Winter Hexagon Asterism: Facts
The Winter Hexagon asterism, also known as the Winter Circle, is a prominent winter asterism formed by seven stars prominent in the winter sky. These are — Sirius, Capella, Rigel, Procyon, Aldebaran, Pollux and Castor. All of the stars in the Winter Hexagon are part of different constellations.
The asterism is visible mostly upon the northern hemisphere’s celestial sphere. It is prominent in the sky during the months of December to March. It cannot be seen from South Island of New Zealand, the south of Chile, Argentine and further south. A part of the Milky Way runs through this asterism. This makes the Winter Hexagon a great guide towards finding deep-sky objects such as galaxies, nebulae and Messier objects.
In the tropics and southern hemisphere, the Winter Hexagon can be extended with the bright star Canopus in the south. Thus creating a “summer hexagon”. Sirius and Procyon are also part of the Winter Triangle. This is a smaller asterism that these two stars form with Betelgeuse in Orion.
One way to find to Winter Hexagon asterism is by locating the Winter Triangle, and though it is smaller, it is easier to find since it contains three of the ten brightest stars in the night sky.
Winter Hexagon Asterism: Stars
Sirius, also known as Alpha Canis Majoris, is the brightest star in the constellation of Canis Major, and overall the brightest star in the night sky. It has an apparent magnitude of 1.46, making it twice as bright as Canopus, the second brightest star in Canis Major. It is around 25 times brighter than the Sun.
Sirius appears so bright partly because it is one of the nearest bright stars to Earth. At a distance of only 8.6 light years, it is the 7th nearest star system to our Solar System.
Sirius is a binary star. The primary star is Sirius A, a main-sequence star of spectral type A0 or A1, which is twice as hot as the Sun and is 200% more massive. It also has 171% of its radius.
The companion is a faint white dwarf, designated Sirius B. It is of the spectral type DA2. Sirius B was actually the very first white dwarf to be discovered, and is around 100,000 times fainter than Sirius A. The distance between the two components varies between 8.2 and 31.5 astronomical units.
Capella, also known as Alpha Aurigae, is the brightest star in Auriga and the sixth brightest star in the night sky. It is also the third brightest in the northern celestial hemisphere and has an apparent magnitude of +0.08.
Capella is a quadruple star system organized into two pairs of two stars. The primary pair is composed out of two bright yellow giant stars. The primary star is around 78.7 times brighter than the Sun, has 256% of its mass and 1,198% of its radius. The companion is 72.7 times brighter than the Sun, has 248% of its mass and 883% of its radius.
Rigel, also known as Beta Orionis, is the seventh brightest star in the night sky, located in Orion constellation, and is a massive blue-white supergiant variable star. It appears as a single star, but is actually a star system that contains four stars.
The primary star, Rigel, is an Alpha Cygni variable star. It is around 61,500 and up to 363,000 times brighter than our Sun and has an apparent magnitude ranging from 0.05 to 0.18.
Rigel is located around 860 light years away from Earth and has a 7,000% of our Sun’s radius and 2,100 % of its mass. As years go on, Rigel will expand to an even greater size and transcend into a red supergiant, possibly exploding into a Supernova one day.
Rigel is occasionally outshone by Betelgeuse, although it is the brightest star in Orion. Alongside Betelgeuse, Bellatrix and Alnilam, Rigel makes up the navigational stars of the Orion constellation.
Procyon is the eighth brightest star in the sky and has an apparent magnitude of 0.34. It is the brightest star in the constellation of Canis Minor. It is located around 11.4 light years away from Earth, making it the 16th nearest star to us. Procyon is a binary star, composed of Procyon A, a white main sequence star of spectral type F5 IV-V, and Procyon B, a white dwarf belonging to the spectral class DQZ.
The name Procyon comes from the ancient Greek Προκύων, which means “before the dog,” referring to the fact that it rises just before the Dog Star (Sirius).
Apart from the Winter Hexagon asterism, Procyon also marks one of the vertices of the smaller Winter Triangle.
Aldebaran, also known as Alpha Tauri, is the brightest star in the constellation of Taurus and the 13th brightest star in the sky. It is an orange giant with an apparent magnitude varying between 0.75 and 0.95. This star got its name, which translates as “the follower”, because it appears to follow the Pleiades cluster, across the sky.
Aldebaran has the stellar classification K5III and is located around 65.1 light years from Earth. It is classified as a slow irregular variable, type LB. Its brightness varies by about 0.2 magnitudes. It has a diameter 44.2 times that of the Sun and is about 425 times more luminous.
This star is fairly easy to find in the sky as it lies close to the Orion constellation. The three bright stars that form Orion’s Belt point in its direction. The star also helps to find Haydes Cluster, although it is not a member of the cluster but merely lies in the same line of sight.
Pollux, also known as Beta Geminorum, is the brightest star in the constellation of Gemini and the 17th brightest star in the night sky. It is an evolved orange giant with the stellar classification K0 III.
It has an apparent magnitude of 1.14 and is located 33.78 light years distant from the Solar System. Pollux has a mass that is twice that of the Sun’s and has a radius that is nine times that of the Sun’s.
In June 2006, an extrasolar planet, Pollux b, was confirmed to be orbiting the star. It has a mass at least 2.3 times that of Jupiter and an orbital period of 590 days.
Castor, also known as Alpha Geminorum, is the second brightest star in Gemini and the 44th brightest star in the sky. It is a binary star with a combined apparent magnitude of 1.58. The two components are separated by 6” and have a revolution period of about 467 years.
The two components of Castor each have a spectroscopic binary star, which makes Castor a four-star system. Castor also has a faint companion about 72” away, which is an eclipsing binary star system with a period of just under a day.
Both components of the system are red (class M) dwarfs making Castor a sextuple star system, as all six of its components are gravitationally bound together. The primary component belongs to the spectral class A1 V and its companion is believed to be of spectral type M5 V. The secondary component stars have the stellar classifications A2 Vm and M2 V.
Winter Hexagon Asterism: Deep Sky Objects
The Winter Hexagon contains several prominent nebulae. These include the Orion Nebula, the Hyades Cluster, Messier 35, The Pleiades and the Horsehead Nebula. Other interesting nebulae in this asterism are the Cone Nebula, the Christmas Tree Cluster, the Rosette Nebula, the Beehive Cluster and the Crab Nebula.
Messier 42, also known as NGC 1976 and the Orion Nebula, is located south of the Orion’s Belt asterism and is a diffuse reflection nebula. It is 1,344 light years away from Earth and is one of the brightest nebulae in the sky. With a visual magnitude of 4.0, it can be seen without binoculars.
The naked eye makes Messier 42 appear as a slightly blurred “star” in the center of Orion’s Sword asterism. It is the nearest known region of massive star formation to the Solar System and is part of the Orion Molecular Cloud Cluster.
Located in the Orion Nebula is the Trapezium or Orion Trapezium Cluster. This is a very young open cluster which is recognized by its four brightest stars that form a trapezium-shaped asterism.
The brightest star in the Orion Trapezium Cluster is Theta-1 Orionis C. It is a blue main-sequence star of spectral class O6pe V and has an apparent magnitude of 5.13. It is located 1,500 light years away from Earth and is one of the most luminous stars with a surface temperature of 45,000 K. This is around 7.7 times hotter than our Sun.
The Horsehead Nebula, also known as Barnard 33, is a dark nebula located south of the star Alnitak, in the bright emission nebula IC 434. It is also located around 1,375 light years away from us and is part of the Orion Molecular Cloud Complex.
The Horsehead Nebula was first discovered in 1888 by Williamina Fleming. It received it’s name thanks to the the shape of its dark dust clouds and gases which resembled the head of a horse.
The Hyades Cluster
The Hyades Cluster, also known as Caldwell 41, Melotte 25 or Collinder 50, is an open cluster that contains hundreds of stars. It is thought to be about 625 million years old and is the nearest open star cluster to the Sun and therefore the most studied one.
The brightest stars in Hyades Cluster are Gamma, Delta, Epsilon and Theta Tauri. They form a V shape with the brightest star in Taurus, Aldebaran. Aldebaran is not of the cluster, but lies in the same line of sight. It is actually much closer to use than Hyades Cluster.
Gamma, Delta, Epsilon and Theta Tauri form the bull’s head and together give Hyades Cluster an apparent magnitude of 0.5. They are around 153 light years away from us.
Messier 35, also known as NGC 2168, is the only Messier object in the constellation of Gemini. It is an open cluster that is located around 3,870 light years away from Earth. It was discovered in 1745 by astronomer Phillippe Loys de Chesaux.
Messier 35 has an apparent magnitude of 5.1, meaning that it can be seen easily with even a small beginners telescope. It has a radius of around 11 light years and is believed to be around 175 million years old. It containing around 1,600 solar masses.
The Pleiades, also known as Messier 45, is an open star cluster that consists of hot, luminous B-class stars. Most of which have formed in the last 100 million years. It is one of the easiest clusters to find in the sky because it is so bright and, therefore, one of the most well known.
The Pleiades cluster has an apparent magnitude of 1.6 and is located between 390 and 460 light years away form the Solar System. The nine brightest stars in the cluster were named after the Pleiades – the Seven Sisters in Greek mythology – Alcyone, Celaeno, Electra, Maia (mother of the god Hermes), Merope (wife of Sisyphus), Sterope, Taygeta – and their parents, the sea nymph Pleione and the titan Atlas.
Messier 45 will exist for another 250 million years or so. After which the gravitational interactions with nearby objects will cause the stars to disperse. The cluster is known in many different cultures and was thought to be first found in the Bronze Age.
- By Elop using Stellarium – Own work, based on a Screenshots with Stellarium, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=58547708
- By NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team – http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/01/https://www.spacetelescope.org/news/heic0601/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1164360
- By Ken Crawford, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31584618
- CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=1480459
- Public Domain, https://commons.wikimedia.org/w/index.php?curid=5873380
- By NASA, ESA, AURA/Caltech, Palomar ObservatoryThe science team consists of: D. Soderblom and E. Nelan (STScI), F. Benedict and B. Arthur (U. Texas), and B. Jones (Lick Obs.) – http://hubblesite.org/newscenter/archive/releases/2004/20/image/a/ (image link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=7805481