Sirius - Etymology, History, System, Mysteries, Sirius in popular culture

The brightest star in our sky, and ninth nearest, at 2·6 parsec; also known as the dog star. It has a faint companion star, which was the first white dwarf star to be recognized as such. Sirius was known as Sothis to the ancient Egyptians, who associated it with the annual flooding of the Nile. Among the ancient Romans, the hottest part of the year was associated with the rising, just before sunrise, of the Dog Star (around 3–11 July), giving rise to the expression, ‘dog-days’.

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.

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.

Under the right conditions, Sirius can be observed in daylight with the naked eye.

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 Chinese the star is known as 天狼星 (Tiānláng xīng literally, “heavenly wolf star”).

History

Historically, many cultures have attached special significance to Sirius.

In the astrology of the Middle Ages, Sirius was a Behenian fixed star, associated with beryl and juniper.

Based on changes in its proper motion, in 1844 Friedrich Wilhelm Bessel deduced that Sirius had a hidden companion.

In 1909 Ejnar Hertzsprung suggested that Sirius was a member of the Ursa Major Moving Group, based on the systems movements across the sky.

Astronomers at the Mount Wilson Observatory determined in 1915 that Sirius B was a white dwarf, the first to be discovered. 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

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. The orbit of Sirius B can take the star in front of Sirius A, briefly lowering the total luminosity.

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. 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. The Sirius star system also emits a higher than expected level of infrared radiation, as measured by IRAS.

The current primary, Sirius A, is expected to have completely exhausted the store of hydrogen at its core within a billion (109) years.

Mysteries

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' Color

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

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.

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.

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.

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.