January 2022 (or any year really) is the time of year that we start to see the constellation Canis Major again. The brightest star in the night sky is Sirius and it is easy to spot, especially if you know where Orion’s Belt is; you just follow the direction of Orion’s Belt towards the South Celestial Pole for about 20 degrees. The stars around Sirius make up the constellation of Canis Major. The name roughly means great dog in Latin. The constellation occupies the bit of the sky next to Orion – towards the South. This is a great part of the sky to see some wonderful objects that show the beauty of star colours as well as their different life stages. Whether you are interested in the aesthetic beauty of the night sky, or the astrophysics – or both, then Canis Major has a little bit for everyone.
Before we get onto the amazing sights in Canis Major it is worth spending a bit of time talking about the wonderful Sirius. The star is in the main sequence, meaning it is fusing hydrogen into helium in its core. Sirius has roughly twice the mass and radius of the Sun. It radiates 26 times more energy than the Sun and has a surface temperature of around 9000 degrees Celsius. The star is only 8.6 light years away – twice the distance of the nearest visible star to our Sun, Alpha Centauri.
It’s not an unusual star but it has an unusual companion. Like most stars in the night sky, Sirius is not alone. It has binary companion a little under the size of the Earth but with a mass of the Sun. The density of material in this tiny star is about 1.7 million tons per cubic metre and it’s surface temperature is around 24,500 degrees Celsius. This strange star is all that remains of Sirius’ once larger binary partner. Astronomers think that the original star was probably about 5 times the mass of the Sun and as nuclear fusion eventually shut down in its core the rest of the mass was lost to space. All that remains is the core of the that star as a white dwarf. It orbits the larger and brighter Sirius though you won’t easily see it unless you have a large telescope and can mask the brightness of Sirius.
The most prominent object in Canis Major, is a large open cluster of stars called M41. In very dark sky locations such as where we observe from at Stonehenge Aotearoa it is visible to the naked eye as a fuzzy patch just to the South of Sirius. The cluster is around 2300 light years away and contains about 100 stars and is between 25 and 26 light years across. Stars are born in clusters and over time the cluster disperses; M41 is likely to disperse in around 500 million years. The stars in the cluster are 190 million years old and few of them have already advanced into the red giant phase having fused all of the hydrogen in their cores.
Visually it is a really nice cluster to observe with a couple of yellow and orange looking stars set against the back ground stars of the Milky Way. Aristotle may have also identified this star cluster as far back as 325 BC though there is some conjecture about this.
To the south of Sirius is three stars that make out a right angle (roughly), they are Adhara, Wezen and Aludra with the nearest two to Sirius being Adhara and Wezen. If you make a line from Adhara through Wezen and then extend that line about the same distance you will see the beautiful open cluster NGC 2362. This cluster is dominated by a very bright blue star and is one of the youngest star clusters in the night sky, possibly being only 5 million years old. The cluster is a little unusual for such as young cluster as there is very little of the gas cloud remaining that would have sparked the birth of the stars.
The cluster is about 4800 light years distant and contains 265 stars. The bright star is an O spectral type which means it is blue big and hot. Surrounding this star are the brighter B spectral class stars with the remaining stars harder to see has they are considerably less brighter. This cluster does have a higher proportion of O and B spectral class stars than other clusters. Visually it is an impressive sight with the O star dominating the view surrounded by the smaller B type stars.
The Winter Albireo
The next highlight of Canis Major is the delightful double known as the Winter Albireo. This pair of stars contrasts stellar colours beautifully and rivals the more famous Albireo double favourably. This double is only two degrees to the north of NGC 2362 so if you’ve been observing that cluster then you only have to move the telescope the distance of about 4 full Moons to find it. The colour contrast of the two stars is really fascinating. The yellow star is a spectral class K3Ib, called a bright giant, these stars are almost supergiants but not quite. The blue looking star is much hotter and is probably in a binary system. The two stars are unrelated other than visually they line up with an observer on Earth. They are almost 2000 light years away from each
The final object, though just outside the official boundary of Canis Major is M93. The open cluster is 3300 light years away and is about 25 light years across. Astronomers have found the cluster to be quite interesting due to the number of variable stars. The age of the cluster is about 390 million years and there’s a bunch of bright B spectral type stars. Visually this cluster is set against a busy background of the galactic plane with some nice colour variations.
You can find this cluster by lining up Aludra with the next two brightest stars increasing in right ascension; the furthermost star is called Asmidiske and you’ll find M93 just North of that star by about 1.5 degrees.
The region in and around Canis Major is a great part of the sky to browse with a pair of binoculars or with a telescope. The region is often overlooked for the brighter and spectacular Orion Nebula and the stunning M46 and M47 clusters, which are great objects to look at. Canis Major is fantastic to view in the Southern Hemisphere especially in Summer when it is high in the sky.
If want to join us to see the sights of Canis Major then come on a Star Safari with us.