Solar System

(The Cross)

Ask the general public what their favourite southern sky object is and they will probably say the Southern Cross because that might be the only southern sky highlight that they are familiar with.

Its fame is quite expansive, it appears on the flags of some southern countries and was used by sailors in the 15th and 16th centuries to guide them on their journeys. It is also very bright with two of its stars being in the top 20 brightest stars list.

Crux is home to two wildly different opposites, the dark inpenetrable void of the Coalsack Nebula and the glittering gemstones of the Jewel Box Cluster.

  Jewel Box Cluster (NGC 4755):
  The wondrous colours of this southern gem sparkle in most commercially available instruments. The cluster is strikingly characterized by a line of three stars at the centre with a ruby red supergiant providing sparkling contrast.
  Coalsack Nebula:

The unimaginable wonder and amazement William Herschel must have felt when he studied this dark nebula. Dark nebula were originally thought of as ancient holes in the sky, they are in fact dust clouds that contain no stars or any nearby stars and are only visible by obstructing the thousands of stars of the Milky Way behind them.

The Coalsack is one of the largest of these dark nebulae and it is so large that parts of it spill over into the constellations Centaurus and Musca. It lies in the bottom left quadrant of Crux and strangely it isn't catalogued in any deep sky catalogues. There are also some catalogues that are completely devoted to nothing but dark nebulae and the most well known ones are the Barnard and the LDN (Lynds Dark Nebula) catalogues. Another dark nebula not to have a catalogued designation is the
Cone Nebula
in Monoceros.

Superimposed in front of the dark cloud is a very reddened blob of an emission nebula, RCW 71. This can be seen with a telescope.

The Southern Cross is a bright highlight of the sky visible from most southern latitudes. It can also be used as a rough pointer to the south celestial pole, since the pole is marked by the very faint star Sigma Octantis. Mouseover the brightest stars for their names. The size of the bright stars is exaggerated and is caused by an atmospheric artifact that makes brighter stars appear larger. The Hubble Space Telescope is above the atmosphere and most of its images don't contain this artifact. The large dark nebula is the Coalsack Nebula.  
Image copyright Marco Lorenzi/G. Favretto
Acrux Mimosa Gacrux Delta Crucis Epsilon Crucis