The sky at night

Like a couple condemned to waltz till they die for the sin of being too intimate, the two stars of Algol perform an incessant dance, circling around each other in gravity's unbreakable embrace. As the duller partner in an unequal match eclipses its more dazzling companion, Algol fades. In four hours it loses well over a magnitude. Maximum eclipse lasts just 20 minutes before the 4-hour recovery commences. The whole performance repeats every two days 20 hours and 53 minutes. More than 100 minima of Algol occur in a year, but only a quarter of them are observable. This week provides two opportunities, both conveniently before midnight: 11.20-11.40pm on Tuesday the 2nd and at 8.10-8.30pm on Friday the 5th. Two stars in nearby Cassiopeiae are useful comparators for judging Algol's progress. Alpha Cassiopeiae shines like Algol at its brightest (magnitude 2.2), while Epsilon resembles Algol's darkest moments (magnitude 3-5).

Jacqueline Mitton