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From March to May you can see the Spring Triangle in northern skies. In summer, the Summer Triangle is most prominent, but may be seen all year round in most of the northern hemisphere. There is also a Winter Triangle. But grandest of all is the Winter Hexagon.
Are they constellations?
The 88 constellations agreed by the International Astronomical Union (IAU) cover the sky with no gaps. Two of the constellations are triangles: Triangulum and Triangulum Australe. The triangles mentioned in this article – plus the hexagon – are asterisms, star groups that are part of one or more constellations. They don't have any official standing.
The Winter Triangle
If you've seen the Summer Triangle – Vega, Altair and Deneb – you know that it's very large. Once you appreciate its scale, its bright stars are easy to pick out. The Winter Triangle is noticeably smaller, yet also composed of three of the top ten brightest stars in the night sky: Sirius, Procyon and Betelgeuse. [Image: Software Bisque 'Tom's Corner']
The Winter Triangle is easy to find if you start at Orion's Belt, an asterism made up of three stars in a row. From there you can see the reddish star Betelgeuse, one of Orion's shoulders. It's a red supergiant around 650 light years away and a thousand times bigger than the Sun. One day it will exhaust its fuel and explode as a supernova, leaving a neutron star behind.
You can find Sirius by following the line of the Belt stars to the southeast. They point to Sirius, which is actually a binary star system. Sirius A is a brilliant hot star about twice the mass of the Sun, and less than nine light years away. Sirius B is a faint white dwarf. Sirius is also known as the Dog Star, being the brightest star of the constellation Canis Major (the Greater Dog). This is the origin of the “dog days of summer” (July 3 to August 11). During this period, Sirius rises at the same time as the Sun, and the ancient Greeks thought the combination of the Sun and the brightest star created the extreme heat of early summer.
The brightest star of Canis Minor (the Lesser Dog) is yellowish white Procyon. As is Sirius, Procyon is a neighbor to us. It's about eleven light years away, and also a binary system. Procyon A is some 1.5 times the Sun's mass, and Procyon B is a white dwarf.
The Winter Hexagon
The Winter Hexagon, also known as the Winter Circle or Winter Oval, encloses a very large area of sky. The six stars that form the hexagon's vertices include Sirius and Procyon. (Betelgeuse is inside the hexagon, but not part of it.) The other four stars are also in the top ten brightest stars we see. They are: Pollux in Gemini, Capella in Auriga, Aldebaran in Taurus, and Rigel in Orion. [Image: constellation-guide.com]
The Winter Hexagon is primarily a northern hemisphere asterism that can be seen high in the night sky from December to March. It isn't visible at all in the higher southern latitudes, and of course, it isn't a “winter” sight in the southern hemisphere.
Some diagrams of the hexagon include both Gemini twins, Castor and Pollux. However only Pollux, which is the brighter, is part of the asterism. It's an orange giant, the closest one to the Sun of this type of star. Pollux is known to have one planet, a giant, over twice the mass of Jupiter.
Capella looks like one bright star to the unaided eye. In fact, it's a system of at least four stars – two binary pairs – though it took until the twentieth century to discover this. Capella Aa and Capella Ab are two yellow giants orbiting each other so closely that only a specialized telescope can separate them. The second pair, two faint red dwarfs, are about 10,000 au from Capella A. That distance in the Solar System would be well beyond the Kuiper Belt, about halfway to the Oort Cloud.
The eye of the bull in the constellation Taurus is the red giant Aldebaran. Although it looks as if it's part of the V-shaped Hyades star cluster, it's merely in our line of sight to the Hyades. Aldebaran is much larger than our Sun. If it were in the Sun's place, it would take up the space out to the orbit of Mercury.
Orion's left foot – on the right side as you view Orion – is the beautiful blue star Rigel. It's around 200,000 times more luminous than the Sun and greatly swollen in size. This supergiant turns out to be Rigel A, for it's a system of between 3-5 stars. Rigel A is a young star, only about 10 million years old. That may not sound young, but compare it to the Sun's 4.5 billion years old. It will never reach the Sun's age, because long before that it will become a red supergiant like Betelgeuse, and then go out with a supernova bang.
It's Rigel that causes the blue glow of the Witch Head Nebula (IC 2118). The blue isn't primarily due to the star's color. The nebula's dust grains reflect blue light more efficiently than red light. [Image: NASA/STScI Digitized Sky Survey/Noel Carboni]
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