Globular cluster M15. Hubble Space Telescope (HST) image of the globular star cluster M15. This is a ball of hundreds of thousands of stars which lies around 40,000 light years from Earth in the constellation Pegasus. Most of the stars are hot, small blue-white stars. Numerous red giants (orange), older stars nearing the ends of their lives, are also seen. The pink blob at upper left is the planetary nebula Kuestner 648. Planetary nebulae are shells of gas cast off by stars at the ends of their lives, after the red giant phase. They are very rare in globular clusters, where their formation is poorly understood.