 The position of Sirius (circled). |
|
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
|---|---|
| Constellation | Canis Major |
| Pronunciation | /ˈsɪriəs/[1] |
| Right ascension | 06h 45m 08.9173s[2][note 1] |
| Declination | −16° 42′ 58.017″[2][note 1] |
| Apparent magnitude (V) | −1.47 (A)[2] / 8.30 (B)[3] |
| Characteristics | |
| Spectral type | A1V (A)[2] / DA2 (B)[3] |
| U−B color index | −0.05 (A)[4] / −1.04 (B)[3] |
| B−V color index | 0.01 (A)[2] / −0.03 (B)[3] |
| Astrometry | |
| Radial velocity (Rv) | −7.6[2] km/s |
| Proper motion (μ) | RA: −546.05[2][note 1] mas/yr Dec.: −1223.14[2][note 1] mas/yr |
| Parallax (π) | 379.21 ± 1.58[2][5] mas |
| Distance | 8.60 ± 0.04 ly (2.64 ± 0.01 pc) |
| Absolute magnitude (MV) | 1.42 (A)[note 2] / 11.18 (B)[3] |
| Orbit[6] | |
| Companion | α CMa B |
| Period (P) | 50.090 ± 0.055 yr |
| Semimajor axis (a) | 7.50 ± 0.04" |
| Eccentricity (e) | 0.5923 ± 0.0019 |
| Inclination (i) | 136.53 ± 0.43° |
| Longitude of the node (Ω) | 44.57 ± 0.44° |
| Periastron epoch (T) | 1894.130 ± 0.015 |
| Argument of periastron (ω) (secondary) |
147.27 ± 0.54° |
| Details | |
| α CMa A | |
| Mass | 2.02[7] M☉ |
| Radius | 1.711[7] R☉ |
| Luminosity | 25.4[7] L☉ |
| Surface gravity (log g) | 4.33[8] |
| Temperature | 9,940[8] K |
| Metallicity [Fe/H] | 0.50[9] dex |
| Rotation | 16 km/s[10] |
| Age | 2–3 × 108[7] years |
| α CMa B | |
| Mass | 0.978[7] M☉ |
| Radius | 0.0084 ± 3%[11] R☉ |
| Luminosity | 0.026[note 3] L☉ |
| Surface gravity (log g) | 8.57[11] |
| Temperature | 25,200[7] K |
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,[5][19][20] the Sirius system is one of Earth's near neighbors; for Northern-hemisphere observers between 30 degrees and 73 degrees of latitude (including almost all of Europe and North America), it is the closest star (after the Sun) that can be seen with a naked eye. 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 recede, but it will continue to be the brightest star in the Earth's sky for the next 210,000 years.[21]
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[7] but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old.[7] 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 120 million years ago.[7]
Sirius is also known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (Greater Dog).[12] 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 it marked winter and was an important star for navigation around the Pacific Ocean.
Observational history
Sirius/Sopdet |
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.[24] 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.[25] The season following the star's appearance came to be known as the Dog Days of summer.[26] 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.[25] 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.[27]
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.[28]
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.[29] 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.[30] 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, Aa and Hoku-Kauopae in Hawaii,[31] 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.[32]
The indigenous Boorong people of northwestern Victoria named Sirius as Warepil.[33]
Kinematics
In 1718, Edmond Halley discovered the proper motion of the hitherto presumed "fixed" stars[34] after comparing contemporary astrometric measurements with those given in Ptolemy's Almagest. The bright stars Aldebaran, Arcturus and Sirius were noted to have moved significantly, the last of which having progressed 30 arc minutes (about the diameter of the moon) southwards in 1,800 years.[35]In 1868, Sirius became the first star to have its velocity measured. Sir William Huggins examined the spectrum of this star and observed a noticeable red shift. He concluded that Sirius was receding from the Solar System at about 40 km/s.[36][37] Compared to the modern value of −7.6 km/s,[2] this both was an overestimate and had the wrong sign; the minus means it is approaching the Sun. However, it is notable for introducing the study of celestial radial velocities.
Discovery of a companion
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 five to ten magnitudes fainter than the white dwarf Sirius B, which would account for the difficulty of observing it.[41] Observations published in 2008 were unable to detect either a third star or a planet. An apparent "third star" observed in the 1920s is now confirmed as a background object.[42]
In 1915, Walter Sydney Adams, using a 60-inch (1.5 m) reflector at Mount Wilson Observatory, observed the spectrum of Sirius B and determined that it was a faint whitish star.[43] This led astronomers to conclude that it was a white dwarf, the second to be discovered.[44] 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.[45] In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has nearly the diameter of the Earth, 12,000 kilometers (7,500 mi), with a mass that is 98% of the Sun.[46][47][48][49]
Red controversy
Around 150 AD, the Greek astronomer of the Roman period Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.[50] The discrepancy was first noted by amateur astronomer Thomas Barker, squire of Lyndon Hall in Rutland, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[51] The existence of other stars changing in brightness gave credence to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[52] Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[53] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as colouring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[54] Seneca, too, had described Sirius as being of a deeper red colour than Mars.[55] However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[56] It 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.[57][58]In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[59] However, other scholars replied that it was likely St. Gregory had been referring to Arcturus instead.[60][61]
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.[55] An interaction with a third star, to date undiscovered, has also been proposed as a possibility for a red appearance.[62] 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.[55]
Visibility
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.[69] These observing conditions are more easily met in the southern hemisphere, due to the southerly declination of Sirius.
The orbital motion of the Sirius binary system brings the two stars to a minimum angular separation of 3 arcseconds and a maximum of 11 arcseconds. At the closest approach, it is an observational challenge to distinguish the white dwarf from its more luminous companion, requiring a telescope with at least 300 mm (12 in) aperture and excellent seeing conditions. A periastron occurred in 1994[note 4] and the pair have since been moving apart, making them easier to separate with a telescope.[70]
At a distance of 2.6 parsecs (8.6 ly), the Sirius system contains two of the eight nearest stars to the Solar System[71] and is the fifth closest stellar system to ours.[71] This proximity is the main 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.[72] However, it is still around 25 times more luminous than the Sun.[7] The closest large neighbouring star to Sirius is Procyon, 1.61 parsecs (5.24 ly) away.[73] 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 (1.3 pc) of Sirius in approximately 296,000 years.[74]
System
Sirius A
Stellar models suggest that 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.[79] It is predicted that Sirius A will have completely exhausted the store of hydrogen at its core within a billion (109) years of its formation. At this point it will pass through a red giant stage, then settle down to become a white dwarf.
Sirius A is classed as an Am star because the spectrum shows deep metallic absorption lines,[82] indicating an enhancement in elements heavier than helium, such as iron.[73][79] When compared to the Sun, the proportion of iron in the atmosphere of Sirius A relative to hydrogen is given by
![\begin{smallmatrix}[\frac{Fe}{H}]=0.5\end{smallmatrix}](http://upload.wikimedia.org/math/8/6/4/864384c478480c81bbe56168baec1840.png)
,[9] which is equivalent to 100.5,
meaning it has 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, rather the iron-peak and heavy metals are radiatively
levitated towards the surface.[79]Sirius B
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, around 120 million years ago. The original star had an estimated 5 solar masses[7] and was a B-type star (roughly B4-5)[85][86] 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.
This star is primarily composed of a carbon–oxygen mixture that was generated by helium fusion in the progenitor star.[7] This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity.[87] Hence the outer atmosphere of Sirius B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in this star's spectrum.[88]
Sirius star cluster
In 1909, Ejnar Hertzsprung was the first to suggest that Sirius was a member of the Ursa Major Moving Group, based on his observations of the system's movements across the sky. The Ursa Major Group is a set of 220 stars that share a common motion through space and were once formed as members of an open cluster, which has since become gravitationally unbound.[89] However, analyses in 2003 and 2005 found Sirius's membership in the group to be questionable: the Ursa Major Group has an estimated age of 500±100 million years, while Sirius, with metallicity similar to the Sun's, has an age that is only half this, making it too young to belong to the group.[7][90][91] 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.[92] This is one of three large clusters located within 500 light-years (150 pc) of the Sun. The other two are the Hyades and the Pleiades, and each of these clusters consists of hundreds of stars.[93]Etymology and cultural significance
See also: Winter triangle
The most commonly used proper name of this star comes from the Latin Sīrius, from the Ancient Greek Σείριος (Seirios, "glowing" or "scorcher"),[94] although the Greek word itself may have been imported from elsewhere before the Archaic period,[95] one authority suggesting a link with the Egyptian god Osiris.[96] The name's earliest recorded use dates from the 7th century BC in Hesiod's poetic work Works and Days.[95] Sirius has over 50 other designations and names attached to it.[64] In Geoffrey Chaucer's essay Treatise on the Astrolabe, it bears the name Alhabor, and is depicted by a hound's head. This name is widely used on medieval astrolabes from Western Europe.[13] In Sanskrit it is known as Mrgavyadha "deer hunter", or Lubdhaka "hunter". As Mrgavyadha, the star represents Rudra (Shiva).[97][98] The star is referred as Makarajyoti in Malayalam and has religious significance to the pilgrim center Sabarimala.[citation needed] In Scandinavia, the star has been known as Lokabrenna ("burning done by Loki", or "Loki's torch").[99] In the astrology of the Middle Ages, Sirius was a Behenian fixed star,[100] associated with beryl and juniper. Its astrological symbol 
was listed by Heinrich Cornelius Agrippa.[101]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 classically depicted as Orion's dog. The Ancient Greeks thought that Sirius's emanations could affect dogs adversely, making them behave abnormally during the "dog days," the hottest days of the summer. The Romans knew these days as dies caniculares, and the star Sirius was called Canicula, "little dog." The excessive panting of dogs in hot weather was thought to place them at risk of desiccation and disease. In extreme cases, a foaming dog might have rabies, which could infect and kill humans whom they had bitten.[25] Homer, in the Iliad, describes the approach of Achilles toward Troy in these words:
In Iranian mythology, especially in Persian mythology and in Zoroastrianism, the ancient religion of Persia, Sirius appears as Tishtrya and is revered as a divinity. Beside passages in the sacred texts of the Avesta, the Avestan language Tishtrya followed by the version Tir in Middle and New Persian is also depicted in the Persian epic Shahnameh of Ferdowsi. Due to the concept of the yazatas, powers which are "worthy of worship", Tishtrya is a divinity of rain and fertility and an antagonist of apaosha, the demon of drought. In this struggle, Tishtrya is beautifully depicted as a white horse.[103][104][105][106]Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion's Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.[102]
In Chinese astronomy the star is known as the star of the "celestial wolf" (Chinese and Japanese: 天狼; ; Chinese romanization: Tiānláng; Japanese romanization: Tenrō;[107] in the Mansion of Jǐng (井宿). Farther afield, many nations among the indigenous peoples of North America also associated Sirius with canines; the Seri and Tohono O'odham of the southwest note the star as a dog that follows mountain sheep, while the Blackfoot called it "Dog-face". The Cherokee paired Sirius with Antares as a dog-star guardian of either end of the "Path of Souls". The Pawnee of Nebraska had several associations; the Wolf (Skidi) tribe knew it as the "Wolf Star", while other branches knew it as the "Coyote Star". Further north, the Alaskan Inuit of the Bering Strait called it "Moon Dog".[108]
Several cultures also associated the star with a bow and arrows. The Ancient Chinese visualized a large bow and arrow across the southern sky, formed by the constellations of Puppis and Canis Major. In this, the arrow tip is pointed at the wolf Sirius. A similar association is depicted at the Temple of Hathor in Dendera, where the goddess Satet has drawn her arrow at Hathor (Sirius). Known as "Tir", the star was portrayed as the arrow itself in later Persian culture.[109]
Sirius is mentioned in Surah, An-Najm ("The Star"), of the Qur'an, where it is given the name الشِّعْرَى (transliteration: aš-ši‘rā or ash-shira; the leader).[110] The verse is: "وأنَّهُ هُوَ رَبُّ الشِّعْرَى", "That He is the Lord of Sirius (the Mighty Star)." (An-Najm:49)[111] Ibn Kathir said in his commentary "Ibn 'Abbas, Mujahid, Qatada and Ibn Zayd said about Ash-Shi`ra that it is the bright star, named Mirzam Al-Jawza' (Sirius), which a group of Arabs used to worship."[112] The alternate name Aschere, used by Johann Bayer, is derived from this.[12]
In Theosophy, it is believed the Seven Stars of the Pleiades transmit the spiritual energy of the Seven Rays from the Galactic Logos to the Seven Stars of the Great Bear, then to Sirius. From there is it sent via the Sun to the god of Earth (Sanat Kumara), and finally through the seven Masters of the Seven Rays to the human race.[113]
Dogon
See also: Nommo
The Dogon people are an ethnic group in Mali, West Africa,
reported to have traditional astronomical knowledge about Sirius that
would normally be considered impossible without the use of telescopes.
According to Marcel Griaule's books Conversations with Ogotemmêli and The Pale Fox
they knew about the fifty-year orbital period of Sirius and its
companion prior to western astronomers. They also refer to a third star
accompanying Sirius A and B. Robert Temple's 1976 book The Sirius Mystery, credits them with knowledge of the four Galilean moons of Jupiter and the rings of Saturn. This has been the subject of controversy and speculation. According to a 1978 Skeptical Inquirer article it is possibly the result of cultural contamination.[114] Some have suggested the contaminators to have been the ethnographers themselves.[115][116] Others see this explanation as being too simplistic.[117]
Serer religion
Main articles: Serer religion and Saltigue
In the religion of the Serer people of Senegal, the Gambia and Mauritania, Sirius is called Yoonir from the Serer language (and some of the Cangin language speakers, who are all ethnically Serers). The star Sirius is one of the most important and sacred stars in Serer religious cosmology and symbolism. The Serer high priests and priestesses, (Saltigues, the hereditary "rain priests"[120]) chart Yoonir
in order to forecast rain fall and enable Serer farmers to start
planting seeds. In Serer religious cosmology, it is the symbol of the
universe.[118][119]Modern legacy
See also: Sirius in fiction
Sirius is frequently a subject used in science fiction and related popular culture,[121] and even the subject of poetry.[122] Sirius is featured on the coat of arms of Macquarie University, and is the name of its alumnae journal.[123] The name of the North American satellite radio company, Satellite CD Radio, Inc., was changed to Sirius Satellite Radio in November 1999, being named after "the brightest star in the night sky".[124] Composer Karlheinz Stockhausen has been claimed to have said on several occasions that he came from a planet in the Sirius system.[125][126] Astronomer Noah Brosch has speculated that the name of the character Sirius Black from the Harry Potter stories might have been inspired by "Sirius B", and notes that the wizard has the ability to transform into a dog.[122] In the BBC Doctor Who series, the Doctor reveals the star actually consists of two smaller ones.Sirius is one of the 27 stars on the flag of Brazil, where it represents the state of Mato Grosso.[127]
Seven ships of Great Britain's 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.[128] The Royal Australian Navy subsequently named a vessel HMAS Sirius in honor of the flagship.[129] American vessels include the USNS Sirius as well as a monoplane model—the Lockheed Sirius, the first of which was flown by Charles Lindbergh.[130] The name was also adopted by Mitsubishi Motors for the Mitsubishi Sirius engine in 1980.[131]
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