Astronomers working with the Hubble Space Telescope have released a revealing new image of a cosmic rarity -- a planetary nebula inside a cluster of stars some 40,000 light-years away from Earth. The planetary nebula, located inside globular cluster M 15, is one of only four such nebulae ever seen.

Globular clusters such as M 15 are extremely common. Thousands have been observed within our own Milky Way Galaxy, and in other galaxies as well. Each cluster is a sphere-like collection of hundreds of thousands of stars, most of which are ancient -- up to 12 billion years old. For comparison, our own fledgling sun is a mere 4.6 billion years old.

What are not common, however, are planetary nebulae residing in these clusters. Astronomers have surveyed 133 globular clusters like M 15, but so far only four planetary nebulae expanding shells of gas that surround dying stars have been found to exist inside such clusters. In the new Hubble image, M 15s planetary nebula, known as Kuestner 648, can be seen as a fuzzy, pink blob to the upper left of M 15s main group of stars.

Planetary nebulae are produced when aging stars use up their inner energy reserves, increase in size to become "red giant" stars, and then start spewing their outer layers into space around them. The resulting shell of gas and dust becomes illuminated as a common planetary nebula when the central star is hot enough to excite its outgassed matter.

"We believe that very probably what happens is that two stars merge together to form a more massive star."

But according to astronomers who study planetary nebulae (which, incidentally, have nothing to do with planets), stars in globular clusters like M 15 are thought to be too lightweight to make the gases around them shine as planetary nebulae. So how can these nebulae exist inside globular clusters at all?

"We believe that very probably what happens is that two stars merge together to form a more massive star," said Space Telescope Science Institute astronomer Mario Livio, who has been studying Kuestner 648 and other planetary nebulae.

When two such stars coalesce, Livio explained, they become one large star thats heavier than typical globular-cluster stars. As a result, the star gets hot enough to make its surrounding gases shine. "We think that the existence of planetary nebulae in some globular clusters actually is evidence of these star mergers," said Livio.

Livio and his colleagues Howard Bond and Keith Noll at the Space Telescope Science Institute obtained the latest data on Kuestner 648 with the help of Hubbles Wide Field Planetary Camera 2. The resulting image represents near true color of the nebula and its surrounding stars.