Arcturus

Arcturus is Alpha Bootis, apparent magnitude -0.05, at 37 light years, spectral type K1.5 III, an orange giant, and the brightest star in the northern celestial hemisphere.

Two things distinguish it. The first is its proper motion, which is among the largest of any bright star: Arcturus is moving across the sky fast enough that its position has shifted noticeably since antiquity. Edmond Halley used exactly this, in 1718, comparing contemporary positions of Arcturus, Sirius and Aldebaran with those in Ptolemy's catalogue, to establish that the fixed stars are not fixed. That was the discovery of proper motion, and Arcturus was one of the three stars that yielded it.

The second follows from the first. Arcturus belongs to the Milky Way's halo rather than to the disc where the Sun and most bright stars lie. It is metal-poor and old, moving at high velocity relative to the local population, and is generally thought to be part of a stream of stars acquired from a dwarf galaxy absorbed by our own. It is a visitor, passing through the solar neighbourhood, and it will not stay.

The name is Greek, Arktouros, guardian of the bear, from its position behind Ursa Major, which the constellation figure of Bootes, the herdsman or ploughman, follows around the pole. It is one of the very few stars named in Homer and in the Book of Job, and its heliacal rising was a seasonal marker in Hesiod's Works and Days.

Arcturus is easy to find: follow the curve of the Plough's handle outward, and the first bright star on that arc is Arcturus. Continue the arc and it reaches Spica. The mnemonic, arc to Arcturus, spike to Spica, is the standard way of orienting oneself in the northern spring sky.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy

Ptolemy gives Arcturus the nature of Mars and Jupiter, and the resulting tradition is unusually consistent: riches, honour and lasting fame, but obtained through effort and self-determination rather than conferred, with a persistent note of self-inflicted trouble.

The two halves divide as they usually do. Jupiter supplies the prosperity and the honour. Mars supplies the danger, the friction and the tendency to create one's own obstacles, which Robson emphasises. What is distinctive is that the tradition consistently attaches work to Arcturus, in a way it does not to Regulus or Spica. Where Regulus confers and Spica blesses, Arcturus is earned. Whether that comes from the constellation figure, Bootes is a herdsman or ploughman, a labouring figure rather than a king or a maiden, is a reasonable guess and no more than a guess.

Arcturus is one of Agrippa's Behenian fifteen, paired with jasper and with plantain.

The star has one modern distinction worth recording, since it is a genuine curiosity rather than a claim. Light from Arcturus was used to open the 1933 Chicago World's Fair: the star's light was focused onto photoelectric cells at several observatories and the resulting current used to switch on the fair's lights. The choice was made on the then-current distance estimate of about forty light years, so that the light arriving in 1933 was taken to have left around the time of Chicago's previous exposition in 1893. The distance estimate has since been revised down to about thirty-seven light years, which spoils the arithmetic slightly.