File:Globular Clusters NGC 1851 and 1904.jpg

English: Like confetti at a party, the diverse stellar populations of globular clusters NGC 1851 and 1904 display a spectacular range of colors in these three-channel composites.

Globular clusters are compact bundles of old stars that date back to the birth of our Milky Way galaxy, approximately 13 billion years ago. The discovery of hot ultraviolet stars in globular clusters proved to be a real surprise to astronomers in the 1970s, who thought that only young, massive stars could shine in the ultraviolet.

As they further investigated this phenomenon, scientists continued to be baffled by the fact that some clusters had ultraviolet sources, while others did not, and that there were variations in the ultraviolet brightness of sources in the same cluster. In the images of NGC 1904 and 1851, for example, a few of the yellow-green specks sprinkled throughout the clusters represent a relatively ultraviolet dim family of stars called "blue stragglers." These stars are formed from collisions or intimate encounters between two closely orbiting stars.

Because blue stragglers shine at wavelengths bordering blue visible light, they can sometimes be confused with very massive, hot, young stars.

Also, like "fools gold," the stragglers can be very tricky. Two stars that are orbiting closely but not interacting can emit the same ultraviolet wavelengths as a blue straggler and fool astronomers into believing that they are looking at a straggler, when they are not. Thus, in both these images, some of the faint, fuzzy, yellow-green dots may actually be very massive "normal" stars. Astronomers might be able to confirm the stellar type by zooming in with other observatories, such as NASA's Hubble Space Telescope.

The blue dots represent a class of stars called Hot Horizontal Branch Stars. In death, these stars shed about 85 percent of their atmosphere and leave behind only a very hot and extremely ultraviolet-bright core. As a contrast, both clusters' populations of "very cold" stars are shown as red points.

NGC 1904 is located approximately 50,000 light years away in the constellation Lepus, and NGC 1851 lives 40,000 light years away in the southern constellation Columba.

These images combine far-ultraviolet (blue) and near-ultraviolet (green) information from NASA's Galaxy Evolution Explorer (GALEX) with infrared J-band (red) data from the Two Micron All Sky Survey (2MASS).