First, What Is a Star?
A star is a luminous sphere of plasma held together by its own gravity. The first example of a star that we can see is our Sun. Stars consist mostly of gas and have a nuclear reaction happening in their core. The heat and light that stars produce is the product of this reaction.
Second, How to Classify Stars
Stars are classified based on their mass and temperature. Each classification has its own letter: O, B, A, F, G, K, or M. O’s being the most massive and hottest. M’s being the smallest and coolest.
Above, our Sun is on the far right and is compared to other known stars. Our Sun is a “G” type star.
Finally, The Life Cycle of Stars
The lifetime of a star depends on how big it is. The bigger it is the quicker it burns up and reversely the smaller it is the slower it burns up. The size of the star determines which cycle the star follows.