Arcturus, a red giant, looks orange to the eye. It is the brightest star on the northern half of Earth’s sky dome. Arcturus is especially noteworthy for its large proper motion, or sideways motion across our sky. Only Alpha Centauri – our sun’s nearest neighbor among the stars – has a higher proper motion among the first-magnitude, or bright, stars in the stellar neighborhood. What can the proper motion of Arcturus be telling us?
It’s telling us that Arcturus is moving at a tremendous rate of speed (122 km/s) relative to our solar system. Moreover, Arcturus is not moving with the general stream of stars in the flat disk of our Milky Way galaxy. Instead, it is cutting perpendicularly through the galactic disk. Arcturus is thought to be an old star. It appears to be moving with a group of at least 52 other such stars, known as the Arcturus stream. Arcturus is likely to be considerably older than our sun. When the sun evolves to become a red giant, the sun might be a star much like Arcturus is now.
As it cuts through the galactic disk, Arcturus will eventually reach a closest point to our sun. The closest approach of Arcturus will happen in about 4,000 years, when the star will be a few hundredths of a light-year closer to Earth than it is today. But remember that Arcturus is not moving with the general stream of stars in the disk of our Milky Way galaxy. Thus, we will lose sight of this star relatively quickly. Millions of years from now, this star will be lost from the view of any future inhabitants of Earth, or at least those who are earthbound and looking with the eye alone.
The red giant Arcturus is roughly 25 times the diameter of our sun. It is not the largest of the red giant, however, as the diagram below shows. Because of its larger size, in visible light Arcturus radiates more than 100 times the light of our sun. If you consider infrared and other forms of radiant energy, Arcturus is about 200 times more powerful than the sun. Its mass is hard to exactly determine, but may be slightly greater than that of our sun (1.1-0.4+0.6 solar mass).
The reddish or orange color of Arcturus signifies its temperature, which is about 7,300 degrees Fahrenheit. That makes it several thousand degrees cooler than the surface of the sun.
How to See It
Arcturus is the alpha star of a cone-shaped constellation called Bootes the Herdsman. It is far enough north on the sky’s dome that – for northern hemisphere observers – it’s visible during some part of the night throughout most of the year.
You might notice this bright orange star passing high overhead on late spring evenings. In summer, Arcturus is high overhead shortly after dark. Autumn observers need to look early because then it sets by mid-evening. The best time to observe it in winter is in the wee hours before dawn.
Our chart shows this constellation as you stand facing south on spring evenings. Seeing a Herdsman in these stars might be difficult, but the constellation is easy to imagine as a kite.
There’s an easy mnemonic for remembering how to identify this bright, orange star. Just remember the phrase: follow the arc to Arcturus.
First, locate the Big Dipper in the northern sky. Notice that the handle of the Big Dipper is a curve or arc. Extend this curve past the end of the Big Dipper’s handle, and you will come to Arcturus.
Arcturus is the brightest star north of the celestial equator. What does that mean? Imagine that Earth’s equator can be projected onto the sky. This line above Earth’s equator is called the celestial equator. It divides the sky into northern and southern hemispheres, just as Earth’s equator does for Earth. The three brightest stars of the sky – Sirius, Canopus and Alpha Centauri – are south of the celestial equator. Meanwhile, Arcturus is the brightest star in the northern part of the sky. It is only marginally brighter than the northern hemisphere’s second-brightest star, Vega. To the eye, Arcturus and Vega reign as co-stars in the northern half of the sky.
History and Myth
Arcturus’ constellation Bootes – the Herdsman – is sometimes pictured as guarding the Great Bear, or Ursa Major, which contains the Big Dipper asterism. We sometimes hear Arcturus called the Bear Guard.
In China, the constellation for which Arcturus is the chief star was sometimes called the Dragon. In some classical Greek stories, Bootes was Icarus, who in another story flew too close to the sun. Passing directly over the Hawaiian Islands, Arcturus was a particularly important navigational star to the islands’ native inhabitants and other Polynesians.
The translation may be questioned, but Arcturus is among the few stars mentioned in the Bible. (“Which maketh Arcturus, Orion, and Pleiades, and the chambers of the south” – Job 9:9 and “Canst thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons?” – Job 38:32, KJV.)
One interesting story about Arcturus relates to the 1933 Century of Progress Exhibition in Chicago. Its promoters wanted a flashy way to open the show, so they decided to have the light from Arcturus pass through a telescope onto a photocell. The photocell in turn worked as the switch that turned on the main spotlights to open the exhibition. There had also been a World’s Fair in Chicago in 1893, 40 years before the 1933 show. At the time, astronomers thought that Arcturus was 40 light-years away. So they thought that light left Arcturus at the end of the 1893 fair and traveled for 40 years through space like an Olympic torch bearer, to open the 1933 show.
The promoters of Chicago’s World’s Fair had a great idea, but today’s astronomers place the distance to Arcturus at just less than 37 light-years.
The position of Arcturus is RA: 14h 15 m 39.7s, dec: +19° 10′ 56″.