Jewel Box - NGC 4755

Located north of the Coalsack dark nebula, 1° SE of Mimosa (β Crucis) and just visible to the naked eye as a slightly extended magnitude 3 star, is the Jewel Box – NGC 4755 – an aggregate of 6-8 bright stars superposed on a rich background of +200 stars. The brightest stars form an A-shaped asterism, with the southern most star being κ Crucis, which sometimes gives an alternate name for this cluster. The standout of this cluster is the red supergiant DU Crucis, which sits almost in the middle of the formation.

The cluster has been discovered in 1751 by Lacaille, and since then, it has been described as one of the most beautiful clusters in the night sky. A unique fact about NGC 4755 is that visual observers report very different colors for its members. While most describe the stars as having shades of blue, some report green, yellow and orange tinges. At one point in time, specifically 1862, the English amateur Francis Abbott even reported significant changes in the structure and number of stars of NGC 4755, which strengthened his belief that the Universe was not static. In 1872, Australian astronomer Henry Russell tried to confirm the reports from Abbott. After some nights where he could not see any changes, he used higher magnification and he reported stars “coming in and out of view”. He also noted that Herschel’s star map of this cluster was missing important stars which he should have been able to see. The only explanation Russell could give is that the Coalsack Nebula, which is nearby, might shift in time, blocking and/or revealing stars behind it. He also wondered if the Earth’s motion through space could alter the line of sight to various stars in the cluster.

After some 100 years of limited research on NGC 4755, the advent of photometric and spectrometric observations in the 1950s allowed for astronomers to focus again on the cluster. In 1958, Halton Arp and Cecil van Sant built the first color-magnitude diagram of the cluster, revealing a dominance of the B-type stars, giants and supergiants. They also classified DU Crucis as a K5 giant, which will be corrected later on to M2-type supergiant. The cluster was found to be free of internal nebulosity and its age was estimated to be 3 million years. The age was significantly changed after analysis done in the 1990s that showed an age closer to 15 million years. Other spectral research showed that NGC 4755 has a similar age and star structure as NGC 3293 in Carina and the Double Cluster in Perseus.