A nebula is a combination of dust particles and gases in space that form a cloud. They are the gaseous and dusty material expelled by an old star just before death. (4) A far better name for these objects would be "ejection nebulae". The term nebula comes from the Latin word for cloud. In Earlier days, astronomers also used the term for distant galaxies outside the earth's galaxy, the Milky Way. These galaxies, called extragalactic nebulae, looked like hazy patches of light among the stars. But modern telescopes showed that extragalactic nebulae are actually systems of stars similar to the Milky Way. Today, the term nebula is used only for the clouds of dust and gases in the Milky Way and other galaxies. These masses are classified into two general types: diffuse nebulae and planetary nebulae. Both types are also called gaseous nebulae. (2).
One of these is created at the beginning of a stars life, when a star first forms from a cloud a gas and dust. A slowly collapsing cloud of hydrogen and helium (and sometimes heavier elements) starts to slowly heat due to gravity. During this process, the object is rotating, and forming a disk around it due to centrifugal force. When the star gets hot enough for nuclear fusion to begin, the star becomes a huge, continually exploding hydrogen bomb. The star then lives most of its life balanced between the nuclear forces pushing it outwards, and gravitation trying to collapse it inwards. It all seems simple enough. In order for such collapse to start it may be triggered by a nearby nova or supernova, or perhaps by a passing object. (3) The two types of nebula will be discussed .
DIFFUSE NEBULAE.
Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain enough dust and gases to form as many as 100,000 stars the size of the sun. (4) A diffuse nebula usually occurs near an extremely hot, bright star. The strong ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to produce light.
