Portal:Stars

Arcturus (/ɑːrkˈtjʊərəs/; α Boo, α Boötis, Alpha Boötis) of the constellation Boötes is the brightest star in the northern celestial hemisphere. With a visual magnitude of −0.04, it is the fourth brightest star in the night sky, after −1.46 magnitude Sirius, −0.86 magnitude Canopus, and −0.27 magnitude Alpha Centauri. It is a relatively close star at only 36.7 light-years from Earth, and, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood.

Arcturus is a type K0 III orange giant star, with an absolute magnitude of −0.30. It has likely exhausted its hydrogen from its core and is currently in its active hydrogen shell burning phase. It will continue to expand before entering horizontal branch stage of its life cycle.

Arcturus is a type K0 III Red giant star. It is at least 110 times more luminous than the Sun in visible light wavelengths, but this underestimates its strength as much of the "light" it gives off is in the infrared; total (bolometric) power output is about 180 times that of the Sun. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun. 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.

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location towards nearest pole for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare). All circumpolar stars are within the circumpolar circle. This was in fact the original meaning of "Arctic Circle", before the current geographical meaning, meaning "Circle of the Bears" (Ursa Major, the Great Bear; and Ursa Minor, the Little Bear), from Greek αρκτικός (arktikos), "near the Bear", from the word άρκτος (arktos) bear.

As Earth spins daily on its axis, the stars appear to rotate in circular paths around one of the celestial poles (the north celestial pole for observers in the Northern Hemisphere, or the south celestial pole for observers in the Southern Hemisphere). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any diurnal motion at all. Depending on the observer's latitude on Earth, some stars — the circumpolar ones — are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the horizon for some portion of their daily circular path (and others remain permanently below the horizon).

The circumpolar stars appear to lie within a circle that is centered at the celestial pole and tangential to the horizon. At the Earth's North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the northern celestial hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "orbit", and the circle containing the remaining circumpolar stars becomes increasingly small. At the Earth's equator this circle vanishes to a single point – the celestial pole itself – which lies on the horizon, and there are therefore effectively no circumpolar stars at all.