Sirius

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For other uses, see Sirius (disambiguation).

Image:Position Alpha Cma.png 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"). As the major star of the "Big Dog" constellation, it is often called 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 (50.3 trillion miles), 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 by the Latin name Canicula ("little dog") and as الشعرى aš-ši’rā in Arabic astronomy, from which the alternate name Aschere derives.

Its closest large neighbour star is Procyon, 1.61 pc or 5.24 ly away.

In 1909, Ejnar Hertzsprung suggested that Sirius was part of the Ursa Major Moving Group; 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.

Binary system

In 1844 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 determined 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.

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, and many ancient Egyptian temples were 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: they called it Σείριος Seirios, often translated "the scorcher." This also explains the phrase "dog days of summer".

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.

Mysteries

There are a few unsolved mysteries regarding Sirius.

• Firstly, apparent orbital irregularities in Sirius B have been observed since 1894, suggesting a third very small companion star, but this has never been definitely confirmed.^[1]
• 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.

The Dogons

It has been suggested that the Dogon tribe of Mali 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. This work has been debunked by astronomer Carl Sagan, among others, as based on confabulation and selective evidence. Careful research reveals there was probably cultural contamination on the part of visiting astronomers who went to the region to observe a transit of Venus.^[2]

Sirius in Fiction

• In the Known Space series of stories by Larry Niven a high gravity world called "Jinx" is the moon of a gas giant in the Sirius system.

• In the fantasy novel "Dogsbody" by Diana Wynne Jones the star Sirius is an intelligent being falsely accused of murdering another star by his peers. As punishment he is sent to Earth in the body of a new born puppy to find the weapon he supposedly used.

• Sirius is also known as the planet/star/vortex/whatever Mental is supposedly on in the Serious Sam games.

• In the Harry Potter book series, the character Sirius Black, who can change shape into a large black dog, takes his name from the star.

See also

• List of nearest stars
• List of brightest stars

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)
• ^ Ridpath, Ian (1978) Investigating the "Sirius Mystery". The Skeptical Inquirer. Retrieved July 11, 2005.
• Detailed information on Sirius
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