Arcturus (Alpha Bootis), facts about the brightest star in the northern hemisphere

Arcturus, also known as Alpha Bootis (Bayer) is a red giant branch star located in the constellation of Bootes, The Bear Driver. Arcturus is a major star and forms part of the constellation.

Based on the spectral type (K2IIIp), Arcturus colour is orange to red, these are stars that are cooler than our own star.

Arcturus temperature is in the range of between 3,700 and 5,200 Kelvin. Arcturus effective temperature is 4,286 Kelvin which is cooler than the Sun's effective temperature which is 5,777 Kelvin.

Arcturus is the brightest star in Bootes and the 4th in the night sky based on the Hipparcos 2007 apparent magnitude. Based on a parallax of 88.83, Arcturus distance from Earth can be calculated at being 36.72 light years away or 11.25 parsecs.

Arcturus is a naked-eye star, so you don't need a telescope or binoculars when you look up on a clear night. The lower the magnitude, the easier it will be to see it.

Arcturus is a variable star, its brightness varies in magnitude between 0.0930 to 0.1400. The smaller number is the brightest the star gets.

Location

The location of the Arcturus in the night sky is determined by the right ascension (R.A.) and declination (Dec.). These are equivalent to the Longitude and Latitude on Earth. The Right Ascension (Longitude) is expressed in time (hh:mm:ss) and is how far the star is along Earth's celestial equator. If the R.A. is positive, then it's eastwards and vice versa.

The Declination (Latitude) is how far north or south the object is compared to the celestial equator and is expressed in degrees. If the value is positive, it is north of the celestial equator. For Arcturus, the location is 14h 15m 40.35 and +19° 11` 14.2 .

Based on the location of Bootes, Arcturus can be located in the northern hemisphere of the celestial sky. The celestial hemisphere is equivalent to the hemispheres on Earth. Arcturus is north of the Ecliptic. The Ecliptic is the path that the Earth takes as it orbits the Sun. As the Earth is titled, we therefore have Celestial and Ecliptic hemispheres and they can be different for a star.

Physical Properties

Spectral Type

Arcturus spectral type of K2IIIp which means its colour and type is orange to red Red Giant Branch Star. There is no relationship between colour and size. For example, a red star can be large or small. Small stars are more energy efficient than larger stars and live longer.

Arcturus Luminosity

Luminosity is the amount of energy a star pumps out relative to the amount that our star, the Sun, gives out. Our star, the Sun's value is 1. Arcturus luminosity figure of 206.84 is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012. The star generates more energy than our star.

Arcturus Size (Radius)

Arcturus radius has been calculated as being 25.4 times bigger than the Sun. The Sun's radius is 695,800km, therefore, the Arcturus radius is an estimated 17,673,320.km. If you need the star's diameter, you just need to multiply the radius by 2. The error range for the radius is between 25.20000 and 25.60000.

Magnitude (Apparent / Absolute / Visible)

A number represents a star’s magnitude, whether apparent/visual or absolute. The smaller the number, the brighter the star is. The Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Arcturus apparent magnitude is -0.05, which is a measure of the star's brightness as seen from Earth. Apparent Magnitude is also known as Visual Magnitude.

If you use the 1997 parallax value, Arcturus' absolute magnitude is -0.31. If you use the 2007 parallax value, Arcturus' absolute magnitude is -0.31. Absolute Magnitude is the star's apparent magnitude from 10 parsecs or 32.6 light years. The magnitude assumes nothing is between the object and the viewer, such as dust clouds. To compare different stars' actual brightness, you would best use Absolute rather than Apparent Magnitude.

Arcturus is visible from Earth without needing binoculars or a telescope. The lower the Apparent Magnitude of a star or other object is, the easier it is to see in the night sky. An object with a magnitude greater than 6.5 cannot be seen without a telescope or other device.