Boötes and Corona Borealis – Arcturus is the star with a circle around it

Arcturus is the brightest star in the constellation Boötes (the Herdsman) and the fourth brightest in the entire sky. It's visible in the northern and southern hemispheres, and has been prominent in the skylore of many cultures for thousands of years.

Through the ages
Arcturus was listed in a Babylonian star catalog around 1100 BCE. It was named Shudun (Yoke) and associated with the high god Enlil who was a patron of farmers. However the name Arcturus has its origins in Greek mythology where it meant bear guard, from to its closeness to Ursa Major (the Great Bear). We know that this name dates back to at least 700 BCE when the poet Hesiod included it in his Works and Days. There are various legends in classical Greek and Roman mythology that relate to Arcturus.

In the far north of Scandanavia, the indigenous Sami people's name for Arcturus is Favdna (the Hunter). It's part of the hefty constellation Sarva (the Elk) which is composed of the classical constellations of Cassiopeia, Perseus, Cepheus and Auriga. Certain stars in the constellation represent elements of the story of a hunt which is played out as the night progresses.

The various nations of the Australian continent had different skylore traditions, though bright Arcturus was likely to feature in some way in them. For example, the star could be a useful calendar. When it began to appear in the dawn sky, the Yolngu people of Arnhem Land in northern Australia knew it was time to harvest the spike-rush. These are plants with edible tubers and reeds that could be made into baskets and fish traps.

Arcturus was a navigation star for early Polynesian navigators. The Hawaiian islands lie at a latitude at which Arcturus appears directly overhead. When the sailors reached this latitude, they could use the star to keep them on the right latitude as they sailed west.

Finding Arcturus
Most people recognize the asterism of the Big Dipper (the Plough). (An asterism is a pattern of stars composed of stars from one or more constellations.) The Dipper is part of the constellation Ursa Major, the Great Bear. The saying “Follow the arc to Arcturus” is your guide. It tells you to follow the arc of the Dipper's handle, which leads to Arcturus. Arcturus is bright and looks a somewhat orange color.

Arcturus and the Sun
The Sun is 4.6 billion years old, but Arcturus is much older, around 7 billion years. It's run out of hydrogen fuel and is now a red giant that's fusing helium into carbon. Arcturus is somewhat more than massive than the Sun, but it's swollen to an amazing 25 times greater in diameter than the Sun. If Arcturus were where the Sun is, a sunset would be lengthy and spectacular. However, Earth would probably way too hot for us, so it wouldn't be worth the spectacle. [Image credit: Roscosmos]

Arcturus opened a World Fair
A World Fair had been held in Chicago, USA in 1893. When the event again came to Chicago in 1933, someone had a cosmic idea for a grand inauguration. Arcturus was 40 light years away, so the light that left the star during the previous fair in 1893 would be arriving in 1933. Telescopes focused the star's light onto photoelectric cells. That generated a current which switched on the floodlights in the exhibition grounds. What a show!

We now have a more precise distance measurement for Arcturus, and it's about 37 light years away. But that doesn't diminish a very clever and original idea.

Ghost of the summer Sun
An unexpected aspect of seeing Arcturus is sometimes referred to as the Ghost of the Summer Sun. Around Halloween (October 31), Arcturus rises at the same time and same place on the horizon as the Sun does during the summer. It also sets at the same time and place as the summer Sun. This means that at high northern latitudes, Arcturus shines all through the night – as the summer Sun does.

In the southern hemisphere however, Arcturus isn't visible at the end of October. It's spring there, and Arcturus is rising and setting as the winter Sun does. That means that it rises after the October Sun and sets before it.

Arcturus may be a galactic alien
Arcturus is zooming through our stellar neighborhood at high speed. It's one of a group of over fifty stars that share a similar motion, known as the Arcturus stream or Arcturus moving group. Although the Sun orbits the Galaxy in the galactic plane, Arcturus has an elliptical orbit that crosses the plane.

It could be that Arcturus and the other stars of the group were born in a star cluster in the spherical halo of stars that surrounds the Galaxy. Other research suggests that Arcturus and its group come from another galaxy, a dwarf galaxy pulled into the Milky Way 5-9 billion years ago, its stars now mostly dispersed.