By Alexander Merz · Updated on · Details checked on 14 September 2026
Arcturus is the brightest star in the northern celestial hemisphere: a red giant in the constellation Boötes, 37 light-years away and about 25 times the Sun’s diameter – the most striking sight of spring evenings.
At a glance: Brightness −0.05 mag · Distance 37 light-years · Constellation Boötes (the Herdsman) · Red giant, about 25 times the Sun’s diameter · Spring and summer sky
The Spring Star
Arcturus (Arktur) in Boötes is the brightest star in the northern celestial hemisphere and, after Sirius, Canopus and Alpha Centauri, number four in the entire sky. Its warm, orange light dominates the evenings of spring and early summer. Arcturus is a red giant: a glimpse into the future of our own Sun, which will swell up in a similar way a few billion years from now.
How to Find Arcturus
The classic star-hopping route: extend the curve of the Big Dipper‘s handle in an arc towards the south – the first very bright, orange star is Arcturus (“Arc to Arcturus”). Following the arc further leads you to Spica in Virgo.
A Fast Mover from Another Era
Arcturus moves unusually fast, crossing the plane of the Milky Way – it most likely belongs to an older stellar population of the galactic “thick disk”. At a good 7 billion years, it is considerably older than the Sun. In 4,000 years its proper motion will have carried it a noticeable distance – a sprint by stellar standards.
Arcturus in Autumn: the Bright Star in the West
In spring Arcturus stands high in the south in the evening, in summer it drifts west – and in September and October you can only see it after sunset low in the west to northwest before it sets during the evening. When people ask about a “bright star in the west” in autumn, they often mean exactly this one. It is easy to tell apart: Arcturus shines distinctly orange and twinkles strongly; Venus, when it is the evening star, is much brighter, pure white and hardly twinkles at all.
Why Does Arcturus Twinkle?
The twinkling is not a property of the star but of the air: stars are so far away that their light arrives as a practically point-like beam, and every bit of turbulence in the atmosphere keeps deflecting it slightly. This is called scintillation. The lower a star is, the more air its light has to pass through – which is why Arcturus twinkles especially strongly low in the western sky in autumn, sometimes flashing briefly red or green. Planets show a tiny disc instead of a point and therefore twinkle much less. In binoculars or a telescope Arcturus remains a point of light, by the way – even a giant star 37 light-years away is too small to show a disc.
Frequently Asked Questions
Is Arcturus the North Star?
No. The North Star (Polaris) sits in the Little Dipper almost exactly above the north celestial pole and appears not to move. Arcturus, on the other hand, rises and sets like most stars; it is simply much brighter than Polaris.
How far away is Arcturus?
About 37 light-years. The light you see today set off in the late 1980s. It became famous in 1933: the Chicago World’s Fair was opened with light from Arcturus, because it was then thought to be 40 light-years away – the light was supposed to date from the year of Chicago’s previous World’s Fair in 1893.
When can you see Arcturus?
From March to August, best on spring evenings high in the south. Find it by following the arc of the Big Dipper’s handle.
Why does Arcturus glow orange?
As a red giant, its surface is only about 4,300 degrees – cooler than the Sun, which gives it its warm colour.
How big is Arcturus?
Around 25 solar diameters with roughly the same mass as the Sun – a bloated star in a late stage of its life.
Is Arcturus the brightest star in the sky?
The brightest in the northern celestial hemisphere. Overall it ranks fourth – Sirius remains unbeaten.
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