Arcturus: The Brightest Star of the Northern Sky

Arcturus at a glance

How to find Arcturus: arc to Arcturus

The classic method uses the Big Dipper (the Plough, or Saptarishi). Follow the gentle curve of the Dipper’s handle away from the bowl and keep going in the same arc. The first very bright star you reach is Arcturus, glowing a warm orange. Continue the curve and you “speed on to Spica”, the blue-white star of Virgo. It is one of the most useful sky-hopping tricks in the Northern Hemisphere.

Arcturus is a star of spring and early summer evenings. In November it is close to the Sun: very low in the west-northwest just after sunset early in the month, then reappearing low in the east before dawn later in the month. From about March to July it is high in the evening sky. It lies north of the celestial equator but is visible from almost the whole world, as far south as about 70°S.

What kind of star is Arcturus?

Arcturus is a red giant (strictly an orange giant, spectral type K1.5): a star a little more massive than the Sun that has used up the hydrogen in its core and swollen to about 25 times the Sun’s diameter. It is cooler than the Sun, about 4,300 K, which gives it its orange colour, but its large surface means it gives out about 170 times as much light. Arcturus shows us roughly what our Sun will look like in about 5 billion years, when it too becomes a red giant.

Arcturus is also an old star, perhaps 7 billion years old, and it is a visitor. It moves across the sky unusually fast, about 2 arcseconds a year, and its path is tilted to the plane of the Milky Way. It belongs to a loose group of old stars, the “Arcturus stream”, that may be the remains of a small galaxy absorbed by the Milky Way long ago. In a few thousand years it will have moved noticeably on the sky.

The star that opened a World’s Fair

In 1933, the Century of Progress World’s Fair in Chicago was officially opened by light from Arcturus. Telescopes at several observatories focused the star’s light onto photoelectric cells, and the electric current switched on the fair’s floodlights. Arcturus was chosen because its light was then thought to have left the star around the time of the previous Chicago World’s Fair in 1893; the modern distance of 36.7 light-years makes that a little less neat.

Name and meaning

The name comes from the ancient Greek Arktouros, “guardian of the bear”, because it seems to follow the Great Bear (Ursa Major) around the pole. In India it is known as Swati, one of the nakshatras (lunar mansions).

Seeing Arcturus in daylight

Arcturus has a special place in the history of astronomy: in 1635 the French astronomer Jean-Baptiste Morin used a telescope to see it in the daytime, the first time a star other than the Sun was observed in daylight. With a telescope and an accurate position, Arcturus and Sirius can still be found in a blue sky today, but never point a telescope near the Sun.

Arcturus anchors the kite-shaped constellation Boötes, the Herdsman, and forms the Spring Triangle with Spica and Regulus. Its warm orange colour stands out clearly against blue-white Spica and Vega, making it one of the easiest stars for beginners to identify on spring and summer evenings, even from light-polluted cities.

Frequently asked questions about Arcturus

Sources: Arcturus overview and references. Star data from standard catalogues; positions and rise/set times are calculated by SpaceTracker.live with the Astronomy Engine library.