Favourite Constellation – Orion

One of my most favourite constellations to observe is the magnificent constellation Orion (The Hunter).

In mythology, Orion is said to be a hunter. The shape of the stars in the sky can be interpreted as a man, with a belt and sword, who is holding out a shield against the neighbouring constellation of Taurus, the Bull.

Orion is near to the celestial equator. Consequently, in the Northern hemisphere, it is a very prominent constellation in Winter, appearing “half-way up” the sky in the South.

However, from northerly latitudes in Summer, Orion is below the horizon at night and cannot be seen.

The main and easily recognised star pattern within Orion, is four stars in a rectangle which is bisected by the slanted “belt” of three stars.

In one direction, the belt points to Sirius, which is the brighest star in the sky and in the constellation of Canis Major. In the other direction, the belt points to the orange-red star Aldebaran in Taurus.

Orion is therefore a very useful “signpost” in the sky.

The “sword” of Orion, which appears to hang down from the belt, is actually a huge nebula (M42/M43), where stars are being created.

Each of the four main stars are bright and interesting.

Betelgeuse

The top left star (for an observer in the northern hemisphere) is the brightest. It is alpha-Orionis, called Betelgeuse.

Betelgeuse is noticably red-coloured to the unaided eye and is a red super-giant, variable star.

Its magnitude fluctuates between O and 1.3 every 6 years and it also pulsates in diameter from 300-400 times the size of the Sun. It is a true giant and it is 430 light years away.

Rigel

The bottom right star is Rigel and it is usually the brightest in Orion at magnitude 0.2. However, because of Betelgeuse’s variability, Rigel is designated beta-Orionis (not alpha-Orionis).

Rigel is a blue-white supergiant that is 800 light years away so it is nearly twice as far from Earth as Betelgeuse.

Bellatrix

The star at the top right (for Northern hemisphere observers) is a gamma-Orionis, called Bellatrix. It is a blue giant of magnitude 1.6

Saiph

The bottom-left star is Saiph, kappa-Orionis, at magnitude 2

The “Belt” of Orion

In some ways, the three stars which form the “belt” are the most interesting.

From left to right, they are zeta-Orionis called Alnitak (“the girdle”), then epsilon-Orionis called Alnilam (“string of pearls”), and lastly delta-Orionis called Mintaka (“belt”).

Alnilam is a blue super-giant, magnitude 1.7. It is 1600 light years away.

Both Mintaka and Alnitak are multiple stars.

Mintaka is an eclipsing binary with a period of 6 days. The pair is a magnitude 2.2-2.35 blue-white star with a magnitude 6 companion you can see in binoculars and small telescopes.

Alnitak is a tight pair of stars with magnitude 1.8 and 4. You need a good telescope to split them.

Sigma-Orionis

Just below Alnitak, is another multiple star, sigma-Orionis. It looks like a double star in binoculars, but a decent small telescope will actually show it to be four stars.

The “Sword”

As mentioned above, the Sword can be seen just under the belt.

It is a swirling mass of gas called the Orion Nebula, where stars are being born and it is excellent to observe through binoculars or telescope.

When Messier compiled his famous catalogue, he recorded this gas cloud as two separate objects, M42 and M43. Astronomers now believe them both to be part of the same cloud, although there is a dark area which makes them look separate, through a small telescope.

The “Trapezium”

In the centre of the Orion Nebula is the multiple star, theta-Orionis. Through a telescope, you can see a trapezium pattern of four stars.

It is believed that these stars have actually been created from the cloud of gas that is the Orion Nebula and that it is their light, that makes the Nebula glow.

Summary

As you can hopefully appreciate and see, the constellation of Orion is fascinating and well-worth some of your observing time, with binoculars or telescope.