August 3, 2000: The globular cluster M15 is shown in this color image obtained with the Hubble telescope. Lying some 40,000 light-years from Earth in the constellation Pegasus, M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. Each of these spherically shaped clusters contains hundreds of thousands of ancient stars. The stars in M15 and other globular clusters are estimated to be about 12 billion years old. They were among the first generations of stars to form in the Milky Way.
The brightest cluster stars are red giants, whose orange-colored appearance is due to surface temperatures lower than our Sun's. Most of the fainter stars are hotter, giving them a bluish-white color. If we lived in the core of M15, our sky would blaze with tens of thousands of brilliant stars both day and night!
Nestled among the myriads of stars visible in the Hubble image is an astronomical oddity. The pinkish object to the upper left of the cluster's core is a gas cloud surrounding a dying star. Known as Kuestner 648, it was the first planetary nebula discovered in a globular cluster. Scientists have identified only three more planetary nebulae in globular clusters since Kuestner 648's discovery in 1928.