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The great Hercules cluster


darrell heath

Darrell Heath

Astronomy Columnist 

Hercules got robbed! You would think that the mighty muscleman of mythology would have a constellation worthy of his fame, right? Wrong. Instead of a constellation that is big and bright, Hercules has a constellation that is big and dim (probably not unlike Hercules himself). How could a mythological character known the world over not have a prominent constellation? It seems that assigning Hercules to this pattern of stars was something of an afterthought for the ancient Greeks. Old star maps simply label it as “The Kneeling Man”, the name “Hercules” didn’t get attached to it until later. The constellation most likely has it its origins much further back in antiquity and the Greeks simply saw it as an opportunity to finally give their hero a place in the sky.
Hercules is one of several summer star patterns and is the fifth largest constellation in our night sky. Its brightest stars are only about third magnitude in brightness. The commonly used scale for denoting the apparent brightness of celestial objects uses smaller numbers for the very brightest while higher numbers are used to indicate the dimmest. Venus, one of the brightest things in our sky, is shining at -5.1 in our western sky right now just after sunset. The dimmest stars that it is possible for you to see with the unaided eye shine at magnitude six. So, while Hercules is dim we should still be able to find it with just a modicum of effort.
Step outside this week at around 10:30 PM and look for the bright star Arcturus. This can be done by extending the arc of the handle of the Big Dipper out one more handle’s length, just “arc to Arcturus”. You can’t miss it, as it really is quite bright. Now, turn a bit to the east-northeast until you encounter another bright star almost the equal of Arcturus. This is Vega in the constellation of Lyra. We are almost there! Draw a line between these two stars; Hercules lays a third of the way between Vega and Arcturus and is pretty close to being overhead. The best way to make sure that you’ve found Hercules is to look for a trapezoid-shaped pattern of four stars known at the Keystone (the line in between Arcturus and Vega passes right through it). The Keystone gets its name from the funny shaped bit of masonry seen at the top of stone archways in old architecture and, in this case, represents Hercules’ pelvis. We aren’t going to worry about picking out the other stars that make up Hercules’ figure, just focus on locating the Keystone. A free star map can be printed out from www.skymaps.com to help you out.
Hercules’ stars may be dim but for the celestial sightseer there are some very lovely things to seek out here; the constellation is especially rich in double stars. The object I want to draw your attention to is Messier 13, the Great Hercules Star Cluster. Look for M13 about a third of the way between the star in the upper left corner of the Keystone (the western side) and the star in the upper right corner (the eastern side). It has an apparent magnitude of 5.8, just on the brink of naked eye visibility, so you will need a very dark sky to see it without any kind of optical aid.
With binoculars the star cluster appears as a spherical, fuzzy blob; with a small telescope you begin to see individual stars around the edges; larger telescopes show, as astronomer Phil Harrington says, “a huge globe of tiny points, almost as if someone dropped a pinch of sugar onto a black-velvet backdrop”.
M13 is what astronomers refer to as a “globular cluster” of stars. All the stars you see in our night sky were born in loosely packed “open clusters” within the spiral arms of our Galaxy’s disc. Globular clusters on the other hand are gargantuan spheres of tightly packed stars, hundreds of thousands to even millions of individual suns all bound together by their mutual gravity. Their exact origins remain a mystery.
M13 is a conglomeration of some one million stars all jammed into a ball that is about 165 light years across. Like most globular clusters M13 does not reside within the galactic disc, it hangs out instead in the Galaxy’s halo (the spherical-shaped outer fringes of a galaxy containing little in the way of gas and dust). We don’t know the exact age of this cluster but some estimates place it at around 11 billion years old, a real celestial geriatric that has been around since shortly after the Big Bang.
If there are any planets within the cluster there are so many stars that any inhabitants would never see a dark sky. Still, in 1974 we sent a friendly greeting to the cluster from the Arecibo Radio Telescope in Puerto Rico on the off chance there might be an intelligent ET waiting on the line. We’ll have a long wait before we get an answer; the cluster is some 23,000 light years away. The message hasn’t even reached M13 yet. If there is an intelligence there capable of replying it will take another 23,000 years before it’s message reaches us. Sorry ET, I won’t be able to take your call.

Darrell Heath works at the University of Arkansas at Little Rock and is a producer and host for the UALR Television show “The Night Sky.”