Hamal

Template:Short description Template:About Template:Starbox begin Template:Starbox image Template:Starbox observe Template:Starbox character Template:Starbox astrometry Template:Starbox detail Template:Starbox catalog Template:Starbox reference Template:Starbox end Hamal, pronounced Template:IPAc-en,<ref name=Kunitzsch/> is a star in the northern zodiacal constellation of Aries.<ref name=bsc2/><ref name=kaler/> It has the Bayer designation Alpha Arietis, which is Latinized from α Arietis and abbreviated Alpha Ari or α Ari. This star is visible to the naked eye with an apparent visual magnitude of 2.0.<ref name=clpl4_99/> Hamal is the brightest star in the constellation and, on average, the 50th-brightest star in the night sky. Based upon parallax measurements made with the Hipparcos astrometry satellite,<ref name=aaa323_L49/><ref name=GSM/> Hamal is about Template:Convert from Earth.<ref name=aaa474_2_653/> It is drifting closer to the Sun with a radial velocity of −14 km/s.<ref name=rgcrv/>

This is an aging giant star that may host an orbiting planet with a mass greater than Jupiter.<ref name=aaa529_A134/>

Nomenclature

File:AriesCC.jpg

Hamal is the brightest star in the constellation of Aries.

Alpha Arietis is the star's Bayer designation. It also bears the Flamsteed designation of 13 Arietis.

The traditional name Hamal (also written Hemal, Hamul, Ras Hammel) derives from the Arabic {{#invoke:Lang|lang}} rās al-ħamal "head of the ram", in turn from the name for the constellation as a whole, Al Ħamal "the ram".<ref name=allen1899/> In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name="WGSN"/> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name="WGSN1"/> included a table of the first two batches of names approved by the WGSN; which included Hamal for this star.<ref name="IAU-CSN"/>

In Chinese, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Bond (asterism), refers to an asterism consisting of Hamal, β Arietis and γ Arietis.<ref>Template:In lang 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, Template:ISBN.</ref> Consequently, the Chinese name for Hamal itself is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref>Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed on line November 23, 2010.</ref>

Properties

The spectrum of this star matches a stellar classification of Template:Nowrap, 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.<ref name=mnras340_3_763/> 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.<ref name=baas25_1319/> It is estimated to have 65% more mass than the Sun,<ref name="gray2019"/> while interferometric measurements using the Navy Precision Optical Interferometer show it to be 15 times larger in diameter.<ref name="npoi"/> 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.<ref name=aaa529_A134/>

Hamal is radiating about 91<ref name=aaa526_A100/> times the Sun's luminosity from its outer envelope at an effective temperature of Template:Val.<ref name="gray2019"/> 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.<ref name=nsv1/> The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is only around 60% that in the Sun.<ref name=aaa526_A100/>Template:Efn The star displays low amplitude pulsations with periods of 0.571 and 0.190 days.<ref name=Kim_et_al_2006/>

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),<ref name=aaa529_A134/> 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.<ref name=orbit_note/>

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.<ref name=ecs/> This is why most astrology columns in modern newspapers begin with Aries.<ref name=raymo1982/> While the vernal equinox has moved to Pisces since then due to precession of the equinoxes,<ref name=ecs/> Hamal has remained in mind as a bright star near what was apparently an important place when people first studied the night sky.<ref name=kaler/> 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, Template:USS.

In 1953, fictional planet orbiting Alpha Arietis, named Lithia, appeared in A Case of Conscience, a classic science fiction novel by James Blish. The planet serves as the arena of the major part of the story, being a homeworld to a fictional sentient race, being studied by humans. Real-life equivalent of Lithia was unknown at the time of the novel's creation.

External links

• Template:Cite encyclopedia
• 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.