Arcturus was the first star besides the Sun seen through a telescope in daylight
In 1635 the French astronomer Jean-Baptiste Morin trained a telescope on Arcturus in broad daylight and saw it, the first recorded daytime sighting of any star apart from the Sun and supernovae. The orange giant is the brightest star in the northern sky, sometimes visible to the naked eye around sunset.
Its name comes from ancient Greek for guardian of the bear, and since the word for bear also came to mean north, it can read as guardian of the north. Ptolemy called it slightly red, and Chaucer mentioned it in his 1391 treatise on the astrolabe. Johann Bayer designated it Alpha Boötis in 1603. To find it, northern observers follow the curve of the Big Dipper's handle: arc to Arcturus, then continue on to Spica.
At magnitude −0.05 it ranks fourth in the night sky behind Sirius, Canopus and Alpha Centauri, but Alpha Centauri is a pair, so Arcturus is the third-brightest single star. It sits 36.7 light-years away, measured by the Hipparcos satellite from a yearly parallax shift of under 89 thousandths of an arcsecond. Pollux is the only giant star nearer to us.
Arcturus is a red giant about 7.1 billion years old that has exhausted the hydrogen in its core. With roughly the Sun's mass, it has swelled to 25 times the Sun's width, about 35 million kilometres. It gives off about 110 times the Sun's visible light, but its cooler surface pours much energy out as infrared, so its total output is some 180 times solar. It shows a magnetic cycle like the Sun's, probably lasting under 14 years, and a hunt for a faint companion star has found none.
It is also on the move, crossing the sky by two arcseconds a year, faster than any first-magnitude star other than Alpha Centauri. It speeds through space at 122 kilometres per second, apparently alongside 52 other old stars in what is called the Arcturus stream. It will make its closest approach to us in about 4,000 years. Seen from its neighbour Muphrid, just 3.3 light-years away, Arcturus would outshine Venus as it appears from Earth.
Source: Arcturus
