Thursday, September 18, 2008

Hercules Revisited

Two weeks ago, we investigated the heart of Hercules, an asterism known as the Keystone. This week, let’s go farther afield in constellation Hercules and locate another asterism (recognizable star pattern) or two, a few more stars, and another globular cluster (dense ball of gravitationally bound stars).

1) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark. The stars in Hercules are not terribly bright, so you’ll have the most success at a dark site, away from city lights.

2) Check the time of moonrise before you observe. The waning (shrinking) Moon will rise a bit later than the night before, each night this week. Because Hercules is a relatively dim constellation, you won’t want moonlight to interfere with your stellar scavenger hunt.

3) Locate the Keystone. If you need assistance to do so, review my earlier post.

4) If you “connect the dots” to a naked-eye star above each of the top (northern) two Keystone stars and a naked-eye star below each of the bottom (southern) two Keystone stars, you'll see the asterism known as the Bowtie. The Keystone becomes the center knot in the Bowtie.

5) The southernmost star of the Bowtie is Kornephoros (core-NAY-for-uss), which is Greek for club bearer, a reference to Hercules’s weapon of choice. Kornephoros is a yellow giant and the brightest star in the constellation.

6) Immediately southwest of Kornephoros are three somewhat dim stars that form a small triangle. This triangle is the asterism known as the Club, and you’ll need dark skies to spot it. The triangle star closest to Kornephoros has no traditional name, so we call it Gamma for its star catalog designation. Continuing in a line from Kornephoros through Gamma, we come to Marsic (MAHR-sick), Arabic for elbow. The third triangle star, to the east, is Cujam (KOO-zhahm), from the Latin for club.

7) Southeast of Kornephoros is a slightly dimmer star called Rasalgethi (rah-sahl-GAYTH-ee). The name of this red supergiant star is from the Arabic for the kneeler’s head. You may recall from my previous post that an early Greek name for this constellation was “Kneeling One.”

Don’t confuse Rasalgethi with Rasalhague (RAH-sahl-hayg), the brightest star in the constellation Ophiuchus the Serpent Handler. Rasalhague is brighter than Rasalgethi and a little farther east.

8) Let’s finish up our survey of the strongman with a deep-sky object. Although M13 is the best known globular cluster in Hercules, he harbors a second glob from the Messier catalog: M92. M92 is perhaps not quite as celebrated as M13, but it’s a very pretty object nonetheless.

You’ll need a telescope for a satisfying view of M92. If you make an imaginary equilateral triangle using the top two stars of the Keystone as the base, and then point your telescope slightly east of the top of the triangle, you’ll run into M92.

If you compare M13 to M92 in your telescope, you’ll note that M92 is a bit smaller and dimmer than M13. However, M92 has an asymmetrical appearance that I find irresistible in a glob. Globular clusters are, after all, the renegades of the Milky Way, arranged in a spherical “halo” around the core of the galaxy rather than huddled in the platter-shaped disk, where all the other stellar material congregates.

Side view of Milky Way, showing halo of globular clusters around galactic core
Diagram by
Richard Powell

Charming imperfection and a wayward nature: a killer combo. As galactic sugar piles go, M92 can always be relied upon to satisfy my sweet tooth.