Algol diagram

Here’s what an eclipse looks like up close. The main eclipse at right occurs when the larger but dimmer companion star, a K2 orange subgiant, partially eclipses Algol A, a more massive but smaller main sequence star. A small secondary eclipse, left, is observed when the B star passes around the back of the primary.

In ancient times it was believed the stars never changed. Today we know this is not true, and there are hints some ancient people also knew it was not true.

One example is a star in the constellation of Perseus that changes brightness every 2.87 days. The star’s name is Algol, the Demon Star. Its change in brightness can be seen with the naked eye; if the weather clears during March, you can see it for yourself.

Algol’s brightness change was most likely noticed by ancient skywatchers, but it was formally discovered by Italian astronomer Geminiano Montanari in 1667. It took another 115 years before someone noticed this bright-dim cycle was regular.

John Goodricke, working with a neighbor in 1782 and 1783, discovered the steady pattern in the cycle. On Nov. 12, 1782, this 18-year-old was amazed to find the brightness of the star had changed. He watched it diligently for an hour and could hardly believe the star’s brightness changed so much in a short time.

The star we see as Algol is really three stars orbiting each other. Two of the stars are in a close binary system; the brighter of the two, Algol A, is a bluish star about 90 times more luminous than our sun. The dimmer one, Algol B, is a yellow star only about three times more luminous than the sun.

These two stars orbit each other very closely, separated by only 4.65 million miles. Our sun is 93 million miles from Earth, so the distance between Algol A and Algol B is just 5 percent of the distance between the Earth and the sun.

Algol A is the primary star in the system and is 3.7 times more massive than the sun. Algol B it the lightweight, at only 0.81 solar masses. Algol B orbits Algol A every 2.87 days, which is why the star appears to dim at that interval. Algol C is a much fainter star that orbits the other two, at a much greater distance, every 1.86 years.

Because the orbit of Algol B around Algol A lies in Earth’s line of sight, B crosses in front of A on every orbit. Every two days, 20 hours, 49 minutes (2.87 days), much of the light of Algol A is blocked by Algol B. With the light of Algol A blocked, the star is eclipsed and appears to dim.

The complete eclipse takes about 10 hours, but the most noticeable change occurs in just two hours. Halfway between these primary eclipses, there is a small dip in brightness when Algol B is behind Algol A.

Normally massive stars age faster than less massive stars, but this is not the case with Algol A and B. Algol A, the massive primary star, is the youthful star of the pair and is less massive. Algol B is much older and more evolved. Astronomers call this the “Algol Paradox.”

It appears Algol B must have been more massive in the past and has somehow lost mass. Where did it go?

Algol A and B are very, very close together—so close spectral analysis reveals the gravitational pull of the more massive Algol A is drawing a stream of matter from its companion, Algol B. This means Algol A is gaining fuel, making it appear younger, while Algol B is losing fuel, making it appear older. That would explain the paradox.

To find Algol in the night sky, we first need to find the constellation Perseus. Look along the north-northwest horizon for the W-shape of the constellation Cassiopeia.

Perseus is a little higher and a little farther south and resembles a lopsided letter K. The lower arm of the K points toward the Pleiades star cluster. Algol is at the end of the upper arm. If you need help finding it, there are several online planetarium programs that could help.

The eclipses of Algol occur every 2.87 days, but many of these eclipses occur during the daylight hours and cannot be seen. The best viewing times in Walla Walla during March are the 13th at 3:36 a.m., the 16th at 12:26 a.m. and the 18th at 9:15 p.m. These times are the midpoints of the eclipses.

The best way to view the change in brightness is to look at Algol about two hours before the above midpoint times to see the star at its brightest. Then look again within a half hour before and after the times given; Algol will be noticeably dimmer.

If you miss seeing Algol in March, use the online calculator at to find another viewing time.

Let’s hope for a clear night on one of the March dates so we can see Algol wink at us.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at  Walla Walla University. He can be reached at

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