M13 - Great Globular Cluster in Hercules
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On a moonless, clear night, with ideal atmospheric conditions, excellent eyesight, and far from cities and other sources of light pollution, a notable detail can be spotted with the naked eye in the constellation Hercules. It appears as a faint, diffuse point of light and lies almost exactly on the line connecting Eta (η Her) and Zeta (ζ Her) of this constellation — approximately one-quarter of the way from Eta toward Zeta. Under normal conditions, however, this object is difficult to see, and observing its structure certainly requires the use of appropriate optical equipment. Using a telescope with a mirror diameter of just 10 cm, one can observe that the diffuse spot is actually a spherical collection of densely packed stars.
This object is the Hercules Cluster, discovered in 1714 by Edmond Halley and later cataloged by Charles Messier as number 13 (M13) in his famous catalog. It is the largest and brightest globular cluster in the northern sky, located approximately 25,000 light-years from Earth, with a diameter of 145 light-years. Measurements and observations indicate that the cluster contains at least several hundred thousand stars. The Hercules Cluster has an apparent magnitude of 5.8m, but as an extended object spanning about 20 arcminutes in diameter, it is much harder to spot than a star of similar brightness.
Unlike stars that independently orbit the core of our galaxy, stars in globular clusters orbit a common center of mass. Some of these stars follow circular orbits, while others have eccentric orbits extending to the cluster's outer regions. Due to gravitational interactions within the cluster, the orbital speed of lighter stars increases, while that of heavier stars decreases. This results in a sort of stellar sorting: lighter stars tend to gather in the cluster’s outer regions, while heavier stars concentrate in orbits closer to the core. Globular clusters like M13, along with the galactic bulge, are considered the oldest components of the Milky Way.
Observations
May 13, 2020 – Jaworzno (Poland), garden
urban conditions, high level of light pollution
The photograph was taken from my garden on a clear night — I marked the cluster as well as two stars with magnitudes of about 7m. However, it is worth noting that the brightness of some of the visible stars is as low as magnitude 11 or even 12, making them impossible to observe with the naked eye.
I must say that the Hercules Cluster is a beautiful object, looking stunning through a telescope eyepiece. The photograph below is the first I have taken of this object and, unfortunately, it is slightly out of focus — so I now know what to improve during my next sessions.
The cluster was also visible using 11x45 binoculars.
April 12, 2025 - Katowice (Poland)
urban conditions, very high level of light pollution
Despite suboptimal sky conditions, the use of more advanced equipment yielded a noticeably better observational result.
In 1974, M13 became famous not just as an astronomical object, but also as the target of a historic scientific experiment. As part of the SETI project, the Message (a coded radio signal containing basic information about humanity) was transmitted in its direction using the Arecibo radio telescope. Although the message will take about 25,000 years to reach M13, and any potential response would take another 25,000 years to return, the act itself symbolized humanity’s desire to reach beyond our world.
Today, M13 remains a favorite target for both amateur and professional astronomers, captivating observers with its symmetry, brilliance, and the cosmic history written in the ancient light of its stars.
Photo 1 Parameters:
- Total exposure time: 10 minutes (stack of 10 RAW frames at 60s each, using an appropriate number of dark, bias, and flat frames)
- ISO: 1600
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system.
Photo 2 Parameters:
- Total exposure time: 60 minutes (stack of 60 RAW frames at 60s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 2000
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Marek Ples