Star classifications

A star is a body massive enough to trigger nuclear reactions at its core.

## Luminosity

There are six categories that stars are divided into according to luminosity.

* 1a — most luminous supergiants
* 1b — less luminous supergiants
* II — luminous giants
* III — normal giants
* IV — subgiants
* V — main sequence and dwarf stars

## Spectral class

There are seven major classes of stars, forming a continuous band of types from O through M.

O B A F G K M

These are divided into numbered subclasses. For example, a star of spectral class "A" could be any one of the following:

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9

Stars are the 0 end of this band are hotter (around 50,000 degrees K); bluer in color and more massive; those at the other end are cooler (around 2,000 degrees K), the redder in color and less massive. A conventional code for a color is:

* O — Violet = White
* B — Blue = White
* A — White
* F — Yellow = White
* G — Yellow
* K — Orange
* M — Red

While a 'giant' star may have a radius of up to 1,000 times of that of Sol and be up to 100,000 times as luminous, most of the stars are in the 'main sequence' portions of their lifetimes and have values near the typical main sequence ones for their type. Sol, Earth's sun, is a type G. Its spectrum as filtered by Earth's atmosphere, is the basis of standard illumination in Human quarters.

## Types of Stars

### Lazarus star

A super nova remnant which, instead of being forced inward into neutron-star mode, survives as a normal star. After expansion into red giant phase, Lazarus stars collapse and undergo supernova for a second time.

### Neutron star

Usually type B-0 and measures only a few kilometres in diametre. An early main sequence star that has completed the nuclear burning processes often explodes. The reactive force of the explosion and the star's self-gravitation eject shell electrons (as in a white dwarf) and nuclear positrons. This leaves a neutronium core, possibly covered by a thin degenerate matter shell.

### Population I

Stars are old stars well down the main sequence (class F, G, K, and M stars) and short on heavier elements. Planetary systems accompanying Population I stars primarily consist of gas giants without accompanying satellites.

### Population 2

Stars are younger stars showing traces of heavier elements, hydrogen, and helium. Planetary systems accompanying Population 2 stars include gas giants, stony worlds, satellite companions and planetoid and comet shells.

### Red Giant Star

The red giant phase is common in the evolution of many less massive stars. When core hydrogen is exhausted, gravitational collapse ignites hydrogen shell burning outside the core. The star's envelope expands far beyond the photosphere limit. The star's atmosphere is relatively cool.

### Runaway Star

A star with a velocity significantly different from its neighboring stars.

### Supernova

When a massive young star exhausts its core hydrogen it undergoes second-stage gravitational collapse. The resulting core temperature increase leads to runaway nuclear burning of helium, carbon, nitrogen and an explosion that blasts the star's outer layer into space. Supernova explosions are the major source of metals and other galactic elements.

### T Tauri Star

One manifestation of a star in formation undergoing initial nuclear burning.

## Dwarf stars

'Dwarf' is a category comprising various small and dim energy-radiating or formerly energy-radiating objects.

### Black Dwarf

An object of stellar mass that has undergone gravitational collapse, reaching minimum potential energy and maximum entropy. Black dwarfs are sub-planetary size and do not radiate.

### Brown Dwarf

A gaseous body producing much more energy through self-gravitation than it receives from the ambient medium, but which is not massive enough to initiate internal fusion reaction and, therefore, not truly a star. Brown dwarfs hot enough to produce visible light (substellar objects) are listed as Class S planets. They are both also known as supergiant gas planets. Some giant gas planets (Class A) may produce slightly more energy than they receive, but they are not generally considered to be brown dwarfs.

### Red Dwarf

Main sequence star of type M. The vast majority of stars in the galaxy are red dwarfs: small, dim and long lived.

### White Dwarfs

Primarily degenerate matter, this main sequence star, usually of type G-late A, has completed nuclear burning processes and has collapsed into a configuration roughly the size of a small planet. White dwarfs radiate at various levels of intensity through self-gravitational collapse. Nuclear burning occurs only on the surface through accretion of unburned matter from other sources; in such cases, nuclear ignition can regularly occur and is the source of the "recurrent nova" effect. The spectral class of white dwarf stars is usually prefixed with a 'D'.