Sirius
This article is about the star. See Sirius (disambiguation) for other uses of the name.
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Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the nighttime sky, with a visual apparent magnitude of −1.46. It is located in the constellation Canis Major. Its name comes from the Latin sīrius, from Greek σείριος (seirios, "glowing"). It is often nicknamed the "Dog Star".
Sirius 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 2.6 pc or 8.57 light years, Sirius is also one of the nearest stars to Earth. The best time of year to view it is around January 1, when it reaches the meridian at midnight.
It is a main sequence star of spectral type A0 or A1 and has a mass about 2.4 times that of the Sun.
It is also known as HD 48915, HR 2491, BD-16°1591, GCTP 1577.00A, LHS 219, ADS 5423 and LTT 2638.
Its closest large neighbour star is Procyon, 1.61 pc or 5.24 ly away.
In 1841[?] Friedrich Wilhelm Bessel deduced that Sirius was actually a binary 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 Mount 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 by far the more massive of the two, since it has already evolved off the main sequence. Robert Hanbury Brown measured the diameter of Sirius for the first time in 1956.
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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, although careful research reveals this was probably cultural contamination on the part of visiting astronomers who went to the region to observe a transit of Venus. This is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert Temple, which has (perhaps arguably) been debunked.
Some facts about Sirius A
- Parallax: 0.379 arcsecs
- Spectral type: A1 V
- Radial velocity: −9 km/s
- Proper motion: 1.339 arcsecs/year
- Apparent visual magnitude: −1.44
- Absolute visual magnitude: 1.45
- Luminosity: 26.1 Solar Luminosities
- Angular diameter: 0.0059 arcsecs
Coordinates for equinox 2000.0:
Some facts about Sirius B
- Parallax: 0.379 arcsecs
- Spectral type: White dwarf
- Radial velocity: −9 km/s
- Proper motion: 1.339 arcsecs/year
- Apparent visual magnitude: 8.44
- Absolute visual magnitude: 11.3
- Luminosity: 2.4e−4 solar luminosities
- Surface temperature: 25000 K
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)