Sirius

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Sirius A / B

The position of Sirius.
Observation data
Epoch J2000.0 (ICRS)
Constellation
(pronunciation)		 Canis Major
Right ascension 06h 45m 08.9173s[1][2]
Declination −16° 42′ 58.017″[1][2]
Apparent magnitude (V) −1.47 (A)[1] / 8.44 (B)[3]
Characteristics
Spectral type A1V (A)[1] / DA2 (B)[3]
U-B color index −0.05 (A)[4] / −1.04 (B)[3]
B-V color index 0.01 (A)[1] / −0.03 (B)[3]
Astrometry
Radial velocity (Rv) −7.6[1] km/s
Proper motion (μ) RA: −546.05[1][2] mas/yr
Dec.: −1223.14[1][2] mas/yr
Parallax (π) 379.21 ± 1.58[1] mas
Distance 8.6 ± 0.04 ly
(2.64 ± 0.01 pc)
Absolute magnitude (MV) 1.42 (A)[5] / 11.33 (B)[5]
Visual binary orbit[6], p. 195
Companion α CMa B
Period (P) 50.09 yr
Semimajor axis (a) 7.56"
Eccentricity (e) 0.592
Inclination (i) 136.5°
Longitude of the node (Ω) 44.6°
Periastron epoch (T) 1894.13
Argument of periastron (ω) 147.3°
Details
Mass 2.02[8] (A) /
0.978[8] (B) M☉
Radius 1.711[8] (A) /
0.0084 ± 3%[9] (B) R☉
Surface gravity (log g) 4.33[11] (A)/8.57[9] (B)
Luminosity 25.4[8] (A) /
0.0024 (B) L☉
Temperature 9,940[11] (A) /
25,200[8] (B) K
Metallicity [Fe/H] =0.50[7] (A)
Rotation 16 km/s[10] (A)
Age 2-3 × 108[8] years
Other designations
System: α Canis Majoris, α CMa, 9 Canis Majoris, 9 CMa, HD 48915, HR 2491, BD -16°1591, GCTP 1577.00 A/B, GJ 244 A/B, LHS 219, ADS 5423, LTT 2638, HIP 32349.
B: EGGR 49, WD 0642-166.[1][12][13]

Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) (IPA: /ˈsɪriəs/) is the brightest star in the night-time sky with a visual apparent magnitude of −1.47. It is also known colloquially as the "Dog Star"; its constellation, Canis Major (English: Big Dog), is named after it.[14] It is the subject of more mythological and folkloric tales than any other star apart from the sun. The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the 'Dog Days' of summer by the Ancient Greeks, while to the Polynesians it was an important star for navigation.

What appears as a single star to the naked eye is actually a binary star system, consisting of a white main sequence star of spectral type A1V, named Sirius A, and a faint white dwarf companion of spectral type DA2, named Sirius B. The reason for its brightness is not its intrinsic luminosity but its closeness to the Sun; at a distance of 2.6 pc or 8.6 light years, Sirius is one of our near neighbours. Sirius A is only about twice as massive as the Sun and, with an absolute magnitude of 1.42, has far less intrinsic luminosity than other bright stars such as Canopus or Rigel.

The Sirius system is between 200 and 300 million years old and comprised two bright bluish stars early in its existence. The more luminous star, 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 120 million years ago.

Contents

X1
N14		 M44

Hieroglyph of Sirius/Sopdet

Sirius is recorded in the earliest astronomical records. The star known in Ancient Egypt as Sopdet (Greek:Sothis). The Middle Kingdom of 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,[15] after a 70 day absence from the skies.[16] The hieroglyph for Sothis features a star and a triangle. Sothis was identified with the great goddess Isis who formed a part of a trinity with her husband Osiris and their son Horus, while the 70 day period symbolised the passing of Isis and Osiris though the duat (Egyptian underworld).[16]

The Ancient Greeks noted the appearance of Sirius heralded the hot and dry summer, and feared its effects on making plants wilt, men weaken and women 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 malign influence. People suffering its effects were said to be astroboletos/αστροβολητος 'star-struck'. It was described as 'burning' or 'flaming' in literature.[17] These came to be known as the dog days of summer.[18] The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes while, and await the reappearance of Sirius in summer; if the star 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.[19]

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 that of the island of Fiji at 17°S and thus passes directly overhead the island 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 Aa and Hoku-Kauopae in Hawaii.[20]

Ptolemy of Alexandria mapped the stars in Book 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. The other five are, in fact, class M and K stars, such as Arcturus and Betelgeuse.[21][22] To the Romans, this meant an angry god, and they are known to have sacrificed red-haired dogs to Sirius.[23][24]

In 1676, Edmund Halley spent a year on the island of St. Helena in the South Atlantic surveying the southern stars. Some 40 years later in 1718 he discovered the proper motion of the hitherto presumed "fixed" stars after comparing his astrometric measurements with those given in Ptolemy's Almagest. Arcturus and Sirius were two noted to have moved significantly, the latter having progressed 30 arc minutes (about the diameter of the moon) southwards in 1800 years.[25]

In 1844, German astronomer Friedrich Wilhelm Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion.[26] Nearly two decades later, on January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup".[27] The visible star is now sometimes known as Sirius A. Since 1894, some apparent orbital irregularities in the Sirius system have been observed, suggesting a third very small companion star, but this has never been definitely confirmed. The best fit to the data indicates a six-year orbit around Sirius A and a mass of only 0.06 solar masses. This star would be 5 to 10 magnitudes fainter than the white dwarf Sirius B, which would account for the difficulty of observing it.[28] More recent observations have failed to confirm the existence of a third member of the Sirius system, but still have not ruled out the possibility that one exists too close to Sirius to be seen. An apparent "third star" observed in the 1920s seems to have been a background object.[29]

In 1909, Ejnar Hertzsprung suggested that Sirius was a member of the Ursa Major Moving Group, based on the system's movements across the sky. However, a more recent analysis in 2003 by Jeremy King and his coworkers found Sirius's membership in the group to be questionable,[30] and a 2005 study concluded that, assuming that Sirius has metallicity similar to the Sun's, its age is only half that of the other members of the group, making it too young to belong to the group.[8][31]

In 1915, Walter Sydney Adams, using a 60-inch (1.5 meter) reflector at Mount Wilson Observatory, observed the spectrum of Sirius B and determined that it was a faint whitish star.[32] This would lead astronomers to conclude that it was a white dwarf, the second to be discovered.[33] The diameter of Sirius A was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using their stellar intensity interferometer.[34] In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has nearly the diameter of the Earth, 12,000 kilometers (7,500 miles), with a mass that is 98% of the Sun.[35][36][37][38]

The Voyager 2 spacecraft, launched in 1977 to study the four Jovian planets in the Solar System, is expected to pass within 4.3 light years of Sirius in approximately 296,000 years.[39]

Today, Sirius A is bluish white. The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy has been 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 and blue hues when near the horizon.[40] Also, not all ancient observers saw Sirius as red. Sirius is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[41][42]

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)
The image of Sirius A and Sirius B taken by Hubble Space Telescope. The white dwarf can be seen to the lower left.[43] (Credit:NASA)

With an apparent magnitude of −1.47, it is the brightest star in the night sky, though is 'outshone' as such by the Moon, Venus, Jupiter and Mars. Sirius can be seen from almost every inhabited region of the Earth's surface, with only those living north of 73 degrees unable to see it. However, it does not rise very high when viewed from some northern cities, reaching only 13° above the horizon from Saint Petersburg.[44] Sirius, along with Procyon and Betelgeuse, forms one of the three vertices of the Winter Triangle to observers in the Northern Hemisphere.[45] Sirius can even be observed in daylight with the naked eye under the right conditions. Ideally, the sky should be very clear, with the observer at a high altitude, the star passing overhead, and the sun low down on the horizon.[46]

At a distance of 2.6 pc or 8.6 light years, the Sirius system contains two of the eight nearest stars to Earth,[47] and is the fifth closest binary system.[10] This proximity is the reason for its brightness, as with other near stars such as Alpha Centauri and in stark contrast to distant, highly luminous supergiants such as Canopus, Rigel or Betelgeuse.[48] The closest large neighbouring star to Sirius is Procyon, 1.61 pc or 5.24 ly away.[49]

Sirius is a binary star system consisting of two white stars orbiting each other with a separation of about 20 AU[50] (roughly the distance between the Sun and Uranus) and a period of just over 50 years. The brighter component, termed Sirius A, is a main sequence star of spectral type A0 or A1, while its companion, Sirius B, is a star that has already evolved off the main sequence and become a white dwarf. Currently 10,000 times less luminous in the visual spectrum, Sirius B was once the more massive of the two. The age of the system has been estimated at around 230 million years. Early in its lifespan it was thought to have been two bluish white stars orbiting each other in an elliptical orbit every 9.1 years.[51] The system emits a higher than expected level of infrared radiation, as measured by IRAS space-based observatory. This may be an indication of dust in the system, and is considered somewhat unusual for a binary star.[52][49]

An artist's impression of Sirius A and Sirius B. Sirius A is the larger of the two stars. (Credit: NASA)
An artist's impression of Sirius A and Sirius B. Sirius A is the larger of the two stars. (Credit: NASA)

Sirius A has a mass about 2.1 times that of the Sun.[53][49] The radius of this star has been measured by an astronomical interferometer, giving an estimated angular diameter of 5.936±0.016 mas. The projected rotational velocity is a relatively low 16 km/s, which does not produce any significant flattening of its disk.[10] Stellar models suggest the star formed during the collapsing of a molecular cloud, and that after 10 million years, its internal energy generation was derived entirely from nuclear reactions. The core became convective and utilized the CNO cycle for energy generation.[10] Sirius A is predicted to have completely exhausted the store of hydrogen at its core within a billion (109) years. At that point it will pass through a red giant stage, then settle down to become a white dwarf.

With a mass nearly equal to the Sun's, Sirius B is one of the more massive white dwarfs known; it is almost double the 0.5–0.6 solar mass average. Yet that same mass is packed into a volume roughly equal to the Earth. A white dwarf forms only after the star has evolved from the main sequence and then passed 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 estimated 5 solar masses[54] and was a B-type star (roughly B4-5)[55][56] when it still was on the main sequence. The outer atmosphere of Sirius B is now almost pure hydrogen, as no other elements are seen in this star's spectrum.[57] While it passed through the red giant stage, Sirius B may have enriched the metallicity of its companion. The spectrum of Sirius A shows deep metallic lines, indicating an enhancement in elements heavier than helium, such as iron.[10][49] The proportion of iron in the atmosphere of Sirius is given by \begin{smallmatrix}[\frac{Fe}{H}]=0.5\end{smallmatrix},[7] which is equivalent to 100.5, or about 316% of the proportion of iron in the Sun's atmosphere. The high surface content of metallic elements is unlikely to be true of the entire star. Instead these may be suspended by a thin convection zone at the surface.[10]

Sirius was once considered to be a member of the Ursa Major Moving Group; a set of 220 stars that share a common motion through space. These stars were once formed as members of an open cluster, which has since become gravitationally unbound.[58] However, recent studies of this moving group given an estimated age of 500±100 million years, which is older than Sirius.[59] Sirius may instead be a member of the proposed Sirius Supercluster, along with other scattered stars such as Beta Aurigae, Alpha Coronae Borealis, Beta Crateris, Beta Eridani and Beta Serpentis.[60] This is one of three large clusters located within 500 light years of the Sun. The other two are the Hyades and the Pleiades, and each of these clusters consists of hundreds of stars.[61]

The most commonly used proper name of this star comes from the Latin Sīrius, from Greek Σείριος (Seirios, "glowing" or "scorcher"),[62] although the word is possibly not of Greek origin. The name's earliest recorded use dates from the 7th century BC in Hesiod's poetic work Works and Days.[63] Sirius has over 50 other designations and names attached to it.[64]

Many cultures have historically attached special significance to Sirius, particularly in relation to dogs. Indeed, it is often colloquially called the "Dog Star" as the brightest star of Canis Major, the "Great Dog" constellation. It was also classically depicted as Orion's dog. The Latin name for this star is Canicula ("little dog"), and in Arabic it is known as الشعرى (transliteration: al-ši‘rā or al-shira; English: "the leader"),[65] from which the alternate name Aschere derives. In Sanskrit, it is known as Mrgavyadha "deer hunter" or Lubdhaka "hunter". As Mrgavyadha, the star represents Rudra (Shiva)[66][67]. In Chinese astronomy the star is known as the star of the 'celestial wolf' (Chinese and Japanese: 天狼; Korean: 천랑; Chinese romanization: Tiānláng; Japanese romanization: Tenrō; Korean romanization: Cheonlang),[68] in the Mansion of Jǐng (井宿), while the Japanese vernacular name of the star is 青星 (Aoboshi, "blue star"). In Scandinavia, the star has been known as Lokabrenna ("burning done by Loki", or "Loki's torch").

Aristotle might have referred to the double star in his Meteorologica, citing African sources:

"... some of the fixed stars too get a tail. For this we must not only accept the authority of the Egyptians who assert it, but we have ourselves observed the fact. For a star in the thigh of the Dog had a tail, though a faint one. If you fixed your sight on it its light was dim, but if you just glanced at it, it appeared brighter."

However, this reference has been attributed to M41 or a train of stars near Delta Canis Majoris.[69]

In the astrology of the Middle Ages, Sirius was a Behenian fixed star,[70] associated with beryl and juniper. Its kabbalistic symbol Image:Agrippa1531 Canismaior.png was listed by Heinrich Cornelius Agrippa.[71] Medieval star charts also listed the alternate names Waghi, Vagieh and Veka for this star.[72]

The Dogon people are a tribal people living in Africa who were reported as having certain traditional astronomical knowledge about Sirius that would normally be considered impossible without the use of telescopes. According to Robert Temple's book The Sirius Mystery, the Dogon knew about the fifty-year orbital period of Sirius, the four Galilean moons of Jupiter, the rings of Saturn, plus a third star accompanying Sirius A and B, to name a few. The reason for this has been the subject of some controversy and speculation, but according to a 1978 "Skeptical Enquirer" article it is likely the result of "cultural contamination."[73]

See also: Sirius in fiction

The name has been utilised widely due to its position as brightest star and hence superlative connotation;[citation needed] It features on the coat of arms of Macquarie University, and is the name of its alumnae journal.[74] Seven ships of the Royal Navy have been called HMS Sirius since the 18th century, with the first being the flagship of the First Fleet to Australia in 1788. The Royal Australian Navy subsequently named a vessel HMAS Sirius in honor of the flagship. American vessels include the USNS Sirius (T-AFS-8) as well as a monoplane model the Lockheed Sirius, the first of which was flown by Charles Lindbergh. The name was also adopted by Mitsubishi Motors as the Mitsubishi Sirius engine in the 1980s. North American satellite radio company Sirius Satellite Radio is named after the star because its three satellites are among the most powerful in orbit.[citation needed] It is also frequently a subject used in science fiction and related popular culture.[75]

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Retrieved from "http://en.wikipedia.org/wiki/Sirius"
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