M92 - Globular Cluster in Hercules
| Polish version is here |
Messier 92 in the constellation Hercules is one of the brightest globular clusters in Earth’s sky and a fascinating target for astrophysical study. It was first observed on December 27, 1777, by Johann Elert Bode and, less than four years later, on March 18, 1781, was independently rediscovered by Charles Messier, who added it to his catalog alongside eight Virgo Cluster galaxies (M84, M85, M86, M87, M88, M89, M90, M91). In 1783 William Herschel confirmed its nature as a star cluster, paving the way for systematic observations of such objects.
The cluster lies about 26,700 light-years from Earth, approaches us at 112 km/s (70 mi/s), and spans about 109 light-years in diameter. Assuming the average mass of a single cluster star is roughly equal to that of the Sun, M92 contains some 330,000 stars concentrated in an extraordinarily dense core where separations between neighbors are measured in hundredths of a light-year.
Observations
May 2, 2024, around 10:00 PM - Jaworzno, Poland
high level of light pollution
The night was relatively clear and warm, and a few isolated clouds did not prevent observing and imaging the cluster, as illustrated in Photo 1.
Despite the cluster’s enormous stellar population, only 16 variable stars have been identified, 14 of which belong to the RR Lyrae class that astronomers use as standard candles for measuring cosmic distances. The presence of a single eclipsing binary of the W Ursae Majoris type makes M92 especially intriguing for studying stellar system evolution in an extremely dense environment. Beyond its modest variable-star count, the cluster is exceptionally metal-poor, indicating that it formed in the early epochs of the Milky Way, when the Universe contained very few elements heavier than helium. Earlier measurements suggested an age of 14.2 ± 1.2 billion years (placing even the lower limit at 13 billion), but more recent photometric and spectroscopic analyses, based on refined isochrone models, lower the figure to roughly 12.5 billion years — still an impressive antiquity.
Studies show that the cluster moves along an elongated orbit cutting through the Galactic halo, hinting at an origin linked to the early accretion of protogalactic fragments. For amateur observers M92 is a spectacular sight: it appears as a diffuse patch in binoculars, while a telescope with at least a 15–20 cm aperture reveals the fine, grainy glow of thousands of ancient suns locked in a tight gravitational embrace.
Photo 1 Parameters:
- ZWO Seestar s50
- Total exposure time: 90 minutes (stack of 540 RAW frames at 10s each, stacked by Siril ver. 1.2.3)
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
Further readings:
- Desselberger J., Kalendarz astronomiczny. W gromadzie raźniej, Urania – Postępy Astronomii, 2016, 3(783), p. 60
- Shapley H., Sawyer H. B., A Classification of Globular Clusters, Harvard College Observatory Bulletin, 1927, 849(849), pp. 11–14
- Marks M., Kroupa P., Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way, Monthly Notices of the Royal Astronomical Society, 2010, 406(3), pp. 2000–2012
- Kopacki G., Variable stars in the globular cluster M 92, Astronomy and Astrophysics, 2003, 369(3), pp. 862–870
Marek Ples