Exploring the dark corners of the Milky Way: What exactly is the mysterious king of globular clusters?

Author | Wang Siliang

Review | Zheng Chengzhuo

Editor | Zhao Jingyuan

Omega Cluster (Image source: ESO)

The Omega Centauri (ω Cen) cluster is the brightest, largest and most spectacular globular cluster in the Milky Way. It is a supreme night sky wonder that can be easily seen with the naked eye. It is numbered NGC 5139 in the New General Catalogue of Nebulae and Clusters, and its Chinese name is Kulou Zengyi. It is also commonly known as the Omega Cluster. It is located in the southern sky, with a declination of about -47 degrees. It is easier to observe in the south of the Yangtze River, but there are also records of enthusiasts taking photos in Beijing. The author has conducted thousands of astronomical observations so far, and the Omega Cluster is the object with the most observations. Its visual total magnitude is about 3.9, and it is the brightest nebulous object except for the Large and Small Magellanic Clouds. Although its visual total magnitude is slightly dimmer than M31 of magnitude 3.4, the Omega Cluster has a smaller apparent diameter and a higher degree of condensation (both relative to M31), which makes it look brighter. Under good transparency conditions, this light spot on the back of Centaur can be easily spotted with the naked eye.

The Omega cluster photographed by the author using a mobile phone is similar to the effect of naked eye observation

The Omega Cluster is one of the easiest deep sky objects to locate in the southern sky. Thanks to its large apparent diameter and high brightness, it can be found quickly with binoculars, without the need for a complicated star bridge. Using binoculars at the right magnification, the first impression of the Omega Cluster is that of a large cotton ball, with a clear transition from the middle to the edge. It is not a perfect sphere in the field of view, but is slightly flattened. Although the stars cannot be resolved using binoculars, it is extremely bright in the field of view, and its large diameter makes it very conspicuous. There are only a few very dim stars around it. The Omega Cluster is like an alien in the field of view, with extremely high contrast - as long as it is in the field of view, the eyes will focus on it involuntarily. Even in the case of a full moon or low transparency, the Omega Cluster can be easily identified with binoculars. Its appearance is like a large round spot of milky color, rich and mellow. If compared with the appearance of the Hercules globular cluster M13 in binoculars, there is no doubt that M13, which is just a small ball of cotton, is far less conspicuous than the Omega cluster, which is a large ball of smoke.

Omega Cluster above the treetops (Photographer: Xiamen Log)

When using an astronomical telescope with a low-power eyepiece to observe, the Omega Cluster, which was previously only a ball of smoke in the binoculars, can now be resolved into dense, fine sand-like stars, with thousands of star points packed from the edge to the center, like a sea of ​​stars! A large number of bright and dense stars in its core and halo are superimposed on an extremely rich background of dark stars. The relatively dense bright core is about 10 arc minutes in diameter, and the star density in this area decreases very slowly until it reaches a diameter of 35 arc minutes. If you change to a 10mm eyepiece, you will feel that its brightness has become slightly dimmer, but the stars in it are resolved more clearly, and the field of vision is full of bright stars, each of which is a blazing sun.

The Omega Cluster in the DOB10+ night vision device (photographed by the author)

After observing the Omega Cluster, let's take a brief look at the history of people's understanding of it. As early as 150 AD, the ancient Greek astronomer Claudius Ptolemy listed it as a star in the Almagest; in the early 17th century, the German astronomer Johann Bayer also regarded it as a star with a brightness of about 4 magnitude, and named it Omega Centauri according to his own star naming method in the Celestial Atlas. It was not until 1677 that the British astronomer Edmond Halley realized that it was not a star when he was observing the southern sky on St. Helena Island. He called it "in dorso Equino Nebula", which means a nebula on the horse's back. In 1826, the Scottish astronomer James Dunlop described it as "a beautiful globe of stars very gradually and moderately compressed to the centre (a beautiful globular cluster, it slowly and just right condenses to the center)". In the 1830s, British astronomer John Herschel called it "the most wonderful object" and "most beautiful in central condensation". In 1896, American astronomer Lewis Swift, who discovered more than 1,000 NGC/IC objects, called it "the most magnificent of all visible clusters, and M13 in Hercules is dull in comparison".

Documentary record of Halley's observation of the Omega Cluster (Source: Reference 4)

Almost everyone who has witnessed this cosmic wonder will agree that there is no more spectacular globular cluster than this one. Many objects may become boring after seeing them too many times, but the Omega Cluster is different. No matter how many times you see it, you will never get tired of it. This beautiful, strange, yet sacred smoke ball has a sense of mystery, and it is solitary and abruptly embedded in the dark gray sky of the southern sky. Although its ω is just a Bayer naming number, ω Cen can indeed represent the end of the visual star journey in the southern sky. If you have not witnessed this wonder in the night sky that is larger than the apparent diameter of the sun and the moon, then no matter how many southern sky objects you have seen, it does not count. Only observers who have experienced its vastness with their naked eyes can put a perfect end to their visual stargazing journey.

References

1. Steinicke W. Observing and Cataloguing Nebulae and Star Clusters[M]. journal of the british astronomical association, 2010: 4.

2. Stellarium contributors. Stellarium DSO Catalog Names[DS/OL]. (2023-12-10) [2024-06-13]. https://github.com/Stellarium/stellarium/blob/v24.1/nebulae/default/names.dat.

3. Herschel JF W. Results of astronomical observations made during the years 1834, 5, 6, 7, 8, at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825[M]. London Smith Elder & Co, 1847.

4. I. An account of several nebulæ or lucid spots like clouds, lately discovered among the fixt stars by help of the telescope. Philosophical Transactions. Royal Society. 1716. 29 (347): 390–392

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