Star Sirius (Alpha Canis Majoris)
|Sirius A and Sirius B captured by the Hubble Space Telescope . Sirius B is the small glowing dot in the lower left.|
dates equinox : J2000.0 , epoch : J2000.0
|right ascension||6h 45m 08.82s _ _ _|
|declination||-16° 42′ 56.9″|
|apparent brightness||−1.46 mag|
|radial velocity||(−8.60 ± 0.72) km/s|
|parallax||(379.2 ± 1.6) mass|
(2.64± 0.01pc )
|proper motion :|
|Rec. share:||(−546 ± 1) mas / a|
|Decl. share:||(−1223±1) mas / a|
|Large semi-axis||7.501″ / approx. 20 AU|
|apparent brightness||A||−1.46 mag|
|B||(8.53 ± 0.05) mag|
|spectral class||A||A1 Vm|
|B−V color index||A||0.01|
|U−B color index||A||−0.05|
brightness M vis
|B||(11.43 ± 0.05) mag|
|Dimensions||A||(2.12 ± 0.06) M ☉|
|B||(0.978 ± 0.005) M ☉|
|radius||A||(1.711±0.013 ) R☉|
|B||(0.00864 ± 0.00012 ) R☉|
|luminosity||A||( 25.4 ±1.3) L☉|
|B||0.027L ☉ _|
|effective temperature||A||(9,900 ± 200) K|
|B||(25,193 ± 37) K|
|rotation duration||A||< 5.5d|
|old||(238 ± 13) million years|
and catalog entries
Sirius , Bayer designation α Canis Majoris ( Alpha Canis Majoris , α CMa ), also Dog Star , formerly also called Ash or Canicula , as a binary star system of the constellation " Canis Major " is the southernmost visible celestial object of the winter hexagon .
Its brighter component has an apparent magnitude of −1.46. This makes Sirius A the brightest star in the night sky , almost twice as bright as the second brightest star , Canopus , with an apparent magnitude of −0.72. Among the stars, only the sun , the moon and the planets Venus , Jupiter , Mars and Mercury are brighter. The brightness of its companion, the white dwarf Sirius B, is only 8.5 mag.
Sirius A is a main sequence star of spectral type A1 with luminosity class V and the suffix m for "metallic". Its mass is about 2.1 times that of the Sun. Interferometric measurements show that its diameter is 1.7 times the diameter of the Sun. Sirius' luminosity is 25 times that of the Sun. The surface temperature is almost 10,000 K (sun: 5,778 K).
The Doppler broadening of the spectral lines caused by the rotation of the star makes it possible to determine a lower limit for the rotation speed at the equator. It is 16 km/s, implying a rotation period of about 5.5 days or less. This low speed means that no measurable flattening of the poles can be expected. In contrast, the similarly sized Vega rotates much faster, at 274 km/s, resulting in a significant bulge at the equator.
The light spectrum of Sirius A shows distinct metallic lines. This indicates an enrichment of elements heavier than helium , such as iron , which is particularly easy to observe spectroscopically . The ratio of iron to hydrogen in the atmosphere is about three times that in the sun's atmosphere (corresponding to a metallicity of [Fe/H] = 0.5). It is suspected that the high proportion of heavier elements observed in the stellar atmosphere is not representative of the entire interior of the star, but is due to the accumulation of the heavier elements on the thin outer convection zone of the star.
According to current star models , the gas and dust cloud from which Sirius A was formed together with Sirius B had reached the stage after about 4.2 million years in which the energy production through the slowly starting nuclear fusion exceeded the energy release through contraction by half. Eventually, after ten million years, all of the energy produced came from nuclear fusion. Since then, Sirius A has been an ordinary, hydrogen-burning main sequence star. At a core temperature of around 22 million Kelvin, it gains its energy mainly via the Bethe-Weizsäcker cycle . Because of the strong temperature dependence of this fusion mechanism, most of the energy generated in the core is transported by convection . Outside the core, the energy is transported by radiation, only just below the surface of the star does convective transport start again (see also stellar structure ).
Sirius B, the faint companion of Sirius A, is the closest white dwarf to the Solar System . It's only about the size of Earth and because of its proximity, it's one of the best-studied white dwarfs ever. He played an important role in the discovery and description of white dwarfs. Observation is made more difficult by being outshined by Sirius A's great brightness. Sirius B has 98% of the Sun's mass but only 2.7% of its luminosity.
In 1844, Friedrich Bessel noticed an irregularity in the proper motion of Sirius when evaluating long-term observation series, which he interpreted as the influence of a binary partner with an orbital period of about half a century. Although the existence of physically related binary stars had been shown four decades earlier by William Herschel , all known to date had two or more visible components. The fact that Sirius' suspected companion had not been seen by anyone before did not deter Bessel: "The fact that countless stars are visible obviously proves nothing against the existence of countless invisible ones".
In 1851, in his habilitation thesis, Christian Peters was able to determine the orbital period of the companion to be 50.093 years and its mass to be more than six Jupiter masses , determine a strong eccentricity of the orbit and give an ephemeris of its expected positions. Despite this assistance, no one succeeded in observing until January 31, 1862, when Alvan Graham Clark , a son of the Boston instrument maker Alvan Clark , examined a newly completed objective lens on Sirius and stated, "Father, Sirius has a companion." Since Sirius B is on As it moved away from Sirius A in its orbit at the time, it could now also be located and measured by numerous other observers.
After several years of position measurements, which also revealed the distances of the two stars from the common center of gravity and thus their mass ratio, Otto von Struve determined in 1866 that the companion was about half as heavy as Sirius itself. With the same structure as Sirius, the companion would have at least 80% of its diameter and therefore only have to have a slightly lower brightness. But because the companion only reached the eighth magnitude, i.e. was 10,000 times fainter than Sirius, Struve concluded "that the two bodies are of very different physical properties".
For decades, Sirius B remained a mere curiosity. After the application of spectral analysis to starlight had allowed stars to be divided into spectral classes , Ejnar Hertzsprung and Henry Russell were able to discover systematic connections between the spectral class of a star and its luminosity from around 1910. In the Hertzsprung-Russell diagram , the stars (studied at the time) formed two groups: "dwarfs" and "giants". However, even then the star 40 Eridani B, a faint companion of 40 Eridani , did not fit into the scheme : it was far too faint in terms of its spectral class. In 1915 a spectrum of Sirius B could be taken, which placed it in the vicinity of 40 Eridani B in the HR diagram and showed that the two apparently belonged to a new class of stars. Their low luminosity despite the high temperature testified to a small radiating surface, i.e. a small radius and thus an immense density.
Starting in the 1920s, Arthur Eddington had developed detailed and successful stellar models by studying spheres of gas in which the gravitational pressure of the gas masses was in equilibrium with the gas pressure and the radiation pressure. However, he was only able to partially describe the so-called "white dwarfs" with his models until Ralph Fowler included the Pauli principle , which had only just been discovered, in 1926 . Inside a white dwarf, the gas is completely ionized , ie it consists of atomic nuclei and free electrons . Since the electrons obey the Pauli principle, no two electrons can have the same quantum numbers . In particular, this means that electrons in an already highly compressed electron gas can only approach each other if the external pressure increases if some of the electrons escape to higher energy levels. Because of this resistance, which is known as degeneracy pressure , additional energy must be expended during compression . While the atomic nuclei provide the main part of the stellar mass, the electrons contribute to the stabilization of the star with the degeneracy pressure caused by quantum mechanics. A consequence of this is that the radius of a white dwarf decreases with increasing mass , while the radius of an ordinary star increases with increasing mass. Subrahmanyan Chandrasekhar showed in 1931 that a white dwarf can no longer be stable above a limit mass of about 1.4 solar masses (“ Chandrasekhar limit ”).
In the course of his preliminary work on the general theory of relativity , Albert Einstein had already predicted in 1911 that photons emitted by a massive body with the wavelength λ 0 would arrive at an observer higher in the gravitational field with a longer, i.e. red- shifted , wavelength. Some attempts to observe this effect on spectral lines of the sun initially failed because of its insignificance. The wavelength shift, expressed as a velocity in km/s (as if it were a Doppler effect due to relative motion) is 0.6· M / R , where M and R are the mass and radius of the body, expressed as multiples of the sun's mass and solar radius . Since the ratio M / R changes little compared to the Sun, even in the case of very massive stars, proof of the effect seemed hopeless until the 1920s.
For white dwarfs, however, the radius decreases with increasing mass. They are therefore massive objects with a small radius, which should show a clear redshift. Eddington, who had already demonstrated the relativistic deflection of light in the sun's gravitational field in 1919, saw this as an opportunity to confirm the extraordinary density of white dwarfs he suspected. Sirius B was chosen because it was part of a binary star system. Therefore, its mass was known, and by comparison with the spectrum of Sirius A it was also possible to distinguish the gravitational component of the redshift from the Doppler shift produced by the system 's radial velocity . Based on the values for temperature and radius assumed at the time, Eddington expected a redshift of around +20 km/s. In 1925, Walter Adams was able to record spectra from Sirius B, which supposedly were only slightly superimposed by light from Sirius A, and obtained a shift of +21 km/s. JH Moore confirmed the measurement in 1928 with a value of (21 ± 5) km/s.
In the following decades, the theoretical models of white dwarfs could be significantly improved. It turned out that Eddington had grossly underestimated Sirius B's temperature and therefore overestimated the radius. The theory now required four times the redshift calculated by Eddington. Indeed, renewed measurements in 1971 showed a redshift of (+89 ± 16) km/s. The authors explained Adams' result by saying that, because of the strong light interference from Sirius A, spectral lines were also measured that were now known to belong to Sirius A. The current value for the gravitational redshift of Sirius B is (80.42 ± 4.83) km/s; the resolving power of the Hubble Space Telescope made it possible in 2004 to record a high-resolution spectrum of Sirius B without significant interference from Sirius A.
According to current stellar evolution scenarios , Sirius A and B formed together as a binary star system about 240 million years ago. Sirius B was originally much heavier and more luminous than Sirius A, with five solar masses and 630 times the luminosity of the Sun, than Sirius A with only two solar masses. Because of its large mass and the associated high fusion rate, Sirius B had burned most of the hydrogen in its core to helium after about 100 million years ( hydrogen burning ). The fusion zone shifted into a shell around the burned out core and Sirius B expanded into a red giant . Eventually, this energy source also dried up, so Sirius B began to fuse the helium produced into carbon and oxygen ( helium burning ). It lost its only weakly bound outer layers because of the strong onset of stellar wind and lost about four-fifths of its original mass. What remained was the burned-out core, consisting mainly of carbon and oxygen, in which practically no more energy production took place. Since the nuclear matter was now completely ionized and the internal pressure for stabilization was missing, the atomic nuclei and free electrons needed much less space. The nucleus was therefore able to shrink to an enormous density until the degeneracy pressure of the electrons prevented further compression. Sirius B has now been at this stage for about 124 million years and is slowly cooling.
Compared to the much brighter component Sirius A, the companion star Sirius B has an apparent magnitude of only 8.5 mag. It is just under one solar mass, making it one of the most massive white dwarfs known (most white dwarfs are concentrated in a narrow band around 0.58 solar masses, with only an estimated 2% or less exceeding one solar mass). With a surface temperature of around 25,000 K, Sirius B is much hotter than the Sun or Sirius A. Despite this high temperature, its brightness is only one ten-thousandth that of Sirius A. Combining the observed temperature and brightness with model calculations gives a diameter of 0.00864 solar diameters (approx. 12,020 km). So Sirius B is even slightly smaller than Earth (mean diameter 12,742 km).
The gravity on the surface of Sirius B is almost 400,000 times that on Earth (log g = 8.556), its mean density is 2.38 tons/cm 3 , the density at its center is 32.36 tons/cm 3 . Sirius B consists of a fully ionized mixture of carbon and oxygen surrounded by a thin atmosphere of ionized hydrogen. Due to the strong surface gravity, almost all of the heavier impurities in the atmosphere have sunk into the core, so that the spectrum of Sirius B shows almost exclusively hydrogen lines. Because the hot hydrogen atmosphere is transparent to X-rays , X-ray emissions originating from deeper, hotter layers can be observed. Sirius B has practically no magnetic field.
In the 1920s, several astronomers repeatedly observed a faint star of about magnitude 12 in the immediate vicinity of Sirius A, but then lost this possible new companion. In 1999, French astronomers were able to examine Sirius A's surroundings more closely for faint stars on a picture taken with it dimmed. They found a background star of matching magnitude that Sirius had passed in the first half of the 20th century, apparently seen by observers at the time. Also, when compared to an earlier image, the astronomers could not find a companion star to within 30 arc seconds that revealed itself by sharing proper motion with Sirius A.
A study of irregularities in the orbital motion of Sirius A and B suggests that there may be a third component in the Sirius system, estimated to be only about 0.06 solar masses, with an orbital period of about 6 years. Since there is no stable orbit around Sirius B with an orbital period longer than 4 years, the potential Sirius C can only orbit around Sirius A.
Sirius A and B as a binary star system
The Sirius star system consists of the two stars described above, which move about their common center of mass with a period of 50.052 years . As with all double stars , each of the two stars moves in an ellipse around this center of mass; for each of the two ellipses one of their foci coincides with the centroid. Since Sirius A is more than twice as heavy as Sirius B, the center of gravity of the system is closer to Sirius A.
For practical reasons only the relative path of Sirius B with respect to Sirius A is usually shown, which therefore occupies a fixed point in the diagram. This relative orbit is also an ellipse, but now with Sirius A at one of its foci. If a terrestrial observer could look perpendicularly at the orbital plane of the binary star system, he would see this ellipse with a 7.501″ long semimajor axis and an eccentricity of 0.5923. Taking into account the distance from Sirius, this results in a length of almost 20 astronomical units (AU) or almost three billion kilometers for the semimajor axis , a smallest distance of 8 AU and a largest distance of 31.5 AU. The smallest and largest distances would appear to this observer at an angle of 3.1″ and 11.9″, respectively.
However, because the orbit is tilted 136.62° from the line of sight, the observer sees the orbit in an oblique view, which again appears as an ellipse, but with slightly more eccentricity. The figure shows this apparent orbit as it appears from Earth. Although Sirius A is at a focus of Sirius B's relative orbit, it is not at a focus of the foreshortened ellipse shown in the diagram because of the oblique view. Due to the oblique view, the largest possible and smallest possible angular distances that Sirius B passes through on this apparent path appear to the observer to be somewhat smaller than the undistorted values given above. Sirius B passed the closest distance to Sirius A (the periastron ) on its true orbit for the last time in 1994, but reached the closest distance on the apparent orbit due to foreshortening as early as 1993.
While Sirius B was in the red giant state, mass conversion may have increased the metallicity of its companion Sirius A. This would explain why Sirius A contains more metals (elements heavier than helium) in its envelope than would be expected for a main sequence star. For example, the iron content is B. 7.4 times that of the Sun. Values measured by the infrared satellite IRAS show higher than expected infrared radiation for the Sirius system. This could indicate dust in this system and is considered unusual in a binary star system.
The nearest star neighbor Prokyon is 5.24 light years away from Sirius A+B. The other larger neighboring star systems are with distances of 7.8 ly Epsilon Eridani , 8.6 ly the Sun and 9.5 ly Alpha Centauri .
Sirius and Arcturus were the first stars to be found to have proper motion , after being considered immobile (“fixed stars”) for thousands of years. In 1717, Edmond Halley , comparing the star positions he had measured himself with the ancient coordinates recorded in the Almagest , noted that Sirius had shifted about half a degree (a full moon diameter) south since the time of Ptolemy .
In 1909, Hertzsprung drew attention to the fact that Sirius, due to its own motion, could also be regarded as a member of the Ursa Major stream . This stellar stream consists of about 100 to 200 stars moving together through space, believed to have formed together as members of an open star cluster , but then drifted far apart. However, research from 2003 and 2005 raised doubts as to whether Sirius could be a member of this group. The age of the Ursa Major group had to be pushed up to about 500 (± 100) million years, while Sirius is only about half that age. That would make him too young to belong to this group.
In addition to the Ursa Major Stream, other motion clusters can be distinguished in the vicinity of the sun, including the Hyades supercluster and the Sirius supercluster. In addition to Sirius and the Ursa Major group, the latter also includes widely scattered stars such as Beta Aurigae , Alpha Coronae Borealis , Zeta Crateris, Beta Eridani and Beta Serpentis . The name "supercluster" is based on the idea that these large asterisms also formed together and - although they have now drifted far apart - have retained a recognizable common movement. Sirius would then be a member, if not of the Ursa Major group, then of the broader Sirius supercluster. However, the scenario of a common origin of the stars in such a supercluster is not undisputed; in particular, it cannot explain why there are stars of very different ages in a supercluster. An alternative interpretation assumes that the Hyades and Sirius streams do not consist of stars of the same origin, but of unrelated stars, which irregularities in the gravitational field of the Milky Way have imprinted a common movement pattern on. We would then not be talking about "superclusters" but about "dynamic flows".
Flyby of the solar system
Sirius will have reached its closest approach to the solar system at about 7.86 light-years from a current distance of 8.6 ly on its course in about 64,000 years. Its apparent magnitude will then be −1.68 mag (−1.46 mag today).
The apparent position of Sirius in the sky depends on where it is observed on Earth and is also influenced by Earth precession and Sirius' own motion. For example, today Sirius cannot be seen from the North Pole to 68 degrees north latitude , while Sirius is constantly in the sky from the South Pole to 67 degrees south latitude. In connection with this, the maximum visible height above the horizon increases from the 67th degree of latitude north to the region of the equator by one degree of altitude to around 83° for each degree of latitude, before reducing again to around 30° at the South Pole. In addition, the point of rise of Sirius changes depending on the degree of latitude . At 67 degrees north latitude, Sirius appears on the horizon in a south-easterly position (172°), while at the equator it appears almost to the east (106°). The different visibility conditions have an effect on the observation time in the sky, which varies between just under six hours at the 67th degree of north latitude and the constant presence at the South Pole, depending on the location.
Similar conditions apply to the latitude for sunrise, which varies in time compared to the previous and following day. The region where Sirius cannot be seen will expand further south in the years to come. In the year 7524 AD, the limit of non-visibility shifted up to the 52nd degree of latitude at the level of Berlin . In about 64,000 years, when Sirius will have reached closest approach, this trend will reverse. In Germany it will then be visible all year round and will never go under.
Because of its brightness, Sirius is also a conspicuous star for the casual sky viewer. Its glaring bluish-white light tends to flicker strongly and often colorfully even when there is little air turbulence .
In temperate northern latitudes it is a star of the winter sky, which it dominates because of its brightness. While it is in the daytime sky throughout the summer and cannot be seen with the naked eye, it first becomes visible at dawn towards the end of August. In autumn it is a star of the second half of the night, in winter it already rises in the evening. At the turn of the year, Sirius culminates around midnight and can therefore be seen throughout the night. In spring, it rises before sunset and can no longer be observed. From May its setting has also shifted to daylight, so that it is no longer visible until late summer. A high altitude observer can see Sirius with the unaided eye even during the day, when the sun is already near the horizon and the star is high above the horizon in a location with very clear skies.
Brightness comparison with other stars
Around AD 235,000 Vega will replace Sirius with a calculated magnitude of −0.7 as the brightest star in the sky, before then in AD 260,000 Canopus with −0.46 mag will again displace Sirius as the second brightest star in third place . The evolution of Sirius' brightness relative to other bright stars between 100,000 B.C. and 100,000 AD is shown in the following chart and accompanying table:
year Sirius Canopus Vega Arcturus procyon altair α Cen −100,000 −0.66 −0.82 +0.33 +0.88 +0.88 +1.69 +2.27 −75,000 −0.86 −0.80 +0.24 +0.58 +0.73 +1.49 +1.84 −50,000 −1.06 −0.77 +0.17 +0.30 +0.58 +1.27 +1.30 −25,000 −1.22 −0.75 +0.08 +0.08 +0.46 +1.03 +0.63 0 −1.43 −0.72 −0.02 +0.37 +0.78 −0.21 25,000 −1.58 −0.69 −0.08 +0.02 +0.33 +0.49 −0.90 50,000 −1.66 −0.67 −0.16 +0.19 +0.32 +0.22 −0.56 75,000 −1.66 −0.65 −0.25 +0.45 +0.37 −0.06 +0.30 100,000 −1.61 −0.62 −0.32 +0.74 +0.46 −0.31 +1.05
Sirius in history
The earliest recorded mention of Sirius ( Ancient Greek Σείριος , Seirios ) is in the 7th century BC. in Hesiod's didactic poem Works and Days . The origin of the name is subject to several interpretations: Leukosia (The White) is one of the sirens (Seirenes) in Greek mythology . A possible connection to Sirius with the naming as The blazing white light is content of controversy as well as the use of the terms searing hot and scorching for Seirios . Finally, another equation with the Indo -European root *tueis-ro for "to be excited" or "to sparkle" is accepted. However, some scholars dispute this derivation.
Sirius from the perspective of other cultures
The apparent magnitude of Sirius was slightly less in ancient times , at −1.41 mag. The distance was 8.8 light years. As a particularly conspicuous star, Sirius has been found in the myths, religions and customs of numerous cultures since prehistoric times, which can only be briefly touched upon here.
|The Egyptians initially did not see Sirius in their language as a single star, but in connection with the triangular constellation Sopdet (
It remains unclear whether the meaning "the tip" was exclusively associated with the constellation Sopdet. As worship focused on Sirius, the other two stars faded in importance. Regarding the flood of the Nile , Sirius held an important place in the course of Egyptian history. Herodot gives the time around the 22./23. June as the beginning of the Nile Flood. Entries in Egyptian administrative documents confirm Herodotus' statements. Historical and astronomical reconstructions show that the first morning sighting of Sirius in the Nile Delta was around 2850 BC. and in the southernmost place Aswan around 2000 BC. with the 22./23. June coincided.
Sirius was therefore considered in the 3rd millennium BC. BC as a herald of the Nile flood and enjoyed an even greater importance in Egyptian religion. In the further course of Egyptian history, the heliacal rising of Sirius took place only after the arrival of the Nile flood. In the Graeco-Roman period of Egypt , mythological account was taken of the changed conditions . Now it was Salet who triggered the flood of the Nile with an arrow shot; her daughter Anukket then took care of the decongestion of the Nile . The heliacal rising of Sirius takes place today in Aswan on August 1st and in the Nile Delta on August 7th.
Sumer and Mesopotamia
Among the Sumerians , Sirius took precedence in the 3rd millennium BC. B.C. in Sumerian religion early on in several central roles. As a calendar star, with the designation MUL KAK.SI.SÁ, it fulfilled an important function in the agricultural cycle. As the MUL KAK.TAG.GA (Heavenly Arrow) , Sirius was considered a chief deity of the Seven and was subject to the ruling star of God over the other celestial objects , Venus , worshiped as the goddess Inanna . In the Akitu - New Year's processions , Sirius was considered to be the one who crossed the seas and received corresponding offerings. It also functioned later in almost unchanged form with the Babylonians and Assyrians , who additionally determined Sirius according to the MUL.APIN clay tablets as a signal giver for the time of leap years .
Greece, Rome and Germany
Among the Greeks and Romans, Sirius was associated with heat, fire, and fever. The Romans called the hottest time of the year (usually from early July to mid-August) the “ dog days ” (Latin dies caniculares , days of the dog star). In German popular belief, the dog days were viewed as a time of bad luck from the 15th century. Sirius was considered by the Greeks to be the forerunner of rabies .
North America and China
Sirius is also associated with dogs or wolves in many North American tribes. For example, among the Cherokee , Sirius and Antares are the Dog Stars guarding the ends of the "Path of Souls" (the Milky Way ): Sirius the eastern end of the winter sky, Antares the western end of the summer sky. A departing soul must carry enough food to appease both dogs lest it wander forever on the path of souls. The Pawnee refer to Sirius as the Wolf Star.
For the Chinese, the stars of today's constellations of Aft Deck and Big Dog formed a constellation representing a bow and arrow. The arrow aimed directly at the "sky wolf" (天狼星), namely Sirius.
South Sea Islands
In Polynesia and Micronesia , the brighter stars of the southern sky, particularly Sirius, have been used to navigate between the scattered archipelagos since prehistoric times. Near the horizon, bright stars like Sirius could be used as guides (with several stars alternating in this role over the course of a night). Stars could also be used to determine latitude. So Sirius, with its declination of 17° South, moves vertically across Fiji with the geographical latitude 17° South and you only had to drive south or north until Sirius passed through the zenith .
Sirius and the Dogon
French ethnologist Marcel Griaule studied the Dogon ethnic group in Mali , West Africa , for two decades starting in 1931 . The extensive Dogon creation myths, which Griaule amassed through interviews, mostly via interpreter, with four high-ranking tribesmen, are said to include information about a strange companion of Sirius:
- the star Sirius (sigu tolo) is orbited by the smaller companion po tolo . Po tolo takes its name from po , the smallest cereal grain ( Digitaria exilis ) known to the Dogon.
- Po tolo moves in an oval orbit around Sirius; Sirius is not at the center of this orbit, but eccentric.
- Po tolo takes 50 years to traverse the orbit once and rotates on itself once a year.
- When po tolo is close to Sirius, Sirius gets brighter. When the distance is at its greatest, Sirius will flicker and may appear as multiple stars.
- Po tolo is the smallest star and indeed the smallest thing conceivable for the Dogon. But at the same time it is so heavy that all people would not be enough to lift it.
- A third member of the Sirius system is the star emme ya tolo (named after a sorghum seed ), which is slightly larger than po tolo but only a quarter as heavy. It orbits Sirius on a larger orbit and also once every 50 years.
The resemblance of these descriptions to Sirius B and a possible Sirius C is all the more astonishing since none of them can be seen with the naked eye. Numerous different speculations try to explain the origin of this alleged knowledge. Two main currents can be found in popular literature: One view, which is mainly represented in Afrocentric literature, even sees the Dogon as a remnant of a once highly developed, scientifically shaped African civilization. R. Temple, on the other hand, suggested in his book The Sirius Mystery that extraterrestrial visitors from the Sirius system initiated the rise of Egyptian and Sumerian civilizations about 5,000 years ago.
The explanation preferred in scientific circles assumes the contamination of Dogon mythology with modern astronomical knowledge. The anthropological variant assumes that the contamination (though not intentional) was done by Griaule himself. Dutch anthropologist Walter van Beek himself worked with the Dogon and attempted to verify some of the Griaule material. However, he could not confirm large parts of the myths reported by Griaule, including Sirius as a binary star system. Van Beek is of the opinion that the myths published by Griaule are not simply reproductions of the stories told by his informants, but have come about through a complex interaction between Griaule, his informants and the translators. Some of them are the result of misunderstandings and overinterpretation by Griaule.
One possible explanation relates to assumed Dogon contacts with European visitors. She points out that the Dogon narratives reflect the state of astronomical knowledge from around 1926 (while Griaule only began working with the Dogons from 1931): the orbital period, elliptical orbit and great mass of Sirius B were already in the 19th century known, its small diameter from about 1910, a possible third companion was suspected in the 1920s, the high density of Sirius B was detected in 1925. The observation of the gravitational redshift on Sirius B went through the popular press as a sensational confirmation of the general theory of relativity. Missionaries , for example, come into consideration as sources , which is also indicated by biblical and Christian motifs in Dogon mythology. Missionary activities among the Dogon took place from 1931, but so far no missionaries have been proven who could be considered a specific source.
Sirius as a red star
Sirius appears bluish-white to the viewer. In the star catalog of the Almagest written by Claudius Ptolemy around 150 AD , Sirius, the main star of the constellation Canis Major, is found with the entry:
|The star in the mouth, the brightest, which is called dog[sstern] and is reddish||Gem 17 2/3||−39 1/6||1|
While Sirius is clearly meant according to the description and coordinates, the reddish color mentioned does not match Sirius' blue-white color. There has been speculation since the 18th century as to whether Sirius could actually have changed color over the past 2000 years. In this case, Ptolemy's remark would provide valuable observational material both in general for stellar evolution and specifically for what is happening around the sun.
However, even with reference to independent sources, it cannot be clearly decided whether Sirius was perceived as red in ancient times or not. An Assyrian text from 1070 BC. describes Sirius as "red as molten copper." Sirius is described as reddish by Aratos in his didactic poem Phainomena and by his later editors. In Pliny Sirius is "fiery" and in Seneca it is even redder than Mars . The early medieval Bishop Gregory of Tours also referred to Sirius as a red star in his work De cursu stellarum ratio (about 580 AD). On the other hand, Manilius refers to Sirius as "sea blue," and four ancient Chinese texts describe the color of some stars as "as white as [Sirius]." In addition, Sirius is often described as having a strong sparkle; but an impressive twinkle would require the full spectral colors of a white star, while the duller twinkle of a red star would have attracted little attention. Five other stars designated by Ptolemy as red (including Betelgeuse , Aldebaran ) are also reddish to today's observer.
According to today's understanding of stellar development, a period of 2000 years is far from sufficient to bring about visible changes in the star types in question. Accordingly, neither a heating of Sirius A from a few thousand Kelvin to today's almost 10,000 K, nor a sighting of Sirius B in its phase as a red giant is conceivable. However, alternative explanations have so far not been entirely convincing:
- An interstellar dust cloud passing between Sirius and Earth may have caused a significant reddening of the light from shining through stars. However, such a cloud would have had to weaken Sirius' light so much that it would have appeared at best as an inconspicuous third-magnitude star and its brightness would not have been sufficient to produce a color impression in the human eye . Traces of such a cloud were not found.
- The terrestrial atmosphere also reddens the light of low-lying stars, but does not weaken it as much. Since the heliacal rise of Sirius was an important calendrical fixed point for many ancient cultures, particular attention could have been paid to Sirius, which is low and then appears reddish. Sirius may then have retained this color as a distinctive attribute. Theoretical calculations suggest that the atmosphere can indeed redden a star's light sufficiently without pushing the brightness below the color perception threshold. However, practical observations have so far not been able to determine any pronounced reddening effect.
- "Reddish" may be merely a symbolic attribute, associating Sirius with the summer heat heralded by its heliacal appearance.
At the beginning of the 19th century , Friedrich Wilhelm Herschel defined the siriometer as the distance from the sun to Sirius. However, the unit failed to catch on and is no longer used today.
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- Reference points of the calendrical Sothis rises in ancient Egypt
- list of stars
- Mount Canicula
- Sirius Knoll
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- Sirius as a precursor of rabies
- Sirius as the harbinger of bad luck in Germany
- The parallax of Sirius is 0.379″. So an AE at that distance appears at an angle of 0.379″. An angle of 7.5″ therefore corresponds to a distance of 7.5/0.379 = 19.8 AU.
- Smallest distance = semi-major axis (1 – eccentricity), largest distance = semi-major axis (1 + eccentricity).
- Calculated from apparent brightness and parallax: M = m + 5 + 5 log(parallax) = −1.46 + 5 + 5 log(0.379″) = +1.43 mag (see distance module ).
- This corresponds to an absolute magnitude of 1.43 mag, see also info box.
- Due to the logarithmic brightness scale, the difference in brightness is not about , but
- Since the apparent brightness is subject to fluctuations, Sirius sometimes appears brighter than the planets mentioned. See also data from NASA Factsheets: Moon (to −12.74 mag), Venus (to −4.6 mag), Jupiter ( Memento of October 5, 2011 on WebCite ) (to −2.94 mag), Mars (to −2.91 mag) and Mercury (to −1.9 mag).
- Chandrasekhar initially received a limit value of 0.91 solar masses because of the only imprecisely known composition of a white dwarf.
- At the earth's surface, the gravitational acceleration g is about 981 cm/s 2 (cgs units!). The gravitational acceleration at the surface of Sirius B is almost 400,000 times higher and amounts to about g = 360 million cm/s 2 d. H. log(g) = 8.556 (a numerical value equation since g carries a unit. By convention g is to be put in cm/s 2 ).
- The name Supercluster should not end here with the English be confused with the term supercluster for superclusters (clusters of galaxies).
- The more often published statements that the heliacal rise of Sirius before 2850 BC. and after 2000 B.C. BC with the onset of the Nile flood cannot be confirmed by the astronomical results and contemporary Egyptian documents.
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