October 7, 2011

M27 - a planetary nebula

Color image of M27.  Photographed by Brandon Knutson, Helen Albea, Lee Nice-Webb, Sarah Fairbanks, and Olivia Crespo

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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