Star Facts: Arcturus

Arcturus by Mike Hankey []

Arcturus is an orange giant star located 37 light years away in the Boötes constellation, with its apparent magnitude of -0.05 making it the 4th brightest star in the night sky. Its name, meaning “Guardian of the Bear”, was a reference to the fact that since Arcturus was the most luminous star in Boötes the Herdsman , it was only reasonable to the ancient Greeks to see it as the “guardian” of both Ursa Major and Ursa Minor, both of which are nearby constellations.

Quick Facts

• Coordinates: RA 14h 15m 39.7s| Dec +19° 10′ 57″
• Star Type: Orange Giant – K Class (K1.5IIIFe-0.5)
• Constellation: Boötes
• Distance to Earth: 37 light years
• Apparent Magnitude: -0.05
• Luminosity: 170 times solar luminosity
• Surface Temp: 4,000C (7,200F)
• Mass: 1.10 solar masses
• Radius: 11 million miles (25.7 solar radii)
• Rotational Velocity: 2.4 km per second
• Age: 10 billion years old
• Designations: Arcturus, Alpha Boötis, Alramech, Abramech


Look for Arcturus by following an imaginary arc line from the Big Dipper’s handle until you see the huge bright, orange star in Bootes. Located just 19 degrees north of the celestial equator, it is therefore visible from both the northern and southern hemispheres most of the year round at some part of the night. In the northern hemisphere, Arcturus is associated with the arrival of spring , and in the southern hemisphere heralds the start of autumn.

Arcturus in Bootes


Arcturus has a visual magnitude of -0.05, making it is the brightest star north of the celestial equator, and the 4th most luminous star in the entire sky. Although Alpha Centauri is brighter than Arcturus, it is only as a result of the combined magnitudes of the Alpha Centauri system, whose individual components are all dimmer to the unaided eye than Arcturus. This fact makes Arcturus the third most luminous solitary star in the entire sky, with Alpha Centauri A following close behind with a visual magnitude of -0.01.


Arcturus is thought to have formed outside of the plane of the Milky Way, in the outer reaches of the galaxy’s thick halo. Some investigators have also posited that Arcturus may actually have been formed in a satellite galaxy that has since been devoured by the Milky Way, and that the 52 stars that share its motion and trajectory through the Milky Way, are in fact the remnants of that galaxy.

Physical Properties

Arcturus is at least 110 times as bright as the Sun, although, much of its light is radiated in infrared wavelengths. If the total energy production of Arcturus is taken into account, the star is about 180 times as energetic as the Sun, with the apparent discrepancy explained by its cooler surface temperature of 4,286 K making it less efficient at producing energy than our Sun (5,778 K) at any given volume.

Star Facts: Arcturus

Arcturus is believed to have only about 1.8 or so solar masses, despite being about 26 times as big as the Sun, and has an estimated age range of between 6 billion and 8.5 billion years. This makes the star old enough to have begun converting helium into oxygen and carbon, and if this is indeed the case, it will continue to expand until it has converted all of its helium supply. It will subsequently blow off its outer layers, and evolve into a white dwarf surrounded by a planetary nebula.

Proper Motion

One notable feature of Arcturus is its high proper motion, which at a break-neck speed of 122 kilometres per second (2 seconds of arc per year), is the fastest of any first-magnitude star except Alpha Centauri. In the case of Arcturus, however, it is not moving in the broad plane of the Milky Way, but almost perpendicularly to the galaxy’s plane, much like a stone dropped into a pool of water. Arcturus is, however, not alone in its headlong plunge through the Milky Way – it is accompanied by 52 other stars, which are collectively known as the Arcturus Stream.

Arcturus in History

The ancient Greeks saw Arcturus in Bootes as the guardian of both Ursa Major and Ursa Minor, while in ancient Rome it was seen as the harbinger of stormy, if not tempestuous weather. Arcturus is one of only a few stars mentioned in the Bible, and by the Middle Ages it was assigned magical properties and lumped together with fourteen other stars which were collectively known as “Behenian fixed stars”, considered especially useful in medieval astrology.

In Australia, the Wotjobaluk Koori aborigines called Arcturus “Marpean-kurrk”, the mother of Djuit (Antares), and Weet-kurrk (Muphrid). The arrival of Arcturus in the north coincided with the arrival of wood-ant larvae, which formed a part of the diet of the Aborigines in south-eastern Australia. Arcturus also marked the beginning of summer when it set in the west with the Sun, which event also marked the disappearance of the wood-ant larvae.

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