The Great Dog is very noticeable thanks to bright Sirius, but even without Sirius, the bright constellation would be easy to find, although it never rises very high above the horizon in North American and Central Europe. It represents one of the dogs of the hunter Orion.
α Canis Majoris is better known as Sirius and at -1.5m the brightest star in our sky, second only to our Sun. However, with an absolute magnitude of +1.5M this impression is only due to its small distance of 8.7 light-years. Only four stars are closer to us than Sirius. Since it is always close to the horizon, its light is refracted as it passes through the Earth’s atmosphere again and again, so that it often seems to flicker in a rainbow of colors.
Only in somewhat larger telescopes you can spot Sirius B, the white dwarf star that is closest to the Sun. It orbits Sirius every 50 years. As a result, Sirius does not move along a straight path across the sky, but moves in a slight pendulum motion, which was first noticed in 1834 by F. Bessel. Sirius B was directly observed by Alvin Clarke no earlier than 1862.
Sirius was regarded as a calendar star in ancient times. In Egypt, the annual flooding of the river Nile happened when Sirius rose just before the Sun, while it announced in Greece the hot season – the “Dog Days”. Sirius was already known in Babylon as the “Dog Star”.
The open cluster M 41 has been described in 325 BC by Aristotle as “cloudy spot” – an impression that you can understand today only if you are far from the cities. You can see two dozen of its roughly 100 stars in binoculars, spread over an area of a full Moon diameter. At a distance of 2,250 light-years, the 100 million year old star cluster has a diameter of about 24 light-years. M 41 is located 4° south of Sirius and is therefore easy to find. There are several groups of stars that form smaller clusters and arcs in it.
NGC 2360 appears only as a slightly grainy, nine minutes of arc large nebula in 10 × 50 binoculars. The brightest stars in this open cluster only reach 10.5m and therefore cannot be resolved. Nevertheless, the group, which can be found circa 3.5° east of γ Canis Majoris in a star-rich part of the Milky Way, is worth visiting – at an age of 1.3 billion years it is extremely old. NGC 2360 is 3,400 light-years away from our solar system and only 0.5° away from the galactic equator.
The open cluster NGC 2362 extends around 4.4m τ Canis Majoris. Its diameter is only eight minutes of arc, so that it can be easily outshone by the star, especially at low magnifications. About a dozen of its 40 stars are visible in binoculars. It is still unclear whether τ Canis Majoris also belongs to the 4,900 light-years distant cluster. If it is part of the cluster, τ Canis Majoris would be 50,000 times brighter than our Sun and one of the most luminous stars known. NGC 2362 was formed only a million years ago, so that the eight light-years large cluster is one of the youngest known representatives of its kind. Its origin nebula is no longer visible.