Spring evenings see the Plough and the rest of Ursa Major spread out across the zenith. Following the line of the Plough's 'handle' down towards the horizon takes the view to the fourth-brightest star in the sky, Arcturus. Arcturus (Alpha Bootis) is of mag -0.04--marginally brighter than Vega in Lyra--and is also one of the most highly-coloured stars. To the naked eye it is pronouncedly orange, reflecting its K spectral class. Arcturus is a giant star, with a surface temperature of 3600K, markedly cooler than the Sun. On a high-inclination orbit relative to our Galaxy's spiral arms, it is just passing through our neighbourhood, heading southwestwards by an apparent 2.3 arcseconds per year; in 500,000 years it will have retreated too far to be visible to the naked eye! Currently, it lies 36 light years away.
Observers using binoculars to enhance the hue of Arcturus may--as I did in 1973--stumble across the star's wide line-of-sight pairing with a non-related star which provides a nice colour contrast. This is mag +6.0 SAO100949, about 20 arcminutes to the SSW of Arcturus. SAO100949 is a spectral class A star, and although it should be white appears greenish in a low power field.
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|Title Annotation:||Sky notes/2009 April & May; observing the Arcturus|
|Publication:||Journal of the British Astronomical Association|
|Article Type:||Brief article|
|Date:||Apr 1, 2009|
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