Physics Photo of the
October 7, 2011
M27 - a planetary nebula
Color image of M27. Photographed by Brandon Knutson, Helen
Albea, Lee Nice-Webb, Sarah Fairbanks, and Olivia
Image of M27 through the H-alpha filter.
Photographed by Olivia
Crespo, Katie Anderson, and Chris O'Leary
M27, also called the Dumbell Nebula, is one of the most
colorful and brightest
planetary nebulas. The nebula consists of a special
class of star in the center called a white dwarf.
The stellar material that forms the nebula consists mostly of
diffuse hydrogen gas. The
extremely hot white dwarf (about 30,000 deg K) emits copious
amounts of highly energetic ultraviolet radiation. The
UV radiation causes the hydrogen gas to glow in its
characteristic colors. This glow is called fluorescence
and emits most of its fluorescent light in one single red
color - the color of the H-alpha emission for hydrogen.
The animated picture at right shows a good comparison of the
ordinary color image and the H-alpha photograph. Most
stars emit their light distributing the light among all colors
- the stars are incandescent sources. The nebula,
however, emits most of its light in the single H-alpha
color. The narrow color filter thus blocks most of the
starlight from the surrounding stars, as well as that from the
central white dwarf. Thus the nebula appears much
brighter with the H-alpha filter than it does in the color photo, but the stars are
highly dimmed using the H-alpha filter. The hydrogen
emission spectrum is shown in the image at the left. The
H-alpha is the bright red emission line.
A white dwarf star is essentially an old star that has spent
all its nuclear fuel, and no-longer can generate energy by
nuclear fusion. With the loss of the nuclear reactions
in the core, the star collapsed and formed the very dense core
called a white dwarf. The white dwarf has such a "hard"
surface that the outer envelope of the star rebounded into
space and expanded. The rebounding stellar material is
the cloud that formed the nebula. Even though
the white dwarf is a "dead" star, not harboring any nuclear
fusion, it is extremely hot (~30,000 deg K) or about 5 times
the temperature of the Sun's surface. Such an extremely
hot object emits copious amounts of energetic ultraviolet
radiation. It is believed that the original star
collapsed to form the white dwarf relatively recently in
astronomical terms (between 10,000 and 40,000 years
ago). The nebula is slowly expanding and the white dwarf
is slowly cooling. Eventually the nebula will
diffuse into the background of space, the white dwarf will
fade, and perhaps the cloud will coalesce with other clouds
and form a new star (give a few billion more years...).
it is believed that the Sun will collapse into a white dwarf
and form a similar planetary nebula in a few billion years.
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