Gamma Cassiopeiae, or just Gamma Cas, is a naked-eye star easily visible from the northern hemisphere; it is the central star in the W (or M) of the iconic constellation Cassiopeia. It’s about 550 light-years from Earth, and is a whopper: it has 15 times the sun’s mass, which means it’s very luminous, about 15,000 times the sun’s energy output. Replace the sun with Gamma Cas and we’d be cooked.

It’s the powerhouse behind the glow in the weird nebula the Ghost of Cassiopeia, which I’ve written about before, too.

But it’s also been at the center of a minor mystery in astronomy that’s bedeviled astronomers for decades. That mystery has finally been solved, but let’s take a step back first to understand what’s what.

It’s a B-type star, which is a classification that means its hot and luminous. But it’s more than that: it’s a Be star (pronounced “Bee Ee”, with the letters spelled out). The “e” stands for emission. In stars like the sun, hydrogen in a star’s atmosphere absorbs light coming up from below at very specific wavelengths (colors), so that when you get a spectrum of the star (spreading its light out like a rainbow) there are dark features at those wavelengths. That’s an absorption spectrum. 

But in some stars the hydrogen is actually excited, pumped up with energy and emitting light at those wavelengths, so we get an emission spectrum. Be stars are like normal B stars but with that emission feature.

We’ve known for a long time that Be stars are rapid rotators, spinning at tremendous speeds — Gamma Cas spins at nearly 400 kilometers per second, while the sun’s rotation is only 2 km/sec! — which is so fast that material at the equator is thrown into space by the centrifugal force. That material forms a disk around the star, called a decretion disk (the opposite of an accretion disk, where material falls onto a star or other object). That’s the material responsible for the hydrogen line emission.

But there’s more! We also know it’s orbited by a white dwarf (called Gamma Cas Ab, making the Be star Gamma Cas Aa^* ): a star once much like the sun but which used up all its nuclear fuel, swelled into a red giant, blew off its outer layers, and revealed its hot, dense core to space. In fact, when that star became a red giant it dumped a lot of material onto Gamma Cas Aa, which is also why it spins so quickly. That material sped it up like a basketball player slapping a ball while balanced on their finger to make it spin faster. 

As it happens, the system also generates a pretty decent amount of high-energy X-rays. The source has been a mystery for decades, though! The X-rays could come from the magnetic field of the Be star interacting with the material in the decretion disk, but if matter is streaming from that disk onto the white dwarf (a reverse of the older situation) it could also generate X-rays.

It hasn’t been possible to distinguish the two scenarios until now. A team of astronomers used the Japanese (with participation from NASA and ESA) XRISM X-ray space observatory to take a look at Gamma Cas. It’s designed to take high-resolution spectra of X-ray emitting objects, which hasn’t been possible before. The importance of this is that when objects move toward or away from us, the wavelengths they emit shift to shorter or longer wavelengths — a Doppler shift. We can measure the object’s motion that way, including its speed. 

What they found is that there is a cyclic shift in the X-rays from the Gamma Cas system, and it matches the orbital period of the white dwarf (including shifting to the shorter wavelengths when the star approaches us in its orbit)! That means it must be coming from the white dwarf, and not the Be star [link to journal paper].

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