Messier 55 (M55)

While Messier 55 (M55) is large and reasonably bright, it is lacking a dense core and many of its stars are quite faint, making it hard to observe in non-optimal conditions. Though this image shows just a portion of M55, the cluster as a whole appears spherical because the stars’ intense gravitational attraction pulls them together. Hubble’s clear view above Earth’s atmosphere resolves individual stars in this cluster. Ground-based telescopes can also resolve individual stars in M55, but fewer stars are visible. The globular cluster is about 20,000 light-years away and has a diameter of about 100 light-years. It contains an estimated 100,000 stars with 55 variable stars whose brightness changes.