Star

A star was an enormous sphere that was primarily composed of super-heated hydrogen and helium that underwent nuclear fusion and produced heat and light. Essentially, stars were composed of plasma. Planetary systems would often form around stars when the gas and dust achieved stability. Stars were often classified by their size, color, and temperature. Most planets orbited yellow-orange stars of medium mass, which were termed as suns.

Types of stars
Stars were classified by sequence, which would include main sequence, pre-main sequence, post-main sequence, and compact stars. The main sequence stars were further classified into seven types:


 * "O"&mdash;Stars in this category were blue and hot, and had a lifespan of less than one million years.
 * "B"&mdash;Stars in this category were white-blue and hot, and had a lifespan of ten million years.
 * "A"&mdash;Stars in this category were white and hot, and had a lifespan of four hundred million to two billion years.
 * "F"&mdash;Stars in this category were yellow-white and of medium-temperature, and had a lifespan of four billion years.
 * "G"&mdash;Stars in this category were yellow and of medium-temperature, and had a lifespan of ten billion years.
 * "K"&mdash;Stars in this category were orange and cool, and had a lifespan of sixty billion years.
 * "M"&mdash;Stars in this category were red and cool, and had a lifespan of approximately one hundred trillion years.

"O" class main sequence stars were the largest of the stars, and the size would gradually decrease to the smallest "M" class stars.

Pre-main sequence stars were smaller than "M" class stars and included brown dwarfs and protostars. Post-main sequence stars were larger than "O" class stars and included red giants, blue giants, and red supergiants. Compact stars included white dwarfs, black dwarfs, neutron stars, and black holes.