The traditional name Hamal (also written Hemal, Hamul, Ras Hammel) derives from the
Arabicرأس الحملrās al-ħamal "head of the ram", in turn from the name for the constellation as a whole, Al Ħamal "the ram".[16] In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[18] included a table of the first two batches of names approved by the WGSN; which included Hamal for this star.[19]
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
evolvedgiant star that has exhausted the supply of hydrogen at its core and is now on the
red-giant branch.[22] 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.[23] It is estimated to have 65% more mass than the Sun,[8] while
interferometric measurements using the
Navy Precision Optical Interferometer show it to be 15 times larger in diameter.[9] 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.[10]
Hamal is radiating about 91[24] times the
Sun's luminosity from its
outer envelope at an
effective temperature of 4,411
K.[8] 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 60% that in the Sun.[24][a]
Planetary system
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),[10] 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.[25]
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.[27] This is why most
astrology columns in modern newspapers begin with Aries.[28] While the vernal equinox has moved to
Pisces since then due to
precession of the equinoxes,[27] Hamal has remained in mind as a bright star near what was apparently an important place when people first studied the night sky.[13] 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 other name of Hamal, Hamul, was used for the name of a
U.S. Navy ship,
USS Hamul.
Notes
^The abundance is estimated by taking [Fe/H] to the power of ten: 10[Fe/H] = 10-0.214 = 0.61
^
abcdJohnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99.
Bibcode:
1966CoLPL...4...99J.
^
abcHR 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.
^Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick (eds.), "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30, vol. 30, University of Toronto: International Astronomical Union, p. 57,
Bibcode:
1967IAUS...30...57E
^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.
ISBN978-1-931559-44-7.
^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
^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.
^
abKaler, James B. (2002), The Ever-changing Sky: A Guide to the Celestial Sphere, Cambridge, UK: Cambridge University Press, pp. 151, 152,
ISBN0-521-49918-6