Algol is part of an eclipsing binary system, plummeting in brightness as its companion passes between it and Earth.
Algol (Beta [β] Persei) puts on one of the most spectacular variable-star shows in the sky. If you want to estimate its changing brightness, compare it to the magnitudes of nearby stars (shown in green). Credit: Astronomy: Roen Kelly
- Algol, the Demon Star, dims noticeably every 2.867 days.
- This dimming is caused by one star eclipsing the other in a binary star system.
- We see this effect because the system is tilted just right relative to Earth.
- Algol’s brightness changes are “not a variable star,” but rather an eclipsing binary.
There’s a demon in the early-morning sky, amid the stars of Perseus the Hero. Second-magnitude Algol (Beta Persei) is also famously known as the Demon Star, and you can spot it two hours before sunrise, standing 30° high in the east, to the upper left of the Pleiades.
Algol is dubbed a demon because every 2.867 days, its brightness changes noticeably — even to the naked eye. It suddenly fades from magnitude 2.1 to magnitude 3.4 in a matter of hours, losing some 70 percent of its brightness for a brief time before recovering once more. And then it happens again, 2.867 days later. And again. And again.
But Algol is not a variable star. Instead, its brightness changes because it is an eclipsing binary: a binary star system that is tilted just so toward Earth, such that we see the companion pass in front of Algol once every orbit. As the companion partially blocks Algol’s light, we see the star appear to plummet in brightness, before brightening again as the secondary moves out of the way. Careful observation also shows secondary eclipses, in which the system appears to dim again — only slightly — as the fainter star passes behind Algol on the other side of its orbit and we lose its contribution to the overall amount of light we see.
The Algol system recently reached a minimum yesterday morning, so it currently appears at its “normal” magnitude of 2.1. It will again reach a minimum the morning of the 13th, so we’ll check in with it at the same time we observe Venus standing near the bright star Aldebaran in the morning sky.
Sunrise: 5:41 A.M.
Sunset: 8:30 P.M.
Moonrise: 9:34 P.M.
Moonset: 6:07 A.M.
Moon Phase: Waning gibbous (99%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.
For a look ahead at more upcoming sky events, check out our full Sky This Week column.