January 08, 2008


To begin the discussion of the constellation Orion, I think it would be best to follow Burnham and quote from the 1929 book "1001 Celestial Wonders" by Charles Edward Barns:

For who would acquire a knowledge of the heavens, let him give up his days and nights to the marvels of Orion. Here may be found every conceivable variation of celestial phenomena: stars, giants and dwarfs; variables, doubles, triples, multiples; binaries visual and spectroscopic; clusters wide and condensed; mysterious rayless rifts and nebulae in boundless variety, with the supreme wonder of all supernal wonders at its heart - the Great Nebula - before which the learned and the laymen alike have stood silent in awe and reverence since the first lens unfolded to man's gaze its true vastness and intricacy, and which offers abundant field for all the geniuses of science ... for generations to come.

The constellation Orion actually originated with the Sumerians, where he was Gilgamesh fighting against the Bull of Heaven. In Greek mythology, Orion had no quarrel with a Bull, though he is often pictured as facing off against Taurus the Bull in the sky. Orion is a great hunter, armed with club and shield with two dogs following at his heels (Canis Major and Canis Minor). In one myth, Orion's target is not Taurus the Bull but rather the affections of the Pleiades which ride on the Bull's back, and so Orion chases them across the sky.

In most versions of the Greek legend, Orion's death comes from the sting of Scorpius, who was placed on the opposite side of the heavens, rising as Orion sets and vice versa. This happened because he either claimed he was a greater hunter than the hunting goddess Artemis (always a mistake to claim greater anything than any Greek god) or he mistakenly tried to ravish Artemis. Another version says that Orion was killed when Artemis' brother Apollo, who disapproved of the love between Orion and Artemis, tricked Artemis into piercing Orion with a deadly arrow.

Another story involving Orion claims that his pursuit of the Pleiades angered their father, who blinded Orion. To regain his sight, Orion was told by an oracle to travel East and face the rising Sun at dawn. Tennant speculates that his reappearance in the Eastern sky at dawn during the Fall signifies the end of his Summer of blindness. When he appears at dawn in the Eastern sky, he is looking upon Aurora, the goddess of the dawn, mother of the winds and of the Morning Star. Aurora fell in love with him and healed his vision.

As for the stars in the constellation Orion, I'll start with the brightest. Betelgeuse (armpit, arm or hand of the giant, depending on your source) is a red supergiant, one of only two red supergiants (besides Antares) that is among the brightest stars in the sky thanks to its proximity and the lack of intervening material. As you can see here, at its largest size, it would easily engulf all of the terrestrial planets in the solar system if it were put in place of our own Sun. It is large enough and close enough that it subtends a slight angle in the sky, allowing us to physically measure its diameter (instead of inferring it from other properties like with most stars).

This star is well along its evolutionary path, probably fusing complex elements in its core on its way to exploding as a Type II Supernova. Like many large stars, Betelgeuse pulsates, changing its size and luminosity over a period of months or years. At times, it is truly the brightest star in Orion, but most of the time, it is slightly fainter than the blue giant Rigel (left leg or foot of the giant), which is normally the 7th brightest star in the sky.

The only thing that makes blue supergiant Rigel's luminosity comparable to Betelgeuse is that it is about twice as far away (about 800 vs 400 light years). Burnham estimates that if Rigel were one of the 5-10 nearest stars to us (like, for example, the blue main sequence star Sirius, which is less than 10 light years away), it would be about 10,000 times brighter, which is about 20% of the brightness of the full moon.

Rigel is one of a number of stars moving away from its presumed birthplace near the Great Nebula in Orion. There's also an expanding bubble of gas centered around this region that I'll talk about later, probably a supernova remnant. Some of these stars were possibly binary companions of very massive stars that exploded and sent their companions shooting off in random directions. Or possibly they were part of a loose cluster of stars that ejected some members through gravitational interactions (a story for another time).

Rigel also has a relatively faint companion, Rigel B, about 9 arcseconds away, easily visible with a 6-inch or better telescope. Rigel B itself is a spectroscopic binary, two B-type main sequence stars orbiting one another about every 10 days. The other two bright stars in the rectangular pattern centered on Orion's Belt are Bellatrix (known as the Amazon Star) and Saiph (sword of the giant). Both Saiph and Bellatrix are hot blue stars like many in Orion, but Bellatrix is probably too close to be of the same group that formed from the Nebula region. Saiph is only dimmer than Rigel because of intervening interstellar gas and dust. Otherwise, they are similar stars at similar distances.

The three stars in Orion's belt are (from East to West) Alnitak, Alnilam and Mintaka, also called "the string of pearls" by Arab astronomers. These are all hot borderline O/B stars, recently formed in the Great Nebula region.

At the heart of the Great Nebula in Orion is the multiple star Theta Orionis, otherwise known as the Trapezium. There are four bright stars here that are very young 15-30 solar mass stars that are providing the majority of the light that reflects back and causes such a pretty sight through the telescope. The nebula is also known as M42 and is one of the youngest star clusters we see in the sky, even younger than the Pleiades.

Meissa marks the head of Orion, though its name doesn't translate as such. It is a very bright star intrinsically, but it is over 1000 light years away so doesn't appear bright to us. It is the brightest star in a small surrounding cluster, and the cluster is surrounded by the Meissa Ring, an illuminated cloud about 150 light years in diameter that is either associated with the formation of Meissa or perhaps the remnant of a supernova that went off from an even more massive star within Meissa's cluster.

There are literally over a dozen multiples in this region of the sky, all part of the same OB association of stars. Perhaps the finest example of many is Eta Orionis (Saif Al Jabbar, sometimes confused with Saiph), which makes sort of a right triangle with the belt of Orion. Through a telescope, one can see a close pair of hot blue stars less than 2 arcseconds apart, The brighter of the pair is actually a triple star, with two stars orbiting very close (about the same distance as it is from the Sun to Mercury) and the third orbiting out near where Saturn would orbit, and it is still debated whether another more distant star is also orbiting the same system. Nearby Sigma Orionis is another similar example, and it provides the light to illuminate the gas clouds in the beautiful Horsehead Nebula region, discussed below.

Among the variable stars in the cluster is W Orionis, a bright red giant and also a carbon star. Carbon stars are in a peculiar phase of their evolution during which Carbon created in the core is being dredged up to the surface by convection in the star's envelope. The carbon atoms are pretty good at blocking blue light, so these are very red stars. This two solar mass star is beginning the process of blowing off its outer envelope, and in a few million years, it will become a planetary nebula with a hot white dwarf left behind at the center.

Another interesting star here is HR 1988, about in the opposite place as Eta Orionis with respect to Orion's Belt. This sun-like star has two planets in orbit around it, discovered via the Doppler wobbling technique. One is a Jupiter-like planet orbiting a mere 0.13 AU from the central star (by comparison, Mercury's orbital distance is 0.40 AU) and the other is right at the border of the star/planet boundary, being around 13 Jupiter masses and orbiting about 3.5 AU away (which would put it squarely in the middle of our asteroid belt if it were here).

One last thing I should mention here is the usefulness of Orion's Belt and other bright stars to act as pointers to other major stars and constellations in the sky. This image shows how to use Orion to find many of the other stars in the Winter Circle.

Finally, on to the deep sky objects in Orion. The obvious first choice is the famous Orion Nebula, also known as M42. Though only visible as a fuzzy 4th magnitude patch to the naked eye just below Orion's Belt, the full extent of the luminosity of this magnificent object is visible in many long-exposure photographs, and it takes up an area of the sky about four times larger than the full moon.

At a little over 1000 light years distant, it is still among the closest active star-forming regions to the Earth. Already, many massive stars have formed and moved out of this region, and some remain, including the brightest stars at the center, also known as the Trapezium. The red color in the nebula comes from emission by Hydrogen gas. The blue color is reflected light by dust from all of the hot, blue stars in the region. The green is forbidden line emission from Oxygen.

As the Hubble Space Telescope has studied this region closely, we have discovered many stars that have dusty disks around them, presumed to be the precursors to planetary systems. Burnham recommends that the best way to observe this nebula with a small telescope is to go to a dark site, get your eyes well-adjusted, then position the telescope and turn off the drive so that the Nebula will slowly drift across your field of view. This makes it easier to see the full extent of the nebulosity.

Though Mayan records clearly show that they observed this object in the sky, no other ancient texts talk about the Great Nebula in Orion, at least not from Western Europeans or Arabic astronomers. Perhaps this region got brighter during the 17th century, when it was first mentioned and studied by many famous astronomers.

Just above the Orion Nebula, a nebula that seems to drop right out of Alnitak (the left end of the belt if Rigel is at the bottom of Orion) is a beautiful reflection nebula lit up by nearby bright stars. The major feature of this nebula is that some of the light is blocked by a very thick, dark dust cloud shaped like the head of a horse. This is Barnard 33, popularly known as the Horsehead Nebula. The feature is about 1500 light years away and 3.5 light years across (roughly the distance from us to the nearest star). Sadly, this is too faint and small to be seen with the naked eye, but it appears easily and beautifully in deep photographs of the region. Nearby to the left of Alnitak is the pretty Flame Nebula (also known as the Christmas Tree Nebula).

Above the belt and very near HR 1988 which I discussed earlier is another pair of objects, M78 and NGC 2071, two more bright cloud complexes lining the front of the gigantic Orion molecular cloud complex that spans nearly the entire area of the constellation on the sky. Circling all of this is Barnard's Loop, which may be an outflow boundary from a supernova explosion that occured long ago in the Orion Nebula region. There are many other smaller nebulae and clusters dotting the front of this giant cloud complex, enough to keep an observer busy for a lifetime.

Posted by Observer at January 8, 2008 04:01 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Great write-up of the Orion constellation! I love this time of year when Orion sits high in the late sky. M42 is brilliant to look at with most scopes, it's quite striking. My new scope does wonders with this nebula.

Keep up the interesting constellation reviews!


Posted by: Phil on January 8, 2008 04:34 PM

That is the one Constellation I can easily identify and always see despite our horrible light pollution at my house.

Even the Big Dipper is sometimes obscured, but Orion is always there. Very nice write up.

Posted by: Liz on January 9, 2008 03:32 PM