An Old Star Gives Up the Ghost
NASA's Hubble Space Telescope has
recently obtained images of the planetary nebula
NGC 6369. This object is known to amateur astronomers
as the "Little Ghost Nebula," because
it appears as a small, ghostly cloud surrounding
the faint, dying central star. NGC 6369 lies in
the direction of the constellation Ophiuchus, at
a distance estimated to be between about 2,000 and
5,000 light-years from Earth.
When a star with a mass similar to
that of our own Sun nears the end of its lifetime,
it expands in size to become a red giant. The red-giant
stage ends when the star expels its outer layers
into space, producing a faintly glowing nebula.
Astronomers call such an object a planetary nebula,
because its round shape resembles that of a planet
when viewed with a small telescope.
The Hubble photograph of NGC 6369,
captured with the Wide Field Planetary Camera 2
(WFPC2) in February 2002, reveals remarkable details
of the ejection process that are not visible from
ground-based telescopes because of the blurring
produced by the Earth's atmosphere.
The remnant stellar core in the center
is now sending out a flood of ultraviolet (UV) light
into the surrounding gas. The prominent blue-green
ring, nearly a light-year in diameter, marks the
location where the energetic UV light has stripped
electrons off of atoms in the gas. This process
is called ionization. In the redder gas at larger
distances from the star, where the UV light is less
intense, the ionization process is less advanced.
Even farther outside the main body of the nebula,
one can see fainter wisps of gas that were lost
from the star at the beginning of the ejection process.
The color image has been produced
by combining WFPC2 pictures taken through filters
that isolate light emitted by three different chemical
elements with different degrees of ionization. The
doughnut-shaped blue-green ring represents light
from ionized oxygen atoms that have lost two electrons
(blue) and from hydrogen atoms that have lost their
single electrons (green). Red marks emission from
nitrogen atoms that have lost only one electron.
Our own Sun may eject a similar nebula, but not
for another 5 billion years.
The gas will expand away from the
star at about 15 miles per second, dissipating into
interstellar space after some 10,000 years. After
that, the remnant stellar ember in the center will
gradually cool off for billions of years as a tiny
white dwarf star, and eventually wink out.
Credits: NASA and The Hubble Heritage