March Constellation of the Month -- Orion

Orion, the Hunter, is the most prominent constellation visible to us in the northern hemisphere at any time of the year. It's easy to identify by the three equally-bright stars, arranged in a line, which form Orion's belt.

The two brightest stars in the constellation are at the upper left and lower right, called Betelgeuse (beetle juice) and Rigel, respectively. Both are giant stars --but of very different types. Betelgeuse is a red supergiant star, it is nearing the end of its life. It has cooled and expanded to a size which is more than twice the width of the orbit of Mars -- about 500 times as wide as our own Sun. (In 5 billion years the Sun will be a red giant, its outer edge will expand out to envelop the Earth.)

Rigel, on the other hand is a blue supergiant. It is relatively young and is 10 times as hot as Betelgeuse -- and 5 times as hot as the Sun. It is 40,000 times as bright as the Sun, while Betelgeuse is 14,000 time brighter. Rigel is actually a triple star. The blue supergiant is orbited by 2 smaller stars, which orbit around each other.

But, the most spectacular objects in Orion are not the stars but nebulae. The Orion Nebula or Great Nebula of Orion is located between the two stars which make up the "sword" -- which are line up vertically below the belt. It is a huge cloud of gasses from which which many stars are being formed. The equally spectacular Horsehead Nebula and the Orion Nebula are just areas within a much larger nebula 1500 light years away from us.