Algol

 

(ăl'gŏl) , famous variable star in the constellation Perseus; Bayer designation β Persei; 1992 position R.A. 3^h07.7^m, Dec. +40°55′. Algol's variation in apparent magnitude, from 2.06 to 3.28, is due to the fact that it is an eclipsing binary star, with one component revolving about the other with a period of 2 days, 20 hr, 49 min. Because the plane of revolution is almost parallel to the line of sight, the star dims noticeably when the dimmer component passes in front of, or eclipses, the brighter component, and dims again very slightly when the brighter component eclipses the dimmer one (see eclipse); the primary minimum, when the brighter component is eclipsed, lasts about 10 hr. Algol is of spectral class B8 V and is about 105 light-years from the earth. The name Algol comes from the Arabic Ras al Ghul, which means “demon's head,” and the star is sometimes called the Demon Star.

Beta Persei A/B/C

The position of Algol.
Observation data Epoch J2000
Constellation	Perseus
Right ascension	03h 08m 10.1315s
Declination	+40° 57′ 20.332″
Apparent magnitude (V)	2.12
Characteristics
Spectral type	B8V/K02IV/A5V
U-B color index	-0.37
B-V color index	-0.05
Variable type	Eclipsing binary
Astrometry
Radial velocity (Rv)	3.7 km/s
Proper motion (μ)	RA: 2.39 mas/yr Dec.: -1.44 mas/yr
Parallax (π)	35.14 ± 0.90 mas
Distance	93 ± 2 ly (28.5 ± 0.7 pc)
Absolute magnitude (MV)	-0.15
Details
Mass	3.59/0.79/1.67 M☉
Radius	2.3/3.0/0.9 R☉
Luminosity	98/3.4/4.1 L☉
Temperature	12,000/4,500/8,500 K
Metallicity	Not available
Rotation	65 km/s.
Age	< 3 × 108 years
Other designations
Algol, 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 (β Per / Beta Persei) is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered. Algol is actually a three star system (Beta Persei A, B and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 during the several hour long eclipses which occur every 2 days, 20 hours and 49 minutes.

History

The name Algol derives from Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) (see "the ghoul") which was probably given due to its peculiar behavior.^[1] In the constellation Perseus, it represents the eye of the Gorgon Medusa.

It is known as 大陵五 (the Fifth Star of the Mausoleum) in Chinese.

The variability of Algol was first recorded in 1667 by Geminiano Montanari, but it is probable that this property was noticed long before this time. The first person to propose a mechanism for the variability of this star was the British amateur astronomer John Goodricke. In May, 1783 he presented his findings to the Royal Society, suggesting that the periodic variability was caused by a dark body passing in front of the star (or else that the star itself has a darker region that is periodically turned toward the Earth.) For his report he was awarded the Copley Medal.^[2]

In 1881, the Harvard astronomer Edward Pickering presented evidence that Algol was actually an eclipsing binary.^[3] This was confirmed a few years later, in 1889, when the Potsdam astronomer Hermann Vogel found periodic doppler shifts in the spectrum of Algol, inferring variations in the radial velocity of this binary system.^[4] Thus Algol became one of the first known spectroscopic binaries.

Star system

As an eclipsing binary, it is actually two stars in close orbit around one another. Because the orbital plane coincidentally contains the Earth's line of sight, the dimmer star (Algol B) passes in front of the brighter star (Algol A) once per orbit, and the amount of light reaching Earth is temporarily decreased. To be more precise, however, Algol happens to be a triple star system: the eclipsing binary pair is separated by only 0.062 AU, while the third star (Algol C) is at an average distance of 2.69 AU from the pair and the mutual orbital period is 681 days (1.86 years). The total mass of the system is about 5.8 solar masses, and the mass ratios of A, B and C are about 4.5 : 1 : 2.

Orbital Elements of the Algol System

Components	Semimajor axis	Ellipticity	Period	Inclination
A—B[5]	0.00218″	0.00	2.87 days	97.69°
(AB)—C[6]	0.09461″	0.225	680.05 days	83.98°

Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can actually be seen.^[7]

This system also exhibits variable activity in the form of x-ray and radio flares. The former is thought to be caused by the magnetic fields of the AB components interacting with the mass transfer.^[8] The radio emissions may be created by magnetic cycles similar to sunspots, but, as the magnetic fields around these stars are up to ten times stronger than that of the Sun, these radio flares are more powerful and longer lasting.^[9]

Algol is 92.8 light years from Earth; however, about 7.3 million years ago it passed within 9.8 light years^[10] and its apparent magnitude was approximately −2.5, considerably brighter than Sirius is today. Because the total mass of the system is 5.8 solar masses, and despite the fairly large distance at closest approach, this may have been enough to slightly perturb the solar system's Oort cloud and increase the number of comets entering the inner solar system. However, the actual increase in net cratering rate is believed to have been quite small.^[11]

Astrology

Astrologically, Algol is considered the most unfortunate star in the sky. In the Middle Ages it was one of the 15 Behenian stars, associated with the diamond and hellebore, and marked with the kabbalistic sign:

Algol in fiction

• The Hitchhiker's Guide to the Galaxy mentions Algol several times.

• The sci-fi comedy book Bikini Planet features aliens from Algol. They are a feminist society and all have lizard-like features.

• In the short story, Beyond the Wall of Sleep, by H.P.Lovecraft, a nova is mentioned as having taken place near Algol on February 22, 1901; this nova is, according to the story, the result of an epic space battle between two incorporeal beings, known in the story as the 'entity' and the 'oppressor.'

• In the Sega video game series Phantasy Star, the star system which the series takes place in is called the Algol Solar System.

References

External links

• HD 19356 -- Eclipsing binary of Algol type (English). SIMBAD. Retrieved on 2006-07-31.
• Beta Persei (Algol) (English). AAVSO (January, 1999). Retrieved on 2006-07-31.
• The Horror-Scope of Algol (English). Skyscript (July 2005). Retrieved on 2006-07-31.
• Algol 3 (English). SolStation. Retrieved on 2006-07-31.
• 4C02517 (English). ARICNS (1998-03-04). Retrieved on 2006-07-31.
• Algol (English). Alcyone ephemeris. Retrieved on 2006-06-08.
• Kaler, Jim. Algol (Beta Persei) (English). Stars. Retrieved on 2006-06-08.
• Bezza, Giuseppe [translated by Daria Dudziak]. Al-ghûl, the ogre. (English). Cielo e Terra. Retrieved on 2006-06-08.
• Artist conception of Algol system
• Czech space metal band "Realm of Algol"

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