Arcturus is the brilliant orange star that can be found by following the curve of the Big Dipper’s handle across the night sky. Its name, meaning “Guardian of the Bear,” reflects its position as the brightest star in Boötes the Herdsman, appearing close to the nearby constellations of Ursa Major and Ursa Minor.
Here are some interesting facts about Arcturus.
Easy to Find Using the Big Dipper
Arcturus is one of the easiest bright stars to find because the Big Dipper points almost directly toward it. Follow the curve made by the three stars in the Dipper’s handle away from the bowl and continue that imaginary arc across the sky until you reach a brilliant orange star. This simple star-hopping method is commonly remembered by the phrase “arc to Arcturus.”
From mid-northern latitudes, Arcturus is especially prominent in the evening sky during spring and early summer, and no telescope or binoculars are needed to see it. Its brightness and distinctive orange color make Arcturus particularly easy to recognize once you know where to look.
Part of the Spring Triangle
Arcturus is one of the three bright stars that form the Spring Triangle, a large seasonal pattern that dominates the evening sky during Northern Hemisphere spring. The other two stars are Spica in Virgo and Regulus in Leo, with orange Arcturus forming an especially distinctive corner of the triangle. The pattern is much larger than many first-time stargazers expect, stretching across a huge area of the spring sky rather than forming a small, compact triangle. Once you realize how widely separated the three stars are, however, the pattern becomes much easier to recognize. The three stars don’t have any physical connection and lie at very different distances from Earth — they simply happen to form a striking triangle from our viewpoint.
Fourth-Brightest Star in the Night Sky
Arcturus is the fourth-brightest star in the night sky, surpassed only by Sirius, Canopus and Alpha Centauri. Yet unlike many of the sky’s most brilliant stars, Arcturus is not an enormously massive or exceptionally luminous supergiant. Instead, much of its apparent brightness comes from being relatively close to us at just 37 light-years away — practically in our stellar neighborhood.
About the Same Mass as the Sun — But 25 Times Wider
Arcturus is a giant star, but surprisingly it contains only about the same mass as the Sun. The enormous difference is its size: Arcturus has expanded to around 25 times the Sun’s radius, making it roughly 50 times wider from side to side. If the Sun were the size of a basketball, Arcturus would be a glowing orange sphere about 6 meters (20 feet) across — despite containing roughly the same amount of stellar material.
An Orange Giant Star
Arcturus is an orange giant star, with a surface temperature of around 4,300 K (7,300°F) compared with about 5,800 K (10,000°F) for the Sun. This cooler surface gives Arcturus its distinctive orange color, which is easily noticeable with the naked eye. Despite being cooler than the Sun, its vastly larger surface area means Arcturus produces around 170 times as much energy as the Sun.
Around 7 Billion Years Old
Arcturus is around 7 billion years old, making it roughly 2.5 billion years older than the Sun. During that enormous span of time it has exhausted the hydrogen fuel that once powered its core and left the main sequence, causing its outer layers to expand as it evolved into the orange giant we see today. Because Arcturus has roughly the same mass as the Sun, it also gives us an intriguing glimpse of the kind of changes that await our own star billions of years from now. Arcturus will eventually shed its outer layers into space, leaving behind its hot, dense core as a white dwarf — the same ultimate fate expected for the Sun.
Moving Rapidly Across the Sky
Arcturus is also unusual because it is moving across the background of the sky much faster than most bright stars. Its large proper motion was famously demonstrated by astronomer Edmond Halley in 1718, when he compared its position with observations recorded by ancient astronomers and realized that the supposedly “fixed” stars could actually move. Arcturus shifts by roughly 2.3 arcseconds each year — over 1,000 years, that’s enough to carry it across the sky by more than the apparent width of the full Moon.