Algol (Beta Persei)
A multiple star system in the constellation Perseus.
Its Arabic name means "Demon Star", apparently a reference to its peculiar
behavior. Algol (Beta Persei) seems to wink slowly, fading for almost 5
hours, reaching a minimum brightness for about 20 minutes, and then gradually
returning to its maximum brightness before starting the whole 2.87-day cycle
again. Its variability was cited in a 1783 paper by William Herschel as evidence of a planet in orbit about the star. However, William Whewell,
in his attack on pluralism in 1853, argued that if the variability were
due to an eclipsing companion then this body would have to be much larger
than a planet. John Goodricke was the
first to realize its true nature.
Algol is now known to be an eclipsing binary,
consisting of a larger (3.5-solar-radius) but fainter subgiant K
star and a slightly smaller (3-solar-radius) but brighter B
star in orbit around each other. The fast decline in brightness takes
place as the K-type component moves in front of and blocks out the light
from its brighter partner. The 20-minute minimum corresponds to the period
when the B-type star is completely eclipsed. A secondary eclipse takes place
when the positions of the stars are reversed, but because the B-type is
responsible for the bulk of the combined light output and can only blot
out a fraction of the larger star's disk, this results in hardly any reduction
in brightness. A third, more remote star exists in the Algol system, with
a period of 1.87 years, which does not take part in the eclipses.
||B8V + K02IV + A5V
||93 light-years (28.5 pc)
||R.A. 03h 08m 10.1s,
Dec. +40° 57' 20.3"
||Gorgona, Gorgonea Prima, Demon Star, El Ghoul, 26
Per, GJ 9110, HR 936, BD +40°673, HD 19356, GCTP 646.00, SAO 38592,
FK5 111, Wo 9110, ADS 2362, WDS 03082+4057A, HIP 14576
|Algol animation. Credit: Larry Molnar