Star Facts: Hamal

Hamal (Alpha Arietis)

Hamal (Alpha Arietis), the most luminous star in the zodiacal constellation Aries, is a red to orange giant located 65.8 light years from Earth that shines with an apparent visual magnitude of +2.00. This also makes it the 50th brightest star in the entire night sky overall. In 2011, a planet was found orbiting Hamal that is around 1.8 times bigger than Jupiter.

Quick Facts

• Constellation: Aries
• Coordinates: RA 02h 07m 10.40570s|Dec. +23° 27′ 44.7032″
• Distance: 65.8 light years
• Star Type: K2 III Ca-1
• Mass: 1.5 sol
• Radius: 14.9 sol
• Apparent Magnitude: +2.00
• Luminosity: 91 sol
• Surface Temperature: 4,480K
• Rotational Velocity: 3.44 km/sec
• Age: 3.4 ± 1.9 billion (109) years
• Other Designations: Hemal, Hamul, Ras Hammel, El Nath, Arietis, a Ari, Alpha Arietis, Alpha Ari, 13 Arietis, 13 Ari, BD+22 306, FK5 74, GC 2538

Visibility

Aries is visible from latitudes of between +90° and -60°, which means that despite being a northern sky constellation it can also be seen from much of the southern hemisphere. Aries lies between the Pleiades in Taurus to its east, and Pisces and the “Great Square” of Pegasus to its west, although it is never a particularly bright or prominent constellation, and a dark sky without moon light is almost a prerequisite to seeing it at all. The constellation is best seen from northern locations during the winter and spring time, and culminates at about 8 PM Local Time over the eastern horizon in late November, at which time the stars Hamal, Sheratan and Mesartim, the three stars that mark out the “bust” of the Ram, become easy to spot.

Physical Properties

Size, Brightness and Velocity

The stellar classification of Hamal is K2 III Ca-1, with the “Ca-1” part referring to the fact that it displays weaker than normal calcium lines in its spectrum. This is perhaps not surprising since Hamal has only about half of the Sun’s metalicity, which is a term used to indicate the amount of elements heavier than hydrogen and helium that is present in a star. Nonetheless, since 1943, Hamal’s spectrum has served as a stable measure against which the spectra of other stars are classified.

The star has reached a stage in its life-cycle in which it has exhausted the supply of hydrogen fuel at its core, and has now evolved onto the red giant branch of the H-R diagram. Despite its bulk, which is about 15 times that of the Sun, the star is still spinning at a rate of 3.44 km/sec at its equator, compared to say the much smaller Sun which is spinning at about 2 km/sec.

Due to its extended bulk, Hamal is about 91 times as luminous as the Sun, although its effective temperature is only 4,480K, which imparts the characteristic orange-red glow of a K-type star. Hamal is also suspected to be a variable, with observed variations in luminosity of only 0.06 magnitudes.

Planet

In 2011, Byeong-Cheol Lee et al announced the possible presence of a planet around the Hamal. Using the radial velocity method, the team from the Bohyunsan Optical Astronomy Observatory in Korea, estimated the putative planet to have a mass of 1.8 times that of Jupiter. According to measurements made between 2003 an 2010, the proposed planet appears to orbit Hamal once every 380 days, with an eccentricity of about 0.25, which translates into a periapsis (closest) distance of 0.9 AU, and an apoapsis (furthest) distance of 1.5 AU from the star. By way of illustrating how hot the planet would be, Hamal has an effective radius of 0.07 AU.

History

The traditional name of the star, Hamal, derives from the Arabic phrase “ras al-hamal” (“head of the ram”), which in turn is taken from Al Hamal (“the ram”), referring to the original Arabic name for the constellation as a whole. It is also worth noting that 4,000 years ago, the apparent path of the Sun through the sky, as seen from Earth, placed Hamal in Aries at the point of the northern vernal equinox, which marks the advent of spring in the northern hemisphere. However, despite the fact precession has since moved the point of the equinox into Pisces, Hamal has remained in the human consciousness as the bright star that once marked an important point in the sky. Currently, Hamal approximately matches the latitude of the Tropic of Cancer, which is a circle marking 23.5 degrees north where the Sun being directly overhead at midday on June 21 heralds the start of summer in the northern hemisphere.

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