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MyCn18
http://imgsrc.hubblesite.org/hu/db/1996/07/images/a/formats/web_print.jpg MyCn18: An Hourglass Nebula Explanation: The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star's life occurs as its outer layers are ejected - its core becoming a cooling, fading White Dwarf. Astronomers have recently used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the "hourglass". The unprecedented sharpness of the HST images has revealed surprising details of the nebula ejection process and may help resolve the outstanding mystery of the variety of complex shapes and symmetries of planetary nebulae. A planetary nebula can result as a star with mass of less than several times the solar mass ("low mass" star) ejects mass in the red giant stage, near the end of the star's life. In this stage, the central part of the star, which is about the size of the earth, consists of a carbon ash core, a shell in which helium is fusing to carbon, and a shell where fusion of hydrogen to helium has temporarily ceased. The rest of the star's envelope has expanded to about 70 times larger than it had during most of its lifetime (almost the size of the orbit of Mars). As more nuclear fuel is consumed, the inner core collapses to a final high density state and the burning in the shell increases in intensity. This state is unstable and the star pulsates, ejecting much of its outer envelope. This leaves a small (less than 1.4 solar masses) hot, dense core of mostly carbon and a helium burning shell. This central star will eventually become a white dwarf. The ejected envelope becomes a spherical shell of cooler thinner matter spread over a volume about the size of our solar system - a planetary nebula. They are called planetary because to early astronomers these fuzzy patches seemed like the disks of planets, but, of course, we now know they have no association with planets. The densities of these nebulae are typically 1.0E8 - 1.0E10 particles per meter3 and the expansion speed is about 20 - 30 km/s. After approximately 10,000 to 50,000 years (a short time, astronomically!) the density becomes too small for the nebula to be seen. About 1000 visual planetary nebulae have been catalogued. The star at the center of the nebula is extremely hot, reaching temperatures of 200,000 K, emitting a large amount of the ultraviolet radiation which is necessary for ionization of hydrogen in the expanding gas. The resulting plasma is similar to that in the HII regions but with more ions in higher energy levels. Therefore the spectral lines from these higher states, such as the green "forbidden" lines of doubly ionized oxygen, are stronger in these nebulae.
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