Feature-Orion

From OttawaRasc

Orion, Winter's Showpiece

Orion, the showpiece constellation of winter, and probably the most-recognized constellation, is visible now in the late evening, and rising earlier each night. By late November it will be visible in the mid- evening sky, and it will grace our clear winter nights for several months. This beautiful constellation is rich with interesting objects, science, and mythology, and is a useful signpost to a large number of other interesting objects.

This article is aimed at the newcomer to Astronomy, and will give you a tour of some of the fascinating objects in and around Orion.

Image copyright David Cortner, used with permission.

Finding Orion

From late fall to late winter, Orion appears in the South East sky, moving through the South, to the South-West. It is always a comfortable distance above the horizon - usually at an altitude (from Ottawa) of 30 to 40 degrees. So, just look toward the south, slightly up from ground level (less than halfway to "straight up"), and look for the familiar shape.

Orion is one of the rare constellations that actually look somewhat like their mythological character - in this case a hunter, with a tight belt, wearing a sword and brandishing a club and a shield.

Look for the distinctive "belt" first. These are closely- and evenly- spaced bright stars in a straight line. From the belt, you can find the 4 corners of the body (shoulders and knees, or shoulders and feet, depending on the interpretation). In darker skies, you will also be able to see the 3-star "sword" hanging from the belt. The hunter's raised club and extended shield are dimmer stars, and require dark skies outside the city light pollution to see.

Orion is not always "vertical", but is rarely far from it. He is a large constellation, stretching about 17 degrees in the sky from foot to shoulder (just under the width of two clenched fists held at arms length).

Mythology

Orion was the son of the god of the sea (Poseidon to the Greeks, Neptune to the Romans). He loved the godess of animals and the moon (Artemis to the Greeks, Diana to the Romans), sister of Apollo. Apollo did not approve.

He was a great hunter, and is located in the sky standing near the river Eridanus, accompanied by his two hunting dogs Canis Major and Canis Minor. He is bradishing his club, and holding his outstretched shield, while fighting nearby Taurus, the bull.

Orion is usually considered related in some way to Scorpius, the giant scorpion, because they are close to 180 degrees apart in the sky, so one is always setting exactly when the other is rising. Myths either say Orion is hunting the scorpion or running from it.

In his youth, Orion was rendered blind by King Oenopion, for chasing his daughter Merope (one of the Pleiades, the Seven Sisters), but his sight was restored by the rising sun.

There are several versions of the story of Orion's death.

Jealous Apollo tricked Artemis into shooting him with arrows while he was swimming; or

Artemis or Apollo produced Scorpius, the giant scorpion Orion could not defeat, who killed him.

Highlights

Orion is of interest to amateur astronomers for many reasons.

First, he is a useful reference for declination. Mintaka, the rightmost star in Orion's belt, is at almost exactly zero declination, which means the belt is a useful reference for Declination 0, the Celestial Equator (the projection of the Earth's equator into the sky.) So, to people on the equator, Orion will pass directly overhead, straight up.

Stars

Orion contains many interesting stars. The two most prominent are Rigel and Betelgeuse.

Rigel

Rigel, the bottom right star, is of interest for being one of the brightest stars in the sky. (It is ranked 6th not including the Sun; between Vega and Procyon.) It is a Blue-White Giant, with visual magnitude 0.1, and is about 55,000 times more luminous than our Sun.

Betelgeuse

Betelgeuse ("beetle juice"), the noticeably red top left star in the main body (the "shoulder" on our left), is an irregularly pulsating Red Supergiant. Its size varies from 300 to 600 times the diameter of our Sun. (If it replaced our Sun, Mercury, Venus, Earth, Mars, and Jupiter would be inside the star.)

Betelgeuse is so large that in 1996 it became the first star to have its actual disk imaged from Earth - by the Hubble Space Telescope.

Our own Sun will become a Red Giant in a few billion years, although it will be smaller than Betelgeuse - only about 200 times its current size.

Nebulae

What Orion is best known for, however, is its collection of nebulae.

Orion Molecular Cloud Complex (courtesy SEDS)

The entire Orion region is engulfed in a huge cloud of Hydrogen - the Orion Molecular Cloud Complex. This cloud is about 1500 light-years away from us, and is hundreds of light-years across. The entire cloud can be seen in wide-field, long exposure photographs of the region.

M42, the Great Orion Nebula, from Hubblesite.Org

Several denser components of the cloud have their own names and are within reach of small telescopes.

M42, the Great Orion Nebula, and nearby M43, are probably the most-viewed and most-photographed nebulae in the entire sky. This cloud is a stellar nursery, and Hubble Space Telescope images confirm that we can see dozens of new stars in the process of forming.

Simulation of the appearance of M42 in a small telescope

M42 is so large and so bright that it is visible to the naked eye in dark skies, in ordinary binoculars, and in even the smallest telescopes. However, you should be aware that a small telescope and eyepiece will not show you the spectacular colours you have seen in photographs. At the eyepiece, nebulae appear in shades of gray.

M42 could not be much easier to find - point your telescope or binoculars at the middle star of Orion's sword.

In a telescope, you will also see that the "star" in the centre of the nebula is actually a multiple star system, with at least 4 stars visible to small scopes.

Amateurs with large aperture telescopes (over 10 inches) and very dark skies, or any amateurs with long-exposure astrophotographic capability, can also see the beautiful Flame Nebula (NGC2024, next to Alnitak, the left star in Orion's belt), and nearby IC434.

Image from Wikipedia

Next door to the Flame Nebula, IC434 is most famous not for its own sake, but because of a blob of dust that sits in front of it, creating a dark region. This "dark nebula", Barnard 33 in the Barnard catalogue of dark nebulae, is called the "Horse Head Nebula" for obvious reasons. Note that the Horse Head is small and very difficult to see. It is a difficult challenge object for the visual observer, requiring large aperture and a narrowband filter.

Ottawa RASC Member Bob Olson won "Best in Show" at 2008 Starfest with this image of NGC2024, the Flame Nebula, IC434, and Barnard 33, the Horse Head nebula.

Orionids

Orion is also the point of origin for the Orionids meteor shower, which occurs annually in the third week of October. While this is not the most spectacular of the year's meteor showers, it is interesting for its origin - it results from the Earth passing through the dusty remnants of the tail of the passing Halley's Comet.

Landmark

Orion is also useful to amateur astronomers as a landmark - it is the starting point to find many other winter stars, constellations, and star clusters.

Nearyby Constellations

Orion is surrounded by famous constellations. Although many of them are familiar as constellations of the Zodiac, Orion himself falls just below the ecliptic, and so is not considered one of the Zodiacal constellations. Among the more interesting and well-known constellations to which Orion can guide you are:

Taurus	Above and to the right of Orion, Taurus the Bull is easy to recognize by the V-shape of his horns. The dense set of stars in the V is actually the open cluster The Hyades.
Auriga	Straight above Orion's head is Auriga, the Charioteer. It is an easy-to-find lopsided pentagon built around the bright star Capella. It contains several open clusters suitable for small telescopes.
Gemini	Just to the left of Orion and slightly above his shoulder height is Gemini, which is easy to find from the bright twin stars Castor and Pollux.
Cancer	Continuing left beyond Gemini, we come to Cancer. The constellation itself is rather uninteresting but it contains M44, the Beehive Cluster, a good target for binoculars.
Leo	Still further left is the constellation Leo the Lion, easy to distinguish by the hook-shaped curve of his head and mane.
Eridanus	To the right of Orion, and slightly lower, is the huge constellation representing the meandering river Eridanus.

Nearby Stars

There are also a number of bright and important stars in other constellations that are easy to find from Orion.

Capella	The brightest star in Auriga, Capella is the first "winter" star that becomes visible in late autumn, rising before Orion.
Castor and Pollux	The stars representing the twins from the Gemini legend. To remember which is which, Pollux, starting with "P", is the one closer to Procyon.
Procyon	The brightest star in Canis Minor (the little dog), Procyon is very bright - the 8th brightest in the sky. However, this is because it is quite nearby (about 11 light-years), not because it is an especially luminous star.
Sirius	The brightest star in the sky, Sirius is both nearby (about 9 light-years) and highly luminous (about 25 times more than the Sun). It is known as the Dog Star, for its placement in the constellation Canis Major. Sirius is a double star, and its white dwarf companion, Sirius B, is a challenging target for small telescopes.
Aldebaran	The eye of Taurus the Bull, Aldebaran is large orange-red star, another of the brightest in the sky.

Nearby Clusters

Open clusters are good targets for beginners with small telescopes or binoculars, and Orion is the starting point to find several.

M45	M45 is above and to the right of Orion - past Aldebaran, the eye of Taurus. Also known as The Pleiades or The Seven Sisters, this beautiful cluster is large enough to be visible with the naked eye. It is sometimes mistakenly thought to be "the Little Dipper" because its bright stars do, in fact, make a small dipper shape. M45 is too large for most telescopes unless you have a very wide angle eyepiece, and looks best in binoculars.
M44	This large cluster in Cancer, called The Beehive, is easy to see in binoculars or a small telescope. Since it is very close to the Ecliptic, it is not uncommon for the Moon or planets to appear inside the cluster, as Mars did in May 2008, and Saturn did in 2006.
M35	Located just above Orion's club, this dense open cluster is slightly more challenging to find, and looks good in a small telescope.
M36,37,38	Scanning around Auriga with binoculars or a small telescope will turn up one or more of these three bright dense clusters.
M47	This cluster is very pretty in a small scope. Slightly harder to find, it can be located by star-hopping from Sirius. Ottawa RASC member Richard uses this cluster in a tutorial on star-hopping (leaves this site).

Conclusion

The arrival of Orion in the mid-evening sky is always a little depressing, as it is a certain indication that we are on the verge of winter. However, this constellation is so beautiful, contains such a wealth of fascinating objects, and is a signpost to so many more, that it quickly becomes your companion and friend while you enjoy the clear skies through the winter months.