Rho Cassiopeiae

Keep an Eye on this Star!

Rho Cassiopeiae, a naked-eye star near one of the most popular and easy-to-find star patterns in the northern sky — the famous W of Cassiopeia — may brighten and dim dramatically this summer. So use the star chart here to locate Rho and monitor its brightness.

Rho Cas is what astronomers call a yellow-white hypergiant; it is so big that if it replaced our Sun at the center of our solar system, its surface would lie beyond the orbit of Mars. Rho is 10,000 light years distant, but we can see it with the naked eye because it shines roughly a million times more brightly than our Sun. Such hypergiant stars are rare — only about seven of them are known in our Galaxy.

Alex J. R. Lobel and Andrea Dupree (Harvard-Smithsonian Center for Astrophysics) have been keeping their eyes on Rho because this star has had a chaotic past — experiencing large and sudden rises and drops in brightness since 1946. Now it appears that Rho is at it again, and it just might do something dramatic in the coming months.

The last time the star behaved erratically was in 2000. In fact, Lobel and Dupree believe that in that year the star had undergone the largest stellar mass ejection ever recorded. In other words, Rho experienced a near catastrophic eruption, spewing matter (roughly 50 Earths worth per day) for 200 days straight! When the entire event was over, roughly a thousandth of Rho Cas’s mass was gone!

Since the event in 2000, Rho Cas’s atmosphere has been pulsating in a strange manner. The star’s gaseous outer layers now seem to be collapsing — a similar event preceded the 2000 outburst, so another eruption may be imminent. Keep your eye on this star — it just may see it POP!

"Rho Cas is in the very last stages of its evolution," says Dupree. It could go supernova in as little as 50,000 years.

How Bright, How Dim?

Look low in north-northeast two hours after sundown for the W-shaped constellation Cassiopeia. Use the night-sky chart here to direct you. Cassiopeia is almost directly beneath the North Pole Star, Polaris — about half way from Polaris to the horizon. Rho is the middle star of three similarly bright stars just to the right of the famous W. Use the photo-illustration to locate the star. Under careful inspection, Rho is the brighter of the three stars.

Night-sky chart for locating Cassiopeia

If you have binoculars, you might want to estimate the brightness of Rho. How else can you be sure if the star is fluctuating in brightness? You can do this by comparing the brightness of Rho with other stars of fixed brightness shown in the photo-illustration below. Normally, Rho shines at about magnitude 4.6. What does that mean?

Sirius, the brightest star in the night sky shines at magnitude -1.5. The faintest stars visible to the unaided eye with a glance under a dark sky are usually about 6th magnitude. So, the larger he number the fainter the star. Rho is near the limit of naked-eye visibility from a suburban sky where there is some sky glow due to light pollution. So the view is best in binoculars.

Note that on the photo-illustration below, the decimal points of the stars’ magnitudes have been omitted because they can be confused with stars. So the star labeled 42 indicates a star of magnitude 4.2.

Photo-illustration of the stars’ magnitudes

To estimate Rho’s brightness, use at least two comparison stars. If Rho looks closer in brightness to one star than the other, figure out the difference between the brightnesses of the comparison stars and record what you see as best you can. Normally the brightness of Rho is about halfway between the 4.2 and the 4.9 stars. If it looks closer to the 4.2 star in brightness (but not as bright) it may be something on the order of 4.3 or 4.4 in brightness. If it looks closer to the 4.9 star, it may be on the order of magnitude 4.7. Be sure to check Rho every clear night, because no one knows when the star will blow. If it doesn’t blow this summer . . . well, you’ve taken the first step to becoming a skilled variable star observer. To learn more about observing stars that change in brightness, check out the Web site of the American Association of Variable Star Observers.

It takes some training to judge a star’s brightness with such precision. But there is no better way to learn than to do. Good luck!