August 15, 2008


Ok, back to constellations! When I last left off, I had covered Ophiuchus, the serpent-bearer, and today I'll start talking about the constellation Serpens, split into two parts on either side of Ophiuchus. The head is Serpens Caput (on the west side), and the tail is Serpens Cauda. I discussed the mythology of Serpens and Ophiuchus previously, so I won't repeat that here. This is one of the faintest constellations, though it is one of the original 48. The brightest star is Unukalhai at 3rd magnitude. There aren't many nearby visible references, but a rough way to find it is to look at the center of a triangle formed by Antares (in Scorpio), Rasalhague (in Ophiuchus) and Arcturus (in Bootes), three relatively bright stars.

The name translates as the serpent's neck, though it is sometimes called Cor Serpentis, for the serpent's heart. Astrophysically, it isn't all that interesting, just another one of the many orange giants within about 100 light years that make up a large fraction of the stars we can see with the naked eye. In fact, none of the naked-eye stars in Serpens are all that remarkable, and I'm going to skip a couple of prototype variables (explaining the light curve of W Serpentis is a little too much inside baseball) but the deep sky objects are a different story, as we have some very famous objects here.

In Serpens Caput, about five degrees Southwest of Unukalhai (or halfway between Antares and Arcturus if you can't find Unukalhai), we find Messier 5, a cluster that rivals the famous Messier 13 cluster in Hercules as one of the brightest and richest in the sky. There are a large number of variable stars here, which make distance determination easy. M5 is in the inner halo but moving very fast, nearly at a speed equal to our galaxy's escape velocity, so it must be on a highly elliptical orbit, much like a comet passing through our solar system as it passes near the galactic center.

Over on the Eastern side of Ophiuchus in Serpens Cauda, we find the brightest of the remaining deep sky objects in the constellation Serpens. I'll start with the most famous, Messier 16, also known as the Eagle Nebula. There is a young open cluster of stars here, lighting up a surrounding cloud of mostly Hydrogen gas and dust 7000 light years away in a neighboring arm of the galaxy (the Sagittarius Arm, closer to the center of the galaxy). The surrounding eagle-shaped dust cloud is seen best in this larger view. In this image, normally red Hydrogen-alpha emission is green simply because it makes it easier for the eye to pick up finer details. Red and blue colors are emission lines from ionized Sulfur and Oxygen, which are indicative of different density and temperature conditions in different parts of the nebula.

Near the center of the image is the famous formation known as the pillars of creation. Near the tips of the pillars, you can see in detail the light from surrounding hot stars burning away the dust cloud, creating a corona of ionized gas. Where the gas is densest, where new stars are forming in the cores of thick, dust clouds, you can see little spires of dust sticking out from the main cloud. These are places where the intense radiation pressure has stripped away the surrounding dust and left only the clump of a forming star along with its "shadow", a trail of dust.

You can find this by looking about 10 degrees due north from the point at the crown of the teapot in Sagittarius, so this is fairly close to the line of sight to the galactic center. About five degrees Northwest of this is a fairly nice 9th magnitude globular cluster, NGC 6539, about 30,000 light years distant.

About 2 degrees North of the globular cluster Messier 5 in Serpens Caput, we find an 11th magnitude face-on barred spiral galaxy, NGC 5921. It made the news several years ago when a type II supernova erupted near the intersection of the bar and the lower spiral arm (seen in this image).

Next is the tilted spiral NGC 6070, about 10 degrees due East from Messier 5, seen here in an image that inaugurated the Sloan Digital Sky Survey telescope. The SDSS image is a little grainy, but the purpose of the survey was not so much for aesthetics but instead completeness, a usable map (including spectra!) of raw data at high resolution for the entire sky. A better image is here from AOP.

Looking now to much fainter objects, there are a couple of really neat targets imaged by the Hubble Space Telescope in this region. NGC 6027 is a little cluster of galaxies nearly 200 million light years away known as Stephan's Sextet. This is actually only four interacting galaxies. The one spiral in the image that isn't distorted is in the distant background about five times further away, and the other "galaxy" is really just a tidal tail that has a concentration that looks a little bit like the nucleus of a galaxy. All four of these galaxies are small enough that this group would fit inside our own Milky Way galaxy, just barely.

Still further away and also in Serpens Caput along the border with Bootes, at a distance of 600 million light years, is Hoag's object, a very nice ring galaxy first noticed in the 1950's. Ring galaxies like this are typically the result of a nearly perpendicular head-on collision that creates a wave of star formation expanding outward through the larger galaxy like ripples on a pond in slow motion.

Posted by Observer at August 15, 2008 10:54 AM

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