Arcturus: Facts About the 'Bear Guard' Star

Arcturus is a red giant star in the Northern Hemisphere of Earth's sky that is the brightest star in the constellation Boötes (the Herdsman). Arcturus is also among the brightest stars that can be seen from Earth. Astronomers say Arcturus will end up as a white dwarf at the end of its life.

Notably, the light of Arcturus was used to help open the Chicago World's Fair in 1933. In science fiction, the name Arcturus has been used several times.

The easiest way to spot Arcturus in the sky by following the curve of Ursa Major's (the Big Dipper) "handle" in the sky, thus revealing the origin of the famous phrase "follow the arc to Arcturus and then speed on to Spica." The latter part of that phrase refers to the bright star Spica (which is actually a binary). Arcturus' location is:

  • Right ascension: 14 hours 15 minutes 39.7 seconds
  • Declination: +19 degrees 10 minutes 57 seconds

Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
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Star Quiz: Test Your Stellar Smarts
Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
Open Star Cluster Messier 50
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With its ruddy orange-red glow, Arcturus is easily visible to the naked eye and thus made it quite visible to astronomers from many cultures. The Inuit in Canada's north, for example, called the star Uttuqalualuk, or "the old man," and the Lakota Sioux referred to it as Itkob u, meaning "going towards."

The name "Arcturus," however, comes from the ancient Greek Arktouros, which means something like Bear Guard or The Watcher of the Bear. The name refers to Arcturus' proximity to Ursa Major and Ursa Minor ("The Great Bear" and "The Lesser Bear.") Arcturus' name appears in Greek literature at least as far back as the time of Hesiod, who wrote about the star in his book "Works and Days."

Other Arcturus references in ancient times:

  • In Roman culture, Arcturus was the narrator in the prologue of Rudens, a comedy by Plautus talking about the adventures of a girl kidnapped by pirates and seeking to return to her father.
  • Arab astronomers called it Al Simāk al Rāmiḥ, sometimes translated as "Leg of the Lance-bearer," according to Star Names: Their Lore and Meaning by Richard Hinckley Allen.
  • In Chinese, Arcturus translates in English characters as something close to "Ta-Kio," or the Great Horn.

Organizers for the Chicago World's Fair in 1933 were searching for a memorable way to open their exhibit. Since Arcturus was believed to be about 40 light-years away from Earth at the time, the organizers hit upon the idea of using the star's light as a part of the exhibition.

They advertised the stunt as a way of commemorating the last world's fair in the city, which took place in 1893 — around the time the light of Arcturus would have started the journey to Earth. At 9:15 p.m. Central Time on May 27, 1933, telescopes focused the star's light on several photoelectric cells. The current was used to flip a switch that turned on the floodlights at the expedition grounds.

As it turned out, later measurements refined the distance to Arcturus as 37 light-years away from Earth. So the light that was used at the Chicago's World Fair actually began its journey to Earth in 1896.

Today, astronomers know Arcturus packs a lot of punch despite it being only about 1.5 times the mass of the sun. To the naked eye, according to Jim Kaler, a professor emeritus at the University of Illinois. Arcturus appears to shine about 113 times more brightly than the sun.

Arcturus, however, has a lower temperature than the sun, which means that a lot of its energy is radiated as heat. Once this is accounted for, Arcturus actually releases 215 times more than the sun's radiation. The star has an apparent magnitude of -0.04 and an absolute magnitude of 0.2. [The Brightest Stars in the Sky: A Starry Countdown]

The star is in the latter stages of its life. Considered a red giant, it has stopped fusing hydrogen in its core as the sun does, and astronomers believe it is now starting to fuse heavier elements such as carbon.

"Such stars are not expected to have magnetic activity like the sun, but very weak X-ray emission suggests that Arcturus indeed is magnetically active and has a hard-to-observe 'buried corona'," wrote Kaler, referring to the shining shell of a star that is most easily visible when eclipsing the light.

Once Arcturus exhausts its helium supply, its outer layers will likely bleed off and a white dwarf remnant will be left behind.

No planets have been found to date surrounding this star, although that subject has been explored in science fiction. One early example is "A Voyage to Arcturus," a book by David Lindsay that was published in 1920. The protagonist travels to Tormance, a fictional planet of Arcturus.

Arcturus — either the star or a planet with the same name — has also appeared in many other science fiction series, including Isaac Asimov's "Foundation" series, "Doctor Who," "Star Trek" and the movie "Aliens." In "The Hitchhiker's Guide to the Galaxy" series, a famous phrase goes: "However fast the body travels, the soul travels at the speed of an Arcturan megacamel."