The Nebula stage is like the pre-stage of the beginning of a star. A Nebula's life was a cloud of scattered dust and gas in space. A Nebula wasn't really close to being a star yet. Nebulas are lonely clouds in space just waiting to become a star.
After the Nebula pre-stage, comes the Protostar pre-stage. A protostar can reach temperatures up to 2000 to 3000 K. With these temperatures, the protostar would be able to produce a glow of dull red with most of the energy infrared. A protostar lives a very hot life.
Main Sequence Star (High Mass)
This stage is the Main Sequence Star stage. The star above is the high mass main sequence star. During this time, the high mass main sequence star starts to produce more nuclear burning, which makes the elements heavier. Also, high mass stars start to use their nuclear fuel rapidly, reducing their lives.
Red Giant Star
This stage is called the Red Giant Star stage. During this stage, the red giant star is continuing to lose its hydrogen fuel from its center. Surprisingly, the red giant star's surface temperature is lower than you would think. You would probably think the star's temperature would be about the same temperature as the sun, but its not. The red giant star is about 2000 to 3000 C.
The picture you see above is called a Supernova. A supernova is an explosion that outshines the entire galaxy. During the explosion, the supernova radiates as much heat as the sun