Almost all planetary nebulae have a remarkably symmetric structure, often in the shape of a ring but sometimes more like an hourglass . The differences in shape are presumably mainly due to variations in the way the gas shell was initially expelled from the star. The speed of rotation of the star, its radiation pressure and its magnetic field are probably also very important, though as yet no adequate theory exists for the dynamics of planetary nebula shells.

There are two pieces of evidence to show that planetary nebulae are expanding away from their central stars. Firstly, photographs of the RING NEBULA taken 40 years apart show that it is slowly increasing its diameter. Secondly, the ‘Doppler shifts of the light emitted from different parts of the shell can be measured. In a symmetrical nebula, light comes partly from the side of the shell nearer the Earth, and partly from the side beyond the central star; if the shell is expanding there will be a difference in wavelength in the light from the front and back. This difference can be measured, and it shows that hi many cases the gas is expanding away from the star at a speed of about 20kms-1. Since a typical planetary nebula is about 0.5pc in diameter the shell must have been produced by the star some 104 years ago.

The variations of colour seen across the faces of some nebulae are due to changes in the degree of ionization hi different regions. For example, the green colour of the inner parts of the Ring Nebula is an indication that in these regions, oxygen is doubly ionized. In the outer regions, oxygen is only singly ionized and the dominant green spectral lines of 0++ are absent. The red colour here (and in many other nebulae) is due to a mixture of the Ha line of hydrogen and of a forbidden line of ionized nitrogen

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