Arcturus (Alpha Boötis)
Arcturus (Alpha Boötis) is a star in the constellation Boötes, magnitude -0.1.
The brightest star of the northern sky — an ancient orange giant just passing through our neighbourhood.
Arcturus is a star in old age. Around seven billion years old — older than the Sun — it has run out of hydrogen in its core and puffed up to about 25 times the Sun’s width. With roughly the Sun’s mass, it is a glimpse of the red giant our own star will one day become. Its cool surface glows orange, and it pours out about 170 times the Sun’s light, much of it as invisible infrared.
It is also just passing through. Arcturus has only about a third of the Sun’s share of heavy elements, a mark of an older generation of stars, and it races past us at about 122 km/s with a loose band of old stars called the Arcturus stream. It is almost at its closest to us now.
Its name is Greek for ‘Guardian of the Bear’: it follows the Great Bear around the pole. Ptolemy described its colour as slightly red, and Chaucer named it Alramih.
Arcturus (Alpha Boötis) is 37 light-years away: the light we see from it set off in 1989, around the time the Hubble Space Telescope was launched.
Did you know?
- Seen in broad daylight. In 1635 Jean-Baptiste Morin turned a telescope on Arcturus during the day and saw it — the first recorded daylight sighting of any star other than the Sun and supernovae.
- The star that would not stay put. Comparing his positions with Ptolemy’s, Edmond Halley found that Arcturus, Sirius and Aldebaran had shifted since ancient times — the discovery that the ‘fixed’ stars move.
- Fast across the sky. Arcturus crosses the sky faster than any other first-magnitude star except Alpha Centauri. In antiquity it sat closer to the middle of its constellation.
A glowing orange star, the brightest in the northern half of the sky. Follow the curve of the Plough’s handle — “arc to Arcturus” — then carry on to Spica. Compare it with white Spica to see its warm colour clearly.
When and where to see it
It is best placed in the evening sky around May, when it crosses the meridian at about 10 pm local time (around midnight in April). It can be seen from everywhere north of 71°S and never sets north of 71°N.
At its highest it stands 59° up from 50°N, 74° up from 35°N, 71° up from the equator, 36° up from 35°S.
Facts
- Type
- Star
- Constellation
- Boötes
- Designations
- Alpha Boötis, α Boo, 16 Boo, HR 5340, HD 124897, HIP 69673
- Magnitude
- -0.1
- Position
- RA 14h 15.7m, Dec +19° 11′ (J2000)
- Spectral type
- K1.5IIIFe-0.5
- Colour
- orange (B−V 1.23)
- Distance
- 37 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Arcturus (Alpha Boötis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.