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Algol

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Revision as of 18:30, 31 July 2006 by RJHall (talk | contribs) (NA on metalicity)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff) For other uses, see Algol (disambiguation).
Beta Persei A/B/C

The position of Algol.
Observation data
Epoch J2000      Equinox J2000
Constellation Perseus
Right ascension 03 08 10.1
Declination +40° 57′ 20.3″
Apparent magnitude (V) 1.58
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
Distance93 ± 2 ly
(28.5 ± 0.7 pc)
Absolute magnitude (MV)-0.69
Details
Mass3.59/0.79/1.67 M☉
Radius2.3/3.0/0.9 R☉
Luminosity98/3.4/4.1 L☉
Temperature12,000/4,500/8,500 K
MetallicityNot available
Rotation65 km/s.
Age< 3 × 10 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 2.3 and 3.5 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 behavior. In the constellation Perseus, it represents the eye of the Gorgon Medusa.

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 File: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 radial velocity a 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.

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.

References

  1. Allen, Richard Hinckley (1963). Star Names: Their Lore and Meaning (revised edition). Dover. ISBN 0486210790.
  2. "John Goodricke, The Discovery of the Occultating Variable Stars". 2003-08-06. Retrieved 2006-07-31.
  3. Pickering, Edward C. (1881). "Dimensions of the Fixed Stars, with especial reference to Binaries and Variables of the Algol type". Astronomical register: 253–256.
  4. A. H. Batten (1989). "Two Centuries of Study of Algol Systems". Space Science Reviews. 50 (1/2): 1–8.
  5. L. A. Molnar, R. L. Mutel (1996). "Dynamical Evolution of the Algol Triple System". Bulletin of the American Astronomical Society. 28: 921.
  6. W.I. Hartkopf, B.D. Mason (2006-07-30). "Sixth Catalog of Orbits of Visual Binary Stars". U.S. Naval Observatory. Retrieved 2006-07-31.
  7. Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy. 4: 64–78.
  8. M.J. Sarna, S.K. Yerli, A.G. Muslimov (1998). "Magnetic activity and evolution of Algol-type stars - II". Monthly Notices of the Royal Astronomical Society. 297: 760–768. {{cite journal}}: Unknown parameter |Issue= ignored (|issue= suggested) (help)CS1 maint: multiple names: authors list (link)
  9. Blue, Charles E. (2002-06-03). "Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals". National Radio Astronomy Observatory. Retrieved 2006-07-31.
  10. J. García-Sánchez, R.A. Preston, D.L. Jones, P.R. Weissman (1999). "Stellar Encounters with the Oort Cloud Based on Hipparcos Data". The Astronomical Journal. 117: 1042–1055.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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