Sirius is the brightest star in the night sky with a visual apparent magnitude of -1.47, almost twice as bright as the next brightest star Canopus.
It is also known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (English: Big Dog).
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 marked winter and was an important star for navigation around the Pacific Ocean.
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, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed 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 (more correctly the Sirius system) 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 Star System is between 200 and 300 million years old and comprised of 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.
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, 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 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).
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.
It was described as 'burning' or 'flaming' in literature.
These 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 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.
The Romans celebrated the heliacal setting of Sirius around the 25th of April, 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 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.
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 Maori, 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.
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.
In 1844, German astronomer Friedrich Wilhelm Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion.
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".
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.
More recent observations have failed to confirm the existence of a third member of the Sirius system, but still have not completely 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.
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.
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.
This would lead astronomers to conclude that it was a white dwarf, the second to be discovered.
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.
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.
In 150 AD, the astronomer 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.
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.
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.
Thomas Jefferson Jackson See resurrected discussion on red Sirius in 1892 with the publication of several papers in 1892, and a final summary in 1926.
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.
Seneca, too, had described Sirius as being of a deeper red colour than Mars.
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.
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.
At a distance of 2.6 pc or 8.6 light years, the Sirius system contains two of the eight nearest stars to Earth, and is the fifth closest stellar system.
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.
The closest large neighbouring star to Sirius is Procyon, 1.61 pc or 5.24 ly away.
Sirius is a binary star system consisting of two white stars orbiting each other with a separation of about 20 AU (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.
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.
Sirius A has a mass about 2.1 times that of the Sun.
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.
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.
Sirius A is predicted to have completely exhausted the store of hydrogen at its core within a billion (109) years after its formation.
At that point it will pass through a red giant stage, then settle down to become a white dwarf.
The spectrum of Sirius A shows deep metallic lines, indicating an enhancement in elements heavier than helium, such as iron.
When compared to the Sun, the proportion of iron in the atmosphere of Sirius A relative to hydrogen is given by, 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.
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. The current surface temperature is 25,200 K.
However, since there is no internal source of energy generation, Sirius B will steadily cool as the remaining heat is radiated into space over a period of more than 2 billion years.
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 and was a B-type star (roughly B4-5) 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.
This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity.
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.
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.
However, recent studies of this moving group given an estimated age of 500±100 million years, which is older than Sirius.
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.
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.
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