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Hubble Finds an Hourglass Nebula
around a Dying Star
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Name: Hourglass Nebula, MyCn18
Description: Planetary Nebula
Position: R.A. 13h 39m 29.68s  Dec. -67° 22' 38.79"
Constellation: Musca
Distance: 8,000 light-years (2,500 parsecs)
Instrument: WFPC2
Exposure Date(s): July 30, 1995
Image Credit: NASA, R. Sahai, J. Trauger (JPL) & the Hubble Heritage team
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This is an image of MyCn18, a young planetary nebula located about 8,000 light-years away, taken with the Wide Field and
Planetary Camera 2 (WFPC2) aboard NASA's Hubble Space Telescope (HST). This Hubble image reveals the true shape of
MyCn18 to be an hourglass with an intricate pattern of "etchings" in its walls. This picture has been composed from three
separate images taken in the light of ionized nitrogen (represented by red), hydrogen (green), and doubly-ionized oxygen (blue).
The results are of great interest because they shed new light on the poorly understood ejection of stellar matter which
accompanies the slow death of Sun-like stars. In previous ground-based images, MyCn18 appears to be a pair of large outer
rings with a smaller central one, but the fine details cannot be seen.

According to one theory for the formation of planetary nebulae, the hourglass shape is produced by the expansion of a fast stellar
wind within a slowly expanding cloud which is more dense near its equator than near its poles. What appears as a bright elliptical
ring in the center, and at first sight might be mistaken for an equatorially dense region, is seen on closer inspection to be a potato
shaped structure with a symmetry axis dramatically different from that of the larger hourglass. The hot star which has been thought
to eject and illuminate the nebula, and therefore expected to lie at its center of symmetry, is clearly off center. Hence MyCn18,
as revealed by Hubble, does not fulfill some crucial theoretical expectations.

Hubble has also revealed other features in MyCn18 which are completely new and unexpected. For example, there is a pair of
intersecting elliptical rings in the central region which appear to be the rims of a smaller hourglass. There are the intricate patterns
of the etchings on the hourglass walls. The arc-like etchings could be the remnants of discrete shells ejected from the star when
it was younger (e.g. as seen in the Egg Nebula), flow instabilities, or could result from the action of a narrow beam of matter
impinging on the hourglass walls. An unseen companion star and accompanying gravitational effects may well be necessary in
order to explain the structure of MyCn18.


When Sun-like stars get old, they become cooler and redder, increasing their sizes and energy output tremendously: they are
called red giants. Most of the carbon (the basis of life) and particulate matter (crucial building blocks of solar systems like ours)
in the universe is manufactured and dispersed by red giant stars. When the red giant star has ejected all of its outer layers, the
ultraviolet radiation from the exposed hot stellar core makes the surrounding cloud of matter created during the red giant phase
glow: the object becomes a planetary nebula. A long-standing puzzle is how planetary nebulae acquire their complex shapes
and symmetries, since red giants and the gas/dust clouds surrounding them are mostly round. Hubble's ability to see very fine
structural details (usually blurred beyond recognition in ground-based images) enables us to look for clues to this puzzle.