Nebula
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. 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 extra galactic nebulae, looked like hazy patches of light among the stars. But modern telescopes showed that extra galactic 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. One of these is created at the beginning of a star’s 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.
