RASNZ logo

Royal Astronomical Society of New Zealand

Crux, a Constellation for May

Contributed by Paul Rodmell, Southland Astronomical Society

CRUX, pronounced CRUCKS, the Southern Cross

Chart showing the constellation.

Crux, the Southern Cross, is the smallest constellation in the sky, but one of the most celebrated. The early Portuguese navigators saw it as a symbol of their faith, and the mystery of the unknown lent it an additional charm in the minds of those from whom the southern skies were hidden. There are other cross patterns formed by stars, but the distinguishing feature of the two bright pointers alpha and beta Centauri make Crux unmistakable.

Crux lies in a dense and brilliant part of the Milky Way, which makes the famous dark nebula known as the Coalsack striking in silhouette against the star background. This is the head of the Emu to the native aborigines of Australia. The rest of the Emu is made from the dark lanes in the Milky Way.

From New Zealand latitudes Crux is circumpolar and always in the sky, rotating about the south celestial pole each day.

Chart showing the constellation orientated as seen about 8.30 pm mid May

Crux and surrounding constellations

VARIABLE stars are shown as open circles.  DOUBLE stars are shown with a bar through their circle.

Constellation Musca Constellation Carina Constellation Chamaeleon Constellation Lupus Constellation Vela Constellation Centaurus

Some stars and interesting objects in Crux

α Crucis (Acrux) appears to the unaided eye as a star of magnitude 0.9 but small telescopes show it to be a close double star with blue-white components of magnitudes 1.4 and 1.9.  Acrux is the 12th brightest star in the sky.
An unrelated star, magnitude 4.8, about 1.5 arc minutes from Acrux, is easily seen as separate in binoculars.

β Cru, is a magnitude 1.3 blue-white star 570 light years away. It is a variable star fluctuating by less than 0.1 magnitudes every 6 hours.

γ Cru, is a magnitude 1.6 red giant star 88 light years away. There is a very wide magnitude 6.4 companion, which is unrelated, visible in binoculars.

δ Cru, is a magnitude 2.8 white star 470 light years away. It is the faintest of the four main stars of the "Cross".

ε Cru, is a magnitude 3.6 yellow star, 125 light years away.

ι Cru is a magnitude 4.7 orange star 280 light years away. This has a wide magnitude 7.8 companion star, visible in small telescopes.

μ Cru is a pair of blue-white stars of magnitudes 4.0 and 5.2, able to split by the smallest of telescopes or even good binoculars.

NGC 4755 (κ Crucis) is an open cluster of stars popularly known as the "Jewel Box", from Sir John Herschel's vivid description of the cluster as a "casket of variously coloured precious stones". It is rich and bright with the stars showing delicate colours accentuated by an orange-red supergiant. It can easily be seen by the unaided eye as a star, and indeed was originally catalogued as such in pre-telescope times.

The Coalsack is a large irregular dark nebula lying nearby at a distance of around 550 light years. It is a cloud of dark gas and dust.

Visibility Crux is visible all the year from New Zealand. It is at its highest at about 9.30 pm in mid May, when the long axis will "point" straight down towards the celestial south pole.

In general Crux is highest during the evening in the early and mid winter months. It is low above the southern horizon in the evening soon after sunset in early summer.


Top of Page      Other Constellations      Return to RASNZ home page.