Hourglass Nebula (MyCn 18)
The Hourglass Nebula (MyCn 18) is a young planetary nebula that about 8,000
light-years away in the constellation Musca,
1.5° SSE of NGC 5189. According to one theory, the hourglass shape of
this object 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 (110,000
K), Wolf-Rayet (WR9) star which has ejected and illuminates the nebula,
and therefore might be supposed to lie at its center of symmetry, is clearly
off center. Hence MyCn 18, as revealed by the Hubble Space Telescope, does
not fulfill some crucial theoretical expectations.
|The Hourglass Nebula seen by the Hubble Space Telescope.
Image credit: R. Sahai and J. Trauger (JPL), the WFPC2 science team,
Hubble has also revealed other features in MyCn 18 that are 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, (as seen, for example, 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 MyCn 18.
Note: part of the Lagoon Nebula (M6,
NGC 6523) is also known as the Hourglass Nebula.
||~8,000 light-years (2,400 pc)
||R.A. 13h 39m 30s,
Dec. 67° 22' 06"
||PK 307-4.1, VV66, SA2-96
AND STAR CLUSTERS