Constellations: Canis Major 'the Great Dog' Content from the guide to life, the universe and everything

Constellations: Canis Major 'the Great Dog'

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Canis Major the Constellation

Name:Canis Major (Latin: 'great dog')
Genitive:Canis Majoris
Short form:CMa
Area:380 sq deg
Co-ordinates1:Right Ascension 07h, Declination −20°

Canis Major was listed by Ptolemy as one of the 48 original constellations handed down from antiquity. It is 43rd in size of the 88 internationally recognised modern constellations.

Bordered by Monoceros, Lepus, Columba and Puppis, Canis Major represents the larger of Orion's two dogs. This one is right on the tail of Lepus, the hare. Canis Major is best known for containing Sirius, the 'Dog Star', which ancient mariners navigated their sailing vessels by.

There is one Messier object in Canis Major, M41 (NGC 2287), an outstanding open star cluster. It also contains HD 47536, a star with a duo-planetary solar system. Then there is the most massive star that we know of, VY Canis Majoris.

There are also two planetary nebulae, IC 2165 and Min 3-1, and the enigmatic emission nebula known as Thor's Helmet.

There is an astonishing feature beyond Canis Major: a pair of colliding galaxies, NGC 2207 and IC 2163. This is an intergalactic combination of cosmic proportions, and it is taking place on an awesome scale, both in terms of size and time.


The scientific star names are simple to understand (if you know your Greek alphabet). For example, 'alpha' means it is the brightest star in that constellation. This is known as the 'Bayer designation'. The next brightest is designated 'beta' etc, or that is the way it is supposed to work. Sometimes the stars are variable or the cataloguing was a little off, so the list has the appearance of a dog's hind leg. In this constellation we have the alpha star in the correct place, then the epsilon, followed by delta, beta, eta, omicron2, zeta, sigma, kappa, omicron1, nu2, omega, theta, then gamma!

Some stars have proper names as well. For example, alpha Canis Majoris is known as Sirius. Others are known by their catalogue number.


Sirius is actually a binary system comprising a white main sequence star, Sirius A, and a dim dwarf star, Sirius B. The apparent magnitude is −1.43, although this is variable. It is so bright because of its nearness to Earth, at 8.58 light years2 distant, Sirius, known as the 'Dog Star', is the fifth-closest to us.

The ancient Egyptians used the pre-dawn (heliacal) rising of Sirius to predict the flooding of the Nile, which they called akhet (the inundation). This was essential to all life in Egypt. They even assigned a special god to oversee this action.

Along with Procyon and Betelgeuse, Sirius forms the Northern Hemisphere's Winter Triangle. Another interesting geometric asterism is the rhombus created by connecting Sirius, Rigel, Aldebaran and Betelgeuse.

VY Canis Majoris

VY Canis Majoris is a red hypergiant. At around 2,000 solar radii, it is the biggest star known to us. Scaling it next to our sun is like comparing a beachball to a pinprick. If this star were at the centre of our solar system instead of the Sun, its surface would be at the orbit of Saturn, and we would be inside it.

Star Table

StarDesignationName or
catalogue number
(light years)
Spectral classification
and/or comments
α CMaalpha Canis MajorisSirius-1.43 var8.58Binary star system
ε CMaepsilon Canis MajorisAdhara+1.5 var431Binary star system
δ CMadelta Canis MajorisWezen+1.81,790Yellow-white supergiant
β CMabeta Canis MajorisMirzam+1.9 var499Blue-white giant
η CMaeta Canis MajorisAludra+2.38 var3,000Blue-white supergiant
ο2 CMaomicron2 Canis Majoris24 Canis Majoris+2.98 var2,560Blue-white supergiant
ζ CMazeta Canis MajorisFurud+3.02336Binary star system
σ CMasigma Canis Majoris22 Canis Majoris+3.49 var1,022Orange supergiant
κ CMakappa Canis Majoris13 Canis Majoris+3.78 var789Blue-white subgiant
ο1 CMaomicron1 Canis Majoris16 Canis Majoris+3.891,976Orange supergiant
ν2 CManu2 Canis Majoris7 Canis Majoris+3.95 var65Orange giant/has a planet
ω CMaomega Canis Majoris28 Canis Majoris+4.0 var924Blue-white subgiant
θ CMatheta Canis Majoris14 Canis Majoris+4.08 var252Orange giant
γ CMagamma Canis Majoris23 Canis Majoris+4.1 var400Blue-white giant
τ CMatau Canis Majoris30 Canis Majoris+4.3 var3,196Blue supergiant, part of NGC 2362
VY CMaVY Canis MajorisHD 58061+9.5 var5,000Red hypergiant

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 - 1916). NGC 2362, also known as the Tau Canis Majoris Cluster, was first noted by Giovanni Hodierna in 1654. It was catalogued by William Herschel in 1783.

A fascinating feature is a post-merger duo-core galaxy, the lenticular NGC 2292 and elliptical NGC 2293. These are given separate catalogue numbers, yet they are now one complete galaxy, verging on spiral.

Thor's Helmet (NGC 2359) is a bright emission nebula with unusual properties. Astronomers suspect it is powered by a rare Wolf-Rayet star; these stars are named after their discoverers Charles Wolf and Georges Rayet. They are blue supergiants which eject stellar wind at a phenomenal speed, causing a 'bubble' effect in the nebula. The stars are massive, over 20 times the mass of our Sun. They have a high rate of mass loss, equivalent to an Earth mass per year. This shortens the star's life and will eventually cause them to go supernova.

NGC Table

(light years)
NGC 2207CW spiral galaxy+10.7114m3Merging with IC 2163
NGC 2217Galaxy+10.463,500ACW barred spiral
NGC 2272Galaxy+11.9120mLenticular
NGC 2280Galaxy+10.975mACW spiral
NGC 2287
Open cluster+4.62,300+80 stars
NGC 2292Lenticular galaxy+11.8100mPost-merger with NGC 2293
NGC 2293Elliptical galaxy+12.1100mPost-merger with NGC 2292
NGC 2325Galaxy+11.285,000Elliptical
NGC 2345Open cluster+7.76,000+70 stars
NGC 2354Open cluster+6.56,000+100 stars
NGC 2359Emission nebula+1115,000Thor's Helmet
NGC 2360Open cluster+7.23,700Caroline's Cluster
NGC 2362Open cluster+4.15,000Tau Canis Majoris Cluster
NGC 2367Open cluster+7.96,000+30 stars
NGC 2384Open cluster+7.46,00015 stars

Canis Major Dwarf Galaxy

The Canis Major Dwarf is an irregular galaxy, possibly our closest neighbour at 25,000 light years distant. Interestingly, it is 42 thousand light years from the Galactic centre. Discovered in November 2003, this galaxy is thought to contain around a billion stars. It is not in the Canis Major constellation (those stars are all in our Milky Way galaxy) - it is far beyond it. We say it is part of the constellation so that we know which direction to look and in what portion of the sky.

Extrasolar Planets in Canis Major

The nomenclature that has been decided upon for extrasolar planet discoveries is to use a lower-case letter after the parent star catalogue number (or name), eg 'HD 47536 b'. This stays with the planet even if more discoveries are made within the same solar system, and despite the position of the new planet relative to the star. Therefore, the first-discovered planet of HD 47536 is HD 47536 b, with HD 47536 c, and so on, having been detected later.

HD 47536 b, was discovered in 2003 and orbits within the system's 'Goldilocks Zone' (habitable range). Unfortunately, it is a gas giant and therefore not a prospect in the search for extra-terrestrial life. However, if it has any rocky moons with enough gravity to sustain an atmosphere then they would be a distinct possibility. The second planet, HD 47536 c, takes almost seven years to orbit the star and, due to a lack of liquid water, life as we know it is beyond the realms of possibility. HD 43197 is a yellow dwarf star which has a gas giant planet orbiting within the system's habitable zone.

The figures given in the table below are the length of the planet's orbital period around its parent star, which we know of as a year. The mass of the extrasolar planet is compared with that of Jupiter, our solar system's largest planet, known to astronomers as the 'Jovian scale'.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 47536HD 47536 b54302003Superjovian; habitable zone
HD 47536HD 47536 c72,5002007Superjovian
HD 47186HD 47186 b0.0714.082008Hot Neptune
HD 47186HD 47186 c0.351,3532008Gas giant
HD 45364HD 45364 b0.187226.92009Gas giant
HD 45364HD 45364 c0.658342.852009Gas giant
HD 43197HD 43197 b0.6327.82009Gas giant; habitable zone
HD 45184HD 45184 b0.045.892011Hot super-Earth
ν2 CMa7 CMa b2.67632011Superjovian; habitable zone
WASP-101WASP-101 b0.53.582013Hot gas giant
HATS-4HATS-4 b1.322.52014Hot superjovian
HD 47366HD 47366 b1.753632016Superjovian; habitable zone
HD 47366HD 47366 c1.866852016Superjovian

Down to Earth

Sirius Black was an important character in the 'Harry Potter' series of books by JK Rowling.

Canis Major in Science Fiction

Many science fiction authors have chosen Canis Major (and Sirius in particular) to base their adventures or write about extraterrestrials who hail from that vicinity. These writers include Lucian of Samosata (the first sci-fi writer that we know of), Douglas Adams, François-Marie Arouet, Isaac Asimov, Arthur C Clarke, Larry Niven and Eric Russell.

The sequel to 101 Dalmations is The Starlight Barking by Dodie Smith - and it involves the 'dog star' Sirius, of course! Popular TV shows also feature the Canis Major constellation stars quite heavily, eg, Star Trek, V and Doctor Who.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.3This means 114 million light years.

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