a more in depth description:
as gasses like hydrogen start to gather, it creates more mass, therefore more gravitational attraction. As the gasses start to collect to considerably large amounts, the gas at the center of the mass starts to compress. As the gas keeps growing, the immense pressure on the core grows and grows, raising the temperature to extreme amounts. As this happens the star starts to undergo a process called nuclear fusion. this process occurs when immense pressure and heat cause the electrons to escape from their nuclei and flow around freely. this is what the fourth state of matter is. this type of matter is called plasma. this alone creates incredible amounts of heat and energy. The main part of the fusion process though is that the nuclei in which the electrons escaped from begin to merge, creating heavier nuclei, therefore creating heavier and denser nuclei. as the nuclei combine they also form denser elements, such as helium (fist merge of nuclei) all the way to eventually iron (last merge before nuclei is at its heaviest. This entire process takes billions of years to go from helium to the extremely dense iron, and stars usually only reach iron if they are big enough (a lot larger than our sun) so that the pressure will be able to keep fusing nuclei. if they dont reach iron, at the end of their life, the fuel from the inside will be used up and the core has to compensate for the gravitational force of the outer star pushing in. this means the star will grow as the outward force becomes greater than the gravitational force. the star will stay like this for a while before finally using up all the fuel and slowly dying off and shrinking. the shrinking star (or dying star) is known as a white dwarf
that is the long answer