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Centaurus, a Constellation for May and June

Contributed by Paul Rodmell, Southland Astronomical Society

CENTAURUS   (Pronounced sen-TAUR-us)

Chart showing the constellation.

Centaurus is a large and rich constellation representing a centaur in Greek mythology, reputedly the scholarly centaur Chiron, who was the tutor of many Greek heroes, and who, according to legend invented the main constellations. He was raised to the sky after being accidentally struck by a poisoned arrow from Hercules.

The constellation is of particular interest because it contains the closest stars to the Sun, α Centauri, a group of three stars linked by gravity. The line of the two stars α and β Centauri (known as "the Pointers") points to Crux, the Southern Cross.

One of the strongest radio sources in the sky, Centaurus A, lies in the constellation, associated with the galaxy NGC 5128.

Centaurus lies in a prominent part of the Milky Way, providing rich star fields and clusters for binoculars.

To find Centaurus look towards the south and up about 70° at this time of the year. You should be able to orient yourself by using the Pointers and Crux the Southern Cross, to help find the constellation.

Chart showing Centaurus as seen high in the sky to the south fairly early evening in June.

Centaurus and surrounding constellations

Constellation Lupus Constellation Triangulum Constellation Circinus Constellation Musca Constellation Carina Constellation Crux

Some stars and interesting objects in Centaurus


α Centauri (Rigel Kentaurus) is the brightest star in Centaurus and 3rd brightest in the sky. It lies very close at 4.3 light years away. To the unaided eye it shines as a star of magnitude -0.27, but small telescopes reveal that it consists of two individual yellow stars of magnitude 0 and 1.4. The brightest of these is very similar in nature to the Sun. These stars orbit each other every 80 years. Associated with these stars is an 11th magnitude red dwarf star called Proxima Centauri. This dim star is slightly closer to the Earth, but lies 2° away from α and is not even in the same telescopic field of view. This star probably takes about a million years to orbit its two brilliant companions.

β Cen (Hadar or Agena) is a magnitude 0.6 blue giant star 460 light years away.

3 Cen, also known as K Cen is a striking pair of blue-white stars of magnitudes 4.7 and 6.2 in a field of scattered stars.

ω Cen (NGC 5139) is the largest and brightest globular cluster visible to the unaided eye from a dark site when the Moon is absent and indeed was originally recorded by Ptolemy as a star. It appears to cover an area over two-thirds that of the full Moon. Small telescopes and binoculars begin to resolve its outer regions into stars. It is a showpiece for all size telescopes provided that the field of view is large enough to include some of the surrounding area.

Edmund Halley of comet fame discovered this globular cluster in 1677. It was often mistaken by the public in 1986 for Halley's Comet when the comet was in the vicinity. It is about 16,000 light years away, which makes it among the nearest globular clusters.

NGC 3766 which is a fine scattered open cluster of about 60 stars 1700 light years away visible to the unaided eye. This cluster is effective even in small aperture telescopes, with a pattern of star loops giving it a lobed appearance and containing orange, yellow, white and bluish stars. It merges into a rich star field.

The Coal sack, in Crux, Centaurus, and Musca, is a dark nebula contrasting well with the bright Milky Way. The Australian aborigines regarded the Coal Sack as the head of the "Emu" with the neck and part of the body being a dark rift in the Milky Way.

NGC 4945 is an edgewise late-type spiral galaxy appearing as a long narrow luminous haze in a beautiful starry field. This galaxy is a member of a small group of galaxies that includes NGC 5128 and M 83 in Hydra.

NGC 5460. This large scattered open cluster is well suited for small telescopes. There is a semi-circle of bright stars north of centre, that includes an orange star and a nice pair of stars. It lies about 2700 light years away. Appearing slightly north-east of the semi-circle is a small conspicuous lenticular galaxy, ESO 221-G26.

NGC 5128 is a remarkable galaxy appearing as a bright round luminous haze bisected by a dark bar seen relatively easily in binoculars from a dark sky site. This galaxy is the famous radio source Centaurus A.

NGC 3918 a small planetary nebula. This was discovered by John Herschel in 1834 who called it the Blue Planetary. Through a telescope, it appears similar to the planet Uranus. It lies in a fine starry field.

Visibility

The more southerly parts of the constellation, including the Pointers, is circumpolar for New Zealand, that is it never sets. The Pointers are low to the South in the evening in Summer.

Central parts of the constellation are highest about 10.30 pm NZST in mid May and 8.30 pm, NZST, in mid June, when northerly parts of the constellation pass overhead in New Zealand. α Cen is highest about an hour later. By the end of June it will be at its highest point at 8.30 pm.


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