Stellar metamorphosis: Hubble pictures of butterfly-shaped nebulae emerging from stellar cocoons 19/03/1998
Studying images of proto-planetary nebulae is important to understanding the process of star death. When it has exhausted its thermonuclear fuel the star becomes bright and cool (red giant phase) and swells to several tens of times its normal size. It then begins puffing thin shells of gas off into space. These become the star's cocoon. In the Hubble images, the shells are the concentric rings seen around each nebula.
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Image Credit: See credits below text descriptions below.  
[TOP LEFT AND RIGHT] The Hubble Space Telescope's Wide Field and Planetary Camera 2 has captured images of the birth of two planetary nebulae as they emerge from wrappings of gas and dust, like butterflies breaking out of their cocoons.

These images highlight a fleeting phase in the stellar burnout process, occurring just before dying stars are transformed into planetary nebulae. The left-hand image is the Cotton Candy nebula, IRAS 17150-3224; the right-hand image, the Silkworm nebula, IRAS 17441-2411. Called proto-planetary nebulae, these dying stars have been caught in a transition phase between a red giant and a planetary nebula. This phase is only about 1,000 years long, very short in comparison to the 1 billion-year lifetime of a star. These images provide the earliest snapshots of the transition process.
Credits: Sun Kwok and Kate Su (University of Calgary), Bruce Hrivnak (Valparaiso University), and NASA.

[BOTTOM LEFT AND RIGHT] This Wide Field and Planetary Camera 2 image of NGC 6818 shows two distinct layers of gas (with dust): a spherical outer region and a brighter, vase-shaped interior "bubble." Astronomers believe that a fast wind - material propelled by radiation from the hot central star - is creating the inner elongated shape. The central star of the planetary nebula appears as a tiny blue dot. The material in the wind is traveling so fast that it smashes through older, slower-moving stellar debris, causing a "blowout" at both ends of the bubble (lower right and upper left).

This nebula looks like a twin of NGC 3918, another planetary nebula that has been observed by the Hubble telescope. The structure of NGC 3918 is remarkably similar to that of NGC 6818. It has an outer spherical envelope and an inner, brighter, elongated bubble. A fast-moving wind also appears to have created an orifice at one end (bottom right-hand corner) of the inner bubble. There are even faint wisps of material that were probably blown out of this hole. In the opposite direction (top left-hand corner), there is a protrusion that seems on the verge of breaking through to form a hole.
Credits: Robert Rubin (NASA Ames Research Center), Reginald Dufour and Matt Browning (Rice University), Patrick Harrington (University of Maryland), and NASA.

 
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