The previous section discussed some of the physical processes taking place in NGC 2024 which allow us to gain an insight into the type of material in the nebula. In effect, the colour of the light provides a fingerprint of the elements that are present there.
 
 

The radiation-producing mechanism is driven by the hot stars that lie within the nebula itself. The association of hot, young stars and a gaseous nebula is not a fortuitous coincidence. Such nebulae are the birthplaces of stars themselves. You can find out more about the processes involved from a wide variety of astronomical reference sources. I will only provide a brief outline here.
 
 

The density of atoms in interstellar space is very low, typically 1 atom per cubic centimetre. However, if the gas cloud is compressed, by the blast wave from a nearby supernova, for example, or from the collision and coalescence of nearby nebulae, the atomic density can increase sharply.
 
 

Whatever the cause, compression of the gas begins and the nebula becomes hot and unstable, fragmenting and forming clumps of material. Although the initial temperature of the fragments may be less than 100K, over thousands of years collapse under gravity causes an increase in core temperature and protostars are formed. Eventually, when the temperature reaches a few million K, fusion reactions occur and the resulting outward radiation pressure is sufficient to balance gravity. An equilibrium is established.
 
 

The fragmentation process determines the size that the star will eventually become, and therefore its luminosity/temperature (there is a strong correlation between mass and luminosity/temperature). Most nebulae produce a range of stars from small, red dwarf stars (with about 0.1 times the mass of the Sun) to giant blue stars (with greater than ten solar masses). There are usually many more smaller ones than larger ones. However, the larger, hotter stars exert a tremendous influence on the gas around them due to the range of photon energies they produce (…the Black body radiation business again !!).
 
 

The H II regions in NGC 2024 are easily observed…a consequence of these hot stars. But the density of this nebula has made it difficult to "see" much below the surface. Astronomers using infra-red telescopes, such as the UKIRT on Mauna Kea in Hawaii, have been probing NGC 2024. IR wavelengths are much less affected (by intervening dust particles within the nebula) than visible light and may eventually lead to the identification of young protostars within this nebula. A similar situation existed 20 years ago with the Orion nebula: many young, developing stars have now been identified within the nebula using IR techniques.