The Stellar Classification System was created by the Kerbal Association for Space Progress and Research (KASPR) as a way to officially categorize different types of stellar mass bodies observed in the Kermes Cluster. It is split into three domains: Stars, Stellar Remnants, and Exotic Objects.
In the spectral classification system, stars are ordered and classified by emission spectrum. From lowest (dimmest and most red-colored) to highest (bright and blue). Classification groups follow this order: M, K, G, F, A, B, O. Within each group, stars are separated by a number between 0 and 9, inclusive. More massive stars have smaller numbers, but only within each spectral group (G2 is hotter and more massive than G8, K9 is hotter than M0).
- M class stars are generally red and dim and usually invisible to the naked eye from Kerbin. They live for hundreds of billions, if not trillions of years and have extremely close in habitable zones. Their emission spectrums peak in the infrared and red part of the electromagnetic spectrum. They are also characterized to behave erratically during the first billion years of their lives, varying their brightness by a factor of two in mere minutes. They are also usually flare stars. This means that life around M dwarfs have to adapt to constant changes in brightness and large amounts of solar activity, especially if the star is young. Nevertheless, life still managed to get a foothold in red dwarf solar systems. (Stars include: Kernim, Kelnis, Kerilim, Kerman's Star, Nessus II, Nessus IV)
- K class stars are smaller than Kerbol, and more orange in color (which is why they are called orange dwarfs). Most K stars are not bright enough to be seen with the naked eye from Kerbin. Small K stars can live up to 50 billion years, and larger K (main sequence) stars may host planets that are not tidally locked. K stars have emission spectrums that peak in the orange and yellow areas of the electromagnetic spectrum, depending on the type of K star. These stars also emit less ionizing radiation than Kerbol, making these stars some of the best candidates for life as we know it. (Stars include: Kelin, Kiro, Korlon, Kilise, Nessus III)
- G class stars are generally the same size as Kerbol, although most of them are smaller (Kerbol is a G2 star, which is relatively large for a G-type). These stars are also generally referred to as yellow dwarf stars, although the spectrum they emit is white. G-type emission spectrums generally peak in the green portion of the electromagnetic spectrum. These stars are also generally regarded as prime candidates for life, as they produce a relatively low amount of radiation and have farther out (and wider) habitable zones than smaller star types. Late G-types are some of the best stars for hosting life (similar to Early K-types), as they emit relatively low levels of ionizing radiation and remain in the main sequence for a long time (up to 20 billion years, 10 billion for Kerbol). (Stars include: Kerbol, Kirb, Kerbos)
- F class stars are referred to as yellow-white stars, although they emit white (and slightly blue for larger F types) light in reality. These stars emit more (and harsher) light than Kerbol, with the emission spectrum peaking in the blue (and violet for larger stars) range of light and with a significant fraction of the spectrum being composed of UV rays. These stars also live for less time than Kerbol: about 3-7 billion years. However, F-type stars are still manageable in terms of UV and lifespan, so these stars are still good candidates for hosting life. (Stars include: Kelaris, Kirlim, Nessus I)
- A class stars are bluish-white in color, and they are the most abundant stars that are visible to the naked eye from Kerbin. These stars typically live for 1-4 billion years, barely enough time for life to get started in some cases. The emission spectrum given by these stars also peak firmly in ultraviolet light. Despite the large amount of radiation and short lifespans, kerbal scientists think life may have a chance of surviving around small A-type star, as they have (barely) tolerable UV emission and lifespans. After all, life usually finds a way. (Stars include: Kerolon, Kerlanes)
- B class stars are pure blue in color, and live for a staggeringly short 100 million to 1 billion years. They also emit a deadly amount of radiation, as the emission spectrum is almost entirely UV and violet light. Needless to say, life probably has a hard time surviving around stars like these. Some kerbal scientists remain optimistic though.
- O class stars are the largest and most dangerous stars in the universe. They have luminosities sometimes millions of times more than Kerbol, and live for less than 100 million years. Most planet forming materials probably get stripped away into deep space by intense solar winds. Even if planetary formation was possible, they most likely wouldn't have enough time to fully grow into true planets. For this reason, many scientists think planets around O-type stars are short-lived and rare. Plus, there are too many harmful X-rays and UV rays being emitted from these stars. Therefore, virtually no kerbal scientists believe O-type stars are capable of supporting life. (Stars include: Kerelus)
Luminosity classes, otherwise known as the Kerkes spectral classification system, is a two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity. Class type is denoted with numerals.
- 0: Hypergiants or extremely luminous supergiants
- I: Supergiants
- Ia: Luminous supergiants
- lab: Intermediate-size luminous supergiants
- lb: Less luminous supergiants
- II: Bright Giants
- III: Normal Giants
- IV: Subgiants
- V: Main Sequence Stars (dwarfs)
- VI: Subdwarfs
- VII: White dwarfs. See "D Class Stellar Remnants".
Stellar classification uses a combination of the two methods listed above to denote star types.
- Kerbol: G2 V
- Kelin: K1 V
- Kelaris: F7 IV
- Kerbos: G8 V
- Kelnis: M8 V
- Z class stellar remnants are black holes; stellar remains that are so dense that the gravity at their surface is so strong that even light cannot escape. (Includes: Karkua)
- N class stellar remnants are known by the more common name neutron stars. These are the hot, dense remains of supermassive stars that weren't quite massive enough to form black holes. Neutron stars are made primarily of neutrons.
- Q class stellar remnants are known as pulsars. These are basically rapidly spinning neutron stars which emit pulses of deadly radiation in all directions. (Includes: Kereksus)
- D class stellar remnants are known as white dwarfs. These are the typical remains of most middle to low mass stars; the glowing hot remains of their cores. (Includes: Kirimo)
- B class stellar remnants are black dwarfs. These are hypothesized remains of white dwarfs after they have cooled down. However, there are currently none in the known universe.
- L class brown dwarfs are large and hot brown dwarfs. These emit a very red glow, and can sustain fusion for short bursts of time. Certain extremely small M-stars may emit an L type glow, and would also be classified in this category.
- T class brown dwarfs are large brown dwarfs with prominent methane bands in their emission spectra. These also glow a dark red. (Includes: Kormin)
- Y class brown dwarfs are smaller brown dwarfs that have never sustained significant fusion reactions or were once L or T class but cooled off over time.
The following objects are celestial bodies that kerbal scientists are still scratching their heads over. There was no way to categorize them so they were put into their own category.
- The All
- W class object (hypothetical)
- This category includes exotic distortions in spacetime such as wormholes: hypothetical extrapolations of Keinstein's Theory of Specific Relativity which kerbal scientists predict are theoretically possible and may exist in the Kerbal universe.
- I class object (hypothetical)
- This is a category reserved for the possible discovery of large scale artificial constructs in space. The Creator is a suspected member of this class.
- C class stars are an exotic type of star more commonly known as Carbon Stars.
- E class stars are known as "Exotic Stars". They are a group that include hypothetical stars such as Strange, Quark, Electroweak, Preon, Boson, and Planck Stars.
- Larger, brighter stars tend to have much wider/thicker habitable zones. For example, the distance between the inner and outer edges of (A-type) Kerolon's habitable zone is more than 2 KAUs across, while for (M-type) Kernim, it's only 0.03 KAUs. This means larger stars can fit more planets into its habitable zone at once.
- However, larger stars like Kerolon change in brightness much more than small stars like Kernim over time. As Kerolon ages, it's habitable zone will move out rapidly. It's been estimated that Kerolon's continuous habitable zone (CHZ: where a planet will remain habitable for the entire main sequence of the star) is probably nonexistent, as the habitable zone will have moved so much in a billion years that no planets currently in the habitable zone will still remain that way. Kernim's habitable zone, on the other hand, will not move very much throughout its main sequence, and so its CHZ will be about the same as its current habitable zone.
- The best balance of habitable zone size and long-term stability lies in high K and low G type stars, as their brightness does not increase dramatically during their main sequence lifespan (and thus have wide CHZ boundaries) and also start off with relatively wide habitable zones.