Arthur (star)
Arthur (Alpha Bootis / α Boo / 16 Bootis) is the third brightest star in the night sky with a visual magnitude of -0.04, after Sirius (α Canis Majoris) and Canopus (α Carinae); Taking together the two main components of Alpha Centauri, which cannot be resolved with the naked eye, Arthur happens to be the fourth brightest star. It is, therefore, one of the brightest stars in the northern celestial hemisphere. Its constellation is Bootes, "El Boyero". It is located in the Local Interstellar Cloud.
Etymology and history
The name Arthur comes from the ancient Greek Ἀρκτοῦρος (Arcturus), "the guardian of the bear" and is related to its proximity to the constellations of the Big Dipper and the Little Dipper. In Arabic he is called Al-Simak al-Ramih (السماك الرامح), translated as "the leg of the one who bears the spear" or "the nobleman who bears the spear". This name, romanized in the past, has given rise to the names Aramec and Azimech, now obsolete. Another Arabic name is Al-Harith as-Sama' (الحارس السماء), "he who keeps the heavens".
In ancient Egypt it seems that it was known as Smat, “the one who reigns” or “the one who rules”, as well as Bau, “the one who comes”. An Egyptian astronomical calendar from the 15th century BCE. C. associates Arthur with Antares (α Scorpii) in an immense celestial figure called Menat. For some authors he was one of the cult stars in the temples of the Nile and in the temple of Venus in Ancona (Italy).
In Hindu astronomy it corresponds to the nakshatra —one of the mansions into which the sky is divided— of Svātī; there it was also called Nishṭya, "outside", possibly due to its northern location far from the zodiac. In China it was known as Ta Kiō, "the great horn", while four small nearby stars were Kang Che, "the lake of drought".
It is possible that, during the Middle Ages, this star and its relationship with the constellations around it, in the case of Virgo and Crater, inspired the legend of the Holy Grail: it would then be identified with King Arthur, due to being the brightest star in the entire circumpolar sky, as proposed in 2019 by the Spanish researcher Fernández Pousada.
Physical characteristics
Arthur is an orange giant of spectral type K1.5III, distant 36.7 light years from the solar system, the second closest giant star after Pollux (β Geminorum). With a surface temperature of 4,290 K, it is visually 113 times more luminous than the Sun; but if the radiation it emits in the infrared is considered, its luminosity is almost double, 215 times greater than that of the sun. Its radius, obtained from the measurement of its angular diameter (0.0210 seconds of arc), is 25.7 times larger than the solar radius. Its mass is about 50% greater than that of the Sun and it is thought that nuclear fusion of helium into carbon has already begun in its inner core. It emits weak X-rays, which suggests that it possesses magnetic activity - and may have a "hidden corona" - something unusual for a star of its characteristics. It is suspected to be a variable star, having been named NSV 6603.
Arthur's velocity relative to the Sun, greater than that of other bright stars, as well as its low metallicity—approximately 28% solar—suggest it may be an old Population II star and a member of the disk galactic bulk. It is part of a group of 53 stars that move together through our galaxy and is called the "Arthur Group". An interesting theory holds that Arturo, as well as the rest of the stars that make up his group, have formed beyond the confines of the Milky Way; the age of some of its members can go back as far as 10-12 billion years, which would imply that they may come from a satellite galaxy absorbed in the past by our own galaxy.