Many of the faint stars in this detailed ultraviolet image of the central part of the 47 Tucanae globular cluster are white dwarfs. These collapsed stars represent the final stage in the evolution of a star such as the Sun. Typically, a white dwarf is about the size of planet Earth and so dense that one cubic centimeter would weight a ton. Its fuel has been exhausted so a white dwarf simply radiates its heat away into space, and gradually cools down over millions of years. Finding a group of white dwarfs in a cluster helps understand this process, since they are all the same age and started with the same chemical composition. White dwarfs are so dim it has not been possible to detect many of them in star clusters with ground-based telescopes.
A globular cluster is a ball-shaped concentration of hundreds of thousands - sometimes millions - of stars. 47 Tucanae, which is 15,000 light years away, is one of several hundred such clusters known in our Galaxy. It is the second-brightest globular cluster in the sky, just detectable to the naked eye. The globular clusters in our Galaxy contain some of its oldest stars.
Ultraviolet HST images of the center of 47 Tucanae also reveal the presence of stars that are unexpectedly hotter and bluer than the rest of the stars in the cluster. Astronomers nick-name them 'blue stragglers'. They seem to be more numerous near the center of the cluster where the stars are packed most closely thogether. No-one knows for sure what they are but a possible explanation is that they are pairs of stars that linked up as close doubles, or perhaps even merged, as a result of close encounters.
Camera: Faint Object Camera with COSTAR
Credit: R. Jedrzejewski (STScI), NASA, and ESA