Hubble Finds an Hourglass Nebula Around a Dying Star
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.
BACKGROUND: PLANETARY NEBULAE
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.
Credits: Raghvendra Sahai and John Trauger (JPL), the WFPC2 science team,
and NASA
