Star of the Month
Hamal proves that being average can be interesting
Aries is often described as looking like a bent line. Although this does the much larger form of the celestial ram a great injustice, it does provide a convenient method of identifying Hamal (Alpha (_) Arietis), the star marking the eastern end of the ‘bent line’.
Hamal is a bright star shining away at mag. +2.0. It lies at the relatively close distance of 65.8 lightyears from Earth and is believed to play host to a large orbiting planet, 1.8x more massive than Jupiter and orbiting its primary every 381 days.
The name Hamal means ‘the lamb’ and it’s the brightest star in Aries. It’s a cool orange giant, 14.9x larger than our Sun and 1.5x as massive. Its bolometric luminosity – that is its luminosity across all wavelengths – is 91x larger than the Sun’s. Hamal has exhausted its core hydrogen and is now evolving along the path of becoming a red giant star.
This is one of the few stars that has had its diameter accurately measured. In this case, Hamal’s disc subtends an angle of 0.00680 arcseconds. This measurement also revealed the presence of limb darkening, a phenomenon that makes the limb regions of a star appear darker than those in the centre. This is a well-known effect that has been seen on our own Sun and results from looking into cooler layers of the star at the edges than when you’re looking straight at its centre.
In this section we often reveal exciting facts about the stars being discussed that give them a hidden persona and make them stand out against their neighbours. In the case of Hamal, the star is characterised by being utterly normal. Although this may sound a little dull, it’s actually an important attribute because it provides a useful baseline against which you can compare other stars.
The only unusual thing about Hamal is that its metallicity is around half that of our Sun’s. A star’s metallicity describes the amount of elements it contains other than hydrogen or helium.