Clusters Nebulae Galaxies

Omega Centauri is the Milky Way’s largest star cluster

Sphere of multicolored stars, less dense toward edges.
Omega Centauri in infrared light via Spitzer Space Telescope/ Wikimedia Commons.

Omega Centauri, the largest globular star cluster of the Milky Way, contains about 10 million stars. This behemoth, with a diameter of 230 light-years, is 10 times more massive than a typical globular cluster. Despite all these stars, scientists released a study in 2018 that said Omega Centauri probably is not home to life.

Stars pack so tightly inside Omega Centauri that the average distance between stars in the cluster’s core is 0.16 light-years, much closer than the sun’s nearest neighbor, Proxima Centauri, at 4.25 light-years. Scientists concluded that stars in Omega Centauri would gravitationally interact with each other too frequently to harbor stable habitable planets.

It is not only Omega Centauri’s largest size that sets it apart from other globular clusters, though. While this type of star cluster generally has stars of similar age and composition, studies of Omega Centauri reveal that it has different stellar populations that formed at varying periods of time. It may be that Omega Centauri is something else: a remnant core of a small galaxy that was absorbed by the Milky Way galaxy in the distant past!

Diagram: Star Spica at the top, with the constellation Corvus, and a small dotted circle below labeled Omega Centauri.
Spica, the brightest star in the constellation Virgo, serves as your guide star to the Omega Centauri globular star cluster. When Spica is highest up for the night, so is Omega Centauri!

How to see Omega Centauri.

Omega Centauri – the most luminous of all globular star clusters – is far to the south on the sky’s dome. It’s visible from the southern half of the United States, or south of 40 degrees north latitude (the latitude of Denver, Colorado). Canadians hasten to remind us that they can 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 Centauri star cluster skimming along the surface of Lake Erie,” they say.

From the Southern Hemisphere, Omega Centauri appears much higher in the sky and is a glorious sight.

If you’re in the Northern Hemisphere and want to spot this cluster, know that Omega Centauri can only be seen at certain times of the year. It’s best seen in the evening sky from the Northern Hemisphere on late April, May and June evenings.

Around mid-May, this wondrous star cluster is highest up and due south around 11 p.m. local daylight saving time.

By mid-June, Omega Centauri is highest up and due south around 10 p.m. local daylight saving time.

Northern Hemisphere residents can see Omega Centauri from January through April as well, but they must be willing to stay up past midnight or get up before dawn.

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 arm’s length approximates 10 degrees of sky. Find Spica by following the arc in the handle of the Big Dipper.

Chart showing how you can extend the handle of the Big Dipper to locate the stars Arcturus and Spica.
In any year, you can use the handle of the Big Dipper to locate the stars Arcturus and Spica.
Star chart of labeled constellations, including globular cluster Omega Centauri.
Even at its highest, Omega Centauri is low in the south for Northern Hemisphere observers. But it’s conveniently just below the bright star Spica. Finding Spica can lead you to this magnificent star cluster. Map via Sky&Telescope/ Stellarium.

Use the search function in to locate sky objects as viewed from your location

Omega Centauri is a globular, not an open, star cluster.

The symmetrical, round appearance of Omega Centauri distinguishes it 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, on the other hand, orbit the Milky Way outside the galactic disk. They harbor tens of thousands to millions of stars. Tightly bound by gravity, globular clusters remain intact after 12 billion years.

Dense ball of millions of stars, fading to less density at the edges.
The globular cluster Omega Centauri seen from ESO’s La Silla Observatory. Image via Wikimedia Commons.

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 16,000 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.

Omega Centauri’s position is at Right Ascension: 13h 26.8m; Declination: 47 degrees 29′ south.

Fuzzy dot in sky with enlargement showing larger fuzzy dot with some star detail.
View larger. | Omega Centauri star cluster, as captured by Greg Hogan of Kathleen, Georgia. Thanks, Greg!

Bottom line: The Milky Way’s largest globular star cluster, Omega Centauri, contains about 10 million stars, but probably no life.

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May 27, 2021
Clusters Nebulae Galaxies

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