101 must-see cosmic objects: Omega Centauri

The most glorious of all globular clusters is Omega Centauri. (NGC 5139 is its more commonplace designation.) It is the 24th brightest “star” in Centaurus, which is the ninth largest of the 88 constellations. It was noted in Ptolemy’s Almagest in AD 150 and designated Omega (ω) by Johann Bayer in his 1603 Uranometria. Edmond Halley is credited with first noting its non-stellar appearance in 1677. Scottish astronomer James Dunlop first described it as a globular cluster in 1826.

At –47° of declination, Omega Centauri is a challenge for mid-northern latitude observers. But it’s bright enough at its peak of 6° above the horizon that observers can catch it from 38° north latitude.

To amateur astronomers far enough south for the cluster to be high in the sky, Omega Centauri makes the rest of the globulars look like pale imitations. Its apparent diameter is 36.3′ — larger than the Full Moon! It’s bright too, at magnitude 3.9. The best and brightest globular from Northern Hemisphere observers is M13 (see #69): magnitude 5.9 and just 20 feet in diameter.

Omega Centauri can be resolved in small telescopes, where other bright globular clusters look like fuzzy balls. A behemoth among the Milky Way’s globular swarm, it contains 10 million stars (with a total mass of 4 million Suns) packed into an area 150 light-years across. M13 is 600,000 solar masses in comparison. Omega Centauri is the second most massive globular cluster in the local group. Only the Mayall II of the M31 is heavier.

Imagine yourself as an astronomer near the heart of this cluster, where the stars are only 0.1 light years apart. Night would look like day. The relative motions of individual stars would be traceable over a few years. Such a sky sounds like the realm of science fiction.

Where does such a massive globular cluster come from? Astronomers believe Omega Centauri is all that remains of a dwarf galaxy engulfed by the Milky Way. Most of this galaxy has since been incorporated into ours. Kapteyn’s star, 13 light-years away, is likely one such former member.

Be sure to explore the full list of 101 cosmic objects you must see on Astronomy. New entries will be added weekly throughout 2022.

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