Star

A **star** was an enormous sphere of immensely hot hydrogen and helium that underwent nuclear fusion to produce heat and light (in essence, it was a giant ball of plasma). Systems of planets would usually form around stars when the gas and dust around them became stable enough. Stars could come in all sorts of sizes, colors, and temperatures. Most planets were seen orbiting medium mass orange-yellow stars. Stars with one or more planets orbiting them were called **suns**.

It was estimated that there were four hundred billion stars in the Milky Way Galaxy. There were 7.1 billion habitable stars in the known galaxy, which made up about 3.2 billion habitable star systems. This means that many systems contained two or more stars. Red dwarfs accounted for approximately 70 percent of the galaxy's stars.

## Characteristics

The parts of a star located above its surface were collectively referred to as the atmosphere. They comprised two principal zones: the chromosphere and the corona. The chromosphere was a gaseous, relatively cool layer located immediately above the surface, and the corona was the tenuous, rarefied gaseous outermost part of the atmosphere.

## Types of stars

There are seven types of main sequence stars in the Milky Way:

* "**O**" stars were blue and hot, and had a lifespan of less than one million years. There were approximately one hundred million habitable O stars in the Milky Way. Example: Soell.
* "**B**" stars were white-blue and hot, and had a lifespan of ten million years. There were approximately one hundred million habitable B stars in the Milky Way. Example: Gorgon.
* "**A**" stars were white and hot, and had a lifespan of four hundred million to two billion years. There were approximately one hundred fifty million habitable A stars in the Milky Way. Example: Kahje.
* "**F**" stars were yellow-white and medium-temperature, and had a lifespan of four billion years. There were approximately two hundred million habitable F stars in the Milky Way. Example: Phoenix.
* "**G**" stars were yellow and medium-temperature, and had a lifespan of ten billion years. There were approximately three billion habitable G stars in the Milky Way. Example: Sol.
* "**K**" stars were orange and cool, and had a lifespan of sixty billion years. There were approximately 5.75 billion habitable K stars in the Milky Way. Example: Malgus.
* "**M**" stars were red and cool, and had a lifespan of approximately one hundred trillion years. They were also called red dwarfs. There were approximately seven hundred million habitable M stars in the Milky Way. Example: Ghibalb.

With O stars being the biggest in the sequence, the size decreased gradually to the smallest M stars. In addition to the main sequence stars, 10 percent of all stars in the Milky Way were non-main sequence, of which five hundred million were habitable. The non-main sequence stars include:

**Pre-main sequence** (smaller than M stars):

* Brown dwarf. Example: Mnemosyne.
* Protostar. Example: Anchora.

**Post main-sequence** (bigger than O stars):

* Red giant. Example: Dholen.
* Blue giant. Example: Nith.
* Red supergiant. Example: Betelgeuse.

**Compact stars**:

* White dwarf. Example: Alpha Draconis.
* Neutron star. Example: Geminga.
* Pulsar. Example: Ghedran.
* Black hole. Example: Sagittarius.