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Sirius

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Sirius A/B

   The position of Sirius.
                             Observation data
   Epoch J2000
   Constellation             Canis Major
   Right ascension           06^h 45^m 08.9173^s
   Declination               −16° 42' 58.017"
   Apparent magnitude (V)    −1.47 (A) / 8.44 (B)
                             Characteristics
   Spectral type             A1V (A) / DA2 (B)
   B-V colour index          0.01 (A) / −0.03 (B)
   U-B colour index          −0.08 (A) / −1.04 (B)
   Variable type             Eclipsing binary
                                Astrometry
   Radial velocity (R[v])    −7.6 km/s
   Proper motion (μ)         RA: −546.01 mas/ yr
                             Dec.: −1223.08 mas/ yr
   Parallax (π)              379.21 ± 1.58 mas
   Distance                  8.6 ± 0.04 ly
                             (2.64 ± 0.01 pc)
   Absolute magnitude (M[V]) 1.47 (A) / 11.35 (B)
                           Visual binary orbit
   Companion                 α CMa B
   Period (P)                50.09 years
   Semimajor axis (a)        7.56"
   Eccentricity (e)          0.5923
   Inclination (i)           136.5°
   Node (Ω)                  44.6°
   Periastron epoch (T)      1894.13
                                 Details
   Mass                      2.14 (A) / 0.98 (B) M[☉]
   Radius                    1.68 (A) / 0.008 (B) R[☉]
   Luminosity                26.1 (A) / 0.0024 (B) L[☉]
   Temperature               9,900 (A) / 25,200 (B) K
   Metallicity               190% Sun (A)
   Rotation
   Age                       2-3 × 10^8 years
                            Other designations
   α Canis Majoris, 9 CMa, HD 48915, HR 2491, BD -16°1591, Gl 119-052,
   GCTP 1577.00 A/B, GJ 244 A/B, LHS 219, ADS 5423, LTT 2638, HIP 32349.
                           Database references
   SIMBAD                    data

   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.
   This binary star system consists of a blue-white main sequence dwarf
   star and a faint white dwarf companion. 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 73.284 degrees cannot see it) and, in
   the Northern Hemisphere, is known as a vertex of the Winter Triangle.
   The best time of year to view it is around January 1, when it reaches
   the meridian at midnight.

   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.

Etymology

   The name of this star comes from the Latin Sīrius, from Greek Σείριος
   (Seirios, "glowing" or "scorcher"). 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:
   الشعرى aš-ši‘rā in Islamic astronomy, from which the alternate name Al
   Shira 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 天狼星 (Tiānláng xīng literally, “heavenly
   wolf star”). The Japanese pronunciation is Tenrōsei and the Korean
   pronunciation 천랑성 Cheonlangseong, while Japanese vernacular name of the
   star is 青星 (Aoboshi).

History

   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. 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."

   In the astrology of the Middle Ages, Sirius was a Behenian fixed star,
   associated with beryl and juniper. Its kabbalistic symbol
   Image:Agrippa1531_Canismaior.png was listed by Heinrich Cornelius
   Agrippa.

   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. 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 first 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.

System

   An artist's impression of Sirius A and Sirius B. Sirius A is the larger
   of the two stars. (Credit: NASA)
   Enlarge
   An artist's impression of Sirius A and Sirius B. Sirius A is the larger
   of the two stars. (Credit: NASA)

   At a distance of 2.6 pc or 8.6 light years, Sirius is one of the
   nearest stars to Earth. Its closest large neighbour star except Sirius
   B is Procyon, 1.61 pc or 5.24 ly away.

   Sirius is a main sequence star of spectral type A0 or A1 and has a mass
   about 2.1 times that of the Sun. 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 (roughly the distance between the Sun and
   Uranus) and a period of close to 50 years. The orbit of Sirius B can
   take the star in front of Sirius A, briefly lowering the total
   luminosity. For this reason the system is considered an eclipsing
   binary variable star.

   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. White dwarfs form only after the star has evolved
   from the main sequence and then pass 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 an
   estimated 5 solar masses and was probably a spectral type B7 V star
   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. 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.

   The current primary, Sirius A, is expected to have completely exhausted
   the store of hydrogen at its core within a billion (10^9) years. At
   that point it will pass through its own red giant stage, then settle
   down to become a white dwarf.

Mysteries

   The image of Sirius A and Sirius B taken by Hubble Space Telescope. The
   white dwarf can be seen to the lower left. (Credit:NASA)
   Enlarge
   The image of Sirius A and Sirius B taken by Hubble Space Telescope. The
   white dwarf can be seen to the lower left. (Credit: NASA)

   There remain a few unsolved mysteries regarding Sirius:

Possible Third Member in System

   Since 1894, some apparent orbital irregularities in Sirius B have been
   observed, suggesting a third very small companion star, but this has
   never been definitely confirmed.

Sirius' Colour

   Curiously, some ancient observations of Sirius describe it as a red
   star. 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.

The Dogon

   The Dogon people are a tribal people living in Africa who are said to
   have known that Sirius had a companion star before its discovery in the
   1840's. Of course, it is impossible to see Sirius's companion except
   with very powerful telescopes and at the right time of Sirius B's
   orbit, which lasts several years. They also supposedly knew that Saturn
   had rings and that Jupiter had 4 moons (Although as of 2003, Jupiter
   had 63 satellites, only Ganymede, Io, Europa, and Callisto can be seen
   with simple telescopes - all the others are only a few hundred miles
   wide at the most).

   This apparent knowledge, which it seems impossible for them to have
   acquired without outside assistance, is a source of speculation for UFO
   enthusiasts and was the subject of the book The Sirius Mystery by
   Robert Temple, who linked this to the legends of the Dogon as well with
   the legends and traditions of the ancient Egyptians and Sumerians,
   conjecturing that aliens from Sirius had visited Earth and influenced
   early civilization.

   This work has been challenged by astronomer Carl Sagan, among others,
   as based on selective evidence. Skeptics believe there may have been
   cultural contamination on the part of visiting astronomers who went to
   the region to observe a transit of Venus.

          see also The Dogon Controversy

Sirius in popular culture

   Being the brightest star in the night sky, Sirius is often mentioned in
   science fiction and other popular culture as well.

   Retrieved from " http://en.wikipedia.org/wiki/Sirius"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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