101 SKY OBJECTS
WWW.ASTRONOMY.COM 1927 Omega Centauri
The most glorious of all globular clusters is Omega Centauri. (NGC 5139 is its
more mundane designation.) It’s the 24th-brightest “star” in Centaurus, which
is the ninth largest of 88 constellations. It was noted in Ptolemy’s Almagest in
A.D. 150 and designated Omega (ω) by Johann Bayer in his 1603 Uranometria.
Edmond Halley is credited for first noting its non-stellar appearance in 1677.
Scottish astronomer James Dunlop first described it as a globular cluster in 1826.
At –47° in declination, Omega Centauri is a challenge for observers at mid-
northern latitudes. But it is bright enough at its apex of 6° above the horizon for
observers to catch it from 38° north latitude.
For amateur astronomers located far enough south that the cluster is 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 mag-
nitude 3.9. Northern Hemisphere observers’ best and brightest globular is M
(see #69): magnitude 5.9 and a mere 20' in diameter.
Omega Centauri is resolvable in small telescopes, where other bright globular
clusters look like fuzzy balls. A behemoth among the Milky Way’s swarm of globu-
lars, it contains 10 million stars (with a total mass of 4 million Suns) crammed in
an area 150 light-years in diameter. M13 is 600,000 solar masses by comparison.
Omega Centauri is the second most massive globular cluster in the Local Group.
Only M31’s Mayall II is heftier.
Imagine yourself as an astronomer near the core of this cluster, where the stars
are only 0.1 light-year 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’s left of a dwarf galaxy gobbled up by the
Milky Way. Most of this galaxy has since been integrated into ours. Kapteyn’s
Star, 13 light-years away, is likely one such former member. — A.G.ALAN DYERthe total star count of the Mizar system to four. Alcor is a binary
system, too, accompanied by a 9th-magnitude red dwarf partner
set just 1" away.
Through a telescope, you might also notice a faint star forming
a broad triangle with Alcor and Mizar. In 1722, German mathemati-
cian Johann Liebknecht thought he saw that star move against the
background from one night to the next. He concluded that it was a
new planet. In his excitement, he christened it Sidus Ludoviciana
(Ludwig’s Star) after Ludwig V, then the king of Germany.
Liebknecht was mistaken, but the star still retains that name. — P. H.
KIM QUICK/TERRY HANCOCK