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This article is about the star. See Sirius
(disambiguation) for other uses of the name.
Sirius (α Canis Majoris, also known as the Dog Star) is the brightest star (-1.46m) in the night sky.
This star can be seen from every inhabited region of the Earth's surface and, in the
northern hemisphere, is known as a vertex of the Winter Triangle. At a distance of
8.6 light years, Sirius is also one of the nearest stars to Earth. It is a main sequence star of
spectral type A0 or A1 and has a mass about 2.4 times that of the
Sun.
In 1841[?] Friedrich Wilhelm Bessel deduced that Sirius was actually a double star. In 1862 Alvan Graham Clark discovered the
companion, which is called Sirius B, or affectionately "the Pup". The visible star is now sometimes known as
Sirius A. The two stars orbit each other with a separation of about 20 AU and a period of close to 50 years. In
1915 astronomers at the Mt. Wilson Observatory discovered that Sirius B was a white dwarf, the first to be discovered. Interestingly, this means that Sirius B must have originally been much
the more massive of the two, since it has already evolved off the main sequence.
History
Historically, many cultures have attached special significance to Sirius. Sirius was worshipped in the valley of the Nile long before Rome was founded, and many ancient Egyptian temples were constructed oriented so that light from the star
could penetrate to their inner altars. The 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: the name Sirius is derived from Seirios meaning "the scorcher." This also explains the phrase "dog days
of summer".
There are a few unsolved mysteries regarding Sirius. Firstly, it has been suggested that there is a third very small companion
star, but it appears that this has not yet been definitely confirmed. Secondly, ancient observations of Sirius describe it as a
red star, when 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. A third mystery is a suggestion that
the Dogon tribe of Africa knew about unseen companion star(s) before they were discovered
in the 19th century. This is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert
Temple.
Some facts about Sirius A
Some facts about Sirius B
See also
References and external links
- 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)
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