Planetary Nebulae represent the late evolutionary stage of low to intermediate mass stars. As these stars reach the final stage of their existence they enter the AGB (asymptotic giant branch) phase. As early AGB stars they begin to lose mass through dense but slow winds. This phase is followed by more profound mass lose through tenuous fast winds or superwind phase. The dynamic interaction between fast and slow winds ultimately forms the complex shell structure of Planetary Nebulae.
M97, more popularly known as the "Owl Nebula" is an older planetary nebula (PN) with a circular morphology and a bland inner structure. It is one of about 1600 planetary nebulae discovered in the Milky Way. The Milky Way has an estimated population of about 10,000 planetary nebulae. The low number is due to the brief time they exist, less than 50,000 years. M97 is at a fairly advanced stage as the superwind from the central star has long since ceased. M97 has a triple shell structure consisting of a round double shell comprising the main optical nebula and a faint bow-shaped outer shell which is very faint. The outer halo formed from material ejected thousands of years ago during the dying stars red giant phase. It continues to interact with the surrounding interstellar medium as the PN moves through space. The central star is a hot dead cinder of about 0.6 solar masses which produces abundant radiation from its 110,000 degree surface temperature.
Astronomers have had increasing success at building three dimensional models of planetary nebulae from two dimensional data. The models are based however on three assumptions: 1) material in the nebula moves exclusively in radial directions 2) each shell is ejected at different time epochs 3) each shell is composed of sub-shells that expand with velocities proportional to their distance from the center. The double shell of the main nebula is about 1.3 light years in diameter and has an expansion velocity of 40 kilometers/second. The inner shell is slightly elongated and the outer shell is round. The outer halo is bow shaped and has no measurable expansion velocity.
There are two distinguishing features of the Owl Nebula. The first is the presence of a central bipolar cavity excavated by the superwind of the central star. The second peculiar feature is the lack of a bright rim. The superwind that carved out the central cavity has since ceased allowing nebula material to backfill the cavity and smear out any bright rim that previously existed. The higher density of the material along the rim of the cavity is responsible for producing the forehead and beak of the owl. The bipolar cavity forms the characteristic eyes of the owl.
A reasonable conjecture regarding the evolutionary
history of M97 begins in the early ABG phase of the dying star.
The first event was the early slow wind which plowed into the
interstellar medium forming the outer halo. During later stages
of the ABG phase high mass loss occurred in the superwind phase
which formed the main body of the planetary nebula.