Stars

Sol, the primary star of the Sol system, is classified as a G2V star: a main-sequence yellow dwarf.

— Example stellar classification

Stars are the slow-motion nuclear fireballs that fleck the inky darkness of space and give light to the cosmos. Their presence allows for life to exist in its many forms, fuels photovoltaic cells, and lends color to the universe. The official definition of a star is a celestial body that is large enough to be spheroid in shape and fuse elements within it at some point in its life cycle.
Stars are just as varied as the planets that orbit them, especially when considering the non-fusing remnants they leave behind after their deaths. Stars and their descendants are generally divided into four categories: protostars (prefusors), substars (subfusors), true stars (fusors), and stellar remnants (postfusors).
Stars are not static objects. They are dynamic; growing, changing, and evolving in a myriad of ways over vast timescales. They exemplify the phrase "dust to dust"; beginning and ending their eons-long lifespans as diffuse nebulae of gas and dust. The stellar lifecycle is long and complex, and the precise pathway of a star's evolution is dependent on many factors.

Contents

Stellar Evolution
Protostars
Substars
> Substar Types

• Violet Substars
• Maroon Substars
• Ruby Substars

Stars
> Star Types

• Red Stars
• Orange Stars
• Yellow Stars
• Pale Stars
• Azure Stars
• Cerulean Stars
• Blue Stars
• Wolf-Rayet Stars
• Carbon Stars

Stellar Remnants
> Remnant Types

• Stable Postfusors
• Metastable Postfusors
• Unstable Postfusors

Stellar Anomalies
> Stellar Anomaly Types

• Hybrid Stars
• Gymnostars
• White Holes

Protostellar objects with more than about 0.09 M_☉ evolve into main sequence stars; those below that mass are not large enough to fuse hydrogen and instead become substars. Stars with a mass below about 8 M_☉ end their lives as red giants, which slowly expel their no-longer-fusing outer layers into space to form spherical or ring-shaped post-stellar nebulae (also called "planetary nebulae," though that is a misnomer). What is left behind is a white dwarf.
Stars which mass between 0.09 and 0.25 M_☉ are fully convective, and thus expected to continue their main sequence fusion for trillions of years until they are unable to continue, at which point they will expel their outermost layer and transition into white dwarfs.
Stars above 8 M_☉ are large enough to end their lives as supernovae, collapsing in on themselves before exploding outward from the rebound shock with more than three times the energy produced across the entire main-sequence lifespan of Sol. Vast amounts of plasma, high-energy particles, and light are expelled into space in all directions over the course of a few days, and over time the plasma will lose energy and become a supernova remnant nebula. The remaining collapsed core of the star can take a few different forms, depending on the initial mass of the star: a neutron star, a more exotic stellar remnant, or even a black hole.

✶ Protostars

Protostars are a rare class of objects: main-sequence stars that are still in the process of forming and have not started fusing protons yet (though in later stages they may fuse deuterium with protons to create helium). Because of this, they are also referred to as "prefusors." These objects are only found in star-forming regions, the closest of which is the Corona Australis cloud complex at a Sol-relative distance of around 430 lightyears. Protostars are always found at the hearts of protoplanetary debris disks (also called "proplyds").
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Protostellar Spectral Classification

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Protostar Class	Peak Radiation Band	Wavelength Range	Age Bracket
I	Submillimeter	1 mm - 100 μm	104 years
II	Far-infrared (I)	100 μm - 30 μm	105 years
III	Near-infrared (II)	30 μm - 700 nm	106 years
IV	Visible (III)	700 μm - 400 μm	107 years

Proplyd in NGC 1333 by R. Hurt

Protostars transition to true stars after a relatively short period of time; a few million years at the most. During their time as prefusors, these objects emit below the visible spectrum and are classified according to the electromagnetic band in which their emission peaks. The peak emission also correlates to the age of the object: prefusors emit steadily shorter wavelengths as they grow and heat up. Protostars transition from radio emission to microwaves, then into infrared, and eventually are energetic enough to emit visible light.
Once they reach Class IV and begin contracting toward critical density, protostars of less than 8 M_☉ transition to one of two late-protostar stages for around 10 million years. Protostars less than 2 M_☉ become Class IVa, commonly called Hayashi (or T Tauri) stars, and are precursors to spectral types M, K, G, and F. Protostars of 2 to 8 M_☉ become Class IVb, commonly called Herbig stars, and are precursors to spectral types A and lower-end B. Protostars of more than 8 M_☉ will transition directly to O-type or higher-end B-type main sequence stars, as the gravitational pressure and resultant friction heating is intense enough to kickstart hydrogen fusion in their cores quite early.

✪ Substars

Luhman 16 A by Doug Marshall

Substars, also commonly called "brown dwarfs," are objects which are large enough to fuse deuterium (and occasionally lithium) in their cores but not large enough to sustain hydrogen-helium fusion. Because of this, they are referred to academically as "subfusors." The upper bound on substar mass is approximately 0.08-0.09 standard solar masses, above which the pressure at their cores is sufficient to fuse lithium and catalyze the proton-proton chain, making them true stars.
Substars emit their own heat, which is why they are subclassified in a manner similar to stars rather than planets and planetoids. Like stars, each spectral subclass is subdivided into more specific numeric groupings (0-9) to better represent the spectrum on which these objects exist. However, since subfusors do not "die" in the same way as fusors, they are not classified by stage or mass in the same way. As of the 2950 CE stellar census, there are 87 substars within thirty-two lightyears of Sol.

Substellar Spectral Classification

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Chromatic Type	MK-Harvard Type	Temperature (K)	Peak Emission (μm)	Mass (M☉)	Radius (M☉)
Violet	Y	< 800	> 3.6	< 0.03	< 0.1
Maroon	T	800 - 1600	3.6 - 1.8	0.03 - 0.06	0.1 - 0.2
Ruby	L	1600 - 2400	1.8 - 1.2	0.06 - 0.08	0.2 - 0.3

✪ Violet

Violet (Y-type) substars blur the line between planet and substar. They are the coolest and dimmest substars, having temperatures of less than 800 K and near-negligible luminosities. The term "violet" is misleading: Y-types emit no ultraviolet or even visible light; only faint infrared. Their atmospheres are cool enough to form ammonia clouds. 20 of the substars within thirty-two lightyears of Sol are Y-type.

✪ Maroon

Maroon (T-type) substars emit almost entirely in the infrared spectrum, though enough visible light is emitted to give them a dull reddish-purple glow. Their atmospheres are cool enough to form methane clouds. T-type substars appear to be the most common type of substellar body in the known universe: 47 T-types lie within thirty-two lightyears of Sol, several of which orbit true stars.

✪ Ruby

Ruby (L-type) substars are, as their name implies, very dark red in color, emitting most strongly in the infrared spectrum. Their atmospheres are cool enough to allow metal hydrides and alkali metals to exist in vapor form. Though they are abundant and do emit in the visible spectrum, they contribute precious little to the overall visible light of the cosmos. 20 of the substars within thirty-two lightyears of Sol are L-type.

✹ Stars

True stars are enormous spheroids of high-energy plasma that spend the bulk of their lifespans fusing hydrogen and helium. Because of this, they are referred to academically as "fusors." Stars transition to stellar remnants at the end of their lifespans, either relatively peacefully by exuding their outermost layers as a planetary nebula or violently in the form of a supernova. Each spectral subclass is subdivided into more specific numeric groupings (0-9) to better represent the spectrum on which these objects exist. Additionally, there is a separate numeric scale to represent where a given star is in its lifespan.

Stellar Luminosity Classification

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Luminosity Class	Abs. Magnitude Range	Size Class	Initial Mass (M☉)
0	> -8	hypergiants	> 25
Ia	-8 to -6	luminous supergiants	25 - 15
I	-6 to -4	supergiants	25 - 10
Ib	-4 to -3	underluminous supergiants	15 - 10
II	-5 to -3	bright giants	10 - 8
III	-3 to +1	normal giants	8 - 5
IV	0 to +4	subgiants	5 - 2
V	-4 to +20	main-sequence dwarfs	2 - 0.5
VI	+6 to +12	subdwarfs	<0.5

Stellar Spectral Classification

The values of mass and radius shown in this table apply to main-sequence stars; late-stage asymptotic giant branch stars exhibit cooler spectra (G-M) at much larger masses and radii due to their overall lower density.
Note^[1] - Indigo and crimson stars are special classes that are (partially) based on characteristics outside the scope of this table; see text for details.
Note^[2] - WR stands for Wolf-Rayet. Originally lumped in with O-types; occasionally shortened to W-type.

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Chromatic Type	MK-Harvard Type	Temperature (K)	Peak Emission (nm)	Mass (M☉)	Radius (R☉)
Red	M	2400 - 3700	1207.4 - 783.2	0.08 - 0.45	< 0.7
Orange	K	3700 - 5200	783.2 - 557.3	0.45 - 0.8	0.7 - 0.96
Yellow	G	5200 - 6000	557.3 - 483.0	0.8 - 1.04	0.96 - 1.15
Pale	F	6000 - 7500	483.0 - 386.4	1.04 - 1.4	1.15 - 1.4
Azure	A	7500 - 10000	386.4 - 289.8	1.4 - 2.1	1.4 - 1.8
Cerulean	B	10000 - 30000	289.8 - 96.6	2.1 - 16	1.8 - 6.6
Blue	O	> 30000	> 96.6	> 16	> 6.6
Indigo[1]	WR[2]	> 144.9	> 20000	> 12	> 4.8
Crimson[1]	C	2600 - 5100	1114.5 - 568.2	1.25 - 3.3	1.3 - 6

✹ Red

Gacrux I by Doug Marshall

Red (M-type) stars are the most abundant stars in the universe, composing approximately 75% of all stars in the galaxy. Most of the light they emit is in the near-infrared, resulting in a dull orange glow to most observers. The majority of red stars are dwarfs, and this diminutive size combined with their cool temperatures results in main sequence lifespans estimated to be several times the current age of the universe. There are 257 M-type main-sequence stars within known space, including Proxima Centauri, Helios, and the Ziirin-Vniir pair.
Although most red stars are dim red dwarfs, most of the largest giant stars are also M-type: once more massive stars reach the end of the main sequence and expand, their increased volume and decreased density work to rapidly cool the outer layers. The closest M-type giant star to known space is Gacrux at a Sol-relative distance of approximately 88.6 lightyears.

✹ Orange

Orange (K-type) stars are far less common than red stars, being larger and hotter than the M-type majority but still cooler than most other stars. Higher-order orange stars are considered favorable for habitable planetary conditions. K-type stars account for roughly 12% of the galactic stellar population. The K stellar type also includes large, dying orange stars, which range from regular giants to the exceedingly rare orange hypergiants. There are 37 K-type main-sequence stars within known space, including Alpha Centauri B, Xiilu, Alai, and Mu. The closest K-type giant star to known space is Pollux at a Sol-relative distance of approximately 33.8 lightyears.

✹ Yellow

Sol (UV) by NASA (SDO)

Yellow (G-type) stars are rarer still, making up just 7.5% of the stars in the galaxy. However, they are one of the most common stars to host life in their orbits; in fact, half of all known yellow stars host at least one planet or moon with endemic life, and four of the Coalition member civilizations originated on worlds orbiting such stars.
G-type stars are an odd liminal phase between the cooler and hotter classes. Yellow stars are virtually always main sequence dwarf, subdwarf, or subgiant stars, as giant stars decay rapidly to the cool classes (K-M). There are 20 G-type main-sequence stars within known space, including Sol, Alpha Centauri A Ra'sen, Aelycah, and Eonuu. The closest G-type giant star to known space is Capella Ab at a Sol-relative distance of approximately 42.9 lightyears.

✹ Pale

Pale (F-type) stars, also called white stars, occupy the middle of the black-body chromatic spectrum, just above yellow G-type stars in temperature. F-type stars are an odd liminal phase between the cooler and hotter classes. Like yellow stars, pale stars are almost always main sequence or subgiants, and make up about 3% of the galactic stellar population. There are seven F-type main-sequence stars within known space, including Procyon A and Tabit. The closest F-type giant star to known space is Caph at a Sol-relative distance of approximately 54.7 lightyears.

✹ Azure

Azure (A-type) stars are blue-white, very luminous, and quite hot. A-type stars are typically subgiants or giants, requiring more mass to reach their characteristic high temperatures. Roughly 1 in 160 stars are of the azure class, making them the most common of the hot chromatic classes. There are four A-type main-sequence stars within known space: Sirius A, Altair, Vega, and Fomalhaut. The closest A-type giant star to known space is Delta Capricorni Aa at a Sol-relative distance of approximately 38.7 lightyears.

✹ Cerulean

Cerulean (B-type) stars are very luminous and hot, and because of this they only live for a relatively short time. Their high mass also implies their rarity: approximately 1 in 800 stars are B-type. Cerulean stars are virtually always giant stars, though the generally accepted theory of stellar evolution predicts the existence of B- and O-type “blue dwarfs” as the final stage of red dwarf evolution. Since red dwarfs are expected to remain on the main sequence for hundreds of billions of years, this is yet unproven. The closest B-type star to known space is Regulus A at a Sol-relative distance of approximately 79.3 lightyears.

✹ Blue

Eta Carinae by NASA (HST)

Blue (O-type) stars are extremely hot and luminous, so much so that their emission peaks in the near-ultraviolet. O-types are by far the rarest of all stars, with a frequency of just 1 in 3000000 across the entire galaxy. Some of the most massive known stars are blue, and typically are found in clusters of other hot, high-mass stars. O-type stars below the supergiant class are unheard of, and because of their extraordinarily large masses, they do not live for very long. The closest O-type star to known space is Zeta Ophiuchi at a Sol-relative distance of approximately 366 lightyears.

✹ Indigo

Indigo (WR-type) stars, also called Wolf-Rayet stars, are an extremely rare type of fusor. WR-type stars are so massive and hot that when they have exhausted all of their hydrogen and helium, rather than dying as a supernova, they begin to fuse heavier elements such as nitrogen and carbon. They are the most luminous stars of all, but their energy output is so intense that they radiate mostly ultraviolet, meaning their visual magnitude is unusually faint. They are also abnormally long-lived for stars of their extreme mass, thanks to their state as "deep-fusors" -fusing increasingly heavier elements instead of dying. The closest Wolf-Rayet star to known space is Gamma Velorum at a Sol-relative distance of approximately 1096 lightyears.

✹ Crimson

CW Leonis by NASA (HST)

Crimson (C-type) stars, more commonly called carbon stars, are a rare type of fusor which have a significant surplus of carbon relative to oxygen. This chemical peculiarity results in a major departure from the expected temperature-mass relationship, signified by the visually-striking shade of red these stars glow.
Another notable feature of carbon stars is their "soot veils": the extreme stellar winds from these abnormal stars result in vast amounts of material -mostly carbon and carbon compounds- drifting off the star and into space. Carbon stars are quite rare; the closest example to known space is CW Leonis at a Sol-relative distance of approximately 310 lightyears.

⍟ Stellar Remnants

Stellar remnants are the exposed cores of stars that have "died" -exited their fusing phase- and metamorphosed into exotic hyper-dense objects. Stellar remnants are of stellar mass and typically still radiate stellar-equivalent heat, but no longer internally fuse elements; hence the alternative term "postfusors."

✦ Stable Postfusors

Aurora over Phantasos by Doug Marshall

Stable postfusors, colloquially known as white dwarfs (typified as WD or D-type), are composed of extremely dense matter compacted into a volume equivalent to a typical telluric planet. These hyper-dense objects resist full collapse into singularities through electron degeneracy pressure: a quantum phenomenon involving the strong tendency of the electrons in atoms to repulse each other. White dwarfs do not undergo fusion; their luminosity is purely leftover thermal energy from their main-sequence fusion.
Stable post-stellar objects are formed by the death of intermediate-size stars, typically of main-sequence classes from type M up to the middle of type B. This makes them the most common type of stellar remnant, and there are 21 known examples within known space including Sirius B, Procyon B, van Maanen, and Simurgh.
After the red giant phase, as the outer layers of the dead star drift off into space, the heavy core collapses in on itself until the strength of electron degeneracy pressure balances out the crushing force of gravity. After tens of billions of years, white dwarfs will cool to the point where they no longer emit radiation, transitioning them to a new type of object called a black dwarf. However, the universe is not old enough for this to have occurred yet; as of now "black dwarfs" are theoretical.

White Dwarf Spectral Classification

[Click to open notes]

Two or more of the type letters may be used to indicate a white dwarf that displays more than one of these spectral features. The type is followed by a number representing the surface temperature of the object, determined by dividing 50400 by the object's effective surface temperature (in Kelvins).

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Standard Type	Spectral Features
DA	hydrogen lines
DB	neutral helium (He I) lines
DO	ionized helium (He II) lines
DQ	carbon lines
DZ	metal lines
DC	no strong lines
DX	ambiguous spectral lines
P	polarized magnetic field
H	unpolarized magnetic field

❋ Metastable Postfusors

Metastable postfusors, generally called "exotic stars," are stellar remnants composed of exotic degenerate matter. Usually these objects are neutron stars, which are composed of matter so dense that the protons and electrons of the remnant core fuse together into neutrons, making a planet-sized object with up to 30 standard solar masses packed inside. Other types of metastable postfusors take this process even further, crushing the neutrons into their component quarks (creating quark stars) and, rarely, even crushing the quarks into neutrinos (the elusive electroweak stars). This matter structure is extremely energetic and hot, far more so than white dwarfs.

Crab Nebula by NASA (JWST)

Like stable postfusors, metastable postfusors are formed from the death of stars. However, neutron stars are the collapsed cores of far larger stars (of the B type) and consequently much less common. They are often found at the hearts of supernova remnant nebulae, such as the Crab Nebula (shown at right). The nearest exotic star to known space is a solitary neutron star fancifully dubbed "Craxar" (Corona Australis X-ray Star).
After a massive star undergoes a supernova, the heavy core collapses in on itself until the neutron degeneracy pressure is reached. Similarly to white dwarfs, exotic stars do not undergo fusion; their remaining luminosity is purely leftover thermal energy from its main-sequence fusion. These objects are metastable in the sense they are able to form and persist naturally, but the addition of too much mass may cause them to degrade further into unstable stellar remnants (black holes).
Pulsars are a type of metastable postfusor that emits jets of radiation along its magnetic axes. These objects rotate at dizzying rates, and the rapid sweep of their jets across an observer gives the illusion of pulsating signals. Magnetars are exotic stars with extraordinarily powerful magnetic fields that emit bursts of high-frequency radiation. Asteroseismic activity within these diminutive yet powerful objects is the source of lethal gamma flares that occasionally sweep across large swathes of space.

✺ Unstable Postfusors

Cygnus X-1 by Doug Marshall

Unstable stellar remnants, colloquially termed "black holes," are among the oddest natural objects in existence: infinitely-steep gravity wells whose surface escape velocities are infinite. The central body of a black hole has a finite mass of infinite density, and are referred to as unstable because their internal pressure is unable to oppose gravitational collapse. These objects are very rare; the nearest black hole to known space is "Apep": a stellar-mass black hole 1560 lightyears away, around which orbits the G-type star "Ma'at."
Like other postfusors, black holes are formed from the death of stars, though their pre-mortem forms are among the largest possible stars (typically blue super-giants.) After the star undergoes a supernova, if the remaining core is heavy enough to overcome all possible quantum degeneracy pressures it will collapse into an infinitesimal point- or circle-like non-volume: a gravitational singularity. This singularity curves spacetime so strongly that, at a certain distance, escape velocity equals the speed of light. Escaping from any point closer to the central singularity than this event horizon requires a velocity greater than c - that is to say, it is impossible.

⚝ Stellar Anomalies

There are some very rare stellar-adjacent objects in the universe which do not neatly fall into the categories discussed here. Hybrid stars, gymnostars, and white holes are the most prominent examples, and each type of object presents a fascinating window into the extreme conditions of the universe. None of these stellar anomalies are found within or anywhere near known space, but they merit mention all the same.

Hybrid Stars

Hybrid stars, also termed "Thorne–Żytkow objects," are cool giant or supergiant stars with exotic stars lurking at their cores. These objects are theorized to form when an exotic star collides with a cool giant or supergiant star, spiraling inward through the diffuse outer layers and eventually merging with the core of the giant. This merger is very energetic, but usually not enough to trigger a supernova or collapse the core into a black hole. No hybrid stars have been confirmed yet, but the most likely candidate is the distant galactic halo star HV 11417.

Gymnostars

The term "gymnostar," meaning "stripped star," refers to an exposed, inert stellar core which has been completely stripped of its outer layers by a very close, ultra-dense companion such as a white dwarf or neutron star. These small, compact objects are no longer massive enough to perpetuate nuclear fusion, but were not subject to normal stellar death and thus cannot be classified as white dwarfs. Gymnostars are incredibly rare; the closest and best-studied example is the secondary object in the EF Eridani system some 520 lightyears distant.

White Holes

"White holes" are an enigmatic spacetime phenomenon which are theorized as the opposite of black holes: regions of spacetime with a positive gravitational gradient so steep that nothing can cross the event horizon of the anti-singularity. Originally proposed as a possible "exit point" of theoretical primordial black holes, the confirmation of wormholes as real spacetime phenomena (and their use in ansibles) gave weight to the idea that one-way traversable wormholes could exist in the natural universe, and the search for white holes was revived.

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