Emission and Absorption nebula14.9 - Understand the principal stages and timescales of stellar evolution for stars of similar mass to the Sun, including:
a) emission and absorption nebula
14.10 - Understand the principal stages and timescales of stellar evolution for stars of much larger mass than the Sun, including:
a) emission and absorption nebulae
Emission nebulae (the plural of nebula) are clouds of high temperature gas.
Stars inside or near the nebulae warm the gas by ultraviolet radiation.
This causes electrons in the gas to ionize and then emit radiation which we see as light and radio waves. Hydrogen is the most common element and this produces a red colour.
Emission nebulae are sometimes known as H II Nebulae and are good indicators of newly born formed stars. The easiest to observe is the Orion Nebula (M42).
Absorption nebulae are usually called dark nebulae for the very reason that they appear as a dark patch against the starry background of the Milky Way.
They are gas and dust that block off the light from the stars behind them and are best seen set against the Milky Way or other nebulae.
A good example of this is the Coal Sack near the Southern Cross.
They were first discovered by William Hershel who described them as a hole in space.
Star Clusters are also areas of star formation.