Star Birth

Demonstrate an understanding that emission nebulae, absorption nebulae and open clusters are associated with the birth of stars

A star forms when gas and dust from a nebula condense. It produces an accretion disc that spins rapidly around it, gathering more material into it.

Eventually it will have such a high temperature that a nuclear reaction takes place.

If there is not enough matter for nuclear reactions to take place, the body will become a brown dwarf.

Excess material is blown away from the force of the initial nuclear reaction and later forms planets and other bodies around the stars.

The star now burns hydrogen and is on the main sequence of stars on the HR diagram.


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.


Drag & Drop
  • Describe the formation of the birth of a star.
  • Where do stars form?
  • Describe these areas.