During these cold, clear winter nights in the Northern Hemisphere, the sky is adorned with a veritable treasure trove of jewels, ranging widely in brilliance, splendor and color.
As seen from mid-northern latitudes, the winter sky has one of the highest densities of bright stars for any given region of the sky. Even a casual observer can appreciate the assortment of bright stars, their splendor, density and brilliance unrivaled. Prominently placed, looking south between December and March, one quickly notices Orion, The Hunter with Canis Major, The Great Dog, to the south and east [of Orion].
Contained in Orion and Canis Major are the stars Rigel and Sirius, both stars providing an excellent object lesson in comparative luminosities.
Standard, unaided view of Orion showing the 3 familiar belt stars with Saiph at the lower left, Rigel at the lower right, Betelgeuse at the upper left and Bellatrix at the upper right. Visible in this image is the famous Orion Nebula, M-42, the pink patch below the belt stars.
At 1.7 solar radii, Sirius is an A1 main sequence star with twice the mass of the sun at a relatively close distance of 8.7 light years.
At 25.4 times the luminosity of the sun, Sirius is not nearly as intrinsically luminous as Rigel, a star with almost 80,000 times the sun’s luminosity. Both stars, however, are of a comparable brightness. So how do we reconcile these widely disparate luminosities? Rigel owes its luminosity to both its size and its temperature. The luminosity of a star is proportional to the second power of its radius and the forth power of its temperature. At 78 solar radii, Rigel is huge, the size alone accounting for 6,100 times the sun’s luminosity.
With a surface temperature of 11,000 K, that result is again multiplied by 13, yielding a comparative luminosity of almost 80,000 solar compared to a paltry 25.4 solar for Sirius.
At 800 light years distant and appearing almost as bright as Sirius, one gets a good sense of the awesome power of Rigel compared to the sun or Sirius. Suppose we were to transpose the positions of Rigel and Sirius, how would they appear in our sky? Rigel would have an apparent magnitude of -9.5, the brightness of the first-quarter moon but concentrated in a point source, a source so bright that you could easily read by its light! Sirius, at the distance of Rigel, would have an apparent magnitude of 8.3 and would be invisible to the unaided eye! Rigel is the most luminous star in our local region of the Milky Way and when the day comes that it ends its life as a supernova, the light show will be spectacular, equal in brightness to the full moon and visible during daylight! Imagine, if you will, all the light of the full moon concentrated into a point source and you have a good idea of just how bright Rigel will appear when it does go. This, at its distance of 800 light years! What if it were at the distance of Sirius when it went supernova? It would rival the brightness of the noon day sun and extinguish all life on earth!
In addition to Rigel and Sirius, this showcase of notable gems includes Betelgeuse, Orion’s right shoulder, an evolved, red supergiant star so bloated and huge that the planet Jupiter would orbit inside the star! Procyon, to the north and east of Sirius is almost as bright as Sirius, all appearing as if someone had scattered a veritable jewel box onto the sky. Sirius, Rigel and Procyon are as diamonds in the sky; Betelgeuse as a ruby; the three stars that make-up Orion’s belt: Mintaka, Alnitak and Alnilam are as 3 lesser sapphires (see unaided-eye view of Orion, above). The ethereal beauty of the winter Milky Way running south through Orion and skirting Canis Major on its eastern boundary as a most-elegant jeweled necklace, more beautiful and sublime than the mind of man can imagine, fills the observer with profound humility and a deep sense of awe. No holiday ornaments, no festive lights, nothing that can be created or imagined by the mind of man can replace these natural jewels in majesty, beauty and splendor. It is no wonder that the winter sky looks as it does, ablaze with these stellar jewels: the sun is located in the “Orion Spur”, a rich, star-forming region of our Milky Way galaxy.
Visible in the wide-field view of Orion the most famous in the winter showcase of celestial jewels is the Great Nebula in Orion, Messier 42. Located in the “Sword of Orion” and flanked by Messier 43 to the left and shining by the intense ultraviolet light of the luminous O and B stars collectively known as the Trapezium (Theta Orionis), the ethereal beauty of this H-II region is legendary.
Aside from the old favorites, I would like to briefly discuss three stunning open star clusters, M-41, M-46, NGC2362 and a stunning, little planetary nebula, NGC 2438 that, while superposed within M-46, is not physically associated with it. South-southwest of Sirius in Canis Major lies the beautiful open cluster, Messier-41 (M-41, for short). It is no accident that, along with the other, brilliant gems scattered throughout this region of the sky, there is no shortage of galactic (open) clusters. Being associated with star-forming regions, these types of star clusters would be found in the arms of the host spiral galaxy. The conditions surrounding their formation playing a critical role in their evolution, these stars will live very short, punctuated lives with many of the more massive ones ending their lives as supernovae.
Stunning in binoculars and visible in the wide-field view (below Sirius towards the right-hand side), and in the telescopic view, M-41 is a winter favorite.
Puppis, south of Canis Major is another constellation that is often overlooked but is, nonetheless, home to a real gem. Messier-46 contains a 10th magnitude planetary nebula (the end-state for stars similar to our sun) and NGC2438, a galactic star cluster that is easily visible in a small to mid-sized amateur telescope. This beautiful little smoke ring appears to float in front of the background stars and is, in fact, a foreground object. M46 can be easily found by scanning due east of Sirius with a wide-field eyepiece. Once found, higher magnification will show the stunning little smoke ring.
Another gem in this treasure chest that is the winter sky in and around Canis Major and Orion is Tau Canis Majoris and the associated open cluster NGC 2362.
While both Rigel and Tau are luminous supergiant stars, Tau, at 4800 light years, is intrinsically more luminous as an O class star (O9), the most luminous. On the other hand, Rigel is a B class star (B8) at 800 light years distant. As was the case with Sirius and Rigel, the comparison of these stars serve as a good object lesson in comparative luminosities. Rigel, at 800 light years, is 6x closer than Tau at 4800 light years but Tau is intrinsically 5x more luminous. If they were the same luminosity Tau would be ~ 36x fainter than Rigel. The apparent magnitude of Rigel at 0.12 compared with the apparent magnitude of Tau at 4.35 is consistent when considering their respective luminosities and distances.
New observations have revealed that this star is a tightly-bound multiple-star system composed of four individual stars with a combined mass of 80 times solar with a relative luminosity of 500,000 solar! Little is known regarding the properties of these component stars. It is also uncertain whether Tau is gravitationally bound to the cluster. If it is, as mounting evidence seems to indicate, Tau Canis Majoris would be one of the most luminous supergiant stars known with an absolute magnitude of -7.2 and an aggregate luminosity of half a million solar! Like most of the other stars discussed in this article, Tau will end it’s life in spectacular fashion as a Type-II supernova!
This beautiful little cluster, about 25 million years old, is a personal favorite with about 60 A class stars, stars similar to Sirius. To locate NGC 2362, scan approximately 1º northeast of Wezen (Delta Canis Majoris – see wide-field view) and the neat little cluster will dance into view. Imagine, if you will, a planet orbiting (Tau) Canis Majoris, a luminous O8 star embedded in NGC 2362 and surrounded by dozens of young, luminous stars. What would your night sky look like? Then also image that this star will live no more than 20 million years and is a likely candidate for a Type II supernova, the same fate that will befall 1 (Theta-1) Orionis, a luminous, blue-white O6 star, the most luminous of the stars in the Trapezium and the principal source of the intense UV light that is causing M-42 to shine. In about 20 million years we had better keep our eyes on this part of the sky because, as the Chinese astronomers of 1054 observed in Taurus, we will be witness to the terrible beauty of a Type II Supernova, the most energetic event in the universe, a spectacle that will be observed in a location none other than what used to be the sword of Orion!
Located in Taurus, the Bull, another object of note is the galactic cluster Messier-45, also known as the Pleiades or seven sisters. Like M-46 discussed above, this young cluster is composed of hot A and B class stars, loosely bound together by gravity. Imagine that this cluster was located at the distance of Sirius, an A class star, the Pleiades would be truly spectacular; our sky would be ablaze with hundreds of brilliant stars such as Sirius!
With the exception of Sirius, most of the bright jewels of Winter’s sky are evolved giant or supergiant stars such as Rigel and Betelgeuse, with the later, like Rigel, a supernova candidate that is nearing the end of its life.
Except for the Witch-head nebula, image credit: the author. Wide-field images of Orion and Canis Major acquired in the Everglades, February 1986 with full, unaided-eye view obtained by the author from his home on Long Island, NY c 12/1970. Hi resolution images of Sirius, M-41, M-42, M-45 and NGC-2362 obtained by the author with his 15 cm (6″) F/4.2 Newtonian or 20 cm (8″) F/7.2 Newtonian reflector, both at Prime focus. As part of an advanced study program, images of M-46 and NGC 2438 were obtained with the 0.61 m (24″) R-C Reflector of LightBuckets on 13 March 2010.