Nebulæ are gas-clouds. They are of very different types. Bright nebulæ, such as M 42 in the Sword of Orion, are stellar nurseries, while planetary nebulæ are dying stars, and there are also supernova remnants, such as the Crab Nebula in Taurus. The galaxies were once called ‘spiral nebulæ’, but this name for them is now obsolete.

PLANETARY NEBULÆ

Planetary nebulæ are very inappropriately named. They were so called by William Herschel because their pale, often greenish discs make them look superficially not unlike Uranus or Neptune when seen through a small telescope, but they are not true nebulæ, and have absolutely nothing to do with planets.

A proto-planetary nebula marks the brief period in a star's history when it has left the asymptotic giant branch of the Hertzsprung–Russell (HR) diagram but has not reached the planetary nebula stage – it has been ‘caught in the act’, so to speak. These nebulæ are faint and rare; a selected list is given in Table 24.1.

(Do not confuse a proto-planetary nebula with a proto-planetary disc. ‘Preplanetary nebula’ has been proposed as an alternative name, and this would be a distinct improvement.)

The Red Rectangle is one of the most notable of the proto-planetaries. It takes the form of a symmetric, bipolar nebula with X-shaped spikes; the central star is a close binary, apparently surrounded by a thick dust torus which forces the otherwise spherical outflow into tip-touching cone shapes.