Arcturus - the Star Content from the guide to life, the universe and everything

Arcturus - the Star

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Seen from Earth, Arcturus is the fourth-brightest star in the sky, and the brightest star in the Northern Hemisphere1. Its name is Greek, meaning 'Bear Guardian', since it appears very close to the Great Bear2  constellation in the night sky.

Arcturus' official name is alpha Boötis. This is because it is the brightest star (the alpha star) in the constellation Boötes (the Herdsman). It has a noticeable orange hue, and it is most prominent in the sky during the spring and early summer. It can be found quite easily if you extend the arc of the three main stars forming the tail of the Great Bear - colloquially known as the Arc to Arcturus.

Arcturus is an orange giant star with a diameter almost 25 times greater than our own Sun, and a mass of just 1.5 times our Sun. Viewed up close, it would be almost 180 times brighter than our Sun. Its colour and size indicates that it is therefore a dying star, having already converted most of its hydrogen to helium. It does not appear to have any smaller companion stars, and no planets have yet been detected in orbit around it.

Along with its size, its proximity to the solar system and its speed make Arcturus an interesting night-sky object. It is just 36 light years away and moving past us at a whopping rate of 500,000 kilometres per hour. It is so close (relatively speaking) that very large telescopes can discern its disk. The fact that it is both close-by and travelling fast means that Arcturus' position in the night sky has changed measurably since scientific observations of the heavens began some 2,000 years ago. It was an observation of this motion3 in the 18th Century by the great astronomer Sir Edmund Halley that led to our present understanding of the universe as ever changing, and not simply fixed and immutable.

Arcturus will remain a bright star in our skies for millennia to come. However, in reality, it is a passing star and over time it will recede farther and farther away from our Sun, eventually disappearing from normal vision in half a million years.

1Unless you are at the North Pole, you will be able to see some of the sky's Southern Hemisphere. Sirius, in the Southern Hemisphere of the sky, is the brightest star and is visible to most people living in the Earth's Northern Hemisphere.2Also known as Ursa Major, The Big Dipper, or the Plough.3Astronomers call it 'Proper Motion'.

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