A globular cluster’s striking red eye

This Picture of the Week shows the colourful globular cluster NGC 2108. The cluster is nestled within the Large Magellanic Cloud , in the constellation of the Swordfish (Dorado) . It was discovered in 1835 by the astronomer, mathematician, chemist and inventor John Herschel , son of the famous William Herschel .

The most striking feature of this globular cluster is the gleaming ruby-red spot at the centre left of the cluster. What looks like the cluster’s watchful eye is actually a carbon star . Carbon stars are almost always cool red giants, with atmospheres containing more carbon than oxygen — the opposite to our Sun. Carbon monoxide forms in the outer layer of the star through a combination of these elements, until there is no more oxygen available. Carbon atoms are then free to form a variety of other carbon compounds, such as C 2 , CH, CN, C 3 and SiC 2 , which scatter blue light within the star, allowing red light to pass through undisturbed.

This image was captured by the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys (ACS) , using three different filters.