Rho Cassiopeiae

August 21, 2020

Rho Cassiopeiae (row casio-pee-eye) is huge. More than 400 times the size of the sun and 40 times as massive. And did I mention it’s bright? Half a million times brighter. Now it’s by no means the MOST luminous star we’ve ever found, but it is one of the brightest yellow stars. Yellow stars, or Class-G stars are fairly common. Our own sun is a Class G. But Rho Cassiopeiae is a yellow hypergiant--a particularly rare and short-lived type of class-G star. They’re thought to be a type of star that evolves from a red supergiant to a hotter blue star. A strange evolution indeed in the celestial world. So even though we’ve known about this particular star for centuries, this huge, bright, yellow stage in its evolution is relatively brief. Strangely massive stars like this have short lives. Rho Cassiopeiae is probably no more than 6 million years old.

To find it, find Caph--the second-brightest star in Cassiopeia. It’s also the highest star in the constellation in our current evening sky--and the one furthest to the right if you were to turn Cassiopeia on its side so it looks like a W. From Caph, scan to the right and up with a telescope or a decent set of binoculars until you see a yellow star. Consult a star chart to make sure you have the right one.

Of the hundreds of billions of stars in the Milky Way, we’ve only found a dozen yellow hypergiants like Rho Cassiopeiae. So if you see it, you’ll be looking at one of the rarest star types we’ve ever discovered.