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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 clusters.
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, and M. Livio, who are interested in the
origin and evolution of planetary nebulae and their central stars.
August 3, 2000
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)