Arcturus, the bright orange star of Boötes

Arcturus is the brightest star north of the celestial equator, a warm orange star in Boötes, the herdsman. It is easy to find, it is moving faster across the sky than almost any other bright star, and it once turned on the lights of a World's Fair.

Where is it from where I am?

Lines and labels Show in my sky

About this picture

NASA/Goddard Space Flight Center Scientific Visualization Studio. Gaia DR2: ESA/Gaia/DPAC. Constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg). NASA's page (opens in a new tab).

The map is a composite of 1.7 billion catalogued stars, not a photograph from one place: it shows stars far too faint for the eye, and no weather, Moon or city light. It is drawn here in this chart's projection, with each star where it was in 2000; the fastest-moving bright star has shifted about a minute of arc since, far less than a pixel here.

How to spot it: Follow the curve of the Big Dipper's handle: 'arc to Arcturus'. Source (opens in a new tab)

What you are looking at

Arcturus is an orange giant 37 light-years away, the brightest star north of the celestial equator.

Arcturus is fourth in ESA's list (opens in a new tab) of the brightest stars, 37 light-years away. NASA calls it (opens in a new tab) the brightest star in the northern sky and puts that down largely to its nearness. By this site's check of the Bright Star Catalogue (opens in a new tab), no star north of the celestial equator is brighter.

It is a red giant (opens in a new tab), a star that has run out of hydrogen in its core and swollen. NASA names Arcturus as one of the red giants you can see with the unaided eye, and notes that such stars look more orange than red, which matches the warm colour you see.

Sources for this section ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab) · ; NASA Science, What's Up: September 2021 (opens in a new tab) · ; Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab) · ; NASA Science, Star Types (opens in a new tab)

How to find it

Follow the curve of the Big Dipper's handle: arc to Arcturus.

NASA's version (opens in a new tab) of the old rhyme: find the Big Dipper, follow the curve of its handle away from the bowl, about the width of an outstretched hand, and you reach Arcturus. In September NASA finds it in the west in the first couple of hours after dark.

By this site's calculation (opens in a new tab), Arcturus stands about 69° up in the south at its highest from 40°N and about 36° up in the north from 35°S, and it is highest at about 9 p.m. local time in mid-June.

Sources for this section NASA Science, What's Up: September 2021 (opens in a new tab) · ; Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab)

A star in a hurry

Arcturus is crossing the sky faster than any other bright star except Alpha Centauri.

Every star has its own motion across the sky, called proper motion, and ESA explains (opens in a new tab) why the constellations still look fixed: the shifts build up over millennia, too slowly to notice in a lifetime, and nearby stars show the largest.

Arcturus is an exception worth noticing. By this site's calculation (opens in a new tab) from its catalogued proper motion it moves 2.3 seconds of arc a year, enough to cross the width of the full Moon in about 800 years. Among the catalogue's stars brighter than magnitude 1.5, only the two bright stars of Alpha Centauri move faster. The record holder, Barnard's Star (opens in a new tab), moves 10.3 seconds of arc a year but is far too faint to see without a telescope.

Sources for this section ESA, How does Gaia study the Milky Way? (opens in a new tab) · ; Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab)

The star that opened a World's Fair

In 1933 Chicago switched on its World's Fair with light from Arcturus.

On 27 May 1933 the lights of Chicago's A Century of Progress exposition were switched on with light from Arcturus (opens in a new tab). Starlight falling on photoelectric plates at four observatories made a weak current, which was amplified and sent by telegraph to the city; when it arrived at 9:15 p.m. the buildings lit up one by one. The idea came from Edwin Frost, a blind astronomer formerly of the University of Chicago's Yerkes Observatory.

Arcturus was chosen because its light was believed to have set out about 40 years earlier, around the time of Chicago's first world's fair in 1893. Today's figure, 37 light-years (opens in a new tab), puts the light's departure nearer 1896.

Sources for this section Paul Durica, Arcturus and Art Deco: The Opening of the 1933 World's Fair, Chicago History Museum (opens in a new tab) · ; ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab)

Where on Earth, and when

It rises from anywhere north of about 71°S.

North of about 71°N, it never sets.

It climbs at least 20° high, the finder's threshold for a suggestion, from latitudes between 51°S and 89°N.

It is highest in the evening sky, at about 9 p.m. local time, in mid-June, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.

Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.

How Arcturus compares with the Sun