The Ghost of Cassiopeia

June 12, 2023   Issue #576

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Pic o’ the Letter

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

One of my favorite things in astronomy is to find something new, some object I’ve never seen in all my decades of studying and writing about the sky. I especially love it when it’s a beautiful and fantastically shaped nebula, a cloud of gas and dust lit up by the forces around and inside it.

I’ve seen a lot of these, with different telescopes and at different wavelengths. But some manage to squeak past me. For example, this magnificent specter of glowing hydrogen:

Isn’t that beautiful? This was taken by Ryan M, who goes by the fun name of OryanAstro (say it out loud if that helps) on Instagram, and who graciously allowed me to use his image here.

The name of this object is a little complicated. The bright part in the middle is called IC 63 (IC = Index Catalog, a supplement to the New General Catalogue of space objects), and the fainter part to the left is called IC 59. They’re clearly part of the same sprawling gas cloud, which in its entirety is called Sharpless 2-185. At some point I’m going to write an article about all the different astronomical catalogues and how things are so confusingly named!

The IC 63 portion is also called the Ghost of Cassiopeia, which is appropriate given its overall shape. It’s located in the northern constellation of Cassiopeia, easily spotted in even mildly light-polluted skies as a big W (or M depending on the time of year) not far from Polaris.

I do see a ghostly apparition here, but I wonder… taking in the whole nebula it looks more like a flaming Phoenix rising from the ashes. And, as it happens, that name makes sense given its physical nature, too.

What you’re seeing is the edge of a molecular cloud about 550 light years from Earth. Molecular clouds are clumps of gas that are generally quite cold and dense. They’re mostly hydrogen — like everything else in the Universe — but are very cold in their interiors, enough so that hydrogen atoms can bond together to form hydrogen molecules: H2.

Some clouds are small, like Barnard 68 (my favorite), and others are immense, like the Orion Molecular Cloud which is hundreds of light-years across. It’s not clear to me how big the Ghost is, but it looks to be on the smaller size, a few light-years across.

Most of the bottom half of the image is the cold molecular cloud. Can’t see it? Well, no, because it’s dark. The material is dense, usually more than a million molecules per cubic centimeter in a cloud. That’s nothing compared to the air we breathe, which is more like 10 million trillion molecules per cc! But molecular clouds have a lot of cubic centimeters in them, so it adds up. Visible light (the kind we see) has a very difficult time penetrating them or getting out, so they’re opaque.

But that’s not always the case. See that bright star at the top of the image? Yeah. That changes everything.

The star is Gamma Cassiopeia, and it’s a beast. It’s what we call a B-type star, much more massive and luminous than the Sun. It weighs in at about 17 solar masses and shines at more than 30,000 the Sun’s brightness! Oof.

It’s actually a multiple star, with several stars orbiting each other, but the most massive one outshines the others so much that it dominates the system. It’s a pretty interesting star; it rotates phenomenally rapidly, nearly 500 kilometers per second at its equator (the Sun rotates at a languid 2 km/sec for comparison). That’s so fast it’s actually squashed from centrifugal force, and blows out material off its equator. This hydrogen then gets lit by the star so it glows, emitting its own light. That technically makes Gamma Cas a Be-star (e for emission).

Stars like this don’t live long, just a few tens of millions of years, so they don’t have enough time to wander far from where they’re born, so it’s likely it formed from the thick molecular cloud. Once it switched on it was so hot it blasted out ultraviolet light, and that profoundly affected the material around it.

For one thing UV light can blast apart molecules of hydrogen, turning them into individual hydrogen atoms. It also gives them a lot of kinetic energy, the energy of motion, as the gas heats up and expands.

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Over time this creates an expanding cavity or bubble in the dense cloud. The bright gas we see in the image is the edge of that bubble, the interface between the dense molecular gas of the cloud and the hot atomic gas filling the bubble surrounding Gamma Cas. Think of it as a wall of material. As UV light hits the dense gas it evaporates away like vapor rising from a puddle of water on a warm sunny day. The atomic gas glows at a characteristic red color called hydrogen-alpha, while the molecular gas is still dark, and that’s what you’re seeing here.

It’s not exactly rising from the ashes, but the name Phoenix Nebula does seem to fit.

In fact I wrote on The Old Blog about something very similar in the Orion Nebula, called the Orion Bar, and the article features images from Keck and JWST, so please check that out for more beauty and physics.

[Zoom in on the Ghost of Cassiopeia. Credit: Hubble, Digitized Sky Survey 2, N. Risinger (skysurvey.org). Music: Astral Electronic.]

It’s interesting that IC 63, the head of the Phoenix if you will, and IC 59, the wing on the left, look so different. IC 63 is much brighter and well defined, even though it looks like both are the same distance from Gamma Cas. I read some research papers on them, and in one astronomers looked at the emission of the gas and dust in both and determined IC 59 is not actually as close to the star as IC 63. We’re seeing a projection effect, where IC 59 is perhaps closer in physical space to us by a bit, but up higher in the image so it looks like it’s the same distance as IC 63. It’s like holding up your thumb next to a distant tree in a forest; it looks like your thumb is close to the tree but in reality it’s much closer to you than the other trees are.

I don’t have a lot of patience for Instagram and some other social media because of the amount of nonsense I see, but I do follow a lot of astrophotographers, and it’s all worth it when a jewel like this shows up in my feed. My thanks again to Ryan for allowing me to use it, and inspiring me to dig into this beauty.

Et alia

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