A star system can contain one star or multiple stars. Humans are more likely to find habitable planets in systems with single stars. In reality, more than half of all star systems have two or more stars, and these systems typically contain planets that are inhospitable to human life.
Stars are classified using a lettering system that describes the star and gives information about its type. Known as the spectral class of a star, a designation of O, B, A, F, G, K, or M is given to the star based on its mass and energy output. Class O stars are the hottest, largest, and brightest stars, and class M stars as the smallest and coldest, with a gradual scale between them. Since a star’s mass determines how hot it burns (as well as how strong its gravity pull is), the star’s classification actually helps extrapolate the kinds of planets that might be in that star’s system. Since larger stars burn hotter and smaller stars burn cooler, the mass of a star determines the climate of the worlds that orbit it.
In addition to the standard array of star types, several other types of stars (or what were once stars) might be found at the center of a star system. Most of these stars (called “non-main sequence stars”) have characteristics that make certain planetary conditions impossible, and no type of non-main sequence star is likely to support worlds hospitable to human life. Types of non-main sequence stars include black holes, neutron stars, white dwarf stars, black dwarf stars, brown dwarf stars, and red supergiants.
Degree of Ionizing Radiation: Ionizing radiation—radiation that breaks down atoms within living tissue—is common in space. All stars produce and emit harmful levels of ionizing radiation, and a star system is considered an “irradiated area” for the purposes of determining radiation exposure, particularly in the vacuum of space. (Planetary atmospheres and protective environment suits can protect a creature from ionizing radiation.) The degree of radiation exposure depends on the nearest star’s classification, as shown in Table: Star Systems. For systems with two or more stars, increase the degree of radiation by one grade (lightly becomes moderately, moderately becomes highly, and highly becomes severely).
Number of Planets: The number of planets in a given star system can be determined by rolling on Table: Star Systems. For systems with multiple stars, use the star with the fewest planets allowable to determine the number of planets in the system.
The chief classifications of hospitable stars are F, G, and K. These stars produce the right amounts of heat and the right types of radiation to allow human-compatible worlds to exist. Not every world around a Class F, G, or K star is hospitable; however, even inhospitable worlds within such systems could be made to support human life with artificial modifications to their ecosystems (a long a painstaking process called “terraforming”).
Class O, B, A, and M stars are the least likely to support planets capable of hosting human life. T he s tars toward the hotter end of the spectrum simply produce too much heat to allow living, breathing organisms to thrive. Class M stars do not give off enough heat to support life at the distance Earth orbits its sun, and these stars are also known to be violently unstable and prone to bursts of stellar activity.
|Table: Star Systems|
|Star’s System’s Classification||Degree of Ionizing Radiation1||Number of Planets|
|Class O (blue-white)||Highly irradiated||1d4+1|
|Class B (blue-white)||Moderately irradiated||1d4+2|
|Class A (blue)||Moderately irradiated||1d6+2|
|Class F (green)||Lightly irradiated||1d6+3|
|Class G (yellow)||Lightly irradiated||1d6+4|
|Class K (orange)||Moderately irradiated||1d6+5|
|Class M (red)||Highly irradiatedv1d8+2|
|Non-Main Sequence Star’s Classification||System’s Degree of Ionizing Radiation1||Number of Planets|
|Black hole||Highly irradiated||—|
|Neutron star||Severely irradiated||1d4–1|
|White dwarf||Moderately irradiated||1d4+1|
|Black dwarf||Lightly irradiated||1d4+2|
|Brown dwarf||Lightly irradiated||1d4+1|
|Red supergiant||Highly irradiated||1d4–1|
|1 Refer to Table: Radiation Exposure for details.|
Black holes are stars that have expended their fuel sources and exploded in a massive supernova. Few, if any, planetary bodies survive the initial death of such a star. Once the star has exploded, its gravity is so great that it collapses in on itself and warps light, time, and space around it. Black holes drag all nearby matter into its center, collecting rings of cosmic debris called accretion discs that can be seen at great distances. Some planets and asteroids might survive being pulled into a black hole long enough for some adventuring, but they are incredibly dangerous places to explore.
A neutron star is a large star that has exhausted its fuel source but hasn’t collapsed in on itself. Instead, the entire star’s remaining matter compresses into a much smaller body mere kilometers in diameter. Within this tightly packed core, the star’s density crushes the atoms into an object composed entirely of subatomic particles known as neutrons. Planets orbiting a neutron star are typically cold, lifeless, and severely irradiated. Another type of neutron star is the pulsar, which emits severe levels of radiation at great distances.
White Dwarf Stars
A white dwarf star is so much smaller than a neutron star that it does not have the mass to collapse in on itself. Instead, white dwarfs are typically small and dense and surrounded by rings of wreckage that were once planetary bodies in its system. White dwarfs emit little light or energy, and the rings surrounding them are usually cold and dark. However, these rings are not bombarded by as high levels of radiation as in a neutron star and could potentially support life, assuming enough heat could be generated.
Black Dwarf Stars
Black dwarf stars completely burn out after expending their fuel. Truly the most stable of dead stars, black dwarfs simply consume their fuel supply and then cool into a cinder that emits no light or heat. Any planetary systems that existed around a black dwarf will remain intact; however, they usually become barren and frozen once their heat and light source is gone.
Brown Dwarf Stars
In many ways, the brown dwarf is not even a star. Brown dwarf stars are stellar bodies that almost coalesced into true stars but never managed to form completely. Brown dwarfs are dim and small. They may have planets in their system, but rarely can these worlds support life due to the lack of heat or light.
Most red supergiants begin their lives as average-sized stars. However, they burn hot and expend their hydrogen fuel supplies quickly. When its hydrogen supply is depleted, a red supergiant begins burning other, heavier elements such as helium, causing the star to expand to enormous size. An expanding red supergiant consumes its innermost planets and then burns so hot and bright that it renders all other planets in its system incapable of supporting life naturally.