How to see it
From our northern temperate latitudes, Omega Centauri – the Milky Way galaxy’s largest and most luminous globular star cluster – can only be seen at certain times of the year. Omega Centauri is best seen in the evening sky from the Northern Hemisphere on late April, May and early June evenings. Around mid-May, this wondrous star cluster is due south and highest in the sky around 10:00 p.m. (11:00 p.m. daylight saving time). By mid-June, Omega Centauri is highest up (and due south) around 9:00 p.m. (10:00 p.m. DST). Northern Hemisphere residents can see Omega Centauri from January through April as well, but they must be willing to stay up past midnight or to get up before dawn.
Omega Centauri is visible from the southern half of the United States, or south of 40 degrees north latitude (the latitude of Denver, CO). Canadians hasten to remind us, though, that they can actually spot Omega Centauri from as far north as Point Pelee in Canada (42 degrees latitude). When seeing conditions are just right, they can catch the Omega Cenaturi star cluster skimming along the surface of Lake Erie.
Spica, the brightest star in the constellation Virgo, serves as your guide star to Omega Centauri. When Spica and Omega Centauri transit – appear due south and reach the highest point in the sky – they do so in unison. However, Omega Centauri transits about 35 degrees south of (or below) sparkling blue-white Spica. For reference, your fist at an arm length approximates 10 degrees of sky. Find Spica’s transit time for your sky at this US Naval Observatory page.
Omega Centauri is a globular cluster
Omega Centauri is a globular star cluster. This disguishes Omega Centauri from clusters such as the Pleiades and Hyades, which are open star clusters. An open star cluster is a loose gathering of dozens to hundreds of young stars within the disk of the Milky Way galaxy. Open clusters are weakly held together by gravity, and tend to disperse after several hundreds of millions of years. Globular clusters orbit the Milky Way outside the galactic disk, and harbor tens of thousands to millions of stars. Tightly bound by gravity, globular clusters remain intact after 12 billion years.
Generally, open clusters visible to the unaided eye are hundreds to a few thousand light-years away. In contrast, globular clusters are generally tens of thousands of light-years distant. At 17,000 to 18,000 light-years, Omega Centauri is one of the few of the galaxy’s 200 or so globular clusters that is visible to the unaided eye. It looks like a faint, fuzzy star, but Omega Centauri’s mere presence testifies to its size and brilliance. Like any globular cluster, Omega Centauri is best appreciated with a telescope.
No ordinary globular
All globular clusters are impressive, but Omega Centauri is in a class by itself. Having a mass of 5 million suns, Omega Centauri is 10 times more massive than a typical globular cluster. Omega Centauri sports a diameter of 230 light-years, this bejewelled stellar city sparkling with perhaps 10 million stars.
Globular clusters generally have stars of similar age and composition. However, studies of Omega Centauri reveal that this cluster has different stellar populations that formed at varying periods of time. It may be that Omega Centauri is a remnant of a small galaxy that merged with the Milky Way.
Omega Centauri’s position is at Right Ascension: 13h 26.8m; Declination: 47 degrees 29′ south