Hamal

Hamal
Diagram showing star positions and boundaries of the Aries constellation and its surroundings


Location of Hamal (α Arietis)

Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aries
Right ascension 02h 07m 10.40570s[1]
Declination +23° 27′ 44.7032″[1]
Apparent magnitude (V) 2.00[2]
Characteristics
Spectral type K1IIIb
U−B color index +1.13[2]
B−V color index +1.15[2]
V−R color index +0.7[3]
R−I color index +0.62[4]
Variable type Suspected[5]
Astrometry
Radial velocity (Rv)−14.2 ± 0.9[6] km/s
Proper motion (μ) RA: +188.55[1] mas/yr
Dec.: −148.08[1] mas/yr
Parallax (π)49.56 ± 0.25[1] mas
Distance65.8 ± 0.3 ly
(20.2 ± 0.1 pc)
Absolute magnitude (MV)+0.47 ± 0.04[7]
Details
Mass1.5 ± 0.2[8] M☉
Radius14.9 ± 0.3[9] R☉
Luminosity (bolometric)91 ± 6[9] L☉
Surface gravity (log g)2.57[9] cgs
Temperature4,480[9] K
Metallicity [Fe/H]–0.25[9] dex
Rotational velocity (v sin i)3.44[8] km/s
Other designations
Hemal, Hamul, Hamal, Ras Hammel, El Nath, Arietis, α Ari, Alpha Arietis, Alpha Ari, 13 Arietis, 13 Ari, BD+22 306, FK5 74, GC 2538, GJ 84.3, GJ 9072, HD 12929, HIP 9884, HR 617, LTT 10711, NLTT 7032, PPM 91373, SAO 75151.[3][4]
Database references
SIMBADdata

Hamal (α Ari, α Arietis, Alpha Arietis) is the brightest star in the northern zodiacal constellation of Aries.[4][10] It has the Bayer designation α Arietis and the Flamsteed designation of 13 Arietis. With an apparent visual magnitude of 2.0,[2] it is among the brightest stars in the night sky. Based upon parallax measurements made with Hipparcos astrometry satellite,[11][12] Hamal is about 65.8 light-years (20.2 parsecs) from Earth.[1] It is a giant star that may host an orbiting planet with a mass greater than Jupiter.[8]

Properties

The spectrum of this star matches a stellar classification of K2 III Ca-1, with the luminosity class of III indicating that it is an evolved giant star that has exhausted the supply of hydrogen at its core and is now on the red giant branch.[13] The 'Ca-1' portion of the classification indicates that it shows weaker than normal lines of calcium in its spectrum. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[14] It is estimated to have about 50%[8] more mass than the Sun, while interferometric measurements show it to be 15[9] times larger in diameter. Despite its enlarged girth, this star is still spinning with a slightly faster equatorial azimuthal velocity than the Sun, having a projected rotational velocity of 3.44 km s−1.[8]

Hamal is radiating about 91[9] times the Sun's luminosity from its outer envelope at an effective temperature of 4,480 K.[9] This is cooler than the surface of the Sun, giving it the orange-hued glow of a K-type star. It is suspected to be slightly variable, with an amplitude of 0.06 magnitude.[5] The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is only around half that in the Sun.[9]

In 2011, the likely presence of a planet in orbit around this star was reported by Byeong-Cheol Lee et al. It was detected using the radial velocity method, based upon measurements made between 2003 and 2010 at the Bohyunsan Optical Astronomy Observatory in Korea. The object has an orbital period of 381 days and an eccentricity of 0.25. The lower bound on this object's mass is about 1.8 times the mass of Jupiter. The estimated semi-major axis of the planet's orbit is 1.2 astronomical units (AU),[8] which would give it a periapsis distance of 0.9 AU and an apoapsis distance of 1.5 AU. By comparison, the star has a radius of 0.07 AU.[15]

The Hamal planetary system[16]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b ≥ 1.8 ± 0.2 MJ 1.2 380.8 ± 0.3 0.25 ± 0.03 — —

In culture

Hamal's orientation with relation to the Earth's orbit around the Sun gives it a certain importance not apparent from its modest brightness. Between 2000 and 100 BCE, the apparent path of the Sun through the Earth's sky placed it in Aries at the northern vernal equinox, the point in time marking the start of spring in the Northern Hemisphere.[17] This is why most astrology columns in modern newspapers begin with Aries.[18] While the vernal equinox has moved to Pisces since then due to precession of the equinoxes,[17] Hamal has remained in mind as a bright star near what was apparently an important place when people first studied the night sky.[10] Currently (epoch J2000) its declination is almost exactly equal to the latitude of the Tropic of Cancer, meaning it can be used to find the position of that imaginary line when the Sun is not nearby.

The name Hamal (also written Hemal, Hamul, Ras Hammel) derives from the Arabic راس الحمل rās al-ħamal "head of the ram", in turn from the name for the constellation as a whole, Al Ħamal "the ram".[19] In Chinese, 婁宿 (Lóu Su), meaning Bond (asterism), refers to an asterism consisting of Hamal, β Arietis and γ Arietis.[20] Consequently, Hamal itself is known as 婁宿三 (Lóu Su sān, English: the Third Star of Bond.)[21] The other name of Hamal, Hamul, is used for the name of United States navy ship, USS Hamul (AD-20).

References

  1. 1 2 3 4 5 6 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  2. 1 2 3 4 Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99). Bibcode:1966CoLPL...4...99J.
  3. 1 2 NAME HAMAL -- Variable Star, database entry, SIMBAD. Accessed on line December 16, 2008.
  4. 1 2 3 HR 617, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line December 16, 2008.
  5. 1 2 NSV 725, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line December 16, 2008.
  6. ↑ Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick, eds., The Revision of the General Catalogue of Radial Velocities, University of Toronto: International Astronomical Union, Bibcode:1967IAUS...30...57E, retrieved 2009-09-10
  7. ↑ Carney, Bruce W.; et al. (March 2008), "Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars", The Astronomical Journal 135 (3): 892–906, arXiv:0711.4984, Bibcode:2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892
  8. 1 2 3 4 5 6 Lee, B.-C.; et al. (May 2011), "A likely exoplanet orbiting the oscillating K-giant α Arietis", Astronomy & Astrophysics 529: A134, arXiv:1104.4431, Bibcode:2011A&A...529A.134L, doi:10.1051/0004-6361/201016293
  9. 1 2 3 4 5 6 7 8 9 Piau, L.; et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442
  10. 1 2 Hamal, Stars, Jim Kaler. Accessed on line December 16, 2008.
  11. ↑ Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode:1997A&A...323L..49P
  12. ↑ Perryman, Michael (2010), "The Making of History's Greatest Star Map", Astronomers’ Universe (Heidelberg: Springer-Verlag), doi:10.1007/978-3-642-11602-5
  13. ↑ Stoesz, Jeffrey A.; Herwig, Falk (April 2003), "Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited", Monthly Notices of the Royal Astronomical Society 340 (3): 763–770, arXiv:astro-ph/0212128, Bibcode:2003MNRAS.340..763S, doi:10.1046/j.1365-8711.2003.06332.x
  14. ↑ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04
  15. ↑ For a = 1.2 AU and e = 0.25, the periapsis is given by a × (1 - e) = 9 AU and the apoapsis is a × (1 + e) = 15 AU. The solar radius is 0.0046491 AU, so the star's radius is 14.9 × 0.0046491 = 0.069 AU.
  16. ↑ Schneider, Jean (2011), "Notes for Planet alf Ari b", Extrasolar Planets Encyclopaedia, retrieved 2011-10-12
  17. 1 2 Kaler, James B. (2002), The Ever-changing Sky: A Guide to the Celestial Sphere, Cambridge, UK: Cambridge University Press, pp. 151, 152, ISBN 0-521-49918-6
  18. ↑ Raymo, Chet (1982), 365 Starry Nights: An Introduction to Astronomy for Every Night of the Year, New York: Simon and Schuster, p. 196, ISBN 0-671-76606-6
  19. ↑ Allen, Richard Hinckley (1899), Star-names and Their Meanings, New York: G. E. Stechert, pp. 78, 80
  20. ↑ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  21. ↑ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.

External links

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