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Globular Cluster in Hercules (M 13)
Sarah Hoekstra

Herculean Cluster

Herculean Cluster

M 13, or Messier 13, is a large globular cluster in the northern hemisphere. It is commonly referred to as the "Great Globular Cluster in Hercules" as it is found about two thirds up the right side of his keystone section. All of the stars are moving together in a rotation giving M 13 its spherical shape. Stars in a globular cluster tend to be concentrated near the center of the sphere and be far older than the stars in the open clusters. This cluster is easily found by amateur astronomers as it is one of the few globular clusters that is out all summer and has a high declination in the sky so it is clearly visible. It was first discovered in 1714 by Sir Edmond Halley. Later in 1764, Charles Messier cataloged it as Messier 13. Messier was a comet chaser and although M 13 is not a comet, its appearance to the human eye is often mistaken to be one. The fuzziness attributed to the cluster is due to the fact that M 13 is home to more than half a million stars. Because of the massive size and popularity of the Great Herculean Cluster, it was chosen to be a target for one of the first radio transmissions designed to communicate with extra-terrestrials. A message was sent out to potential life forms residing within this cluster. With over half a million stars and the potential for exponentially more planets, M 13 was seen as an ideal target. Unfortunately for all of the E.T. fans out there, the whole exercise was a show of technology more than anything else. The actual message will take more than 25 thousand years to arrive. By this time, the cluster will have moved and the message will have missed it.

Messier 13 is seen without aid as a fuzzy little star part way up Hercules. This fuzzy little blot is mistaken for a comet until the telescopes or binoculars come into play. M 13 is a cluster of mostly white stars orbiting together in a large sphere. It is visible all summer long and in the fall it is often still up in the sky near midnight. The main concentration of stars is near the center but as the line of sight moves to the edge the stars drastically thin out making it hard to spot where exactly the cluster comes to an end. In the photo a concentrated white core can be seen with smaller stars moving towards the outer ring intermixing with some tiny blue stars as well. The cluster is home to more than 500 thousand stars. Although it is over 23,157 light years away the brightness of the stars orbiting so closely together allows us to see it with the eye alone.

This cluster is estimated at nearly 150 light years across. The distance to M 13 is estimated higher than 23 thousand light years with the angular size measuring in at 133.5 arc seconds or 647*10^(-6) radians. The stars in this cluster are tightly bound together making this cluster easily found in the sky but fairly tricky to image because of the amount of starlight entering the telescope camera lens. The exposure times need to be long enough to capture the light through the filter but when dealing with so many start exuding light, over exposure is also a factor. Four different filters were used (Clear, Red, Green and Blue) and several exposures were taken through each filter. This effect allows shorter exposure photos to combine light sources. Star matching assures that the points of light line up right and don’t duplicate stars in the final photo. Once each filter has a final photo these are combined by color. This final photo shows stars of differing sizes and varying colors but very little dust clouds. The stars near the center are large and have a white and almost yellowish tinge. Near the outer edge the stars are much smaller with a blue tint. The stars in the blue spectrum are much hotter and the brightness of the center stars implies that they are super giants.

Non optical Imaging

Non Optical

Photos taken from a telescope can only tell us so much about the stellar objects that we are observing. There are several different methods to look at the light and energy traveling through space. Sources like infrared and ultraviolet imaging can show us the light at different wavelengths. They can tell us heat distribution as well as energy emissions and dark matter variations. That can tell us more about the composition of the cluster and more about its evolution thus far.

Calculations

Distance of Cluster:

7.1 kiloparsecs

23157.5974 light years

Angular size of Cluster:

133.4875 arcseconds

0.0006471657 radians

Physical size of Cluster:

149.868 light years

The distance of the cluster was looked up at the physics mc master to assure that we were getting the right numbers. From there we looked at the pictures that we took from our Calvin telescope and measured the pixel length across our respective objects. We know the scope of our telescope so we are able to calculate that there are 0.59 arcseconds per pixel in our photos. Using our new found angular size and our distance we are able to calculate the physical size of our star clusters.The equation is, angular size (rad) = physical size / distance. This gave me the span of close to 150 light years as the diameter of my cluster M 13. The Size suprised me a bit because I was expecting something a bit larger. I was estimating something close to 200 light years given that you can spot this cluster without the aid of telescopes if you know where to look in the sky. Half a million stars have to concentrate into that small area and that luminosity is what gives M 13 the edge.

References:

http://earthsky.org/clusters-nebulae-galaxies/m13-finest-globular-cluster-in-northern-skies

http://messier.seds.org/m/m013.html

http://www.universetoday.com/31430/messier-13/

http://apod.nasa.gov/apod/ap120614.html

http://freestarcharts.com/20-guides/messier/91-messier-13-m13-great-hercules-globular-cluster

http://scienceblogs.com/startswithabang/2012/12/31/messier-monday-the-great-globular-cluster-in-hercules-m13/

Right Ascension (J2000) 16:41:41.24
Declination (J2000) +36:27:35.5
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B, V, and R ( seconds); C ( )
Image dimension 540 x 513 pixels; 318 x 302 arc minutes
Date/time observed 19 February 2015

 

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