Messier 13: The Hercules Cluster

Last night, I learned how to find the Hercules Cluster. I selected Messier 13 from a menu on a Celestron AVX computerized telescope mount, and in a 102mm refractor I viewed it at 47x. I then spotted it in the red-dot finder scope. From there, I went to a 130mm reflector mounted on a manual drive and placed that red-dot finder in about the same location. And with the same 14mm ocular there it was at 46x, not noticeably smaller and yet a little brighter in the larger aperture. 

Taken with the NASA Harvard 6-inch
Maksutov f/3.47

 An active amateur astronomer will tell you that it is easy to locate this huge sphere of stars. It is nominally a summer constellation near the zenith when the sky is finally dark from mid-June to mid-July though rising with nightfall from April and May and setting at sundown in October and November. That is nice to know. 

Midnight 18 June
(Philip's Planisphere Octopus Publishing
and Cambridge University Press 
(part of a set found at Half Price Books).

 

The fact remains that only four constellations look anything like what they are named for: Scorpius, Leo, Orion, and Taurus. We tell extended and involved stories about the others to help us remember them because otherwise they are just a dense random scattering of small lights. From the city, they are a thin random scattering. I once read that the southern sky is so dense with stars, including the lesser galaxies that we call the Magellanic clouds, that the Incas noted the dark lanes as more interesting. 

 

The stars are pretty at any magnification. I like to understand what I am looking at. Burnham’s Celestial Handbook provides four pages of history and explanation on M13 as an introduction to another four pages about globular clusters in general. 

 

 Octopus Publishing
and Cambridge University Press 
(part of a set found at Half Price Books). 

They orbit above the pinwheel disk of our galaxy and are “extremely ancient.” William Herschel estimated 14000 stars in the Hercules cluster. A hundred years later, Mount Wilson doubled the estimate. But the central core is so dense that Burnham suggests one million stars as the upper limit. 

 

The Virial Theorem lecture from the École
polytechnique fédérale de Lausanne and edX

Newton’s powerful mathematics fails with three bodies. Modeling an extra-galactic globular cluster of stars works best by considering them as point masses in a gas cloud. The Virial Theorem allows us to analyze them from considerations of the temperature of a gas. That also works from the considerations of kinetic versus potential energy as the stars move about their common center of mass. 

 

PREVIOUSLY ON NECESSARY FACTS

 

An Online Class in Astrophysics 

In Suspect Terrain 

The Map that Changed the World 

Observing with NASA: A Platform for Citizen Science 

(Not) Observing with NASA and Harvard