Arcturus
What is Arcturus?
Stars· Distance 36.7 ly · size 25× Sun. The brightest star in the northern half of the sky, an orange giant. On spring evenings, follow the curve of the Big Dipper's handle to find it.
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Distance and spectral type
Arcturus is about 37 light-years away, a orange bright giant (spectral type K2IIIp), in the constellation Cancer. Its apparent magnitude is 0, visible to the naked eye.
Stellar figures
| Distance | 36.7 ly |
|---|---|
| Magnitude | −0.05 — brightest in the northern sky |
| Type | K1.5 red giant |
| Constellation | Boötes |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Greek "guardian of the bear". Its light, focused on photocells, switched on the 1933 Chicago World's Fair.
The 1933 fair lights switched on by Arcturus
On 27 May 1933 the opening lights of the Chicago World's Fair were switched on by the light of Arcturus. Telescopes at several observatories gathered the star's light and focused it onto photocells, devices that turn light into electricity. The faint current they produced was amplified and used to throw the switch at the fairground.
Arcturus was chosen for a reason. At the time its distance was thought to be about 40 light-years. That meant the light arriving that night had left the star forty years earlier, around the time of Chicago's previous world's fair in 1893. The idea was that light setting out during one fair would open the next.
Using today's distance of 36.7 light-years, that light actually left around 1896. The numbers were slightly off, but the event still showed a huge audience that starlight travels for years across space before it reaches us. Photocells work on the same principle as today's solar cells, and the event was also a showcase for that technology.
The star in depth
Arcturus is the brightest star in the constellation Boötes. At magnitude −0.05 it is the brightest star in the northern half of the sky. It lies 36.7 light-years away and is an orange red giant. It has about the same mass as the Sun, but as it aged it swelled until its radius reached about 25 times the Sun's.
A star shines by burning hydrogen at its centre, and when that hydrogen runs out its outer layers puff up into a giant. Arcturus has already reached that stage. Because it is so large, it gives off about 170 times as much light as the Sun in total, yet its surface is only about 4,300 K, cooler than the Sun's, which is why it looks orange. The Sun is expected to look much like this in about five billion years.
The name comes from Greek for guardian of the bear, because it follows the Great Bear, Ursa Major, around the sky. On spring evenings, follow the curve of the Big Dipper's handle onward and you reach Arcturus; keep going and you reach Spica in Virgo. This curve is known as the Spring Arc.
Arcturus moves at 122 km/s relative to the Sun. It is an old star travelling in a different direction from most of its neighbours, together with a group of similar stars called the Arcturus stream. It is almost at its closest to the Sun now and will be nearest in about 4,000 years. In the Sky view, look high in the east on a May evening for the obvious orange star.
Frequently asked
- How far is Arcturus from Earth?
- The distance of Arcturus is 36.7 ly.
- How bright is Arcturus (apparent magnitude)?
- The magnitude of Arcturus is −0.05 — brightest in the northern sky.
- What kind of object is Arcturus?
- Arcturus is a K1.5 red giant.
- Which constellation is Arcturus in?
- Arcturus lies in the constellation Cancer.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.