Algol at last

October 29, 2013


Whew! I just now–well, about an hour ago–made my final observation for the Astronomical League Urban Observing Program. The final target was Algol, the “demon star” in Perseus, and one of the finest naked-eye variable stars in the heavens.

Now, I have seen Algol hundreds of times. It is also known as Beta Persei, because when it’s not in eclipse it is the second-brightest star in the constellation. But I had never tracked its brightness through one of its eclipses until the All-Arizona Star Party this year. The eclipses happen every 2.87 days when the dimmer star of this close binary passes in front of the brighter member. The effect is pretty striking–over the space of about three hours, Algol goes from being a twin of Almach (Gamma Andromedae, mag 2.1), to nearly as dim as Kappa Persei (mag 3.8).


Chart from Sky & Telescope

Now, the Urban Club rules say nothing about observing Algol more than once, but I figured the only reason it was on the list is because it’s such a noted variable star, and therefore the only respectable thing to do was to observe it both in and out of eclipse. My observations from this year’s AASP didn’t count because they weren’t made from in town. So I have been waiting. On Oct. 8 I was clouded out. By the 11th, it was not yet dark when Algol was in mid-eclipse, and it was probably below the horizon, to boot. Three nights ago I was clouded out again. Three nights hence it will probably be too low and too early to see clearly. So I either had to bag it tonight or wait until late November.

I didn’t think it was going to happen tonight. Mid-eclipse was supposed to be at 10:46 PM. At 10:15 it was still raining. But by 10:45 it had stopped, so I popped outside for a quick peek. The sky was full of clouds but there was a big sucker hole rolling in from the west, aimed right at Perseus (or rather, aimed right at the blank wall of clouds that I knew Perseus was lurking behind). But the sucker hole started closing up as it crossed the zenith and I got just a brief glimpse of Alpha Persei before the clouds knit themselves together completely. Curses!

Still, sucker holes are to stargazing what nibbles are to fishing–or maybe more accurately, what the occasional small winning hands are to poker. So I grabbed the old Tasco 7×35 binoculars that I got back in high school, pulled a folding chair out of the garage, and sat down to wait. I didn’t wait terribly long–at 11:14, the clouds tore open over Persei for just a bit. I couldn’t see the whole constellation, not by a long shot. But there was a bright star farther up the sky–Almach, surely–and a dim one closer to the horizon–Epsilon Persei, I reckoned, and a couple in the middle about equally dim–Algol and Rho Persei, just possibly? I snatched up the binoculars and found my putative Algol in a squashed trapezoid of stars, with an arc of three slightly dimmer ones just off to the north. Then the clouds rolled back in.

Well, I’d seen something, and had a fair idea of the relative brightnesses of the different objects, but had I seen Algol? I dashed inside for my Pocket Sky Atlas and breathed a big sigh of relief. There was Algol in the squashed trapezoid. The arc of three slightly dimmer stars to the north is anchored on Kappa Persei, one of the better comparison stars for estimating Algol’s brightness. At the time I saw it, Algol was midway between Kappa and Epsilon Persei in brightness, which is about right for half an hour past max eclipse.

Incidentally, the squashed trapezoid and arc of three stars that I used to identify Algol and Kappa Persei are not visible in the simple finder chart above, nor are all of the members visible to the naked eye under less than excellently dark and clear skies. I would have been hosed without the binos to confirm where I was in the sky–not the first time that binos have saved my butt, and almost certainly not the last, either.

So, here’s some homework. Don’t print out that Sky & Tel chart above. Instead, just grab your favorite atlas and a pencil and write in the brightnesses of the following stars:

  • Almach (Gamma Andromedae) – 2.1
  • Algol (Beta Persei) – 2.1-3.4
  • Epsilon Persei – 2.9
  • Kappa Persei – 3.8

Now you’ll have the brightnesses of all of the most useful comparison stars in your atlas, and you’ll never be without them (if you don’t have an atlas, use the Evening Sky Map, or download one of the free atlases listed on the sidebar to the right). For finding the eclipse times, use the calculator at Sky&Tel.com. If you track Algol through one of its cycles, report back with your observations. I’m going to sleep…with the Urban Program finally laid to rest.


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