A protostar is what forms before a star. A protostar is a bunch of gas that collapsed from a molecular cloud ( molecular cloud sometimes, called a stellar nursery, is a type of interstellar cloud.) This period of stellar evolution usually occurs for about 100,000 years. During this time, the gravity and pressure increase resulting in the protostar collapsing. The process of increasing pressure and gravity results in the pre-man-sequence star becoming a main-sequence star when hydrogen fusion begins.
T Tauri Stars
T Tauri Stars (pre-main-sequence) stars -form after protostars and before main-sequence stars. T Tauri stars are held together by gravitational pressure. However, the gravitational pressure is also the energy source because, at this stage, the star does not have enough pressure or heat in the core to begin nuclear fusion.
Main Sequence Stars
The majority of stars in our sky and the entire universe are main-sequence stars. According to space.com, about 90% of the stars in the universe are main-sequence. For example, our Sun is a main-sequence star. Main-sequence stars can vary in size, brightness, and mass. However, all of their cores are turning hydrogen into helium and creating tons of energy.
Red Giant Stars
Red Giants occur when all the hydrogen in a star is utilized & fusion stops. When a star runs out of hydrogen and fusion stops the shell of hydrogen around the core ignites and causes the star to surge in size. The Red Giant will be 100 times bigger than it was during its main-sequence stage. This stage of stellar evolution will only last a few hundred million years. Once the Red Giant runs out of helium to consume it becomes a white dwarf.
White Dwarf Stars
White dwarfs occur when a star consumes all the layers of helium in its core and runs out of other elements for fuel. When the fusion reaction stops, the stars collapse inward from their gravity. However, the stars still shine because of their immense heat. The white dwarf will take hundreds of billions of years to cool down completely. However, when the stars cool we still don't know what will happen.
Red Dwarf Stars
The mass of a red dwarf star is so low they have a lower temperature than stars similar to our sun.
Red dwarfs can keep the hydrogen fuel mixing in their cores. Therefore, red dwarfs can conserve their fuel much longer. This conservation of fuel results in astronomers predicting red dwarfs will burn up to 10 trillion years.