Stars · a giant

Arcturus: the brightest star of the northern sky

Arcturus, a large orange-white star with a mottled surface, drawn in Exo-View

Arcturus is the fourth brightest star in the night sky and the brightest north of the celestial equator, 36.7 light-years away in the constellation Boötes. It is an orange giant: a star like the Sun some seven billion years on, whose outer layers have swollen to 25 times the Sun's width. The Sun will be one. Exo-View flies you to it.

Fly to Arcturus See the Milky Way

Distance: 36.7 light-years
Kind: Orange giant, spectral type K1.5
Size: 25 times the Sun's width
Surface temperature: About 4,300 K, against the Sun's 5,800 K
Light: About 170 times the Sun's
Spin: Once in about two years
Seen in: The constellation Boötes, the Herdsman

Finding it

Follow the curve of the handle of the Plough, or Big Dipper, away from the pan: 'arc to Arcturus', the old saying goes, then 'speed on to Spica', the next bright star. Arcturus is a warm orange, clearly different from the white of most bright stars, and it is high in the sky on spring evenings in the north.

A star at the end of its fuel

Arcturus has used up the hydrogen at its core. Its centre has shrunk and heated while its outer layers have swollen and cooled, which is why it is both huge and orange. It is about as massive as the Sun, but it gives out 170 times as much light. In about five billion years the Sun will go the same way, swelling to something like Arcturus's size, and swallowing Mercury and Venus.

A fast star from the Galaxy's older population

Arcturus is moving fast, at about 120 km/s relative to the Sun, in a different direction from most nearby stars: it belongs to an older population of stars in the Milky Way's thick disc, with fewer heavy elements than the Sun. When the Century of Progress world's fair opened in Chicago in 1933, the light of Arcturus, then thought to have left the star at about the time of the previous Chicago fair, in 1893, was used to switch on the lights.

In Exo-View

The button below flies from the Sun to Arcturus. Its surface is drawn as a mottled pattern of huge convection cells, an illustration of what the physics says, and the plate says so.