Star of the Month
Alpha Boötis (Arcturus)
Position: 14 hrs 15.7 min 19 deg 11 min.
Due south at 23:41 (BST) on 15 May 2020
Arcturus
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
Like last month’s star V Hydrae and Betelgeuse in Orion, Arcturus is a red giant. But with a B-V of 1.2 rather than 5.4 it is much less red than V Hydrae. To the naked eye—like Betelgeuse—it looks orange rather than red. It is easy to find, as it is the fourth brightest star in the entire sky and the second brightest in our skies (after Sirius). To find it follow the arc formed by the handle of the Plough down until you see a very bright orange (or yellow) star. Above it you should see a dented kite of stars which is the constellation of Boötes, the herdsman. It is appropriate that Boötes dominates the Spring sky as it is the time of year that the herdsman would traditionally drive his animals to the higher summer pastures after the winter has passed. The animals would be in danger of being attacked by bears in the high pastures and Arcturus means the bear-watcher in Ancient Greek, as he is looking up warily at Ursa Major.
Arcturus is relatively close to the Sun being 36.7 light years distant and it is moving towards the Sun reaching its closest approach in 4000 years’ time. It is a part of a fast moving group of stars called the Arcturus stream which was previously thought to be a dissolved open cluster, but this hypothesis was disproved in November 2019 by the Gaia Data Release 2, because of the range of chemical abundances, and it is probably a phase-space wave formed by a merger event (see https://www.aanda.org/articles/aa/abs/2019/11/aa35234-19/aa35234-19.html). At around 7 billion years old, Arcturus—like Betelgeuse—is an elderly red giant near the end of its life when it will experience a helium flash supernova and become a white dwarf.