SIRIUS

'Sirius' (α CMa / α Canis Majoris / Alpha Canis Majoris) () is the brightest star in the night-time sky with a visual apparent magnitude of −1.47. It is located in the constellation Canis Major. Sirius can be seen from almost every inhabited region of the Earth's surface (those living north of 73 degrees cannot see it) and, in the Northern Hemisphere, is known as a vertex of the Winter Triangle.
Under the right conditions, Sirius can be observed in daylight with the naked eye. Ideally the sky must be very clear, with the observer at a high altitude, the star passing overhead, and the sun low down on the horizon.^[1]
What to the eye appears as a single star is actually a large binary star system, consisting of a bright white main sequence star of spectral type A1V, named 'Sirius A', and a faint white dwarf companion of spectral type DA named 'Sirius B'.

Contents

Discoveries

System

Etymology and cultural significance

Mysteries

Color

Possible third member

Dogon beliefs

See also

References

External links

Discoveries

Based on changes in its proper motion, in 1844 Friedrich Wilhelm Bessel deduced that Sirius had a hidden companion. Nearly two decades later in 1862 Alvan Graham Clark discovered the faint companion, which is called 'Sirius B', or affectionately "the Pup". The visible star is now sometimes known as 'Sirius A'.
In 1909 Ejnar Hertzsprung suggested that Sirius was a member of the Ursa Major Moving Group, based on the systems movements across the sky. However, more recent research by Jeremy King et al. at Clemson University in 2003 questions whether that is true, since the two components of Sirius appear to be too young.^[2] Sirius is roughly half the age of the other members of the stream, so their common motion is most likely a coincidence.
Astronomers at the Mount Wilson Observatory determined in 1915 that Sirius B was a white dwarf, the second such star to be discovered.^[3] 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.^[4] 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.^[5]
The ''Voyager 2'' spacecraft, launched in 1977 to study the four Jovian planets in the Solar System, is expected to pass within 4.3 light years of Sirius in approximately 296,000 years time.^[6]

System

At a distance of 2.6 pc or 8.6 light years, Sirius is one of the nearest stars to Earth.^[7] Its closest large neighbour star except Sirius B is Procyon, 1.61 pc or 5.24 ly away.⁹
Sirius is a main sequence star of spectral type A0 or A1 and has a mass about 2.1 times that of the Sun.^[8][9] It has a companion star that has already evolved off the main sequence into a white dwarf, and hence was once the more massive of the two. These two stars orbit each other with a separation of about 20 AU^[10] (roughly the distance between the Sun and Uranus) and a period of close to 50 years.
A typical white dwarf has a mass equal to 0.5–0.6 solar masses. With a mass nearly equal to the Sun's, Sirius B is one of the more massive white dwarfs known. Yet that same mass is packed into a volume roughly equal to the Earth. A white dwarf forms only after the star has evolved from the main sequence and then passed through a red giant stage. This occurred when Sirius B was less than half its current age, approximately 120 million years ago. The original star had an estimated 5 solar masses^[11] and was a B-type star (roughly B4-5)^[12][13] 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. For this reason Sirius A now has a higher than normal abundance of elements heavier than helium, such as iron.⁹ The Sirius star system also emits a higher than expected level of infrared radiation, as measured by IRAS. This may be an indication of dust in the system, and is considered somewhat unusual for a binary star.⁹
The current primary, Sirius A, is expected to have completely exhausted the store of hydrogen at its core within a billion (10⁹) years. At that point it will pass through its own red giant stage, then settle down to become a white dwarf.

Etymology and cultural significance

The name of this star comes from the Latin ''Sīrius'', from Greek ''Σείριος'' (''Seirios'', "glowing" or "scorcher").^[14] As the major star of the "Big Dog" constellation, it is often called the "Dog Star".
The Latin name for this star is 'Canicula' ("little dog"), and in Arabic it is known as 'الشعرى' (transliteration: ''al-ši‘rā'' or ''al-shira''; English: "the leader")^[15], from which the alternate name 'Aschere' derives. In Sanskrit, it is known as 'Mrgavyadha' ("deer hunter") or 'Lubdhaka' ("hunter"). As Mrgavyadha, the star represents Shiva. In Chinese the star is known as the star of the 'celestial wolf' (; Chinese romanization: Tiānláng; Japanese romanization: Tenrō; Korean romanization: Cheonlang) in the Mansion of Jǐng (井宿), while the Japanese vernacular name of the star is '青星' ('Aoboshi', "blue star"). In Scandinavia, the star has been known as 'Lokabrenna' ("burning done by Loki", or "Loki's torch").
Historically, many cultures have attached special significance to Sirius. Sirius was worshipped as Sothis in the valley of the Nile long before Rome was founded. The Middle Kingdom of Egyptians based their calendar on the heliacal rising of Sirius, which occurred just before the annual flooding of the Nile and the summer solstice.^[16] In Greek mythology, Orion's dog became Sirius. The Greeks also associated Sirius with the heat of summer: they called it Σείριος ''Seirios'', often translated "the scorcher." The dog days of summer were also connected to Sirius.
Aristotle might have referred to the double star in his Meteorologica, citing African sources: ''"... some of the fixed stars too get a tail. For this we must not only accept the authority of the Egyptians who assert it, but we have ourselves observed the fact. For a star in the thigh of the Dog had a tail, though a faint one. If you fixed your sight on it its light was dim, but if you just glanced at it, it appeared brighter."'' However, this reference has been attributed to M41 or a train of stars near Delta Canis Majoris.^[17]
In the astrology of the Middle Ages, Sirius was a Behenian fixed star, associated with beryl and juniper. Its kabbalistic symbol was listed by Heinrich Cornelius Agrippa.

Mysteries

There remain a few unsolved mysteries regarding Sirius:

Color

Curiously, some ancient observations of Sirius describe it as a red star.^[18][19] To the Romans this meant an angry god, and they are known to have sacrificed red dogs to this star. Today, Sirius A is bluish white.
The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy is rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when it was observed rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red/white/blue hues when near the horizon.^[20]
Sirius is the standard star for the color white in ancient China. Multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.^[21]

Possible third member

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 and a mass of only 0.06 solar masses. The magnitude of this star would be 5 to 10 times fainter than the white dwarf Sirius B, which would account for the difficulty of observation. While this supposed companion could orbit around either star, an orbit around Sirius A would be more stable.^[22]

Dogon beliefs

The Dogon people are a tribal people living in Africa who were reported as having certain traditional astronomical knowledge about Sirius that would normally be considered impossible without the use of telescopes. The reason for this has been the subject of some controversy and speculation, but may be the result of cultural contamination.^[23] However, others see this explanation as being too simplistic.^[24][25]

See also

★ List of nearest stars

★ List of brightest stars

★ Sirius in fiction

References

1. On the Visibility of Sirius in Daylight, C. Henshaw, , , Journal of the British Astronomical Association, 1984
2. Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group, J.R. King, A.R. Villarreal, D.R. Soderblom, A.F. Gulliver, S.J. Adelman, , , The Astronomical Journal, 1999
3. How Degenerate Stars Came to be Known as White Dwarfs, , Holberg, J. B., Bulletin of the American Astronomical Society, 2005
4. Interferometry of the Intensity Fluctuations in Light. IV. A Test of an Intensity Interferometer on Sirius A, R.H. Brown, R.Q. Twiss, , , Proceedings of the Royal Society of London, 1958
5. Astronomers Use Hubble to 'Weigh' Dog Star's Companion
6. Interstellar Mission
7. The One Hundred Nearest Star Systems
8. The 10 Brightest Stars
9. Sirius 2
10. 1 light year = 63,241 AU; semi-major axis = distance × tan(subtended angle) = 8.6 × 63,241 × tan(7.56″) = 19.9 A.U., approximately
11. The Age and Progenitor Mass of Sirius B, J. Liebert, P.A. Young, D. Arnett, J.B. Holberg, K. A. Williams, , , The Astrophysical Journal, 2005
12. Computation of Isochrones
13.
14. Greek-English Lexicon, , Henry G., Liddell, Oxford University Press,, 1980, ISBN 0-19-910207-4
15. Sirius, entry in the Encyclopedia Britannica. Accessed on line September 7, 2007.
16. The Sun Behind The Sun — A Special Day In Egypt's Remote Past
17. M41 possibly recorded by Aristotle Frommert, H.; Kronberg, C.
18. Claim CE351
19. A Recent Transformation of Sirius?
20. A physical interpretation of the `red Sirius' anomaly, , D. C. B., Whittet, Monthly Notices of the Royal Astronomical Society, 1999
21. 中国古籍中天狼星颜色之记载, 江晓原, , , 天文学报, 1992
22. Is Sirius a triple star?, Benest, D., & Duvent, J. L., , , Astronomy and Astrophysics, 1995
23. Investigating the Sirius "Mystery"
24. Sirius, Mystery Of Red Color
25. The Sirius Mystery

External links

★ Getting Sirius about time