Sirius (Alpha Canis Majoris)
dates equinox : J2000.0 , epoch : J2000.0
|Right ascension||6 h 45 m 08.82 s|
|declination||-16 ° 42 ′ 56.9 ″|
|Apparent brightness||−1.46 mag|
|Radial velocity||(−8.60 ± 0.72) km / s|
|parallax||(379.2 ± 1.6) mas|
|distance||(8.60 ± 0.04) Ly
(2.64 ± 0.01 pc )
|Proper movement :|
|Rec. Share:||(−546 ± 1) mas / a|
|Dec. portion:||(−1223 ± 1) mas / a|
|Major 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.||0.027 L ☉|
|Effective temperature||A.||(9,900 ± 200) K.|
|B.||(25,193 ± 37) K.|
|Metallicity [Fe / H]||A.||0.5|
|Rotation time||A.||<5.5 d|
|Age||(238 ± 13) million years|
and catalog entries
Sirius , Bayer designation α Canis Majoris ( Alpha Canis Majoris , α CMa ), also Hundsstern , Aschere or Canicula called, is a binary system of the constellation " Large dog " the southernmost visible sky object of the Winter Hexagon .
Its brighter component has an apparent magnitude of −1.46 mag. 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 mag. Among the stars only the sun , moon and the planets Venus , Jupiter , Mars and Mercury are brighter. The brightness of its companion, the white dwarf Sirius B, on the other hand, is only 8.5 mag.
Sirius A is a main sequence star of the spectral type A1 with luminosity class V and the addition m for "rich in metal". Its mass is about 2.1 times that of the sun . Interferometric measurements show that its diameter is 1.7 times the solar diameter. 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 rotational speed at the equator. It is 16 km / s, which results in a rotation period of around 5.5 days or less. This low speed does not suggest any measurable flattening of the poles. In contrast, the similarly large Vega rotates much faster at 274 km / s, which results in a considerable bulge at the equator.
The light spectrum of Sirius A shows pronounced metallic lines. This indicates an accumulation of heavier elements 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 as large as in the sun's atmosphere (corresponding to a metallicity of [Fe / H] = 0.5). It is assumed 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.
The gas and dust cloud , from which Sirius A and Sirius B arose , had, according to common star models, reached the stage after about 4.2 million years in which the energy generated by the slowly starting nuclear fusion exceeded the energy released by the contraction by half. After ten million years, all of the energy produced came from nuclear fusion. Sirius A has since been a common, hydrogen-burning main sequence star. At a core temperature of around 22 million Kelvin, it generates 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 star surface, convective transport starts again (see also star structure ).
Sirius B, Sirius A's faint companion, is the closest white dwarf to the solar system . It is only about the size of Earth and, because of its proximity, one of the best-studied white dwarfs ever. He played an important role in the discovery and description of white dwarfs. The observation is made difficult by the fact that it is outshone by the great brightness of Sirius A. Sirius B has 98% the mass of the sun, but only 2.7% of its luminosity.
In 1844, while evaluating long-term series of observations, Friedrich Bessel noticed an irregularity in Sirius' own motion , which he interpreted as the influence of a binary star partner with a period of about half a century. Although William Herschel had shown the existence of physically related binary stars four decades earlier , all known to date had two or more visible components. The fact that the presumed companion of Sirius had not yet been seen by anyone did not deter Bessel: "The fact that countless stars are visible obviously does not prove anything against the existence of countless invisible ones".
In his habilitation thesis in 1851, Christian Peters was able to determine the orbital period of the companion as 50.093 years and its mass as more than six Jupiter masses , determine a strong eccentricity of the orbit and give an ephemeris of its expected positions. Despite this help anyone succeeded in the observation until the 31 January 1862 Alvan Graham Clark , a son of the Boston instrument maker Alvan Clark , a recently completed objective lens to Sirius reviewed and stated: "Father, Sirius has a companion." Since Sirius B to its orbit was then increasingly removed from Sirius A, it could now also be located and measured by numerous other observers.
After several years of position measurements , from which the distances of the two stars from the common center of gravity and thus their mass ratio resulted, Otto von Struve found in 1866 that the companion was about half as heavy as Sirius himself. With the same structure as Sirius, the companion would have it at least 80% of its diameter and therefore only have to have a slightly lower brightness. But because the companion only reached the eighth size class, i.e. 10,000 times less luminous 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 allowed the stars to be divided into spectral classes , Ejnar Hertzsprung and Henry Russell were able to uncover systematic relationships between the spectral class of a star and its luminosity from around 1910. In the Hertzsprung-Russell diagram , the stars (examined at the time) formed two groups: "dwarfs" and "giants". Even then, the star 40 Eridani B, a weak companion of 40 Eridani, did not fit into the scheme : measured against its spectral class, it was far too faint. In 1915 a spectrum of Sirius B could be recorded, which moved him in the HR diagram close to 40 Eridani B and showed that the two apparently belonged to a new star class. Their low luminosity in spite of the high temperature testifies to a low radiating surface, i.e. a small radius and thus an immense density.
Arthur Eddington had worked out detailed and successful star models from the 1920s by looking at gas spheres 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 incorporated the Pauli principle, which had only recently been discovered, in 1926 . Inside a white dwarf, the gas is completely ionized , i.e. it consists of atomic nuclei and free electrons . Since the electrons are subject to the Pauli principle, no two electrons can match in all quantum numbers . This means in particular that electrons in an electron gas that is already highly compressed can only approach each other further when the external pressure is increased if some of the electrons move to higher energy levels. During the compression, additional energy has to be expended because of this resistance, known as degenerative pressure . While the atomic nuclei provide the bulk of the star's mass, the electrons contribute to the stabilization of the star with the degeneracy pressure caused by quantum mechanics. One 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 above a limit mass of about 1.4 solar masses (" Chandrasekhar limit ") can no longer be stable.
In the course of his preparatory work on the general theory of relativity, Albert Einstein predicted as early as 1911 that photons emitted by a massive body with the wavelength λ o would arrive at an observer higher in the gravitational field with a larger, i.e. red-shifted, wavelength. Some attempts to observe this effect on the sun's spectral lines initially failed because of its insignificance. The wavelength shift, expressed as a speed 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 as a multiple of the solar mass and the solar radius . Since the M / R ratio changes little compared to the Sun, even with very massive stars, proof of the effect appeared to be hopeless until the 1920s.
In white dwarfs, however, the radius decreases with increasing mass. They are therefore massive objects with a small radius that should show a clear redshift. Eddington, who had already proven the relativistic deflection of light in the gravitational field of the sun in 1919 , saw this as an opportunity to confirm the extraordinary density of white dwarfs he suspected. Sirius B was chosen because he was part of a binary star system. Hence its mass was known, and by comparing it with the spectrum of Sirius A it was also possible to distinguish the gravitational part of the redshift from the Doppler shift produced by the radial velocity of the system. Based on the values for temperature and radius assumed at that time, Eddington expected a redshift of about +20 km / s. Walter Adams was able to record spectra from Sirius B in 1925, which were supposedly only slightly overlaid by light from Sirius A, and received 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 improved considerably. It turned out that Eddington had greatly underestimated the temperature of Sirius B and therefore overestimated the radius. The theory now demanded four times the redshift calculated by Eddington. In fact, new measurements in 1971 showed a redshift of (+89 ± 16) km / s. The authors explained Adams' result by saying that because of the strong light scattering 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 resolution of the Hubble space telescope made it possible in 2004 to record a high-resolution spectrum from Sirius B without significant interference from Sirius A.
According to the current scenarios for stellar evolution , Sirius A and B emerged together as a binary star system around 240 million years ago. Sirius B was originally with five solar masses and 630 times the luminosity of the sun much heavier and more luminous than Sirius A with only two solar masses. Because of its large mass and the associated high rate of fusion , 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 puffed up into a red giant . Eventually this energy source also dried up, so that Sirius B began to fuse the generated helium into carbon and oxygen ( helium burning ). It lost its only weakly bound outer layers due to the strong onset of the stellar wind and lost about four fifths of its original mass. What was left was the burned-out core, consisting mainly of carbon and oxygen, in which there was practically no more energy generation. 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 degenerative pressure of the electrons prevented further compression. Sirius B has been in this state for about 124 million years and is slowly cooling off.
Compared to the much brighter component Sirius A, the companion star Sirius B only has an apparent brightness of 8.5 mag. It has just under solar mass, making it one of the most massive white dwarfs known (most white dwarfs are concentrated in a narrow range around 0.58 solar masses, only an estimated 2% or less exceed 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. The combination of the observed temperature and brightness with model calculations results in a diameter of 0.00864 Sun diameters (approx. 12,020 km). So Sirius B is even slightly smaller than the earth (mean diameter 12,742 km).
The gravity on the surface of Sirius B is almost 400,000 times as high as 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. Because of 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 practically exclusively hydrogen lines. Since the hot hydrogen atmosphere is transparent to X-rays , X-ray emissions can be observed that originate from deeper, hotter layers. Sirius B has practically no magnetic field.
In the 1920s, several astronomers repeatedly observed a faint star of about the 12th magnitude in the immediate vicinity of Sirius A, but then lost this possible new companion again. In 1999, French astronomers were able to examine the surroundings of Sirius A more closely for faint stars on a picture taken with the screen down. They found a background star of suitable brightness that Sirius had passed in the first half of the 20th century and that had apparently been seen by observers at the time. When compared with an earlier image, the astronomers were also unable to find a companion star within 30 arc seconds that would have given itself away through a proper motion shared with Sirius A.
An investigation of irregularities in the orbital motion of Sirius A and B suggests that there could be a third component in the Sirius system, the mass of which is 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 orbit period of more than 4 years, the potential Sirius C can only orbit Sirius A.
Sirius A and B as a binary star system
The star system Sirius consists of the two stars described above, which move around their common center of mass with a period of 50.052 years . As with all double stars , each of the two stars moves on an ellipse around this center of gravity; for each of the two ellipses one of its focal points coincides with the center of gravity. 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 trajectory 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 in one of its focal points. If an earthly observer could look perpendicular to the plane of the orbit of the binary star system, he would see this ellipse with a 7.501 ″ long major semi-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, a smallest distance of 8 AU and a largest distance of 31.5 AU for the semi-major axis . The smallest or largest distance would appear to this observer at an angle of 3.1 "or 11.9".
However, since the path is inclined by 136.62 ° against the line of sight, the observer sees the path in an oblique view, which in turn is an ellipse, but with a slightly larger eccentricity. The illustration shows this apparent orbit as it appears from Earth. Although Sirius A lies in a focal point of the relative orbit of Sirius B, it is not in a focal point of the ellipse shown in the diagram foreshortened because of the oblique view. Due to the oblique view, the greatest and smallest possible angular distances that Sirius B traverses on this apparent path appear to the observer to be somewhat smaller than the undistorted values given above. Sirius B passed the smallest distance to Sirius A (the periastron ) on its true orbit for the last time in 1994, but reached the shortest distance on the apparent orbit as early as 1993.
While Sirius B was in the state of the red giant, the metallicity of his companion Sirius A could have increased through mass crossing . This would explain why Sirius A contains more metals (elements heavier than helium) in its shell than would be expected for a main sequence star. The iron content is e.g. B. 7.4 times above that of the sun. Values measured by the infrared satellite IRAS show a higher infrared radiation than expected for the Sirius system. This could indicate dust in this system and is considered unusual in a binary star system.
The nearest neighbor star Prokyon is 5.24 light years away from Sirius A + B. The other larger neighboring star systems are Epsilon Eridani with distances of 7.8 Ly , the Sun 8.6 Ly and Alpha Centauri 9.5 Ly .
Sirius and Arcturus were the first stars to detect proper motion after being considered immobile ("fixed stars") for millennia. In 1717, when Edmond Halley compared the star positions he had measured himself with the ancient coordinates handed down in the Almagest , Sirius had shifted about half a degree (one full moon diameter) to the south since the time of Ptolemy .
In 1909, Hertzsprung drew attention to the fact that Sirius should also be regarded as a member of the Ursa Major Current due to his own movement . This stellar stream consists of about 100 to 200 stars that show a common movement through space and presumably formed together as members of an open star cluster , but then drifted far apart. However, research from 2003 and 2005 cast doubt on whether Sirius could be a member of this group. The age of the Ursa Major group had to be raised to about 500 (± 100) million years, while Sirius is only about half that old. That would make him too young to belong to this group.
In addition to the Ursa Major Current, other clusters of movement can be distinguished in the vicinity of the sun, including the Hyaden 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 term “supercluster” is based on the idea that these large groups of stars each formed together and - although they have since drifted far apart - have retained a recognizable joint movement. Sirius would be, if not a member of the Ursa Major group, then at least of the larger Sirius Supercluster. The scenario of a common origin of the stars in such a supercluster is not without controversy; in particular, it cannot explain why there are stars of very different ages in a supercluster. An alternative interpretation assumes that the Hyaden and Sirius currents do not consist of stars of common origin, but of stars without relationship, on which irregularities in the gravitational field of the Milky Way have impressed a common movement pattern. We would then not speak of “superclusters” but of “dynamic flows”.
Flyby the solar system
On its course, from a current distance of 8.6 years, Sirius will have reached its closest approach to the solar system in about 64,000 years with about 7.86 light years. Its apparent brightness will then be −1.68 mag (today −1.46 mag).
The apparent position of Sirius in the sky depends on the place of observation on earth and is additionally influenced by the earth's precession and Sirius' own movement. For example, in today's Sirius from the North Pole to the 68th northern latitude can not be seen, while Sirius from the South Pole to the 67th southern latitude is constantly in the sky and never sets. Associated with this, starting from the 67th degree of north latitude, the maximum visible height above the horizon up to the region of the equator increases by one degree of altitude per degree of latitude to around 83 °, only to reduce to around 30 ° up to the south pole. In addition, the place of origin of Sirius changes depending on the latitude . At the 67th north latitude, Sirius appears on the horizon in a south-easterly position (172 °), at the equator, however, almost in the east (106 °). The different visibility conditions affect the observation time in the sky, which varies between almost six hours at the 67th parallel north and the constant presence at the south pole, depending on the location.
Similar conditions apply with regard to the latitude for the sunrise, which varies in time compared to the previous and the following day. The region where Sirius cannot be seen will continue to expand south in the following years. In the year 7524 AD the limit of invisibility shifts up to the 52nd parallel to the height of Berlin . This trend will be reversed in about 64,000 years, when Sirius will have reached closest approximation. In Germany it will then be visible all year round and will no longer go under.
Because of its brightness, Sirius is a noticeable star even for the casual observer of the sky. Its bright, bluish-white light tends to have a strong and often colorful flickering , even when there is little air disturbance . In temperate northern latitudes, it is a star of the winter sky, which it dominates because of its brightness. While it stands in the daytime sky during the summer and cannot be seen with the naked eye, it becomes visible for the first time at dawn towards the end of August. In autumn it is a star of the second half of the night, in winter it rises in the evening. At the turn of the year , Sirius culminates around midnight and can therefore be seen all night. In spring, it rises before sunset and can no longer be observed. From May onwards, its downfall has shifted to the bright time of the day, so that it is no longer visible until late summer. An observer can see Sirius with the naked eye at high altitude even during the day, when the sun is already close to the horizon and the star is in a location with a very clear sky high above the horizon.
Comparison of brightness with other stars
Wega is about 235,000 n. Chr. Sirius with a calculated brightness of -0.7 may replace the sky as the brightest star before then 260,000 n. Chr. Canopus with -0.46 like again will displace as zweithellster star Sirius in third . The evolution of the brightness of Sirius compared to other bright stars in the period between 100,000 BC. And 100,000 AD is shown in the following diagram and table:
year Sirius Canopus Vega Arcturus Prokyon 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 the story
Origin of name
The earliest recorded mention of Sirius ( ancient Greek Σείριος , Seirios ) can be found 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 one) is one of the sirens (Seirenes) in Greek mythology . A possible connection to Sirius with the designation as the glistening white light is also the content of controversial discussions as is the use of the terms glaring hot and scorching for Seirios . Finally, another equation with the Indo-European root * tueis-ro for "to be excited" or "to sparkle" is assumed. However, some scientists dispute this derivation.
Sirius from the perspective of other cultures
The apparent brightness of Sirius was insignificantly lower in ancient times and was −1.41 mag. The distance was 8.8 light years. As a particularly noticeable star, Sirius has been found in the myths, religions and customs of numerous cultures since prehistoric times, which can only be touched on here.
|The Egyptians initially saw Sirius in their language not 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 the worship focused on Sirius, the other two stars faded in meaning more and more. With regard to the flood of the Nile , Sirius had an important place in the course of Egyptian history. Herodotus gives the time around the 22./23. June as the beginning of the Nile flood. Entries in Egyptian administrative documents confirm Herodotus information. Historical and astronomical reconstructions prove that the first morning visibility of Sirius in the Nile Delta around 2850 BC. And in the southernmost town of Aswan around 2000 BC. With the 22./23. June coincided.
Sirius was therefore valid in the 3rd millennium BC. BC as herald of the flood of the Nile and enjoyed even greater importance in the 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 Greco-Roman times of Egypt , the changed conditions were mythologically taken into account. Now it was Salet who triggered the flood of the Nile with an arrow shot; her daughter Anukket then made the Nile swell . The heliacal rise 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 part in the 3rd millennium BC. Chr. In the Sumerian religion played several central roles early on. As a calendar star he fulfilled an important function in the agricultural cycle with the designation MUL KAK.SI.SÁ. As MUL KAK.TAG.GA (heavenly arrow) , Sirius was considered a main deity of the seven and was subordinate to the ruling star of God over the other celestial objects , Venus , who was worshiped as the goddess Inanna . In the Akitu - New Year processions Sirius finally was considered a topcoat over the seas and received appropriate offerings. Almost in unchanged form, it also later functioned with the Babylonians and Assyrians , who additionally determined Sirius as a signal generator for the time of the leap years according to the MUL.APIN clay tablets .
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 seen as a bad time from the 15th century onwards. Sirius was regarded by the Greeks as a pioneer of rabies .
North America and China
Many North American tribes also associate Sirius with dogs or wolves. With the Cherokee, for example, Sirius and Antares are the dog stars that guard the ends of the "Path of Souls" (the Milky Way ): Sirius is the eastern end of the winter sky, Antares the western end of the summer sky. A soul departing from the world must carry enough food with it to appease both dogs if it does not want to wander around forever on the path of souls. The Pawnee refer to Sirius as the Wolf Star.
For the Chinese, the stars of the current constellations, the aft deck and the Great Dog, formed a constellation representing a bow and arrow. The arrow was aimed directly at the "sky wolf", namely Sirius.
South Sea Islands
In Polynesia and Micronesia , the brighter stars of the southern sky, especially Sirius, have been used for navigation between the scattered archipelagos since prehistoric times. Close to the horizon, bright stars like Sirius could be used as direction indicators (with several stars replacing each other in this role over the course of a night). Stars could also be used to determine latitude. With its declination of 17 ° south, Sirius moves vertically over Fiji with the geographical latitude 17 ° south and you only had to drive south or north until Sirius went through the zenith .
Sirius and the Dogon
The French ethnologist Marcel Griaule studied the Dogon ethnic group in Mali, West Africa, for two decades from 1931 . The extensive creation myths of the Dogon, which Griaule gathered in conversations with four high-ranking tribesmen, mainly conducted by interpreters, allegedly contain 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 grain known to the Dogon ( Digitaria exilis ).
- Po tolo moves on an oval path around Sirius; Sirius is not in the center of this orbit, but eccentrically.
- Po tolo takes 50 years to travel the track once and turns around once a year.
- When po tolo is close to Sirius, Sirius becomes brighter. When the distance is greatest, Sirius will flicker and may appear as multiple stars.
- Po tolo is the smallest star and the smallest thing that the Dogon can think of . 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 millet ), 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 similarity of these descriptions with Sirius B and a possible Sirius C is all the more astonishing as none of them can be seen with the naked eye. Numerous different speculations try to explain the origin of this alleged knowledge. There are two main currents in popular literature: one view, 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, in his book The Sirius Mystery , suggested that extraterrestrial visitors from the Sirius system initiated the rise of Egyptian and Sumerian civilizations about 5000 years ago .
The explanation preferred in scientific circles is based on the contamination of Dogon mythology with modern astronomical knowledge. The anthropological variant assumes that the contamination (though not intentionally) was caused by Griaule himself. The Dutch anthropologist Walter van Beek himself worked with the Dogon and tried to verify parts of Griaule's material. However, he could not confirm large parts of the myths reproduced by Griaule, including Sirius as a binary star system. Van Beek takes the view that the myths published by Griaule are not simply reproductions of the stories of his sources, but rather came about in a complex interplay between Griaule, his informants and the translators. Some of them are the result of misunderstandings and over-interpretation by Griaule.
One possible explanation relates to assumed contacts between the Dogon and European visitors. She points out that the Dogon narratives reflect the state of astronomical knowledge from around 1926 (while Griaule did not start working for the Dogon until 1931): the orbital period, the elliptical orbit and the great mass of Sirius B were already in the 19th century known, its small diameter from around 1910, a possible third companion was suspected in the 1920s, the high density of Sirius B was proven in 1925. The observation of the gravitational redshift on Sirius B went as a sensational confirmation of the general theory of relativity by the popular press. 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 onwards, but so far no missionaries have been identified who could actually be used as sources.
Sirius as a red star
Sirius appears brightly 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 Big Dog constellation, can still be found with the entry:
|The star in the mouth, the brightest, which is called the dog and is reddish||According to 17 2/3||−39 1/6||1|
While the description and coordinates clearly mean Sirius, the reddish color mentioned does not match Sirius' blue-white color. Since the 18th century there has been speculation as to whether Sirius might actually have changed its color over the past 2,000 years. In this case Ptolemy's remark would provide valuable observational material both on stellar evolution in general and specifically on the processes in the solar environment.
However, even with reference to independent sources, it cannot be clearly determined whether Sirius was perceived as red in ancient times or not. An Assyrian text from 1070 BC. BC describes Sirius as "red as molten copper." Sirius is described by Aratos in his didactic poem Phainomena and by his later editors as reddish. With Pliny , Sirius is "fiery" and with 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 (approx. 580 AD). On the other hand, Manilius calls Sirius “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 very sparkling; an impressive sparkle, however, requires the full spectral colors of a white star, while the duller sparkle of a red star would hardly have attracted any attention. Five other stars that Ptolemy referred to as red (including Betelgeuse , Aldebaran ) are reddish even for today's observer.
According to today's understanding of stellar evolution, a period of 2000 years is by far not sufficient to be able to bring about visible changes in the relevant star types. 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 his phase as a red giant is conceivable. However, alternative explanations have not yet been completely convincing:
- An interstellar cloud of dust passing between Sirius and Earth could have caused a significant reddening of light from translucent stars. Such a cloud would have had to weaken Sirius' light so much that it would have appeared as an inconspicuous star of the third magnitude and its brightness would not have been sufficient to produce a color impression in the human eye . No traces of such a cloud were found.
- The earthly atmosphere also reddens the light of lower stars, but does not weaken it so much. Since the heliacal rise of Sirius was an important calendar fixed point for many ancient cultures, the attention may have been directed particularly to the low and then reddish Sirius. Sirius could then have retained this color as a distinguishing attribute. Theoretical calculations suggest that the atmosphere can indeed redden the light of a star sufficiently without pushing the brightness below the color perception threshold. Practical observations have so far not been able to determine any pronounced reddening effect.
- “Reddish” could be a purely symbolic attribute that Sirius associates with the summer heat heralded by his heliacal appearance.
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- The parallax of Sirius is 0.379 ″. So an AE at this 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 = major semi-axis (1 - eccentricity), largest distance = major semi-axis (1 + eccentricity).
- Calculated from the 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 brightness of 1.43 mag, see also info box.
- Due to the logarithmic brightness the difference in brightness is not about , but
- Since the apparent brightness is subject to fluctuations, Sirius appears brighter than the planets mentioned at some times. See also the data from NASA factsheets: Moon (up to −12.74 mag), Venus (up to −4.6 mag), Jupiter ( Memento from October 5, 2011 on WebCite ) (up to −2.94 mag), Mars (up to −2.91 mag) and Mercury (up to −1.9 mag).
- Chandrasekhar initially received a limit value of 0.91 solar masses because of the poorly known composition of a white dwarf.
- On the earth's surface, the acceleration due to gravity g is around 981 cm / s 2 (cgs units!). The acceleration due to gravity on the surface of Sirius B is almost 400,000 times higher and is around g = 360 million cm / s 2 d. H. log (g) = 8,556 (a numerical equation, since g has a unit. By convention, g is to be used in cm / s 2 ).
- The name Supercluster should not be used with the English. Designation Supercluster for superclusters (clusters of galaxies) are confused.
- The frequently published statements that the heliacal rise of Sirius before 2850 BC And after 2000 BC BC with the onset of the Nile flood cannot be confirmed by the astronomical results and contemporary Egyptian documents.
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