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Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the night-time sky, with a visual apparent magnitude of −1.47. This binary star system consists of a blue-white main sequence dwarf star and a faint white dwarf companion. It is located in the constellation Canis Major. Sirius can be seen from almost every inhabited region of the Earth's surface (those living north of 73.284 degrees can't see it) and, in the Northern Hemisphere, is known as a vertex of the Winter Triangle. The best time of year to view it is around January 1, when it reaches the meridian at midnight. Under the right conditions, Sirius can be observed in daylight with the naked eye. Ideally the sky must be very clear, with the observer at a high altitude, the star passing overhead, and the sun low down on the horizon.•
Etymology The name of this star comes from the Latin Sīrius, from Greek Σείριος (Seirios, "glowing" or "scorcher"). As the major star of the "Big Dog" constellation, it is often called the "Dog Star". The Latin name for this star is Canicula ("little dog") and in Arabic: aš-ši‘rā in Islamic astronomy, from which the alternate name Al Shira derives. In Sanskrit, it is known as Mrgavyadha ("deer hunter") or Lubdhaka ("hunter"). As Mrgavyadha, the star represents Shiva. In Chinese the star is known as ( literally, “heavenly wolf star”). The Japanese pronunciation is and the Korean pronunciation Cheonlangseong. History
System At a distance of 2.6 pc or 8.6 light years, Sirius is one of the nearest stars to Earth. Its closest large neighbour star except Sirius B is Procyon, 1.61 pc or 5.24 ly away. Sirius is a main sequence star of spectral type A0 or A1 and has a mass about 2.1 times that of the Sun. It has a companion star that has already evolved off the main sequence into a white dwarf, and hence was once the more massive of the two. These two stars orbit each other with a separation of about 20 AU (roughly the distance between the Sun and Uranus) and a period of close to 50 years. The orbit of Sirius B can take the star in front of Sirius A, briefly lowering the total luminosity. For this reason the system is considered an eclipsing binary variable star. A typical white dwarf has a mass equal to 0.5–0.6 solar masses. With a mass nearly equal to the Sun's, Sirius B is one of the more massive white dwarfs known. Yet that same mass is packed into a volume roughly equal to the Earth. White dwarfs form only after the star has evolved from the main sequence and then pass through a red giant stage. This occurred when Sirius B was less than half its current age, approximately 120 million years ago. The original star had an an estimated 5 solar masses and was probably a spectral type B7 V star when it still was on the main sequence. While it passed through the red giant stage, Sirius B may have enriched the metallicity of its companion. For this reason Sirius A now has a higher than normal abundance of elements heavier than helium, such as iron. The Sirius star system also emits a higher than expected level of infrared radiation, as measured by IRAS. This may be an indication of dust in the system, and is considered somewhat unusual for a binary star. The current primary, Sirius A, is expected to have completely exhausted the store of hydrogen at its core within a billion (109) years. At that point it will pass through its own red giant stage, then settle down to become a white dwarf. Mysteries • (Credit:NASA)">A and B Hubble photo.jpg|thumb|left|220px|The image of Sirius A and Sirius B taken by Hubble Space Telescope. The white dwarf can be seen to the lower left.• (Credit:NASA) There remain a few unsolved mysteries regarding Sirius: Possible Third Member in System Since 1894, some apparent orbital irregularities in Sirius B have been observed, suggesting a third very small companion star, but this has never been definitely confirmed. Sirius Color Curiously, some ancient observations of Sirius describe it as a red star.•• To the Romans this meant an angry god, and they are known to have sacrificed red dogs to this star. Today, Sirius A is bluish white. The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy is rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when it was observed rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red/white/blue hues when near the horizon. The Dogon The Dogon people are a tribal people living in Africa who are said to have known that Sirius had a companion star before its discovery in the 1840's. Of course, it is impossible to see Sirius's companion except with very powerful telescopes and at the right time of Sirius B's orbit, which lasts several years. They also supposedly knew that Saturn had rings and that Jupiter had 4 moons (As of 2003 Jupiter is known to have at least 63 satellites. Most are a few miles, or less, in diameter. Only 4 can be seen with simple telescopes). This apparent knowledge, which it seems impossible for them to have acquired without outside assistance, is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert Temple, who linked this to the legends of the Dogon as well with the legends and traditions of the ancient Egyptians and Sumerians, conjecturing that aliens from Sirius had visited Earth and influenced early civilization. This work has been challenged by astronomer Carl Sagan, among others, as based on selective evidence. Skeptics believe there may have been cultural contamination on the part of visiting astronomers who went to the region to observe a transit of Venus. see also The Dogon Controversy Sirius in popular culture Being the brightest star in the night sky, Sirius is often mentioned in science fiction and other popular culture as well. See also | ||||||||||
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