What Is a Planetary Nebula?

Planetary nebulae are essentially dust and gas shells thrown out by some dying stars, and their diameter is generally about one light year. Consisting of matter that is ten times less massive than the Sun at the end of its evolution, the core's hydrogen fuel is exhausted and then continuously ejects.

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The distance of planetary nebulae is often difficult to measure ^[1] . Closer planetary nebulae can be measured by measuring their expansion speed. High-resolution observations taken several years apart can show the expansion of the nebula in the vertical line of sight, and the Doppler shift of the observed spectrum can be used to determine the speed in the line of sight. Comparing the angle of expansion and the speed of expansion, we can reveal the distance to the nebula.

The question is how to produce a wide variety of planetary nebulae in various shapes, which is a controversial topic. Theoretically, the matter leaving the star at different speeds can interact with each other to produce the observed shapes. However, some astronomers have assumed that more complex and more extreme planetary nebulae should be caused by nearby binaries. Several exhibit strong magnetic fields, and their interaction with ionized gases can explain the shape of some planetary nebulae.

There are two main methods for measuring the abundance of metals in the nebula, both relying on composite lines and spectral lines excited by collisions. However, the results of these two methods sometimes differ greatly. This may be explained by the existence of some temperature disturbances inside the planetary nebula; however, some too large differences cannot be explained by temperature. Some assumptions exist that very small hydrogen condensation nodes exist to explain the observed phenomena. However, no such node has been observed so far.