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Sirius () is the brightest star (in fact, a star system) in the Earth's night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek Σείριος (''Seirios''), meaning "glowing" or "scorcher". The system has the Bayer designation Alpha Canis Majoris (α CMa). What the naked eye perceives as a single star is actually a binary star system, consisting of a white main-sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, called Sirius B. The distance separating Sirius A from its companion varies between 8.2 and 31.5 AU.〔 Sirius appears bright because of both its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs (8.6 ly), as determined by the Hipparcos astrometry satellite,〔 the Sirius system is one of Earth's near neighbors. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to increase, but it will continue to be the brightest star in the Earth's sky for the next 210,000 years.〔''Sky and Telescope'', April 1998 (p60), based on computations from Hipparcos data.〕 Sirius A is about twice as massive as the Sun () and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun〔 but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and years old.〔 It was originally composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around years ago.〔 Sirius is also known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (Greater Dog).〔 The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the "dog days" of summer for the ancient Greeks, while to the Polynesians in the Southern Hemisphere the star marked winter and was an important reference for their navigation around the Pacific Ocean. == Observational history == Sirius, known in ancient Egypt as ''Sopdet'' (Greek: Σῶθις ''Sothis''), is recorded in the earliest astronomical records. During the era of the Middle Kingdom, Egyptians based their calendar on the heliacal rising of Sirius, namely the day it becomes visible just before sunrise after moving far enough away from the glare of the Sun. This occurred just before the annual flooding of the Nile and the summer solstice,〔 after a 70-day absence from the skies.〔 The hieroglyph for Sothis features a star and a triangle. Sothis was identified with the great goddess Isis, who formed a part of a triad with her husband Osiris and their son Horus, while the 70-day period symbolised the passing of Isis and Osiris through the ''duat'' (Egyptian underworld).〔 The ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer, and feared that it caused plants to wilt, men to weaken, and women to become aroused.〔 Due to its brightness, Sirius would have been noted to twinkle more in the unsettled weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence. Anyone suffering its effects was said to be ''astroboletos'' (ἀστροβόλητος) or "star-struck". It was described as "burning" or "flaming" in literature.〔 The season following the star's heliacal rising (i.e. rising with the Sun) came to be known as the Dog Days of summer.〔 The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes, and would await the reappearance of the star in summer. If it rose clear, it would portend good fortune; if it was misty or faint then it foretold (or emanated) pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, highlighting Sirius' importance.〔 The Romans celebrated the heliacal setting of Sirius around April 25, sacrificing a dog, along with incense, wine, and a sheep, to the goddess Robigo so that the star's emanations would not cause wheat rust on wheat crops that year.〔 Ptolemy of Alexandria mapped the stars in Books VII and VIII of his ''Almagest'', in which he used Sirius as the location for the globe's central meridian. He curiously depicted it as one of six red-coloured stars (see the Red controversy section below). The other five are class M and K stars, such as Arcturus and Betelgeuse.〔 Bright stars were important to the ancient Polynesians for navigation between the many islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses to assist mariners in charting courses to particular destinations. They also served as latitude markers; the declination of Sirius matches the latitude of the archipelago of Fiji at 17°S and thus passes directly over the islands each night.〔 Sirius served as the body of a "Great Bird" constellation called ''Manu'', with Canopus as the southern wingtip and Procyon the northern wingtip, which divided the Polynesian night sky into two hemispheres.〔 Just as the appearance of Sirius in the morning sky marked summer in Greece, so it marked the chilly onset of winter for the Māori, whose name ''Takurua'' described both the star and the season. Its culmination at the winter solstice was marked by celebration in Hawaii, where it was known as ''Ka'ulua'', "Queen of Heaven". Many other Polynesian names have been recorded, including ''Tau-ua'' in the Marquesas Islands, ''Rehua'' in New Zealand, and ''Ta'urua-fau-papa'' "Festivity of original high chiefs" and ''Ta'urua-e-hiti-i-te-tara-te-feiai'' "Festivity who rises with prayers and religious ceremonies" in Tahiti. The Hawaiian people had many names for Sirius, including ''Aa'' ("glowing"), ''Hoku-kauopae'', ''Kau-ano-meha'' (also ''Kaulanomeha''), "Standing-alone-and-sacred", ''Hiki-kauelia'' or ''Hiki-kauilia'' (the navigational name), ''Hiki-kau-lono-meha'' ("star of solitary Lono", the astrological name), ''Kaulua'' (also ''Kaulua-ihai-mohai'', "flower of the heavens"), ''Hiki-kauelia'', ''Hoku-hoo-kele-waa'' ("star which causes the canoe to sail", a marine navigation name), and ''Kaulua-lena'' ("yellow star"). The people of the Society Islands called Sirius variously ''Taurua-fau-papa'', ''Taurua-nui-te-amo-aha'', and ''Taurua-e-hiti-i-tara-te-feiai''. Other names for Sirius included ''Palolo-mua'' (Futuna), ''Mere'' (Mangaia), ''Apura'' (Manihiki), ''Taku-ua'' (Marquesas Islands), and ''Tokiva'' (Pukapuka). In the cosmology of the Tuamotus, Sirius had various names, including ''Takurua-te-upuupu'', ''Te Kaha'' ("coconut fiber"), ''Te Upuupu'', ''Taranga'', and ''Vero-ma-torutoru'' ("flaming and diminishing"). The indigenous Boorong people of northwestern Victoria named Sirius as ''Warepil''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sirius」の詳細全文を読む スポンサード リンク
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