Planetary nebulae

M57 (Ring Nebula)

M57 (Ring Nebula), an iconic planetary nebula in the northern celestial hemisphere     [Source]

A planetary nebula is the quiet death of a low mass star. It is what our own Sun will eventually become. Not all stars are massive enough to fuse into heavier elements—including eventually iron—to end their lives as a massive supernova. Instead their cores become degenerate (a collection of non-interacting particles) before reaching the required ignition temperature for additional nuclear fusion into heavier elements. The fuel-exhausted star’s outer gas layers are then ejected into space— where they can become ionized as a planetary nebula—and all that remains is the degenerate core: a white dwarf star.

The radiation emitted by the ionized gas of a planetary nebula features a distinctive spectrum, characterized by two especially bright emission lines at 500.7 nm and 495.9 nm, both in the green part of the visual spectrum. First noticed in 1864, these were initially thought to be indicative of a new element—later referred to as “nebulium”—as the spectral lines did not match any known elements at the time. However, this was found not to be the case six decades later, when they were instead revealed to be the forbidden lines of doubly ionized oxygen (O-III), which can only materialize in conditions of extremely low density (incapable of simulation in laboratories on Earth).

The spectrum of M57

The spectrum of M57, exemplifying the emission lines characteristic of a planetary nebula     [Source]

Planetary nebulae are some of the most diverse astronomical objects. For this reason, many observers find planetary nebulae to be their favorite kind of deep sky object.


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