Doorstep Astronomy: See Mighty Orion
Looking east around 8 p.m. on an early December evening.
Credit: Starry Night® Software

You know Orion always comes up sideways.

Throwing a leg up over our fence of mountains,

And rising on his hands, he looks in on me

Thus Robert Frost describes Orion's appearance in the eastern sky in early December.

If you go out any night this month around 8 p.m. and look east, you will see Orion rising. His shoulders are marked with the bright stars Betelgeuse and Bellatrix and his knees by Saiph and Rigel [See Map].

In between are three stars in a line which mark his belt. When Orion is high overhead, this belt is at a jaunty angle, but when rising in the Northern Hemisphere the three stars are in almost a vertical line; when setting nine hours later they are almost horizontal. Incidentally, Orion is an "equal opportunity" constellation: located on the celestial equator, it is readily observed from anywhere on Earth other than the poles.

One of the first things to notice about Orion is the contrast in color between its two brightest stars: Betelgeuse in the upper left corner is a old red giant while Rigel in the lower right is a young blue star. (Left and right, and up and down are reversed if you are observing from the Southern Hemisphere.) Like most red giants, Betelgeuse is a variable star. Its brightness can be seen to change over a period of months as it expands and contracts in size.

Many of the stars in Orion are double or multiple when viewed with a small telescope. Rigel is particularly interesting because its bright 1st magnitude primary star is accompanied by a faint 7th magnitude secondary star. In a telescope this pair looks like a star and accompanying planet, though the "planet" in this case is a white dwarf star.

The most interesting area in Orion is the Orion Nebula and its associated stars. You find this by starting with the line of three 2nd magnitude stars which form the Belt of Orion: from left to right, Alnitak, Alnilam and Mintaka. Below these stars is a line of three fainter stars which form the Sword of Orion. The middle of these three, known as Theta or The Trapezium, is the star we are interested in.

In a small telescope, Theta is seen to be a quadruple star: four stars arranged to form a trapezium. With a larger telescope, at least two more tiny components can be seen in the Trapezium.

The most wonderful thing about the Trapezium? is that it is engulfed in nebulosity: the Great Nebula of Orion, number 45 in Messier's catalog. In a dark sky, this birthplace of new stars can be readily seen in binoculars as a faint glow around Theta. In any sky it is visible in telescopes of all sizes. In fact, it is arguably the most beautiful object visible anywhere in the sky in any telescope, large or small. It is interesting to look at with any magnification, also with or without various nebula filters. Even experienced observers can lose themselves for hours in its glowing swirling clouds of gas and dust.

One famous deep sky object in Orion which beginners will look for in vain is the Horsehead Nebula. A popular subject for astrophotographers, visually this is one of the most challenging objects in the sky. Many experienced observers with hundreds of deep sky objects to their credit have never seen the Horsehead. The reason is that this is a dark nebula (a dense cloud of dust) visible only as a silhouette against a very faint emission nebula. This emission nebula can only be seen with the help of a special filter, which passes only the beta spectral line of hydrogen. Even with this special filter, it requires an extremely dark sky and a very skilled eye.

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.