Nebula is of various types. Some Nebula are formed as the result of the supernova explosion in a star. Many nebula are also formed by the gravitational collapse of the different gases and interstellar dusts.
Different types of Nebula are :
Nebula are the birth place of any star. Hydrogen gas clouds collapse under its own gravity. Molecules collide with each other and gets heated up to fusion temperature ( 10^7 K ). As the fusion starts a new star born. If there are remaining dusts and rocks then they form planets which revolve around the star at its center, thus forming a solar system.
Stars are the huge ball of Hydrogen gas cloud. That cloud is known as Nebula. Gas and dust particles collapse under its own gravity and forms a Protostar. These young stars further collapse to form Star.
Hydrogen molecules collide with each other and gets heated up to few crore degree celsius. Now Nuclear Fusion starts and Hydrogen molecules combine to form Helium. As the star runs out of Hydrogen then Helium comes into play. Helium combines to form Lithium further Lithium to higher elements. This process continues till the formation of Iron.
Stars expand as they grow old. As the core runs out of Hydrogen and then Helium, the core contracts and the outer layers expands, cool and become less bright. This is a Red Giant or a Red Super Giant (depending on the initial mass of the star).
It will eventually collapse and explode. Its fate is determined by the original mass of the star relative to the mass of the Sun that it will become either a Black Dwarf, Neutron Star, or Black Hole. The picture above can tell you more about the topic.
