Arcturus

ArcturiansArcturianAlpha BooArcturanArcturos (mythology)Great Hornα Booα Boo (Arcturus)
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Boötes

BootesBoötes constellationBoo
Arcturus, designation α Boötis (Latinized to Alpha Boötis, abbreviated Alpha Boo, α Boo), is the brightest star in the constellation of Boötes, the fourth-brightest in the night sky, and the brightest in the northern celestial hemisphere.
It contains the fourth-brightest star in the night sky, the orange giant Arcturus.

Spica

α VirAzimechAlaezel
Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli.
Spica, along with Arcturus and Denebola or Regulus depending on the source, is part of the Spring Triangle asterism, and by extension, also of the Great Diamond together with the star Cor Caroli.

Red giant

red giant starred giantsred giant stars
Relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K0III—an ageing star around 7.1 billion years old that has used up its core hydrogen and moved off the main sequence. Arcturus is an evolved red giant star with a stellar classification of K0 III.
The K0 RGB star Arcturus is 36 light-years away, and Gamma Crucis is the nearest M-class giant at 88 light-years' distance.

Spring Triangle

Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli.
The Spring Triangle is an astronomical asterism involving an imaginary triangle drawn upon the celestial sphere, with its defining vertices at Arcturus, Spica, and Regulus.

Asterism (astronomy)

asterismasterismsFalse Cross
Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli.

Sirius

Sirius BSirius superclusterDog Star
With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27).
The other five are class M and K stars, such as Arcturus and Betelgeuse.

Vega

Alpha Lyrae2828Botercadent
With an absolute magnitude of −0.30, Arcturus is, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood.
This star is relatively close at only 25 light-years from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood.

Denebola

Beta Leonisβ Leoβ Leo (Denebola)
Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli.
Denebola, along with Spica and Arcturus, is part of the Spring Triangle asterism, and by extension, also of the Great Diamond together with the star Cor Caroli.

Alpha Centauri

Alpha Centauri BAlpha Centauri Aα Centauri
With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27).
When considered among the individual brightest stars in the sky (excluding the Sun), it is the fourth brightest at an apparent magnitude of −0.01, being slightly fainter than Arcturus at an apparent magnitude of −0.04.

Aldebaran

Alpha Tauribrightest starRohini
Ptolemy described Arcturus as subrufa ("slightly red"): it has a B-V color index of +1.23, roughly midway between Pollux (B-V +1.00) and Aldebaran (B-V +1.54).
With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter at that wavelength.

Arcturus moving group

Arcturus stream
) Arcturus is thought to be an old-disk star, and appears to be moving with a group of 52 other such stars, known as the Arcturus stream.
In astronomy, the Arcturus moving group or Arcturus stream is a moving group or stellar stream which includes the nearby bright star Arcturus.

Deneb

Alpha CygniAridedα Cyg
Observations of the stars Arcturus and Deneb (Alpha Cygni) were conducted in the summer of 1984.
Observations of the stars Deneb and Arcturus (Alpha Boötis) were conducted in the summer of 1984.

Stellar classification

spectral typeK-typeG-type
Arcturus is an evolved red giant star with a stellar classification of K0 III.
There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.

Great Diamond

Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism and, by extension, also of the Great Diamond along with the star Cor Caroli.

First-magnitude star

first magnitude starfirst magnitudebrightest
Because of its proximity, Arcturus has a high proper motion, two arcseconds a year, greater than any first magnitude star other than α Centauri.

Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27).

Magnitude (astronomy)

magnitudemagnitudesmag
With a near-infrared J band magnitude of −2.2, only Betelgeuse (−2.9) and R Doradus (−2.6) are brighter.
To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor.

Stellar evolution

evolvedevolvingevolution
Arcturus is an evolved red giant star with a stellar classification of K0 III.
Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation of Boötes.

Svati

SwathiSwātiSvātī
In Indian Astrology or Vedic Astrology or Sidereal Astrology, Arcturus is called Swati which is a word meaning "very beneficent" derived from the language Sanskrit.
It was the name of one of the wives of the Sun in Hindu Epics and the Sanskrit name of Arcturus as well as of the nakshatra (lunar mansion) associated with Arcturus in Hindu astrology.

Big Dipper

Northern DipperThe PloughPlough
From the northern hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper (or Plough).

Yapp telescope

36-inch Yapp reflectorYapp 36 Inch reflector
In 1984, the 90 cm (36-inch) reflecting Yapp telescope at Herstmonceux was tested with an echelle spectrograph from Queen's University Belfast and a CCD camera.
Observations of the stars Alpha Cygni (Deneb) and Alpha Boo (Acturus) using this instrumentation on the 90 cm (36-inch) Yapp reflector were conducted in the summer of 1984.

Proper motion

proper motionsproper-motionhigh proper motion star
Because of its proximity, Arcturus has a high proper motion, two arcseconds a year, greater than any first magnitude star other than α Centauri.
Proper motion was suspected by early astronomers (according to Macrobius, AD 400) but a proof was not provided until 1718 by Edmund Halley, who noticed that Sirius, Arcturus and Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly 1850 years earlier.

Eta Boötis

η Booη BoötisEta Bootis
η Boötis, or Muphrid, is only 3.3 light-years distant from Arcturus, and would have a visual magnitude −2.5, about as bright as Mercury from Earth, whereas an observer on the former system would find Arcturus as bright as Venus as seen from Earth.
Eta Boötis appears close to the prominent star Arcturus (Alpha Bootis) in Earth's sky, and Arcturus is in fact its closest stellar neighbor, as both stars are nearly identical in distance from the Sun.

Giant star

giantorange giantgiants
As the brightest K-type giant in the sky, it was the subject of an atlas of its visible spectrum, made from photographic spectra taken with the coudé spectrograph of the Mt. Wilson 2.5m telescope published in 1968, a key reference work for stellar spectroscopy.

J band (infrared)

J bandJJ-band
With a near-infrared J band magnitude of −2.2, only Betelgeuse (−2.9) and R Doradus (−2.6) are brighter.
The next brightest stars in the J band are Antares (−2.7), R Doradus (−2.6), Arcturus (−2.2), and Aldebaran (−2.1).