NGC6302



Fine Structure in the Butterfly Nebula

          VLT UT1 First Light Photo No. 4 

This splendid colour image of a famous southern Planetary Nebula, the
Butterfly (NGC 6302), was obtained by combining blue, yellow and red
images obtained on May 22, 1998, with 10 minute exposures and an image
quality better than 0.6 arcseconds.

Towards the end of their life, some massive stars expand to giant
dimensions. They shed most of the hydrogen in their outer layers as a
strong "stellar wind", before they contract towards a final compact
stage as "white dwarfs".

After this ejection process, the star remains thousands of times
brighter and also much hotter than the Sun during a few thousand
years. Its strong ultraviolet radiation has the effect of ionizing the
previously ejected gas, which then shines before it disperses into
interstellar space. The resulting nebulae (traditionally referred to
as Planetary Nebulae, because of their resemblance to a planet in a
small telescope) often exhibit very complex morphologies.

The Butterfly Nebula belongs to the class of bipolar nebulae, as this
picture clearly illustrates. A dark, dusty and disc-like structure -
seen edge-on in this image - obscures the central star from our
view. However, its strong radiation escapes perpendicular to the disk
and heats and illuminates the material deposited there by the stellar
wind.

The origin of the dark disk may be due to the central star being a
member of a double star system. This has been shown to be the case in
some other bipolar nebulae in which, contrary to the Butterfly Nebula,
there is a direct view towards the star.