Globular clusters are fascinating objects to look at. Globular clusters consist of tens of thousands to millions of stars that are held together by their own gravity.
The space image of the week is globular star cluster NGC 6355. This image was taken by the Hubble Space Telescope and includes data from both the Wide Field Camera 3 and the Advanced Camera For Surveys.
NGC 6355 is less than 50,000 light-years from Earth and lies in the constellation Serpent Bearer. You cannot see this globular cluster with the naked eye. The globular cluster has a magnitude (brightness) of 8.6, so you need at least good binoculars.
What you can clearly see in the photo is that the density of the globular cluster increases near the center. There are many more stars in the busy center than outside. The stars seem to be like herrings in a barrel, but appearances can be deceiving. In reality, there is still a relatively large amount of space between the stars, even though the distances between stars are much smaller than outside globular clusters. Stars in a globular cluster are average one light-year apart. Outside globular clusters, the average distance to neighboring stars is five light-years.
Scientists believe that globular star clusters are a good place to look for extraterrestrial civilizations. In a globular cluster, the development of life may have started earlier than on Earth, because the Earth is only 4.5 billion years old, while there are planets in globular clusters that may be ten billion years old. Partly because of this advantage – and because it is easier to travel from star to star – an intelligent civilization in a globular cluster may be able to develop further than humanity.
