There are at least 6,000 stars in the sky that are bright enough to be visible with the naked eye. Of these we could expect 3,000 or so to be seen at any one time under dark skies (since one half of the Earth is in daylight). However, none can rival the glory of the brightest night-time star of all, Sirius.

Located in the constellation of Canis Major "the Great Dog", Sirius shines with an apparent magnitude of -1.46. It's easily 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" and what's immediately noticeable is it twinkles! The star flashes through many colours of the rainbow - especially when close to the horizon - but of course this has nothing to do with Sirius itself; it's a pure white star and the colour variations are solely caused by the Earth's unsteady atmosphere. In practice all stars do twinkle, also to a lesser extent the planets, but the effect is most obvious with Sirius.

Sirius (Fred Espenak)

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 termed Sirius A. This star - the one we see with the naked eye - has a radius of 1.7 times that of the Sun and is 25 times more luminous than our star. However, the secondary component is much fainter.

In 1844, German astronomer Friedrich Bessel noticed that Sirius "weaved" its way through the sky instead of moving in a straight line. He deduced that there must be an unseen companion at least as massive as the Sun and with an orbital period of 50 years. Bessel worked out where the companion should be but despite extensive observations he couldn't find it.

It was on January 31, 1862 when American telescope-maker and astronomer Alvan Graham Clark finally discovered the companion, now named Sirius B or the "Pup". The reason why it took so long was because it's a white dwarf star, 10,000 dimmer than its neighbour. Clark discovered Sirius B with an 18.5-inch (470 mm) aperture refractor telescope, which happened to be the largest refracting telescope in the World at that time. At magnitude +8.6, Sirius B isn't particularly faint and if located on it's own, it would even be visible with binoculars.

Today, Sirius B can be spotted with a much small sized scope than Clark's. The separation between the two stars varies between 3 and 11.5 arc seconds. It's even possible to glimpse the Pup with a good quality 100mm (4-inch) refractor on nights of excellent seeing when close to maximum separation. When at minimum separation the B star is incredibly difficult to observe.

In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has a diameter of 12,000 kilometres (7,500 miles) almost equal to that of the Earth and a mass 98% that of the Sun. The star system is close to Earth at 8.6 light-years distant. Perhaps surprising, Sirius is one of the least powerful first magnitude stars, its brilliance is simply due to its proximity.

Apparent positions of Sirius B (Astronomical Society of South Africa -

Naturally Sirius has its place in ancient history. It's name has Greek origins which is unusual as most other named stars are Arabic based. The Greek translation is "sparkling" or "scorching" and along with the Romans they regarded it 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 Nile River.

With a declination of 16S, Sirius is visible from almost the entire World. The exceptions are latitudes 74N or greater where the star never rises above the southern horizon. However, from cities just inside the Artic Circle such as Murmansk in Russia, Tromsø in Norway and Barrow in Alaska it can be seen, though admittedly not for very long.

Finding Sirius is easy but in case of uncertainty it's located along an imaginary line extending from the three stars of Orion's belt southwards.

Sirius and Orion

There is an old mystery surrounding the colour of Sirius. It was often described in ancient times as red or orange although today it's clearly 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 all seems very strange.

Changes to Sirius A or B over a period of 2,000 years can be rejected as the timescale is too short and there is no sign of nebulosity in the system that would be expected had such a change taken place. Sir John Herschel suggest that a space cloud may have passed between us and Sirius therefore reddening the light as seen from our perspective but this seems unlikely as we would almost certainly be able to still detect such a cloud today. It also seems unlikely that the twinkling of Sirius would have fooled Ptolemy as any red colour is never permanent and the star was relatively high in the sky from his location and above the thicker layers of the atmosphere.

Most likely there was no change in the colour of the star but this remains a curious mystery even today!

Finder Chart for Sirius (Alpha Canis Majoris)

Finder Chart for Sirius (Alpha Canis Majoris) - pdf format

Sirius A Data Table

Bayeralpha (α) CMa
Flamsteed9 CMa
ConstellationCanis Major
RA (J2000)06h 45m 09s
DEC (J2000)-16d 42m 58s
Distance (ly)8.60
Apparent Mag.-1.46
Absolute Mag.1.42
Spectral TypeA1V
Radius (Sol)1.71
Surface Temp (K).9,940
Luminosity (Sol)25
Age (million years)250
Other designationsHR 2491, HD 48915, HIP 32349
Notable featuresBrightest star in the night-time sky. Has a white dwarf companion.

Sky Highlights - April 2017

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