There are at least 6,000 stars bright enough to be visible with the naked eye. Under dark skies, up to about 3,000 can be seen at any one time (since one half of the Earth is in daylight). However, none can rival the brilliance of Sirius, the brightest night time star.
Located in the constellation of Canis Major, Sirius shines with an apparent magnitude of -1.46. It's noticeably brighter than its nearest rival Canopus (α Car mag. -0.72) and four times more brilliant than Arcturus, the brightest star in the northern section of sky. Sirius is also known as the "Dog Star". When close to the horizon it twinkles. Of course, the colour variations are only a result of the Earth's unsteady atmosphere. In practice, all stars do twinkle and to a lesser extent the planets as well.
To the naked eye Sirius appears as a single star, but it's actually a binary system. The primary component is a white main sequence star of spectral type, A1V, named Sirius A. This star is the one visible to the naked eye. It has a radius of 1.7 times that of the Sun and is 25 times more luminous.
In 1844, German astronomer Friedrich Bessel noticed that Sirius "weaved" its way through the sky instead of moving in a straight line. He deduced there must be an unseen companion as massive as the Sun and with an orbital period of 50 years. Bessel also worked out where it should be, but despite numerous attempts he couldn't spot it.
It was American telescope-maker and astronomer Alvan Graham Clark who finally found the companion, now named Sirius B or the "Pup", on January 31, 1862. This star is a white dwarf about 10,000 times dimmer than its neighbour. Clark discovered it using an 18.5-inch (470 mm) aperture refractor, which happened to be the world's largest refracting telescope at that time. At magnitude +8.6, the star isn't particularly faint and would actually be within binocular range, if it weren't positioned so close to the primary star.
Today, Sirius B can be spotted with much smaller scopes. The separation of the two stars varies between 3 and 11.5 arc seconds. When close to maximum separation it's possible to glimpse the "Pup" with good quality 100mm (4-inch) refractors on nights of excellent seeing. At minimum separation, the B star is virtually impossible to visually spot.
In 2005, astronomers using the Hubble Space Telescope determined that Sirius B has a diameter of 12,000 kilometres (7,500 miles) which is almost exactly the same as the Earth. It has a mass 98% of the Sun. The star system is close at 8.6 light-years away. Perhaps surprising, Sirius is one of the least powerful first magnitude stars with its brilliance due simply to its proximity.
Naturally Sirius has its place in ancient history. Its name has Greek origins, which is unusual as most other named stars have Arabic origins. The Greek translation is "sparkling" or "scorching" and along with the Romans they regarded the star as an unlucky omen. The Egyptians placed much significance on Sirius. It was worshipped as Sothis or the "Nile Star" and it's first appearance in the dawn sky marked the important annual flooding of the River Nile.
With a declination of 16S, Sirius is visible from almost the entire globe. The exceptions are latitudes north of 74N. However, even from cities inside the Artic Circle such as Murmansk in Russia, Tromsø in Norway and Barrow in Alaska the star can be spotted low down during the Winter months.
Locating Sirius is easy but in case of uncertainty, it's at the end of an imaginary line extending southwards from the three stars of Orion's belt.
There is an old mystery regarding the colour of Sirius. In ancient times it was often described as red or orange, although today it's pure white. Around AD 150, the great astronomer of his time, Ptolemy, described Sirius as reddish along with five other stars (Betelgeuse, Antares, Aldebaran, Arcturus and Pollux). Some astronomers agreed with Ptolemy's observations, but others noticed only a white star. However, it's all a bit strange. Possible changes to either Sirius A or B over a period of 2,000 years can be rejected as the timescale is too short. There is also no sign of associated nebulosity, which would be expected had such a change taken place. Sir John Herschel suggested that a space cloud may have passed between Sirius and us and therefore reddened the light, but this seems unlikely as we certainly would be able to detect such a cloud today. It also seems unlikely that the twinkling of Sirius would have fooled Ptolemy. The star was relatively high in the sky from his location and therefore above the thicker layers of the atmosphere. Most likely there was no change in the colour, but this remains a curious mystery.
Sirius A Data Table
|Bayer||alpha (α) CMa|
|RA (J2000)||06h 45m 09s|
|DEC (J2000)||-16d 42m 58s|
|Surface Temp (K).||9,940|
|Age (years)||250 Million|
|Other designations||HR 2491, HD 48915, HIP 32349|
|Notable features||Brightest star in the night time sky. Has a white dwarf companion.|