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Algol

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Beta Persei A/B/C

   The position of Algol.
                              Observation data
   Epoch J2000
   Constellation             Perseus
   Right ascension           03^h 08^m 10.1315^s
   Declination               +40° 57′ 20.332″
   Apparent magnitude (V)    2.12
                               Characteristics
   Spectral type             B8V/K02IV/A5V
   B-V colour index          -0.05
   U-B colour index          -0.37
   Variable type             Eclipsing binary
                                 Astrometry
   Radial velocity (R[v])    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 (M[V]) -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 × 10^8 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's magnitude changes regularly between 2.1
   and 3.4 over a period of 2 days, 20 hours and 49 minutes.

History

   The name Algol means "demon star," (from Arabic الغول al-ghūl, "the
   ghoul") which was probably given due to its peculiar behaviour. 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.

   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
   Image:Agrippa1531_caputAlgol.png .

   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.

   In 1881, the Harvard astronomer Edward Pickering presented evidence
   that Algol was actually an eclipsing binary. This was confirmed a few
   years later, in 1889, when the Potsdam astronomer Hermann Vogel found
   periodic doppler shifts in the spectrum of Algol, infering variations
   in the radial velocity of this binary system. 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.

   CAPTION: Orbital Elements of the Algol System

   Components Semimajor axis Ellipticity   Period    Inclination
      A—B        0.00218″       0.00      2.87 days    97.69°
     (AB)—C      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.

   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. 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.

   Algol is 92.8 light years from Earth; however, about 7.3 million years
   ago it passed within 9.8 light years 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.
   Retrieved from " http://en.wikipedia.org/wiki/Algol"
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