A DYING STAR IN GLOBULAR CLUSTER
The globular cluster Messier 15 is
shown in this color image obtained with the NASA
Hubble Space Telescope's Wide Field Planetary Camera
2 (WFPC2). Lying some 40,000 light-years from Earth
in the direction of 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 clusters is a spherical association
of hundreds of thousands of ancient stars.
The image, prepared by the Hubble
Heritage team, attempts to show the stars in M15
in their true colors. The brightest cluster stars
are red giants, with an orange color 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
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, this was the first planetary nebula
to be identified in a globular cluster. In 1928,
F. G. Pease, working at the 100-inch telescope of
California's Mount Wilson Observatory, photographed
the spectrum of K 648 and discovered the telltale
bright emission of a nebular gas cloud rather than
a normal star. In the ensuing 70 years, only three
more planetary nebulae have been discovered in globular
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. Our Sun, by comparison,
is a youthful 4.6 billion years old. As a star like
the Sun ages, it exhausts the hydrogen that fuels
its nuclear fusion, and increases in size to become
a red giant. Then it ejects its outer layers into
space, producing a planetary nebula. The remnant
star at the center of the nebula gradually dies
away as a white dwarf.
Planetary nebulae are so named because
their shapes reminded 18th-century astronomers with
small telescopes of the round disks of planets.
They are actually huge clouds of gas, glowing because
of ultraviolet light emitted by the stars in their
centers. The surface temperature of the central
star of K 648 is about 70,000 degrees Fahrenheit
(40,000 degrees Celsius), and analysis of the Hubble
data indicates that the star's remaining mass is
only 60 percent that of our Sun. The star's outer
layers were ejected some 4,000 years ago.
The most massive stars use up their
hydrogen first, and then less-massive stars in turn
run out of fuel, become red giants, and fade away.
For stars less massive than the Sun, some astronomers
believe the evolutionary process to be so gradual
that a visible planetary nebula will not form. At
the present time, the most massive stars remaining
in M15 have about 80 percent of the mass of our
Sun, a fact that makes the existence of a planetary
nebula like K 648 something of a mystery. The Hubble
images used to make this image were taken to test
the idea that the progenitor of K 648 may have "borrowed"
some mass from a nearby stellar companion. No such
companion was revealed by Hubble, so the mystery
remains unsolved. One possibility is that the progenitor
of K 648 was two stars, which then merged together
to become the single star now seen at the center
of the nebula.
The Hubble data on K 648 were obtained
and analyzed by a team of Space Telescope Science
Institute astronomers, including H. E. Bond, D.
R. Alves, M. Livio, L. K. Fullton, and K. G. Schaefer,
who are interested in the origin and evolution of
planetary nebulae and their central stars.
Credit: NASA and The Hubble
Heritage Team (STScI/AURA)
Acknowledgment: H. Bond (STScI)