BY LETTER
Star
 Image from Trolligi | |
| Most stars in the universe today are red dwarfs: cold, dim, low mass, and extremely long-lived stars. | |
Stars are celestial bodies which are sufficiently massive to naturally achieve a significant rate of nuclear fusion of hydrogen to helium in their interior at some point in their lives. The majority of stars are composed mostly of hydrogen and helium, the two most abundant elements in the universe, although there are exceptions, such as Wolf-Rayet stars or white dwarfs.
Table of Contents
1. Structure and dynamics1.1 Structure
1.2 Dynamics
2. Types
2.1 Main sequence stars
2.1.1 M-type and L-type stars
2.1.2 F, G, and K-type stars
2.1.3 O, B, and A-type stars
3. Variability
4. Evolution
5. Star systems
6. Uses
6.1 Light
6.2 Materials
6.3 Archailect uses
6.4 Weapons
7. Modification
8. Cultural significance
Structure and dynamics
Structure
At the center of a star lies the core, where temperature and pressure are the greatest. In main sequence stars, this is the region where hydrogen fusion happens by either the proton-proton chain, which dominates in less massive stars, or the carbon-nitrogen-oxygen (CNO) cycle, which dominates in more massive stars. Helium ash builds up within this region as the star ages, eventually causing it to become nearly hydrogen-free by the end of the main sequence phase. In more massive stars, the core may continue fusing helium and heavier elements up to iron.Energy produced by fusion at the center of the star is transported to the surface via two methods: radiation and convection. This manifests as the radiation zone, where energy is transported via radiation and conduction, and the convection zone, where energy is transported via convection. The placement and thickness of both zones differ depending on stellar mass and the current stage in stellar evolution, and affects the dynamics of the star. For example, main sequence stars less massive than roughly 7E29 kg (0.35 solar masses) are fully convective. This means, among other things, that nearly all hydrogen present within the star can enter the core and become fused over its lifetime.
Once energy produced at the core reaches the photosphere, the layer where the star becomes transparent and generally regarded as the lowest layer of the stellar atmosphere, it is finally emitted as radiation. Additionally, cooler stars containing sufficiently deep surface convection zones have two additional layers of atmosphere: the chromosphere and the corona.
Dynamics
Two main types of stellar magnetic fields play a dominant role in the dynamics of a star. Hotter stars with thin or nonexistent surface convection zones usually possess simple, stable, and dipolar fields. Cooler stars possessing deep surface convection zones generally have complex and dynamic fields, resulting in various phenomena within the stellar atmosphere. In the photosphere, granules (convection cells of plasma), faculae (bright valleys between granules), and starspots (dark or bright spots) can be observed, while spicules (short-lived vertical jets of plasma) and plages (bright regions analogous to faculae) may be found in the chromosphere. Additional phenomena include flares (bursts of light emission), prominences (large plasma and magnetic structures extending from the surface), and coronal mass ejections (ejection of stellar mass).Stars emit a constant stream of particles known as the stellar wind. The strength of the stellar wind varies from star to star, and in extreme cases can lead to mass loss on the order of 1E30 kg over millions of years or less, as happens in extremely massive stars or post-asymptotic giant branch (post-AGB) stars. The stellar wind pushes back the interstellar medium, forming a bubble surrounding the star that is known as the astrosphere. The boundary separating the astrosphere from interstellar space, where the stellar wind collides with the interstellar medium, is the astropause.
Types
Main sequence stars
The main sequence is a stable, and generally long stage in a star's life when the outward pressure created by energy generation via hydrogen nuclear fusion within the core balances with the inward pressure exerted by the star's own mass. Stars that are in this stage are referred to as main sequence stars, or dwarf stars. Main sequence stars comprise the majority of stars that exist today.A main sequence star's radius, luminosity, and temperature (and consequently, color) are all strongly dependent on its mass. This causes them to show up on a single, long band on the Hertzsprung-Russell diagram, which goes from the least massive (characterized by being small, cold, and dim) to the most massive (large, hot, and bright) stars. Counterintuitively, the larger the mass of a star, the shorter its lifespan; this can largely be attributed to the disproportionately fast increase in fusion rate (which is reflected in luminosity) compared to the increase in mass.
A common feature of main sequence stars is to gradually brighten over the course of their lifetime. As stars burn hydrogen into helium 'ash', the mean molecular weight increases, causing pressure within the core to drop. The core contracts in response, which in turn increases the density and temperature, boosting the fusion rate of the core. The increased energy production pushes the outer layers of the star outward, causing it to become hotter and larger.
M-type and L-type stars
The dimmest, least massive main sequence stars are associated with the M-type and much rarer L-type (stars today can be as cold as approximately 2,000 K, roughly corresponding to type L3 V). Generally known as red dwarfs, they are by far the most common type of star, making up around three-fourths of all stellar objects that populate the Terragen Sphere. These stars are very long-lived, with the largest examples being able to persist for many tens of billions of years while the smallest can continue to shine for several trillion. They also evolve very slowly, remaining youthful and highly active for the first few billions of years, and often display large starspots that induce significant changes in brightness over the star's rotation period. At their temperature, output radiation peaks in the infrared range, but the star remains luminous enough to be visible to baseline humans as warm white on the hotter side, to orange on the cooler side.M and L-type stars form with relatively little material in their protoplanetary disks, and thus planetary systems around them generally consist of proportionately small planets. Worlds smaller than Neptune make up the bulk of the population, while planets in the jovian mass range only begin to appear around the more massive examples of the type. However, giant worlds formed from disk fragmentation can occasionally be found around lighter stars.
Due to their low mass, relatively few M and L-type stars are connected to the Wormhole Nexus; most are only accessible via the Lightways, interstellar cyclers, or starships. Reclusive communities that desire to separate themselves from the rest of the Terragen Sphere are often found in these places, as well as the Deeper Covenant which preferentially claims these stars in addition to brown dwarfs and free-floating planets. Some factions concerned about their survival into deep time, including some early ahumans of the Diamond Belt, once preferred claiming stars belonging to this group because of their long natural lifespan, but as starlifting and conversion technologies became common, allowing the transformation of any star into red dwarfs or mattercaches to be used in a controlled, less wasteful way, this tendency became rarer.
F, G, and K-type stars
Main sequence stars with masses close to Sol belong to the F, G, and K-type, which make up around one-fifth of all stars within the Terragen Sphere. In terms of their properties, they are the in-betweens; FGK stars are moderately massive and bright, with lifespans between a few billion years to several tens of billions of years, depending on mass. Starspots are still present, but in general these are smaller in size relative to the star compared to what can be found on the M-types. Stellar brightness peaks in the visible light range for baseline humans, and their appearance ranges from bluish white to warm white.In the absence of jovian planets, superterrestrial and subneptunian planets with masses in the range of 1E25 to 1E26 kg (1-10 Earth masses) are common, somewhat reminiscent of a scaled up version of a common planetary system arrangement found around stars from the previous group. However, jovian planets form more frequently around these stars, where the materials required for them are more abundant, and their presence can dramatically alter a planetary system. For example, a jovian formed early on near the ice line can prevent excess icy materials from arriving in the inner system, easing the formation of small, dry terrestrial worlds.
FGK stars are frequently connected to the Wormhole Nexus, but tend to be left relatively undeveloped by the archai, who generally focus on more massive bodies. Such systems tend to gather fairly large modosophont populations, as well as communities of lower transapients. In addition, certain groups of Earth anemoics, low tech societies, luddites, semperists, and terraformation hobbyists often desire a planet around this type of star, feeling that such offer very Earth-like environments and require no sophont maintenance. Finally, the similarity of stars in this group to Earth's sun was very appealing to the early interstellar colonization efforts of the Federation Era and earlier periods which had a tendency to seek stars similar to Sol.
O, B, and A-type stars
The few remaining, massive main sequence stars are members of the O, B, and A-types. Together they account for less than a percent of all stars, but make up for it with overwhelming luminosity, which they trade for with their lifespan. The least massive A-type stars live for a few billion years, while the most massive O-type linger for only a few million years, departing the main sequence almost immediately after formation. In fact, most O and B-type main sequence stars can be found in or near a star-forming region, as they never have enough time to migrate far from their own birth clusters. Most spin rapidly enough to become noticeably oblate.Large jovian worlds are most common around OBA-type main sequence stars, which usually form surrounded by massive protoplanetary disks, but Earth-like dry terrestrial planets are rare, becoming virtually impossible if the host star is more massive than 6E30 kg (3 solar masses). For very massive stars, planets that are anomalously large for their class can be found.
Outputting a generous amount of energy and offering large quantities of mass waiting to be extracted, OBA stars are some of the most highly sought after locations in the Terragen Sphere, and are usually targeted by a large number of colonization expeditions looking to grab at least a portion of the system for themselves. For these same reasons, they are usually the first stars in their neighborhood to be connected to the Wormhole Nexus, permitting the archai to utilize the bountiful resources of the system more easily; as a result they tend to be bustling with activity and people. Starlifting and archailect-level operations are frequently performed on these stars, both to harvest materials for construction of macrostructures such as Matrioshka hypernodes, and to prolong their lifespan.
- Variability
- Evolution
Uses
Stars are of immense utility to Terragen civilization. They are some of the most massive singular objects that can be found in the universe, and together they contain a large fraction of all mass-energy available in the Terragen Sphere. The two primary uses of stars are as sources of light and materials.Light
Stars passively generate energy in the form of light that can be utilized even by simple beings, and is the primary source of energy for many biospheres, natural or artificial, directly or indirectly. Early Terragens on Earth were dependent on their home star, Sol, to survive, and this is also the case for many low technology cultures across the Terragen Sphere. While the development of nuclear fission, fusion and conversion reactors rendered illumination from stellar objects redundant long ago, many Terragens today still prefer to rely on sunlight and starlight to power themselves, their technologies, and their environments.Materials
Aside from light and warmth, stars are also useful as sources of materials. Most stars primarily consist of hydrogen and helium, with heavier elements (collectively referred to as metals) making up just a small fraction of the total. However, due to their high mass, stars often contain the vast majority of the heavier elements in a system. Although small-scale harvesting and usage of mass via stellar wind became possible early in Terragen history, large-scale utilization of stellar resources involving direct extraction of matter from the star itself is more difficult due to the extreme temperatures and gravity found around stellar objects. Therefore, stars were rarely used for materials until the third millennium when the development of more durable hardware, improved methods, and Deep Well Industrial Zones combined to make the practice much more common.Archailect uses
Stars appeal to the greatest minds of the Terragen Sphere, who require a large amount of mass-energy to operate; it is no surprise that the archailects often seek to claim the larger stars and connect them to the Wormhole Nexus. Complete disassembly of stars is commonly performed by archailects, who often convert them and their accompanying planetary systems into artifacts such as computation nodes, industrial machinery or weapons. Some archailects may also inhabit very large stars, converting them into Godstars.Weapons
Stars are rich in mass and energy, which means they can be used to create immensely powerful weaponry for dealing with great threats. Fortunately, however, their great destructive potential means such weapons are rarely ever utilized.Modification
Intentional modification of stellar properties is known as stellar engineering. Stellar engineering can be divided into two types: star lifting and stellar husbandry. In addition to the modification of their physical properties, their kinematic properties can also be altered by stellar propulsion systems.A related technology are stellification engines, which are used on non-stellar objects, especially jovian planets and brown dwarfs, to convert them into star-like objects.
Cultural significance
In addition to tangible usage, stars are also featured prominently in Terragen cultures. Many low tech cultures on Earth used stars for various purposes, such as timekeeping and navigation. Many cultures gave the brightest stars visible to them proper names, associated them with mythological figures and objects, and grouped them into asterisms and constellations. Various religions and metaphysical beliefs such as astrology also revolve around stars.As more information regarding stars became available, their roles in Terragen cultures transformed from people and items into locations, usually simply serving as narrative anchors for the planets around them, where most stories took place. But many old traditions have survived and see continued usage. Many ancient star names have become widely used. Constellations from antiquity were codified and would later serve as ways to specify direction, and eventually, volumes of space as Terragens became an interstellar civilisation.
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Development Notes
Text by The Astronomer 2023
From an original article by M. Alan Kazlev.
Initially published on 31 December 2001.
Article overhaul phase 1: Structure and dynamics, Uses, Modification, and Cultural significance (2023-02-05, by The Astronomer)
Classification section renamed to Types, Main sequence stars sub-section added (2023-02-26, by The Astronomer)
From an original article by M. Alan Kazlev.
Initially published on 31 December 2001.
Article overhaul phase 1: Structure and dynamics, Uses, Modification, and Cultural significance (2023-02-05, by The Astronomer)
Classification section renamed to Types, Main sequence stars sub-section added (2023-02-26, by The Astronomer)
