The Great Cluster in Hercules

From Scope F70076

Current revision as of 14:33, 6 February 2017

April 26, 2008. Tonight I found another deep sky object, the Great Cluster in Hercules, or M13. The visibility was bad, there were even some small clouds in the sky. So I had to move ahead from Vega again, because this was the only star nearby which I saw, no stars in Hercules were visible.

The star hopping guides often instruct to "move in the direction" of some star. This means that you should know in what direction from your starting point is the star towards which you should move, you should find out what this direction is in your location at the time when you do your observation. I use Linux Stellarium to find that out, it can also be derived from the star maps, though it is a bit more difficult. You though have to move only approximately into some direction, because you always have to move until you reach something very distinguished, mostly a bright star, which you easily see when it would appear into your field of view.

So, using the finder scope, I moved ahead from Vega in the direction of Theta Herculis, which is the closest star to Vega in the constellation Hercules, until I reached Theta Herculis, or more exactly, the first bright star which I saw. From Theta Herculis I moved in the direction of Pi Herculis, which is almost the same direction as before. Pi Herculis is one of the stars of the keystone of Hercules, four stars of almost equal brightness which form a well distinguished tetragon in the sky. I moved into that direction until I saw three bright stars quite close together, these stars are Rho Herculis, e Herculis and Pi Herculis. I moved the first of these stars in my direction, Rho Herculis, into the center of the crosshair, and looked at it through the eyepiece (I always use the 20mm eyepiece with 35x magnification when I search the objects). This star should have a double star close to it, and indeed there was a double star, made of two stars whith equal brightness, near that star. From that I knew, that these three stars which I saw, were really Rho, e and Pi Herculis. From these three stars I moved towards Eta Herculis, another star of the Hercules keystone, until I reached Eta Herculis. This direction was again almost the same as before.

But from that star I couldn't move ahead with the finder scope, as finder scope did show a complete darkness, no stars whatsoever, between Eta Herculis and Zeta Herculis, where the Great Cluster in Hercules is located. So I looked at Eta Herculis through the eyepiece. Close to that star there should be an almost equilateral triangle made of small stars of equal brightness, but with the conditions of bad visibility I were not able to see even these stars. But in the same direction a little further from that equilateral triangle there should be an isosceles triangle of brighter stars with equal brightness, with its shortest side towards the Eta Herculis. I indeed saw that triangle. The shortest side of that isosceles triangle is a part of seven stars in a row in almost a straight line. I saw this line of stars, and from that I knew that the bright star which I looked at was indeed Eta Herculis. Then I moved in the direction of that line of stars, until I reached three stars of almost equal brightness in almost a straight line. Look at the brightest star nearby, which you should see in the same field of view together with these three stars. These three stars together with that brighter star form an almost isosceles triangle, the base of which is formed from these three stars, and in the center of that isosceles triangle is the Great Cluster in Hercules.

But if you followed that track, you most likely did notice that object already earlier. I observed the Great Cluster in Hercules through the 12.5mm eyepiece (56x magnification). I saw a dim hazy patch, which was cleary round, and was brighter in the center. The patch which I saw was quite big, some 1/8 of the field of view of my 12.5mm eyepiece. I saw though no stars in it or anything brighter.

Several times during my journey to that object, looking through the eyepiece, I saw satellites, and once I saw a meteorite. The Great Cluster in Hercules is quite far from Vega, so I proved with that, that with star hopping using only a finder scope, and sometimes eyepiece of course, it is possible to find most of the deep sky objects, proceeding from a visible bright star which is closest to them, even when only a few stars are visible in the whole sky. I should say that I did like the process of finding that object, sitting near the telescope in the darkness of the night sky, and all the observation during that, seeing so many stars during all that time. It is somehow relaxing, just to see the stars, and once you saw them through the telescope, you need to see them again. That the objects which I saw looked bad, is not the fault of the telescope, but it was because of visibility -- when you see only a few brighter stars in the sky, then the visibility is likely below the magnitude 2.

The Great Cluster in Hercules was discovered by Edmond Halley in 1714.

I have also observed star clusters through 10x50 and 10x25 binoculars. Through the binoculars, you don't see directly the star clusters, what you see is a different and more diffuse interference pattern, than that of the stars. Due to the reason that these objects are not point of light sources such as stars. One can see that difference in interference pattern, it is described as fuzzy, also called a fuzzy star. Seeing that, one unmistakably knows that this object is not a star, and may be a star cluster or even some nebula.