Hubble investigates mystery of a dying star

larger   K648, the first planetary nebula discovered in a globular cluster, is the pinkish object to the upper left of the cluster core

By Richard Stenger
CNN.com Writer

(CNN) -- The Hubble Space Telescope has been enlisted to do some detective work on a case that has baffled scientists for years: a star dying a puzzling death in a globular cluster.

The star in question generated the first known planetary nebula in a globular cluster, one of more than 100 dense collections of stars that surround the Milky Way galaxy, Hubble researchers said Thursday.

Planetary nebulae are huge clouds of glowing gas that certain kinds of stars emit near death. They are so named because their shapes reminded 18th century astronomers using primitive telescopes of planetary disks.

"It's a stage of life for stars at the very end. They become very large giants and eject their outer layers, which exposes the hot core of the star," said Hubble astronomer Mario Livio. "Because it's hot, it ionizes the nebula to make it shine, like a neon light."

While common in some parts of the universe, they remain almost unknown in globular clusters because the clusters possess few stars with enough mass to generate them.

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Low-mass dying stars lack the firepower necessary to heat their nebulas enough to glow during the brief time such gas clouds exist, about 10,000 years or so.

"They never become hot enough quickly to shine," Livio said.

Only four such nebulas are known to exist in globular clusters, the first of which astronomer F.G. Pease discovered in 1929 -- K648. It is located in globular cluster M 15, about 40,000 light-years away in the direction of the constellation Pegasus.

In this Hubble image, taken in December 1998, K648 is the pinkish object to the upper left of the cluster core.

The remaining mass of K648 is about 60 percent of the sun, barely enough to create a nebula, but much greater than most stars in the cluster.

To found out why it has more mass, Hubble scientists used the orbiting observatory to look for a nearby star that could have donated a significant amount of mass.

"Presently, there is no known companion to this star," said Livio, who offered an alternative explanation. "But we believe in the past this star formed by the merger of two stars."

The Hubble has spotted plenty of candidates for stellar mergers in globular clusters, notably those known as blue stragglers, which possess much more mass than astronomers had expected, Livio said.

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