The Owl Nebula, a planetary nebula in Ursa Major
Image Credit & Copyright: Don Goldman, Astrodon Imaging (http://www.astrodonimaging.com)
The Owl Nebula (designated Messier 97 or NGC 3587) is a fairly faint planetary nebula of some 3 light-years across that formed about 6,000 years ago. It is located within our Milky Way galaxy, about 2,600 light-years away from Earth in the constellation of Ursa Major (the Big Bear or the Big Dipper).
Its round shape along with the placement of two large, dark “eyes” do suggest the face of a staring owl, hence its nickname.
Planetary nebulae represent the final brief stage in the life of a medium-sized star like our Sun. While consuming the last of the fuel in its core, the dying star (collapsing from a red giant to a white dwarf) expels a large portion of its outer envelope. This material then becomes heated by the radiation from the stellar remnant and radiates, producing glowing clouds of gas.
Over the next several thousand years, the Owl Nebula will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.
The Owl Nebula is one of the more complex planetary nebulae. The expanding bubble is confined by the interstellar medium, the bubble slows down and the outflow of gas builds up at the leading edge producing the bright reddish rim that surrounds the bluish core of the nebula. The red rim is in turn enveloped by a round halo that is about 10,000 times fainter than the core of the nebula, and doesn’t appear in most images.
However, Don was aware that such a halo exists around the Owl Nebula and was determined to photograph it. The resulting color image from the 21 total hours of exposures is shown above. The extended halo is clearly shown.
The nebula is illuminated by its 16th magnitude, central star, which also carved the two “eyes”. This dying central star (seen between the Owl’s eyes) is a small (about the size of Earth ) and hot white dwarf with a temperature of around 100,000o Kelvin at the surface. The original star was somewhere between 150%-to-200% the mass of our Sun, and is now believed to be only about 0.7 solar masses, while the mass of the nebula has been estimated to be 0.15 solar masses.
As other planetary nebulae, the glow of the Owl Nebula comes mainly from the emissions from ionized hydrogen, H-alpha (red), and ionized oxygen (O-III), what gives these objects their dominant reddish and greenish hues. But the Owl has, also, a considerable portion of light from the white dwarf that has been reflected by nebular particles. This mechanism makes its color bluer, and distinguishes this from other planetaries with a smaller reflection component.
The Owl Nebula is significantly brighter visually than photographically, as most light is emitted in one green spectral line. It can be seen with amateur telescopes in dark nights, but large telescopes, or photographic techniques, are required in order to perceive the two cavities that justify its name.
The small nebulous region near the star below the Owl Nebula is a spiral galaxy called PGC 34279.
This composite image was taken on April 29, 2011 and May 3, 2011, from the Sierra-Remote Observatories, Shaver Lake, California, with the Apogee U16M Camera, using narrowband filters and different color mappings. North is to the upper left.
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