Algol
Where is Algol?
Look south, about 80° up at 03:14 in Perseus.
- Rises
- Always up
- Highest
- 03:11
- Sets
- Never
- Best after dark
- 03:14 · 80°
Can I see Algol tonight?
From London tonight, Algol is above the horizon all night. It’s highest after dark at 03:14, in the south at 80°.
At magnitude 2.1 it is visible by eye, even from many towns, when the sky is clear.
This is position and timing only: clouds, haze and light pollution aren’t included.
What does Algol look like?
Algol looks blue-white to the eye (color is easier to see in binoculars). Its spectral type is B8V: hot blue-white main-sequence star (much hotter than the Sun).
Eyes
A moderately bright white star. Compare it with nearby stars over a few hours during an eclipse to see it fade.
Binoculars
Not needed, but helpful to compare it with neighboring stars of known brightness.
Telescope
A single white point. The eclipse is best followed by eye.
About Algol
Algol, the Demon Star, is the most famous eclipsing binary. Every 2.87 days, a fainter companion passes in front of the brighter star, and Algol drops from magnitude 2.1 to 3.4 for a few hours before recovering.
You can watch this happen with your own eyes. The whole eclipse takes about ten hours, and during that time Algol visibly fades compared with nearby stars.
Algol is about 90 light-years away. Its name comes from the Arabic for “the demon’s head”, from the head of Medusa that Perseus holds in the old star stories.
The brightness changes were first recorded in the 1660s. In 1783 the young English astronomer John Goodricke suggested that a darker companion was eclipsing the star, which turned out to be correct.
Algol is in fact a triple system. Its two eclipsing stars have exchanged material over time, which explains why the less massive one is the more evolved, a puzzle once called the Algol paradox.
Algol in Perseus
Questions
Can I see Algol change brightness?
Why is Algol called the Demon Star?
Sources: SkyMate; Wikidata (CC0). Positions computed for your place and time with astronomy-engine. Data & Licences