Arcturus | Guardian of the Bear
Arcturus, also known as Alpha Boötis, is one of the brightest stars in the northern hemisphere. In fact, it is the third brightest individual star in the entire night sky behind Sirius and Canopus. At times, it is referred to as the fourth brightest star in the night sky, behind Alpha Centauri, when binary stars are included.
Arcturus is Greek meaning ‘guardian or keeper of the bear’. The Greeks saw this star as the guardian of Ursa Major and Ursa Minor, which are both nearby the constellation Boötes, where Arcturus is located.
The star also forms one corner of the Spring Triangle asterism, with Spica, and Regulus forming the other two vertices. The triangle connects the constellations of Boötes, Virgo, and Leo and is visible between March and May in the northern hemisphere.
Arcturus looks orange in appearance in the night sky. Photo by Trevor Jones.
- Constellation: Boötes, the Herdsman
- Type: Red Giant (K1.5III)
- Distance to Earth: ~37 light-years
- Radius: 25.4±0.2 R☉
- Apparent magnitude: -0.30
- Surface Temperature: 4286±30 K
- Coordinates: 14h 15m 39s RA, +19° 10′ 56” DE
Arcturus The Red Giant
Arcturus is an evolved and aging red giant star that is 25 times larger than our sun. Since the energy of this star is spread across such a large surface area, the surface temperature is generally cooler. This is signified by its orangish appearance.
At 37 light-years away, Arcturus is much closer to us than some of the other red giants, like Betelgeuse or Antares. It also means that its rapid movement through the sky (122 km/s), in comparison with our solar system, is noticeable at approximately 2.3″ per year.
Arcturus has an early K-type stellar classification and is the brightest K-type giant in the sky.
When can you see Arcturus?
Since it is far enough north, Arcturus is visible throughout most of the year. In the spring and summer, the orange star is high in the southwestern sky shortly after dark. By autumn, Arcturus is low in the western sky. By mid-evening and in winter, it can be found during the early hours before dawn.
There are no bright neighbors until the Summer Triangle appears in the eastern night sky. It’s this ‘isolation’ from nearby bright stars that make Arcturus so easy to identify in the sky.
How to Find Arcturus
Arcturus is visible from both the northern and southern hemispheres, located at 19°north of the celestial equator.
To locate the Arcturus, find the Big Dipper asterism in the northern hemisphere. Use the curve or arc of the Big Dipper handle to guide you and extend the curve past the end of the handle and you’ll find the brightly orange-colored star. The phrase ‘follow the arc to Arcturus can be used to help you find the star.
It can also be found by locating the constellation Boötes, which belongs to the Ursa Major family of constellations. This particular constellation forms a kite-like shape, near the Big Dipper and Corona Borealis. Arcturus can be found at the bottom of the kite.
I like to use Arcturus as an alignment star when setting up my equatorial telescope mount for astrophotography. Arcturus is the perfect star to use for this purpose, as its orange, bright color makes it stand out from dimmer stars nearby.
It is easy to spot Arcturus in the sky because it is noticeably warmer in color temperature than other stars, and it is very bright. If you are still having trouble identifying Arcturus in the night sky, use the Big Dipper as a guide.
Constellation Boötes and Arcturus | Sky & Telescope
Boötes is the 13th largest constellation in the night sky and is dominated by a diamond-shaped asterism formed by its brightest stars.
There are ten named stars within the constellation and three meteor showers: the January Bootids, the June Bootids, and the Quadrantids.
This particular constellation does not contain any Messier objects.
Boötes constellation | Sky and Telescope
Like many constellations and stars, there are many different myth variations or meanings depending on the culture. Arcturus is greek meaning: ‘guardian of the bear’, ‘bear follower’, ‘bear keeper’. The common theme is that the star is following or tending to the Big Dipper.
This is particularly true for those in UK, who see the Boötes constellation as the Herdsman and the Big Dipper section of Ursa Major as a plow.
Another common variation is the Greek myth where Boötes and Ursa Major represent Arcas (the son of Zeus) and Callisto (a follower of the goddess Artemis). Arcas eventually became king and Callisto was turned into a bear after being unfaithful.
As the country’s best hunter, Arcas came across Callisto, recognizing her only as a bear to be hunted. Before Arcas could shoot an arrow, Zeus turned the mother and son into constellations. One version has Arcas becoming Boötes, as guardian of the bear. In another, he becomes a bear before being placed in the sky as Ursa Minor. Callisto becomes Ursa Major in both versions.