Current Issues in Planetary Nebula Studies
A long standing problem in the study of planetary nebulae is that in most cases, their distances are very poorly determined. For the nearest planetary nebulae, it is possible to determine distances by measuring their expansion parallax. High resolution observations taken several years apart will show the expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the Doppler shift will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula.
The issue of how such a diverse range of nebular shapes can be produced is a controversial topic. It is believed that interactions between material moving away from the star at different speeds gives rise to most observed shapes. However, some astronomers believe that double central stars must be responsible for the more complex and extreme planetary nebulae. Several planetary nebulae have been shown to contain strong magnetic fields, something which has been hypothesized by Grigor Gurzadyan in the 1960s. Magnetic interactions with ionized gas could be responsible for shaping some planetary nebulae.
There are two methods of determining metal abundances in nebulae. These rely on different types of spectral lines—recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between the results derived from the two methods. Some astronomers explain this by presence of small temperature fluctuations within planetary nebulae; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesize the existence of cold knots containing very little hydrogen to explain the observations. However, no such knots have yet been observed.
Read more about this topic: Planetary Nebula
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