Sirius
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"Sirius B" redirects here. For Therion's musical album, see Sirius B (album).For information on Sirius satellite radio, see Sirius Satellite Radio.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 can't 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.
The name of this star name 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:
aš-ši‘rā in
Islamic astronomy, from which the alternate name
Al Shira derives.
In
Chinese the star is known as
(
' literally, "heavenly wolf star"). The Japanese pronunciation is and the Korean pronunciation Cheonlangseong'''.
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
 |
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 Sirius B to be 12,000 kilometers (7,500 miles) in diameter, with a mass that is 98% of the
Sun.
|
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 occured 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.
There remain a few unsolved mysteries regarding Sirius. Since 1894, for example, some apparent orbital irregularities in Sirius B have been observed, suggesting a third very small companion star, but this has never been definitely confirmed.
[Benest, D., & Duvent, J. L. (1995, July). Is Sirius a triple star? Astronomy and Astrophysics, 299, 621-628. (available at The NASA Astrophysics Data System)]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 (Jupiter actually has many more, but only 4 can be seen with simple telescopes).
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. Careful research reveals there may have been
cultural contamination on the part of visiting astronomers who went to the region to observe a
transit of Venus, though this is still a matter of some dispute.
[Ridpath, Ian (1978) Investigating the "Sirius Mystery". The Skeptical Inquirer. Retrieved July 11, 2005.]see also The Dogon ControversyBeing the brightest star in the night sky, Sirius is often mentioned in
science fiction and other
popular culture as well.
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