, more bubbles of coronal gas will form, but what about the already existing bubbles? Are there any out there that have already cooled off?

Yes, there are—and I’ll discuss that a little in my next post.

Although coronal gas has too low a density and too high a temperature to be any use for forming new stars and solar systems, it plays an instrumental role.

Sometimes, neighboring bubbles of coronal gas might expand into one another and merge into more massive superbubbles. (I know, the terminology here is kind of ridiculous. Bubbles and superbubbles? Really?)

Anyway, when superbubbles form, the expanding gas can compress nearby GMCs (giant molecular clouds). And these, as I explained in my last post, are the places of star birth.


The magnificent “Pillars of Creation” are a region of such GMCs. Just the club-like glob on the top of the leftmost “pillar” has enough mass to form many new solar systems. There’s just one problem, though. Something has got to trigger that.

In order to form new stars, GMCs have to begin to collapse in on themselves. They are by no means uniform in density, containing denser clumps of material here and there, and these dense clumps eventually form the cores of the new stars.

The problem GMCs face is that the interstellar medium remains, for the most part, in equilibrium—essentially, in balance. The forces pushing each cloud to expand are balanced by the forces of pressure from the surrounding clouds.


The balance of the interstellar medium is a good thing. Our solar system itself is encased within a local bubble or local void of coronal gas, which was inflated by a nearby supernova within the last few million years.

Who knows if there are other regions of the universe where life could form, or has already formed, within a cloud of interstellar gas? The gas itself would be harmless, but if the fragile equilibrium of the interstellar medium were disturbed, we have no idea what the consequences would be.

We do know that life is fragile, delicate, and humans depend on the ability of their planet—which depends on the local environment of the space surrounding it—in order to survive.

But nonetheless, the equilibrium of the interstellar medium is a problem for GMCs, which need to be disturbed in order to collapse and begin star formation. And that’s where coronal gas comes into play.

Superbubbles of coronal gas have been known to compress nearby clouds—and that’s exactly what GMCs need to trigger star formation.

So…we’ve covered cool clouds, HII clouds, molecular clouds, and coronal gas…all that’s left is the gas-star-gas cycle, next up, and we’ll be done with the interstellar medium!

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