Arcturus, mag. -0.04, is an orange giant that's usually regarded as the fourth brightest star in the night sky. However, it does have justifiable claims for third position since it's marginally brighter than both main components of the Alpha Centauri system. What's clear is that Arcturus is the brightest star in the northern section of the celestial hemisphere.

Arcturus is the stand out star in the large constellation of Boötes, the Herdsman or Plowman. A vague legend has it that the herdsman was placed in the heavens for successfully inventing the plough. The constellations next brightest star Izar (ε Boo) shines much fainter than Arcturus at magnitude +2.35. As one of the nighttime's brightest stars Arcturus has been significant to observers since antiquity. The star is mentioned in the Bible and was featured on old Chinese star maps, named Dajido. In India it was sometimes referred to as Nishtya or the Outcast, presumably because of its position in the sky far away from the zodiac and Milky Way band.

Arcturus (Fred Espenak)

The name Arcturus derives from Arktouros, which means in ancient Greek the "Bear's Tail" or equally could refer to "the Keeper of the Bear". The name in Greek literature goes back to at least the time of Hesiod, who wrote about the star in his book "Works and Days." Despite Arcturus being a beautiful star it has not always been held in high regard. For example, seamen of ancient times regarded it as an unlucky omen.

More recently, Ptolemy called it "golden red" and curiously in 1852 some well-respected astronomers observed a change in the star's colour before it reverted back to normal a few years later. It's difficult to believe any intrinsic changes to Arcturus had occurred - it's not that type of star - and it's likely that the colour changes were due to atmospheric effects.

Finding the star is easy, just follow the tail of the Plough or Big Dipper asterism of Ursa Major southwards to arrive at the distinct orange hue of Arcturus. Located at a declination of 19N it's visible from every inhabited location on Earth except from Antarctica science stations. It appears highest in the sky from tropical and Northern Hemisphere latitudes during the months of April, May and June.

Arcturus and Ursa Major

Arcturus is a giant type K0 III star with a diameter 25 times larger than the Sun. If placed at the centre of the Solar System it would extend more than half the way to the orbit of Mercury. Although large, it's nowhere near as vast as supergiant stars such as Betelgeuse (α Ori) and Antares (α Sco).

Arcturus is relatively close at 37 light-years distant and 170 times more luminous than the Sun. It's 7.1 billion years old and therefore 2.5 billion years older than our star. The star is single and noted for its high proper motion, two arc seconds a year, greater than any first magnitude star other than alpha (α) Centauri. It's currently at about its closest point to the Sun and to date no planets have been identified surrounding it.

In 1933, the light from Arcturus was used to open the "Century of Progress" exposition in Chicago. The star was selected at that time as it was though to be 40 light-years distant and therefore the light arriving had left the star at the time of the previous exposition.

Finder Chart for Arcturus (Alpha Boötis)

Finder Chart for Arcturus (Alpha Boötis) - pdf format

Arcturus Data Table

Bayeralpha (α) Boo
Flamsteed16 Boo
RA (J2000)14h 15m 40s
DEC (J2000)+19d 10m 56s
Distance (ly)37
Apparent Mag.-0.04
Absolute Mag.-0.30
Spectral TypeK0III
Radius (Sol)25.4
Surface Temp (K).4,300
Luminosity (Sol)170
Age (million years)7,100
Other designationsHR 5340, HD 124897 , HIP 69673
Notable featureBrightest star in northern section of the sky

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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