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. 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 constellation's next brightest star, Izar (ε Boo), shines at magnitude +2.35 and is much fainter than Arcturus. As a bright night time star, Arcturus has been significant to observers since antiquity. It was mentioned in the Bible and featured on old Chinese maps with the name 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.
The name Arcturus derives from Arktouros, which means in ancient Greek the "Bear's Tail" or the "Keeper of the Bear". The name in Greek literature goes back to at least the time of Hesiod, who wrote about this star in his book "Works and Days." Despite being a beautiful star, Arcturus 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 anything intrinsically had changed as Arcturus is not that type of star and it's more likely the "colour changes" were simply due to atmospheric effects.
Finding the star is easy, just follow the tail of the Plough or Big Dipper asterism of Ursa Major southwards. Located at a declination of 19N, Arcturus is visible from every inhabited location on Earth except Antarctica science stations. It appears highest in the sky from tropical and Northern Hemisphere latitudes during the months of April, May and June.
Arcturus is a giant type K0III star with a diameter of 25 times that of the Sun. If placed at the centre of our Solar System, it would extend more than half the way to the orbit of Mercury. Although intrinsically large, it's nowhere near as vast as red supergiant stars such as Betelgeuse and Antares.
Arcturus is relatively close at 37 light-years and is 170 times more luminous than the Sun. It's 7.1 billion years old and therefore 2.5 billion years older than the Sun. The star is single and noted for its high proper motion of two arc seconds per year, which is greater than any first magnitude star except Alpha Centauri. Arcturus is currently at about its closest point to the Sun. To date, no planets surrounding it have been identified.
In 1933, the light from Arcturus was used to open the "Century of Progress" exposition in Chicago. The star was selected as it was thought to be 40 light-years distant and therefore the arriving light had left the star at the time of the previous exposition.
Arcturus Data Table
|Bayer||alpha (α) Boo|
|RA (J2000)||14h 15m 40s|
|DEC (J2000)||+19d 10m 56s|
|Surface Temp (K).||4,300|
|Age (years)||7.1 Billion|
|Other designations||HR 5340, HD 124897 , HIP 69673|
|Notable feature||Brightest star in northern section of the sky|