MORE MATTER ASTRONOMICAL:SIRIUS- THE "DOG STAR"

These late winter evening are a great time to observe Sirius, the brightest star in the sky. Sirius is actually at least a binary star system with an apparent magnitude of -1.47(1). Sirius is visible from any part of the Earth lower than about 73 degrees north latitude. At this time of year to see Sirius draw an imaginary line downwards from the stars in Orion's belt. At about 20 degrees to the southeast you'll come to a very bright object that seems to exhibit a faintly blueish white twinkling. When the star is near to the horizon the actual apparent colour may vary (2). This star's name is derived from the Latin "Sirius", a term borrowed from Greek meaning "glowing" or "scorcher". This is because in ancient Greek times this star rose at dawn during the hottest time of the summer. Because of precession Sirius now rises much later in the year.

The ancient Latin name for this star is 'Canicula', the "little star", and the Arabic name is 'Al Shira'. Many other culture have other names for Sirius. In Sanscrit it was known as 'Mrgavyadha' (the deer hunter) or Lubdhaka (hunter). The former name is a representation of the God Shiva. The Chinese knew Sirius as the "celestial wolf", and the Japanese common name for Sirius is Aoboshi (blue star). In ancient Egypt this star was worshipped under the name of Sothis. The Middle Kingdom of Egypt actually based their calender on the rising of Sirius which occurred just before both the summer solstice and the annual flooding of the Nile.

It was as late as 1844 when astronomer Friedrich Wilhelm Bessel deduced the existence of a companion star to Sirius by means of analysis of its proper motion(the way that stars move in relation to each other-first described in 1718 by William Halley). This companion was confirmed in 1862 by Alvan Graham Clark. Nowadays this companion star is called Sirius B while the star that we see with the naked eye is called Sirius A. Sirius B is a white dwarf with a diameter only that of the Earth even though its mass is 98% that of our Sun. Sirius A is a main sequence star with a mass of about 2.1 times that of the Sun. The two stars are separated by a distance of about 20 AU ie about 2o times the distance from the Sun to Earth. There is a possibility that there is a third member of this system, but this has never been definitely confirmed.

The stars Betelguese (at the upper left of Orion), Sirius and Procyon in Canis Minor form what is called 'The Winter Triangle' in the southern sky. Sirius lays about 8.5 light years from Earth and is the fifth closest star to our system (and the second closest "visible star" after Alpha Centauri). It intrinsically about 23 times as bright as our Sun.

Sirius has found its place in pseudoscience as well. Early ethnographers of the Dogon people in West Africa reported that "had knowledge" of the companion star to Sirius A. In actual fact this claim was based solely on the observation that the Dogon had a mask ceremony in which a new mask was created every 60 years, supposedly showing knowledge of the periodic dimming of Sirius A as its companion passes in front of it - every every 50 years actually- the star seems to dim, but this luminosity change is imperceptible to the human eye. A crooked book writer by the name of Robert Temple, inspired by the financial success of people such as Von Daniken, used this along with other pseudofactoids to concoct an "ancient astronauts" argument in 'The Sirius Mystery'. The whole matter has been thoroughly debunked by Carl Sagan, but it's doubtful that Sagan made 1/1000th of the money that Temple did with his book.

MOLLY NOTES

1)The astronomical scale for apparent luminosity is an inverse logarithmic scale. This is like the better known Richter scale for earthquakes where one integer equals a ten fold change. In astronomy, however, the lower the number the greater the luminosity. The Sun, for instance, has a luminosity of about +26, and the full Moon shines with a luminosity of about +11. This scale was originally devised by Hipparchus, and it assigns magnitudes such that the faintest stars in the sky have a magnitude of about -6.

2)This has led to some puzzles in interpretation as several ancient Latin texts refer to Sirius' colour as "red", even though Chinese texts of the same time describe unequivocally as white. Some have suggested that this represents a recent stellar evolution change in the Sirius system, but the position on the horizon is a more likely explanation.

Molly