Alpha Arietis

α Arietis

α Arietis shown in its constellation, Aries.
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Aries
Right ascension	02h 07m 10.4071s[1]
Declination	+23° 27′ 44.723″[1]
Apparent magnitude (V)	1.98 to 2.04[2]
Characteristics
Spectral type	K2IIICa-1[3]
U−B color index	+1.12[3]
B−V color index	+1.15[3]
V−R color index	0.7[1]
R−I color index	+0.62[3]
Variable type	Suspected[2]
Astrometry
Radial velocity (Rv)	−14.2 ± 0.9[1] km/s
Proper motion (μ)	RA: 190.73[1] mas/yr Dec.: −145.77[1] mas/yr
Parallax (π)	49.48 ± 0.99[1] mas
Distance	66 ± 1 ly (20.2 ± 0.4 pc)
Absolute magnitude (MV)	0.48[note 1]
Details
Mass	2[4] M☉
Radius	15[4] R☉
Luminosity (bolometric)	90[4] L☉
Temperature	4,590[4] K
Rotational velocity (v sin i)	< 17[3] 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.[1][3]
Database references
SIMBAD	data

Alpha Arietis (Alpha Ari, α Ari, α Arietis), which also has the traditional name Hamal, is the brightest star in the constellation Aries.^[3][4] Its Flamsteed designation is 13 Arietis.

α Arietis has spectral type K2 IIICa-1, which means that it is a K-type giant star, slightly cooler than the Sun and much larger. The Ca portion of the spectral type indicates that it shows calcium lines in its spectrum. It is estimated to be approximately twice as massive as the Sun and 15 times larger in diameter, and has around 90 times the Sun's bolometric luminosity.^[4] The Hipparcos satellite indicates that α Arietis is about 66 light-years from Earth. Combined with its intrinsic brightness, this relatively small distance makes it shine at an apparent visual magnitude of about 2.00; it is the 48th brightest star in the night sky.^[1] It has been reported to be slightly variable, by about 0.06 magnitude.^[2]

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.^[5] This is why most astrology columns in modern newspapers begin with Aries.^[6] While the vernal equinox has moved to Pisces since then due to precession of the equinoxes,^[5] Hamal has remained in mind as a bright star near what was apparently an important place when people first studied the night sky.^[4]

James Kaler^{[year needed]} notes that Hamal: "...has the honor of having (along with Shedar, Alpha Cassiopeiae) the most accurately-measured angular diameter, 0.00680 seconds of arc (a penny 60 kilometers away), from which we find another value for true diameter of 14.7 solar, beautifully in accord with that found from temperature and luminosity. This precise measurement allowed the detection of Hamal's "limb darkening." As a gaseous sphere, the Sun is slightly darker at the edge (its "limb") than at the center, the result of our not looking as deeply into the solar gases. Similar limb darkening can be detected in the members of eclipsing stars as they get in front of each other. Hamal is one of the select few single stars for which limb darkening has been seen, showing that we are not far from observing features on the surfaces of at least some of our stellar neighbors."

Names

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".^[7]

In Chinese, 婁宿 (Lóu Su), meaning Bond (asterism), refers to an asterism consisting of α Arietis, β Arietis and γ Arietis.^[8] Consequently, α Arietis itself is known as 婁宿三 (Lóu Su sān, English: the Third Star of Bond.)^[9]

Planetary system

The Alpha Arietis system^[10]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity
b	1.8 ± 0.2 MJ	1.2	380.8 ± 0.3	0.25 ± 0.03

Hamal as the name

The other name of Hamal, Hamul, is used for the name of United States navy ship, USS Hamul (AD-20).

Notes

1. ^ From apparent magnitude and parallax.

References

1. ^ ^{a b c d e f g h i} NAME HAMAL -- Variable Star, database entry, SIMBAD. Accessed on line December 16, 2008.
2. ^ ^{a b c} NSV 725, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line December 16, 2008.
3. ^ ^{a b c d e f g} 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.
4. ^ ^{a b c d e f g} Hamal, Stars, Jim Kaler. Accessed on line December 16, 2008.
5. ^ ^{a b} pp. 151, 152, The Ever-changing Sky: A Guide to the Celestial Sphere, James B. Kaler, Cambridge, UK: Cambridge University Press, 2002, ISBN 0-521-49918-6.
6. ^ p. 196, 365 Starry Nights: An Introduction to Astronomy for Every Night of the Year, Chet Raymo, New York: Simon and Schuster, 1982, ISBN 0-671-76606-6.
7. ^ pp. 78, 80, Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899.
8. ^ (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.
9. ^ (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.
10. ^ Jean Schneider (2011). "Notes for Planet alf Ari b". Extrasolar Planets Encyclopaedia. http://exoplanet.eu/planet.php?p1=alf+Ari&p2=b. Retrieved 12 October 2011. 

External links

• Jean Schneider (2011). "Notes for star alf Ari". Extrasolar Planets Encyclopaedia. http://exoplanet.eu/star.php?st=alf+Ari. Retrieved 12 October 2011. 
• GJ 84.3, entry in the Gliese-Jahreiss catalogue (Preliminary Version of the Third Catalogue of Nearby Stars, W. Gliese and H. Jahreiss, 1991, CDS ID V/70A.)
• Image of Hamal from Aladin.
• The Constellations and Named Stars, Purple Hell.
• Kaler's "Stars" website
• SIMBAD: HD 12929