Date: January 4, 2011

Title: Algol

Podcaster: Laura Higginbotham

Description: A look at the eclipsing star Algol in the constellation of Perseus.

Bio: A wife, mother, and 12th year high school teacher of all things physics, and amateur astronomer. My goal is to inspire people to look up in the sky and wonder. . . .

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

The Demon Star. The Gorgon’s Head. A Pile of Corpses.

Cultures throughout the ages have feared it. The ancient Babylonians saw Lilith, the storm demoness wife of Adam. To the Hebrews, it was Satan’s Head; to the Chinese, a pile of corpses; to the Greeks, the severed head of Medusa in Perseus’ hand. To the Arabs, the Ghoul’s Head.

So, what is this harbinger of doom, this malicious malcontent? The unfortunate star of history , Beta Persei.

This second brightest star in the constellation of Perseus puzzled astronomers for millenia. On a very short period of 2.87 days, it dims from its normally bright 2.1 magnitude to an unremarkable 3.4 and brightens back to normal in about 10 hours. The short variance gave this poor star its bad repuation.

Actually, Algol is an eclipsing binary star system. Eclipsing binaries have been thoroughly described in previous podcasts, so I’ll just give a short description here: Two stars are gravitationally bound together so that they orbit each other. Their orbital planes happen to be positioned edge-on to our view, so when one passes in front of the other, we see an overall dimming and brightening as the front star blocks the light from the back star. An eclipsing binary pair have two “minima,” times at which the light is dimmed. The primary minimum, the more drastic drop in luminosity, occurs when the dimmer of the two stars blocks the light of the brighter. The secondary minimum, then, is the brighter star eclipsing the dimmer star. In the case of Algol, only the primary minimum is visible to the unaided eye; the secondary minimum was not recorded until the advent of photoelectric devices.

In reality, the Algol system contains three stars located about 93 light years from our solar system. In astronomical terms, that’s just our neighbor down the street. The third star, Beta C, has an orbital period of 1.86 years, and orbits the other two at a distance of 2.69 AU from the main pair. Since the third star does not play into the eclipse, we won’t dwell anymore on that one here.

Beta A classified as a B8V star. That means that it is a bluish-white star, with a surface temperature of 12,500K, a diameter nearly three times larger than our sun, and located on the main sequence of the HR Diagram. It’s partner, Beta B, is a K2IV star, a red sub-giant 3½ times larger than our Sun, but a cool 4500K on its surface. It is important to note, that though B is larger than A, A is the brighter star because of its higher temperature
http://stars.astro.illinois.edu/sow/Algol.html

The two stars are about 0.062 AU apart, about 16.7 million miles; that is more or less half the distance from the Sun to Mercury. To help you visualize that distance, if we dropped Beta A and B into our solar system in place of our Sun, the two would be well within the orbit of Mercury. And if we added Beta C to the mix, it would orbit along the outside edges of the asteroid belt.

Although generations of astronomers had observed Algol’s unusual nature, discovery of its variability is credited to an Italian astronomer, Geminiano Montanari in 1667. Then in 1783, an Englishman by the name of John Goodricke determined the period of eclipse to be 2.87 days; he also suggested at the time that the change in brightness was caused by a dark body passing in front of the star. Not until 1881 did astronomers begin to theorize that the star might be an eclipsing binary; this idea was based on the work of Edward Pickering, the director of the Harvard College Observatory. Finally in 1889, using spectral analysis, Hermann Carl Vogel proved that the star was, indeed, two stars playing hide and seek. (http://www.aavso.org/vstar/vsots/0199.shtml)
And being from Illinois, I must add that the primary and secondary eclipses were first recorded photoelectrically at the University of Illinois Observatory using a 12 inch refracting telescope. Joel Stebbins published this work in Astrophysical Journal in 1910.

Algol presents a wonderful target for amateur astronomers with a desire to observe the unusual. I began observing Algol as a project for a college astronomy class, and now I’d like to share with you a few tips and resources that I find helpful if you would like to observe Algol through an eclipse.

First, Sky and Telescope website ends its report on Algol with a calculator that will tell you the date and time of the next eight minima. Find a date and local time that will put Algol at minima in the depths of night and plan your observing session either starting 5 hours ahead or continuing 5 hours after the time; Perseus is in the sky in the late fall and winter, so please don’t plan to try this as a summer project. Check a planisphere or helpful website like Heavens-above.com to be sure the constellation of Perseus will be visible from your location at the time of minimum and for the 5 hours before or after.

You will need to compare the brightness of Algol to surrounding stars; the American Association of Variable Star Observers (or AAVSO) provides comparison charts for observing eclipsing stars. It is very important that you have the proper charts. Many stars in the sky vary in brightness, and your observations will be skewed if you are comparing your eclipse to another star that is also at a minima. The AAVSO Eclipsing Binary Chart identifies a half dozen or so stars marked with their magnitudes that can be used for comparison purposes. I suggest observing Perseus and Algol a few times before the night of the eclipse when the star is at full brightness to be sure you are familiar with its location.

On the night of the big event, load yourself up with charts, red-filtered flashlight, notebook, chairs, blankets, and hot drinks; also convenient, though not necessary, are telescope and camera, if you have one that can image the night sky. Stake out your observing site, preferably far away from city lights and plan to arrive and set up your equipment before night fall. After full dark falls, locate the constellation Perseus and the star Algol. Using your AAVSO comparison chart, decide which star in the region is most similar to Algol in brightness. Record your time and magnitude of Algol as you’ve found it compared to other stars.

For the next five hours, compare the star to others in its vicinity every ½ hour, recording your observations each time. As the hour of the eclipse approaches, check it more often; as the deepest minimum approaches, the magnitude will change more quickly. The time for the deepest minimum lasts for approximately 20 minutes while star B is directly in front of star A. At each observation, you may then make use of that camera to digitally record the changes; photograph the entire constellation of Perseus and later you will see the changes in magnitude very clearly on your images.

If you are a real trooper, and have the time before sunrise, you may choose to continue your observations until Algol returns to full brightness. Otherwise, head for home; the rest of your work can be done on a computer or by hand. Create a chart plotting Time vs. Magnitude. If you only observed half of the eclipse, then you will chart only half the curve. For your own benefit, you can assume the opposite half of the curve will be symmetrical to the half that you observed.

Finally, on a side note, while you are waiting for your next observation of Algol, it’s a great time to get in some observing of constellations or other night sky objects using that telescope you might have brought along.

Well, that’s as far as I can take you in this podcast. I’m Laura Higginbotham, astronomy and physics teacher at Rosehill Christian School in Tomball, TX, and member of North Houston Astronomy Club. You can find my club at www.astronomyclub.org

http://www.daviddarling.info/encyclopedia/A/Algol.html

http://www.skyandtelescope.com/observing/objects/variablestars/Minima_of_Algol.html

http://www.aavso.org/vstar/vsots/0199.shtml

http://www.aavso.org/images/Betaper-aa.gif

End of podcast:

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