Celestial Body Classification in The Midnight Sea | World Anvil

Celestial Body Classification

The astral sea is littered with objects of all kinds, from verdant planets to blazing stars to ravenous singularities. The myriad bodies that fill the void are wildly varied, and as such, several complex classification systems have been developed to assist in cataloguing efforts. The dominant system for categorizing celestial bodies has been standardized by the Astronomicon.

Minor Planetoids

Minor planetoids are solid celestial bodies that are too small to be spheroid in shape.


Asteroids are minor planetoids that are composed of silicate rock and/or metallic compounds. Unless magically altered, most asteroids are incapable of retaining an atmosphere; although large, unusually dense clusters of asteroids may share a collective atmosphere and occasionally even a collective water cycle.


Comets are minor planetoids that are composed of volatiles -chemical compounds with freezing points above 100 degrees Universal Centigrade (K). Unless magically altered, most comets are incapable of retaining an atmosphere. However, comets closer to stars begin to sublimate, creating their iconic vaporous comae.


Planets are the comparatively tiny motes of dust and gas that provide a platform for the vibrant verdancy of the living cosmos. Planets are defined as celestial bodies that are large enough to be spheroid in shape but not large enough to naturally fuse elements within them at any point in their life cycles. The upper bound on planet mass is approximately 4200 standard planetary masses (~0.012 solar masses), above which the pressure at their cores is sufficient to fuse deuterium and transfer them into the phaetona class.


Planetary Size Classification

Size ClassMass RangeDominant Phase
Microtellura/Microaquaria< 0.002 MTerrasolid/liquid
Minitellura/Miniaquaria0.002 MTerra to 0.02 MTerrasolid/liquid
Subtellura/Subaquaria0.02 MTerra to 0.2 MTerrasolid/liquid
Tellura/Aquaria0.2 MTerra to 2 MTerrasolid/liquid
Supertellura/Superaquaria2 MTerra to 10 MTerrasolid/liquid
Megatellura/Megaquaria> 10 MTerrasolid/liquid
Mininephela< 5 MTerrasupercritical
Subnephela5 MTerra to 10 MTerrasupercritical
Nephela10 MTerra to 25 MTerrasupercritical
Supernephela25 MTerra to 50 MTerrasupercritical
Meganephela> 50 MTerrasupercritical
Miniaeola< 50 MTerragaseous
Subaeola50 MTerra to 200 MTerragaseous
Aeola200 MTerra to 600 MTerragaseous
Superaeola600 MTerra to 1800 MTerragaseous
Megaeola> 1800 MTerragaseous


All planetary bodies can be classified as one of the following based on their average surface temperature:
  • Frigid (< 90 K)
  • Cold (90 K to 170 K)
  • Cool (170 K to 250 K)
  • Temperate (250 K to 330 K)
  • Warm (330 K to 500 K)
  • Hot (500 K to 1000 K)
  • Torrid (> 1000 K)


All solid planetary bodies can be classified as one of the following based on their average ambient surface pressure:
  • Airless (< 1e-6 bar)
  • Infrabaric (1e-6 bar to 1e-3 bar)
  • Hypobaric (1e-3 bar to 1e-1 bar)
  • Mesobaric (1e-1 bar to 1e1 bar)
  • Hyperbaric (1e1 bar to 1e3 bar)
  • Ultrabaric (> 1e3 bar)


All solid planetary bodies can be classified as one of the following based on the percentage of the surface that is liquid solvent:
  • Arid (< 1%)
  • Lacustrine (1% - 50%)
  • Marine (50% - 90%)
  • Oceanic (> 90%)
  • Superoceanic (ocean transitions directly to mantle)


All planetary bodies can be classified as one of the following four types based on their overall composition and geophysical structure. Solid planets are generally divided into two phyla depending on their bulk composition: tellurae are worlds that are >50% silicate mineral or metallic; aquariae are worlds that are >50% volatiles. These two solid planet phyla are further classified by the atmosphere and liquisphere properties they exhibit. The other two major classes of planet are nephelae and aeolae, supercritical and gaseous planets respectively.  


Tellurae are typically stratified into three or more layers: a solid inner core, a molten mantle, and a solid crust. They are composed mostly of stable transition metals and silicates, though this is concentrated in the inner layers (especially the core). The crust of telluric planets, by contrast, consist largely of lighter metalloids and nonmetals (such as silicon, carbon, and aluminum). Telluric planets often have tectonic activity caused by interior magma cycles or, in some cases, extreme heating and cooling of the surface.  


Aquaric planets, by contrast, are typically enveloped in glacial shells rather than rocky crusts, which may span all the way to the planets' smaller, lighter cores, often of porous rock; some aquarae even have undifferentiated interiors of jumbled rock in ice. Occasionally, however, these icy crusts hide global liquid oceans, akin to the molten mantles of tellurae, that produce tectonic activity in the glacial crust. These subsurface oceans are warmed by either a core dynamo effect between the core and whatever thin mantle may exist, or tidal heating in certain cases.  


Nephelae are planets that are predominately composed of volatiles in a supercritical state -simultaneously liquid and gas. Typically the structure of nephelae includes a solid metallic core at the center, surrounded by a mantle of high-pressure ice which is in turn enveloped by a supercritical atmosphere of hydrogen, helium, and volatiles. Unlike solid worlds, however, nephelae are characterized by a density gradient toward their cores rather than rigid stratification.  


Aeolae are planets that are predominantly composed of hydrogen and helium in a gaseous state. Aeolae, like nephelae, are characterized by a density gradient toward their cores, though even less stratified. Their superheated cores are surrounded by large, hot mantle of liquid metallic hydrogen, which is in turn enveloped by an atmosphere of gaseous hydrogen, helium, and trace volatiles.


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. Stars are defined as celestial bodies that are large enough to be spheroid in shape and fuse elements within them at some point in their life cycles.


Phaetonae are objects commonly called "substars" -they are large enough to fuse deuterium 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 standard solar masses, above which the pressure at their cores is sufficient to fuse lithium and catalyze the proton-proton chain, making them heliae. Phaetonae emit their own heat, which is why they are subclassified in a manner similar to stars rather than planets and planetoids. Like heliae, each spectral subclass is subdivided into more specific numeric groupings (0-9) to better represent the spectrum on which these objects exist. However, since phaetonae do not "die" in the same way as heliae, they are not classified by stage or mass in the same way.

Substellar Chromatic Classification

The standard protostellar spectral classification system used by the Astronomicon.

Standard Chromatic TypeTemperature (K)Mass (SSM)Radius (SSR)MK-Harvard Type
Violet (V)< 800< 0.03< 0.1Y
Magenta (F)800 - 15000.03 - 0.060.1 - 0.2T
Maroon (M)1500 - 24000.06 - 0.080.2 - 0.3L


Violet substars are the coolest and dimmest substars, having temperatures of less than 800 K and near-negligible luminosities. Their atmospheres are cool enough to form ammonia clouds.


Magenta substars emit almost entirely in the infrared spectrum, though enough visible light is emitted to give them a dull magenta glow. Their atmospheres are cool enough to form methane clouds.


Maroon stars 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. M-stars are by far the most common type of substellar body in the known universe, but contribute precious little to the overall visible light of the cosmos.


Proteae are a rare class of objects more frequently called "protostars": 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 referred to academically as "prefusors." Proteae transition to heliae after a relatively short period of time; a few million years at the most. Protostars are classified according to the electromagnetic band in which their emission peaks; this also correlates to the age of the object.

Protostellar Spectral Classification

The standard protostellar spectral classification system used by the Astronomicon.

Protostar ClassPeak Radiation BandWavelength RangeAge Bracket
ISubmillimeter1 mm - 100 μm104 years
IIFar-infrared (I)100 μm - 30 μm105 years
IIINear-infrared (II)30 μm - 700 nm106 years
IVVisible (III)700 μm - 400 μm107 years


Heliae are stars proper: 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." Heliae transition to thanatae 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

Luminosity ClassAbs. Magnitude RangeSize ClassInitial Mass (SSM)
0> -8hypergiants> 25
Ia-8 to -6luminous supergiants25 - 15
I-6 to -4supergiants25 - 10
Ib-4 to -3underluminous supergiants15 - 10
II-5 to -3bright giants10 - 8
III-3 to +1normal giants8 - 5
IV0 to +4subgiants5 - 2
V-4 to +20main-sequence dwarfs2 - 0.5
VI+6 to +12subdwarfs<0.5

Stellar Chromatic Classification

The standard stellar spectral classification system used by the Astronomicon.

Standard Chromatic TypeTemperature (K)Mass (SSM)Radius (SSR)MK-Harvard Type
Red (R)2400 - 37000.08 - 0.45< 0.7M
Orange (O)3700 - 52000.45 - 0.80.7 - 0.96K
Yellow (Y)5200 - 60000.8 - 1.040.96 - 1.15G
Pale (P)6000 - 75001.04 - 1.41.15 - 1.4F
Azure (A)7500 - 100001.4 - 2.11.4 - 1.8A
Cerulean (C)10000 - 300002.1 - 161.8 - 6.6B
Blue (B)> 32000> 16> 6.6O
Indigo (I)> 20000> 12> 4.8W


Red stars are the most abundant stars in the universe, composing approximately 75% of all heliae. 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. Although most red stars are dim red dwarfs, most of the largest supergiant stars in the galaxy are red, because 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.


Orange stars are far less common than red stars, being larger and hotter than the R-type majority but still cooler than most other stars. Higher-order orange stars are considered favorable for habitable planetary conditions. O-type stars account for roughly 12% of the galaxy’s stellar population. The O stellar type also includes large, dying orange stars, which range from regular giants to the exceedingly rare hypergiants.


Yellow stars are rarer still, making up just 7.5% of the galaxy’s stars. However, they are one of the most common stars to host life in their orbits. Y-type stars are an odd liminal phase between the cooler and hotter classes. Yellow stars are virtually always main sequence or subgiant stars, as giant stars decay rapidly from the hot classes (A-C) to the cool classes (O/R).


Pale stars, also called white stars, occupy the middle of the black-body chromatic spectrum, just above yellow stars in temperature. P-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 galaxy’s stars.


Azure 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.


Cerulean 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 C-type. Cerulean stars are virtually always giant stars, though the generally accepted theory of stellar evolution predicts the existence of C- and B-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.


Blue stars are extremely hot and luminous, so much so that their emission peaks in the ultraviolet. B-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. B-type stars below the supergiant class are unheard of, and because of their extraordinarily large masses, they do not live for very long.


Indigo stars are an extremely rare type of star. I-type stars are so massive and hot that when they have exhausted all of their hydrogen, 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.


Thanatae are the remains of stars that have "died" -exited their fusing phase- and metamorphosed into post-stellar objects. Thanatae are of stellar mass and typically still radiate stellar heat, but no longer internally fuse elements.


Stable thanatae are more commonly known as white dwarfs, as they are composed of extremely dense matter compacted into the volume of a typical telluric planet, prevented from full singular collapse by electron pressure alone. White dwarfs do not undergo fusion; their remaining luminosity is purely leftover thermal energy from its main-sequence fusion.   Stable post-stellar objects are formed by the death of intermediate-size stars, typically of main-sequence classes from M up to the middle of C. After the red giant phase, as the outer layers of the dead star drift off into space, the heavier core collapses in on itself until the electron degeneracy pressure is reached. 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.


Metastable thanatae are also called exotic stars as they are composed of exotic degenerate matter. The matter that makes up neutron stars is 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 thanatae 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, like that of white dwarfs, is extremely energetic and hot.   Like stable thanatae, metastable thanatae are formed from the death of stars, though neutron stars are the collapsed cores of far larger stars of the B type. After the star undergoes a supernova, the heavier core collapses in on itself until the neutron degeneracy pressure is reached. Similarly, 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 adjuncts.   Pulsars are a type of metastable thanata 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 rare, lethal gamma flares that occasionally sweep across large swathes of space.


Unstable thanatae are among the oddest natural objects in existence, being the overlap between stellar objects and anomalous space phenomena. Colloquially termed black holes, unstable thanatae are singularities of the Null-Negative (N-) type; that is, they are infinitely deep 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.   Like other stellar degenerates, black holes are formed from the death of stars, though their pre-mortem forms are among the largest possible stars (typically blue supergiants.) After the star undergoes a supernova, the heavier core collapses in on itself until a singularity is formed, and by this point it has created an event horizon in a spherical radius around itself. The event horizon of a black hole is the point at which the escape velocity equals the speed of light, and escaping from any point closer to the central object than the horizon requires a velocity greater than c.


Nebulae are vast clouds of gas and dust that drift in the astral sea; primordial matter left over from creation. They blur the lines between object and natural phenomenon. Three kinds of nebulae exist, classified according to how they interact with electromagnetic radiation.


Emission nebulae are a class of nebula composed largely of ionized plasma that emit radiation, usually ultraviolet to radio. Most often, the source of this ionization is ultraviolet light given off by nearby stars, likely among the hotter types. Both stellar-birth cradles and post-stellar planetary nebulae tend to fall under this category, as both kinds are ionized by local stellar activity.


Reflection nebulae are a class of nebula composed largely of molecular gas and dust that reflect and scatter radiation, typically ultraviolet and visible light from stars within or around them. Some stellar cradles classify as reflection nebulae, if the local stars are not energetic enough to ionize the nebula.


Absorption nebulae are a class of nebula composed largely of molecular gas and dust that is dense enough to obscure radiation of most wavelengths, hence their colloquial name: "dark nebulae." Absorption nebulae are typically vast molecular clouds stretching for hundreds to thousands of lightyears, and may contain star-forming regions but are mostly inert.


Constructs are celestial bodies that are not naturally formed from abiotic cosmic processes. Though unbelievably more varied than natural celestial bodies, these objects can all be categorized into one of two broad phyla based on the nature of their origins.


Organic constructs are objects created by the natural processes of life in the universe. Constructs that fall into this category include clusters of space trees, the shells of vast void-nautili, and the remains of dead divinity.


Artificial constructs are objects created by the will and work of intelligent life in the universe. Constructs of this category are typically subclassified by their general form, including cylinders, discworlds, and worldshells.


Space is not without its oddities. Across the known cosmos, numerous peculiarities in the fabric of spacetime itself have been observed and studied, both natural and artificial. Known spacetime anomalies can be sorted into three categories.  


Singularities are objects whose gravitational fields are infinite, warping the fabric of spacetime into non-euclidean cones with nonexistent vertices. These are rather common natural phenomena; at least, one subclass of singularity is. Type N-Negative (N-) singularities are more commonly labeled as unstable thanatae: black holes. These objects, formed by the death of supermassive stars, have infinitely deep and inescapable gravity wells.   White holes, or N-Positive (N+) singularities, have infinitely steep gravity wells. The energy required to mount the gravity hill is greater than the speed of light, and thus the objects appear to radiate energy and mass. These objects, unlike their counterparts, are typically rare and ephemeral, existing for brief flashes before collapsing back into euclidean space. Though not wholly understood, white holes are hypothesized to be temporal inverse echoes of black holes.   Rarest of all in the natural cosmos are type N-Neutral (N0) singularities: wormholes, also called portals or gates. These objects, formed by rapidly rotating ring-shaped singularities, allow near-instantaneous passage between two remote points in space without risk of being crushed by a core singularity. While infinite, their gravity wells are toroidal rather than spherical. Temporary wormholes are commonly generated by magic or technology -or more often a mix of both- as a means of faster-than-light travel distinct from spelljamming.  

Timelike Curves

Certain regions of space are warped in peculiar ways which, due to time-space duality, result in strange localized temporal effects. There are two well-documented types of timelike curves, a.k.a. "dents": open and closed. Open timelike curves are regions where objects moving across the warp experience intense time dilation. Closed timelike curves, on the other hand, are more like localized paradoxes, where space is warped to the point that time moves in an isolated loop within the region. Objects moving across closed dents may become trapped in an endless time-loop, and any transmissions sent inside the loop ricochet back and forth through time.  


Rifts are undoubtedly the rarest phenomena in the universe, as few known rifts have been determined to have occurred by natural means. These celestial objects are quite literally gateways to other planes of the cosmos or even wholly separate universes, differing from wormholes in both structure and exit point. Because of their structure, rifts are not spheroid or toroidal in nature; rather, they take elongated and often irregular forms.

Hazardous Conditions

There are a wide array of dangerous environments to be found in the universe, from the mundane concerns of planetary atmospheric conditions to the presence of hostile forces outside the scope of the mortal mind. The Astronomicon standardises these threats into thirty types of environmental dangers and also standardises the symbology used in warnings.
Radiation Hazard
High Solar Radiation
High Temperature
Low Temperature
High Pressure
Low Pressure
Toxic Atmosphere
Caustic Hydrosphere
Particulate Hazard
Electrostatic Hazard
Magnetic Hazard
Extreme Weather
Impact Hazard
High Gravity
Low Gravity
Zero Gravity
Anomalous Gravity
Temporal Anomaly
Arcane Hazard
Arcane Null Zone
Planar Bleed
Hostile Lifeforms
Hostile Inhabitants
Undead Hazard
Psionic Hazard
Fey Hazard
Eldritch Hazard
Divine Hazard


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