Image from Bernd Helfert
Astronomy is the investigation of the cosmos through observation of stars, planets, nebula, galaxies, and other celestial bodies. This science includes some of the oldest data available within the Terragen Sphere, as observations that were recorded more than thirteen thousand years before present. Observations are available that were made by the Muuh xenosophonts several million years ago, although some of these may be unreliable.
This science has many specialized subdivisions including astrogation, astrometry, astrophysics, cosmology, heliology (including stellar structure and evolution), galactic structure, gravity wave astronomy, high-energy sources like cosmic rays, the nature of the interstellar medium, neutrino astronomy, optical astronomy, planetology, radio astronomy, spectrography, virtual astronomy, and the study of distant objects like quasars.
- 51 Peg planet - Text by M. Alan Kazlev
A planet, that shows orbital periods shorter than 15 terran days. These were among the first extrasolar planets discovered by Terragen astronomers, during the pre-singularity period (early Information Age). Included in this category are 51 Peg itself, Tau Bootis, 55 Cancri, and Upsilon Andromedae, which have orbital periods of just 4.2, 3.3, 14.7 and 4.6 days, respectively. The orbits are all small, with radii less than 0.1 A.U. They commonly range in mass from 0.44 Jupiters (for 51 Peg) to 3.64 Jupiters (Tau Bootis). They move in circular or near-circular orbits.
- A-Type Star - Text by M. Alan Kazlev
A hot white dwarf star near the brighter end of the sequence of spectral types.
- Accretion Disk - Text by M. Alan Kazlev and Steve Bowers
Whirling disk of hot gas and dust around a large compact object such as a white dwarf, neutron star, or black hole, usually material that has been thrown off one star and is being drawn into the companion object.
- Albedo - Text by M. Alan Kazlev, amended by Steve Bowers
The ratio of the amount of stellar/solar radiation or other megascale light source reflected from an object (such as planet, moon, orbital or habitat) to the total amount it receives. The higher the albedo the brighter the object. A white, completely reflecting object has an albedo of 1.0; a black object with no reflectivity has an albedo of 0.0. Icy bodies tend to have a high albedo, sooty bodies a low albedo.
- Aphelion - Text by M. Alan Kazlev
For an object (a planet or comet) orbiting the Sun (or any star), the aphelion is the point in its orbit which is farthest from the primary. See also apoapsis, apogee.
- Apoapsis - Text by M. Alan Kazlev
For an object orbiting a celestial body in general, the apoapsis is the point in its orbit which is farthest from that body. Now generally synonymous with apogee.
- Apogee - Text by M. Alan Kazlev
Originally a term used only for objects orbiting Old Earth, but from the Interplanetary Age onward a term used for any object orbiting any planet or moon. Thee point in an object's orbit at which is farthest from the world it is orbiting. Apoapsis is a more general term yet again. The opposite of perigee.
- Apparent Brightness of the Local Sun - Text by Steve Bowers
To standard Terragen eyes, suns with a different spectral type from Sol do not seem as bright from the surface of a terraformed planet, since less of their radiation falls within the visible range.
- Argus Array - Text by Todd Drashner, Peter Kisner, and M. Alan Kazlev
Distributed telescopic network occupying a spherical volume approximately 1000 ly across in the Inner and Middle Regions.
- Arity - Text by Steve Bowers
The number of stars in a gravitationally-bound system of stars.
- Astrography - Text by Anders Sandberg
Interstellar cartography, determining one's position in space, naming of interplanetary and interstellar bodies and regions, the science of interplanetary and interstellar map-making.
- B-Type Star - Text by M. Alan Kazlev
Hot blue-white star that is among the brightest in the sequence of spectral types, second only to the O-type star. They are distinguished by neutral helium lines, with hydrogen lines strengthening in B6 through B9.
- Barred Spiral Galaxy - Text by M. Alan Kazlev
A spiral galaxy whose spiral arms attach to a bar-like feature containing the nucleus. It appears to occur when an instability in the stellar distribution alters the orbits of the inner stars and generates and elongated nucleus. In all other respects resembles a normal spiral galaxy.
- Barycentre - Text by M. Alan Kazlev
The centre of mass around which a system of objects rotate. For example, Earth and Luna both revolve around their barycentre, which is 1707 km inside the Earth along the line between them. Pluto and its moon Charon revolve around their barycentre, which is 700 km inside Pluto along the line between them.
- Belt, Asteroid - Text by M. Alan Kazlev
Generally, a band of asteroids which encircles a star, often where a planet would normally have formed. It may form the main "world" in a stellar system, as in the Barnard Belt, or it may simply be a region between planets where no planet coalesced, as in the Sol Belt.
- Binary Stars - Text by M. Alan Kazlev and Steve Bowers
Two stars rotating around a common centre of mass. About half of all stars are binaries. Three categories of binary stars, depending on whether neither, one, or both fill their Lagrangian lobes.
- Black Dwarf - Text by M. Alan Kazlev
A small, very dense, cold, dead star. It is made mostly of carbon. This dark star is what remains after a red giant loses its outer layers, forming a planetary nebula and then a white dwarf. The nuclear core of a black dwarf is depleted. Black dwarfs are about the size of a terrestrial class planet. Most typical stars will eventually turn into black dwarfs.
- Black Hole - Text by M. Alan Kazlev, with comments by Todd Drashner
Any massive object (or region) in space that is so dense that its gravitational field does not let even light escape.
- Blue Giant - Text by M. Alan Kazlev
A huge, very hot, very luminous, blue post-main-sequence star; spectral type O or B. These rare, extremely hot stars burn helium. and are very rare and very bright. Blue giants are at least 18 Sol or more in mass. Examples include Rigel and Regulus.
- Blue Straggler - Text by Steve Bowers
Rare class of hot, young, blue stars that are formed in old clusters by close encounters and collisions, and by mergers between binary stars.
- Bode's Law - Text by M. Alan Kazlev
Also known as the Titus-Bode Law. An interesting mathematical coincidence, and not a physical law. However, the general form of geometric spacing is valid in Sol-type stable stellar systems, hence the numerical series that matches planetary distances from Sol and many other stars.
- Bok Globule - Text by M. Alan Kazlev, amended by Steve Bowers
Dark concentrated nebula cloud about 1 ly in diameter, containing some ten to a thousand Sol masses of dust and gas, and pertaining to the early stages of star formation. Bok globules are very popular with some cloud harvesting and cloud engineering clades and civilizations.
- Brahe, Tycho - Text by M. Alan Kazlev
Late Agricultural Age (1546 to 1601 AD; 423 to 368 BT) Old Earth Danish astronomer who made extensive and seminal calculations of the orbits of the planets.
- Brown Dwarf - Text by M. Alan Kazlev, amended by Steve Bowers
A massive object midway between gas giant and star; a starlike object too small to achieve nuclear reactions in its centre; any stellar object smaller than about 0.08 Sol masses.
- Bubble (astronomy) - Text by M. Alan Kazlev
A roughly spheroidal shell of interstellar gas blown outward from a star by a stellar explosion or strong stellar wind. Large bubbles are caused by supernova explosions and contain gas that is much hotter but also much more diffuse (about one atom per cubic cm) than that of the surrunding interstellar medium. These are also called loops.
- C stars - Text by Chris Clowes and M. Alan Kazlev
Stars rich in carbon. C stars, known as carbon stars, overlap with late G, K and M stars in most respects but are distinguished by compositional differences, typically an unusually high concentration of carbon. They are sometimes mined for carbon for large scale diamondoid or adamant megastructures.
- Cepheid Variable - Text by M. Alan Kazlev and Chris Clowes
A giant star that regularly pulsates in size and changes in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs.
- Chandrasekhar, Subrahmanyan - Text by M. Alan Kazlev
Old Earth Indian-American astrophysicist, 59 BT - 26 AT (1910 -1995 c.e.), who studied stellar physics, evolution, and black holes. He realized that the fate of dying stars depended upon their mass, and above a certain point (1.4 times Sol mass, known as the "Chandrasekhar limit"), a star will undergo extreme collapse and not simply becomes a white dwarf. There are a number of asteroids, habitats, ships, one black hole, and several black hole observatories named in his honor.
- Classical Cepheids - Text by Chris Clowes
Population I yellow or red supergiant stars (type A to F at maximum, G to K at minimum) with periods ranging from 2 to 40 days and varying in brightness by up to one magnitude.
- Close Binary Stars - Text by Anders Sandberg - additional notes by Chris Clowes and Steve Bowers
Generally, a binary pair close enough that a terrestrial planet in the comfort zone orbits both.
- Closed Universe - Text by M. Alan Kazlev
A model of the universe in which the average density is great enough to reverse the expansion and cause it to contract to a physical singularity. Although a closed universe is a matter of faith among Tiplerites (Omega Point), Theosophists, Buddhists, and some NuAgers (cyclic cosmology), it has been known since pre-singularity time that the universe is open.
- Cluster, Stellar - Text by Chris Clowes and Steve Bowers
Association of stars moving through space together in small groups or larger clusters, probably because they were all formed out of the same initial gas cloud.
- Comet - Text by M. Alan Kazlev
A minor celestial icy body that orbits a star in a highly elliptical path.
- Contact Binary - Text by M. Alan Kazlev
A co-orbiting pair of stars whose inner atmospheres or surfaces touch.
- Copernicus, Nicolaus - Text by M. Alan Kazlev
Old Earth amateur Polish astronomer who developed the Copernican system, a model of the solar system in which all the planets orbit the Sun, thus overturning the earlier Ptolemaic System. His seminal work was De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orb"), published in 430 BT (1543 AD).
- Core Collapse - Text by M. Alan Kazlev
Occurs when a star loses its pressure support because no more energy can be released by nuclear reactions. The core collapse drives the explosion and heavy-element creation of a supernova.
- Cosmic Rays - Text by M. Alan Kazlev
High energy interstellar particles that travel at relativistic speed. The 10 most abundant elements in cosmic rays are hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, and sulphur. Cosmic rays many sources, including local stars, supernovae, and distant galaxies, and make life hazardous for a biont in any unshielded habitat. Some cosmic rays penetrate through planetary atmospheres.
- Cosmological Principle - Text by M. Alan Kazlev
The assumption that the universe is homogeneous and isotropic (the same everywhere). This assumption is central to all scientific cosmology, and ten and a half thousand years of Terragen observation has shown it to be a good approximation to the observed state of the universe.
- Cosmology - Text by M. Alan Kazlev
The study of the nature, structure, and origin of the universe; the study of the cosmos.
- Cosmos - Text by M. Alan Kazlev
The universe, the totality of physical creation; generally including not only the observable universe but also that which is beyond the light-cone (although for some cosmos only means the visible cosmos). In some memeticities the cosmos also includes non-physical or supraphysical realities; while others refer to a series of cosmoses. There is also disagreement among philosophers, eschatologists and others over whether the cosmos had a beginning and whether it will have an end.
- Cyclic Cosmology - Text by M. Alan Kazlev
In a number of memeticities and religions, the idea that the universe undergoes periodic cycles of emergence and dissolution. This implies that the cosmos is a closed universe, for which the evidence seems to be lacking. Some cyclo-omegists have suggested that there were a number of previous cycles, but now the universe will expand forever, in order to guarantee a perpetual divinised consummation.
- Dark Nebulae - Text by M. Alan Kazlev
Generic term for non-glowing nebula. These are concentrations of dust only visible if they block out the light of stars that lie behind them. Many dark nebula have already been claimed by hider and cloudharvesting civilizations.
- Distance - Text by M. Alan Kazlev
When dealing with astronomical objects, distances are seldom specified in kilometers, because the numbers rapidly get too big to be manageable. Instead, AU, LY, and Parsecs are used.
- Drake Equation - Text by M. Alan Kazlev
An early (pre-interstellar age) attempt to estimate the number of civilizations in the galaxy, according to the statement that the fraction of stars harboring intelligent life equals the number of all stars times a sequence of fractions, such as the fraction of all stars having planets, the fraction of planets that are habitable, and so on.
- Dust Lane - Text by M. Alan Kazlev
A lane of dark interstellar dust in a galaxy or emission nebula that obscures the light of stars behind it.
- Dwarf Star - Text by M. Alan Kazlev
Generic term for relatively small stars, no more than 20 solar masses (and often less). Most stars in the galaxy belong to this category.
- Eagle Nebula - Text by M. Alan Kazlev
Huge emission nebula 7,000 light-years from Sol, Serpens Outer Volume.
- Eccentricity - Text by M. Alan Kazlev
The measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an elliptical orbit an eccentricity between 0 and 1; a parabolic orbit an eccentricity equal to 1; and a hyperbolic orbit an eccentricity greater than 1.
- Emission Nebula - Text by M. Alan Kazlev
A nebula that emits light. Emission nebulae are formed when energetic ultraviolet light from a very hot star excites a cloud of hydrogen gas; the UV radiation ionizes the hydrogen, stripping the electrons from the hydrogen atoms. The free electrons combine with protons, reddish light being produced when electrons and protons combine to reform hydrogen atoms.
- Epoch (Astronomy) - Text by Steve Bowers
A point in time from which astronomical quantities are measured.
- Event Horizon - Text by M. Alan Kazlev
The Schwarzschild radius; the radius from a black hole or other physical singularity within which even light cannot escape. It is also the radius to which a mass must be compressed down to in order to turn it into a black hole.
- Evolutionary Track - Text by M. Alan Kazlev
The change in location of a star on the Hertzsprung - Russell (H-R) Diagram. As a star ages and evolves, you can trace out its history on the H-R diagram.
- Evolved Star - Text by M. Alan Kazlev
An old star that is near the end of its existence, having exhausted most of its nuclear fuel. The star loses mass from its surface, producing a stellar wind.
- Extended World Classification System (EWoCS) - Text by Dangerous Safety
A system used to classify planets and other non-luminous bodies according to their mass, temperature, composition, cloud coverings, liquid layers and orbital parameters.
- Eyes of Neptune, The - Text by Tom Mazanec, updated by The Astronomer in 2021
Long baseline telescope in First Federation era Solsys.
- F-type Star - Text by M. Alan Kazlev
A bright yellow-white star in the middle of the spectral sequence.
- G-Type Star - Text by M. Alan Kazlev
The category of yellow G-type star includes as its most famous representative Sol, the original home star of terragenkind. These stars, which are located in the middle of the spectral sequence, are distinguished by very strong ionized calcium lines, with the hydrogen lines weaker. Metal lines especially iron, become prominent.
- Galactic Core - Text by M. Alan Kazlev
The dense central region of a galaxy. The central region of the Milky Way, like many other galaxies, is densely crowded with stars, pulsars, white dwarfs, and other objects. It contains a massive central object, Sagittarius A*.
- Galactic Directions - Text by Anders Sandberg, amended by Steve Bowers
Planetary terms like north, south, east, west, are insufficient for referring to directions within the galaxy. Instead, the a different set of conventions have achieved widespread acceptance when referring to direction.
- Galaxy - Text by M. Alan Kazlev
A gravitationally bound collection of stars, dust, and gas with a mass ranging from 100 million to 10 billion times that of the sun. Types include spiral, elliptical, and irregular.
- Galaxy, Irregular - Text by M. Alan Kazlev
A galaxy of amorphous or chaotic appearance, possessing large clouds of gas and dust, and both Population I and Population II stars, but no spiral arms or nucleus. They are thought to result from a collision with a larger galaxy, pulling them out of shape. Most have relatively low mass (about a hundred million to ten billion stars). About 25% of all galaxies are irregular.
- Galaxy, Spiral - Text by M. Alan Kazlev
A disk-shaped galaxy with a spiral pattern of arms, typically containing 1010 or more solar masses of stars, dust, and gas.
- Galileo Galilei - Text by M. Alan Kazlev
Italian mathematician, astronomer, and physicist, Old Earth 405-327 BT (1564-1642 AD); one of the creators of the scientific method of hypothesis, experiment and theory formation.
- Gamma Ray Bursters - Text by Steve Bowers
High intensity sources of gamma radiation. The two different types of gamma ray bursters can be distinguished by the length and brightness of the burst.
- Gas Cloud - Text by M. Alan Kazlev
Region of interstellar space, generally termed nebula, in which the density of atoms per cubic centimetre range from around 10 to 1000. The temperature of these clouds is very cold, between 10 and 50° Kelvin, and may be in the form of wispy infrared cirrus. Gas clouds are about three quarters hydrogen and one quarter helium, with traces of other elements occurring. Although the density of gas is a vacuum by any planetary standards, space adapted clades and neogens are able to harvest the gas for various purposes.
- Globular Cluster - Text by Chris Clowes
Gravitationally bound, spherical grouping of 105 to 106 stars, usually having a marked concentration toward the center.
- Graviational Wave Detector Argus Co-Function - Text by Steve Bowers
The widely spaced gravitational wave detectors developed for the Reality Intertextualization Project have also been useful in more conventional astronomy. The laser and neutrino interferometer instruments involved are now linked with the Argus Array information net to produce a gravitational wave map of the universe, showing neutron stars and black holes throughout this and neighboring galaxies. The Gravitational Wave Detector Argus co-function as it is called has also produced detailed information concerning the early years, minutes and even seconds of the post-Big Bang universe, showing the fine detail of this period and the surprising complexity of the high energy cosmos. Some cosmologists and esotericists refer to this period of complexity as the Alpha, postulating an advanced intelligence or civilisation at this time of unknown origin.
- Gravitational Collapse - Text by M. Alan Kazlev
The contraction of an object like a star or an interstellar dust-cloud under its own gravitational attraction.
- Gravitational Interferometers - Text by Adam Getchell
Instruments for recording the passing of gravitational waves.
- Gravitational Lens - Text by M. Alan Kazlev
A massive object in space, such as a galaxy, that generates sufficient gravity to bend light passing through it.
- Great Annihilator - Text by M. Alan Kazlev
Powerful x-ray source located near the center of the Milky Way Galaxy. The Great Annihilator is almost certainly a huge black hole. This object produces anti-matter in the form of positrons which, when they collide with ordinary electrons, mutually annihilate, resulting in the production of x-rays and other radiation. A number of neumanns and other clades have launched missions to the galactic core, although it will be many tens of thousands of years before any news is forthcoming.
- Great Attractor - Text by M. Alan Kazlev
Huge mass equivalent to some 100,000 galaxies exerting a gravitational pull on nearby galaxy clusters. Unambiguous evidence of the source of this effect has not yet been detected, and it has been suggested that this is actually an artifact left by an ancient civilization.
- Great Rift, Great River - Text by M. Alan Kazlev
Series of dark, obscuring dust clouds in the Milky Way Galaxy. As seen from the Inner Sphere, These clouds stretch from the constellation Sagittarius to the constellation Cygnus.
- Gum Nebula - Text by Steve Bowers
Very large, diffuse Middle Regions Nebula
- H I region - Text by M. Alan Kazlev
Interstellar region in which the hydrogen gas is predominantly neutral.
- H II region - Text by M. Alan Kazlev
Interstellar region in which the hydrogen gas is predominantly neutral. is predominantly ionized.
- Halo - Text by M. Alan Kazlev
Pertaining to the outer sphere or "halo" of a solar system or galaxy, not part of the main disc or system of planets, an object (like a globular cluster, oort body or haloist habitat) pursuing its own orbit that is highly eccentric relative to the plane of the ecliptic.
- Heliology - Text by M. Alan Kazlev
Science that studies stars, their composition, evolution, solar emissions, and so on. Applied heliology includes such disciplines as stellar engineering, sun mining, and starlifting.
- Helium Flash - Text by M. Alan Kazlev
Runaway and explosive helium fusion inside a star as it evolves off the main sequence and into the giant phase of evolution. It occurs when degenerate gas at the star's center reaches a temperature of about 108 K. It can be prevented, but this is costly and requires extensive stellar engineering
- Hertzsprung-Russell (H-R) Diagram - Text by M. Alan Kazlev
Graph that plots a star's temperature and spectral type against its luminosity and brightness, providing a complete typology and evolutionary chart. Most stars are "main sequence stars," and form the classic diagonal sequence on the diagram. Giants and Supergiants are above the main sequence, and white dwarfs are below it.
- Hohmann Transfer Orbit - Text by Martin Andreas Cieslik
The most energy efficient way between planets or other bodies in fixed orbits (e.g. lagrange points); an elliptic orbit tangent to both orbits (start and finish).
- Hot Luminous Stars - Text by Chris Clowes
Stars occupying the top left corner of the Hertzsprung-Russell Diagram. They are among the most spectacular inhabitants of the galaxy. They evolve quickly and encompass a range of stellar types during their lifetimes. They include Type O, Type Of, Ofpe/WN9, Wolf-Rayet, WNL, LBV, B[e] stars.
- Hot Neptunes - Text by Steve Bowers
Planets between 10 x Earth's mass and 0.08 x Jupiter's mass which orbit close enough to have a temperature of 800 Kelvin or more
- Hydrogen Burning - Text by M. Alan Kazlev
The process by which stars through nuclear fusion burn hydrogen, releasing vast quantities of energy and converting the hydrogen atoms into helium.
- Hyperbola (astronomy) - Text by M. Alan Kazlev
The orbital curve followed by any free-falling body moving faster than escape velocity.
- Hyperbolic Orbit - Text by M. Alan Kazlev
An orbit in which the eccentricity is greater than 1.
- Infrared Cirrus - Text by M. Alan Kazlev
Whispy interstellar dust clouds associated with the coldest parts of the interstellar medium, about 15 to 30° Kelvin.
- Interferometry - Text by M. Alan Kazlev
A means of obtaining high-resolution astronomical observations by linking several physically separated telescopes by datalink (e.g. buckytube cable, laser link, nanoguage wormhole), thus creating a single, much larger telescope. The largest interferometry device in the galaxy is the Argus Array.
- Interstellar Medium - Text by M. Alan Kazlev
Distributed gas, dust and other matter that is found throughout interstellar space.
- Interstellar Obscuration - Text by M. Alan Kazlev
Absorption of starlight by interstellar dust, causing distant objects to appear fainter.
- Interstellar Reddening - Text by M. Alan Kazlev
The process whereby interstellar dust scatters blue starlight, causing distant objects to appear redder and fainter.
- Iron Meteor/Meteorite - Text by M. Alan Kazlev
A meteor/meteorite composed of a nearly pure nickel-iron alloy.
- Janusian type worlds - Text by Steve Bowers
Named after the moon of Saturn, these are pairs of worlds with 1:1 resonant orbits which regularly exchange momentum.
- Kepler, Johannes - Text by M. Alan Kazlev
Old Earth German mathematician (298-339 BT; 1671-1630 AD) who first postulated that the planets revolve around the sun in elliptical orbits, rather than (as had previously been believed) spherical ones.
- Kuiper Belt - Text by M. Alan Kazlev
In the Solar System, a belt of icy objects beyond Neptune but closer than the Oort Cloud. In general, any icy belt outside the main system but closer than the local Oort cloud, if present.
- Kuiperian Type Planetoid - Text by John M. Dollan in his Planetery Classification List
Ice dwarf; Planetoid Class. Found in the outer reserves of a solar system, these are the remnants of planetary formation.
- Lagrange Points - Text by M. Alan Kazlev
In any system with one large and one small orbiting body (e.g. a planet and a moon, star and a planet, or a double planet or binary star), a set of five points where a still smaller body (e.g. a ship or habitat or smaller moon) would retain a fixed position with respect to the other two.
- Large Magellanic Cloud (LMC) - Text by M. Alan Kazlev
Irregular-shaped galaxy in the Local Group, some 163,000 light-years from Sol. It is thought that the shape is the result of a disturbance, perhaps a collision of two galaxies. A number of optimistic relativistic probes and linelayers have been launched towards the LMC by several empires and ai clades.
- Life-zone - Text by M. Alan Kazlev
The region around a star within which a planet can have a temperature allowing liquid water on the surface. Also known as bio-zone.
- Light-year - Text by M. Alan Kazlev
The distance light travels in one standard or metric (Earth) year, viz. 9.46x1012 km. Standard unit of measurement. It is equal to 6.323x104 astronomical units (AU) or 0.307 parsecs.
- Linde Scenario - Text by M. Alan Kazlev after Mitch Porter in Anders Sandberg's Transhuman Terminology
Scenario for indefinite survival of intelligent life. It assumes it is possible to either create basement universes connected to the original universe with a wormhole or the existence of other cosmological domains. Intelligent life continually migrates to the new domains as the old grow too entropic to sustain life. It is believed that a number of hyperturings, powers, and archailects may have or have already embarked on projects such as this; in fact the disappearance of some transcended powers may actually have been a transference to another universe. [After Andre Linde - early Information Age physicist who developed a chaotic inflation cosmology, where new universes are continually spawned.]
- Local Arm - Text by M. Alan Kazlev
The arm of the Milky Way Galaxy where most of Terragen civilization is still located, and which includes the Sol System. It is also called the Orion Arm.
- Local Bubble - Text by M. Alan Kazlev
"Low density" plasma region of space left by former supernova some tens of millions of year ago. Approximately equivalent to the Inner Sphere.
- Local Group - Text by M. Alan Kazlev
The cluster of galaxies to which the Milky Way and a number of nearby galaxies belong.
- Loop - Text by M. Alan Kazlev
A roughly spheroidal shell of interstellar gas blown outward from a star by a stellar explosion or strong stellar wind. Large bubbles are caused by supernova explosions and contain gas that is much hotter but also much more diffuse (about one atom per cubic cm) than that of the surrounding interstellar medium.
- Luminosity Class - Text by M. Alan Kazlev
In astronomy, the total brightness of a star; the total amount of energy in all wavelengths of light per second.
- Luminous Blue Variables - Text by Chris Clowes
Massive, intrinsically bright stars which display different scales of light and color variability, ranging from rapid micro-variations to rare outbreaks of catastrophic mass loss. They represent a very short-lived (perhaps as little as 40,000 years) strongly mass-losing phase in the evolution of massive stars, during which they undergo deep erosion of the outer layers before they enter the Wolf-Rayet phase.
- M-Type Star - Text by M. Alan Kazlev
The small, cool, red dwarf is the faintest in the sequence of spectral types, but also by far the most common type of star in the galaxy. These dim stars are distinguished by strong absorption bands of titanium oxide and many metallic lines.
- Magellanic Clouds - Text by M. Alan Kazlev
Irregular-shaped nearby galaxies. The Large Magellanic Cloud (LMC) is near the constellation Dorado (as seen from SolSys), and is 163,000 light-years away. The Small Magellanic Cloud (SMC) is near the constellation Tucana, and is under 200,000 light-years away. There have been at least two thousand major, and several million minor, relativistic expeditions mounted to each of the Clouds to date.
- Magnetar - Text by M. Alan Kazlev
A type of highly active neutron star with magnetic fields of about 10e15 Gauss.
- Main Sequence - Text by Steve Bowers
Main sequence stars are those which shine by the fusion of hydrogen into helium. These stars are generally dwarfs. Most main sequence stars are small red dwarfs of class M v; these have lifetimes of tens of billions of years. Sol , the star of the Old Solar System, is a larger G class yellow dwarf, and has a lifetime on the main sequence of ten billion years.
- Metallicity - Text by M. Alan Kazlev
When referring to stars this term is used slightly differently to the traditional chemical sense and refers to elements heavier than helium.
- Meteor - Text by M. Alan Kazlev
A small piece of stone or piece of ore travelling through space, almost always in a solar system. Most are left over from asteroidal or cometary debris. Large meteors are rare.
- Micrometeoroids - Text by M. Alan Kazlev
Dust-grain-sized stony or metallic particles found in deep space. They are not uncommon and a hazard to relativistic vessels.
- Milky Way - Text by M. Alan Kazlev
The spiral galaxy in which we live.
- Multiple Systems - Text by Chris Clowes
About 85% of all stars are in double or multiple systems.
- Nebula - Text by M. Alan Kazlev
A huge, diffuse cloud of gas and dust in interstellar space. The gas is mostly hydrogen (H2).
- Neutron Star - Text by M. Alan Kazlev
A very small dense star that is composed mostly of tightly-packed neutrons (neutronium).
- Non-Luminary World Classification Scheme - Text by John M. Dollan, with additions by Steve Bowers
The Non-Luminary World Classification Scheme, or NoLWoCS, is a standard classification method used to identify planetary bodies, minor worlds, and artificial structures.
- Norma Arm - Text by M. Alan Kazlev
The innermost arm of the Milky Way Galaxy, close to the central bar.
- Nova - Text by M. Alan Kazlev
A star that undergoes an explosion or sudden surge of energy, temporarily increasing its luminosity by up to 10 to 14 magnitudes. Unlike supernovae, novae retain their form and most of their substance after the explosion. Most novae are found in close binary systems with one component a red giant and the other a smaller hot, star which is the seat of the instability.
- Nucleus, Galactic - Text by M. Alan Kazlev
The central region of a galaxy; the galactic core.
- O-type Star - Text by M. Alan Kazlev
The O type star is the first and brightest in the sequence of spectral types, and is distinguished by lines of ionized helium. Often neutral helium and weak hydrogen lines are also visible. An O5 type star generally has a mass of about 40 times that of Sol, a luminosity 405,000 times as great, and and a surface temperature of some 40,000° Kelvin. The star is so hot it burns blue-white in colour.
- Omega Centauri - Text by Steve Bowers
Large globular cluster, target for several long range missions.
- Oort Cloud - Text by M. Alan Kazlev
A spherical shell around most stars containing numerous small bodies consisting of ice and dust. A typical Oort cloud is about 0.5 to 1 light years from its primary and has an aggregate mass about that of Earth.
- Open Star Clusters - Text by Chris Clowes
Loose structures that contain anywhere from a few dozen to a tens of thousand stars.
- Parabola - Text by M. Alan Kazlev; amended by Stephen Inniss
A conic section, a curve that is a set of points (P) such that the distance from a line (the directrix) to P is equal to the distance from P to focus F. A parabolic mirror will concentrate incoming light at a single point, or send out light from a source at that point in a collimated beam. In celestial mechanics parabolas have an eccentricity of 1, and an object in a parabolic orbit will swing past and change course but will not return since it is moving at escape velocity.
- Parsec - Text by M. Alan Kazlev
Unit of distance at which a star would have a parallax of 1 second of arc from a planet of exactly 1 astronomical unit (AU) distance from its primary. One parsec is equal to 3.262 light-years, 2.063x105 AU, or 3.085678 x1013 kilometers.
- Periapsis - Text by Stephen Inniss
In orbital dynamics, the point at which two objects in an elliptical orbit around their centre of gravity are at their closest approach. The opposite of apoapsis. Derivative terms are sometimes used for particular situations, as for instance perigee for a hab or moon orbiting a planet or perihelion for a planet orbiting a star.
- Perigee - Text by M. Alan Kazlev
For an object orbiting the Earth (or, since the Interplanetary Age, for any planet), the perigee is the point in each orbit which is closest to the primary. The perigee generally varies a small amount from orbit to orbit. The closest perigee is called the proxigee. The opposite term is apogee.
- Perimelasma - Text by Geoffrey Landis, in Anders Sandberg's Transhuman Terminology
The closest approach on an orbit around a black hole.
- Perseus Arm - Text by M. Alan Kazlev
The immediately rimward spiral arm of our galaxy.
- Planet - Text by M. Alan Kazlev
A solid (or partially liquid) body usually found orbiting a star. Planets are too small to generate energy by nuclear reactions. One of six categories of matter that make up solar systems, nebula, and galaxies, and can be harvested for interplanetary or interstellar development (the other categories are interstellar gas, dust, planetisimals, brown dwarfs, and stars).
- Planetary Nebula - Text by M. Alan Kazlev
An expanding shell of gas - actually the surface layers - ejected from a medium-mass star during its last stages of evolution.
- Planetesimal - Text by M. Alan Kazlev
One of the small bodies in a solar nebula, from which protoplanets may form. They usually range from micrometers to kilometers in diameter. On the larger scale, planetesimals shade into asteroidal class planetary type bodies.
- Population I - Text by M. Alan Kazlev
Stars with a few percent heavy elements (heavier than helium), found in the disks of spiral galaxies and in irregular galaxies. Includes "metal- rich" stars, which are the only ones around which planets form. They were formed from Population II stars.
- Population I, Extreme - Text by M. Alan Kazlev
Short-lived bright, metal-rich stars, representing the most extreme form of Population I. Stars of this type are found mainly in the spiral arms of the Galaxy, with generally circular orbits around the galactic core. about the central nucleus. Extreme Population I stars are young with age ranges of less than 100 million, and often only around 20 to 50 million years. They have average metallicities of about 3%. Spectral classes O, B1 and B2 as well as including T Tauri stars and certain supergiants and high mass Cepheid variables those with periods in excess of 10 to 13 metric days are included here.
- Population I, Intermediate - Text by M. Alan Kazlev
Typical, moderately metal-rich stars. Stars of this type (which includes Sol) move in mostly circular (slightly elliptical) orbits around the galactic core, and are distributed throughout the entire galactic disc. Age ranges from 200 million to 10 billion years. They have average metallicities of about 1 to 2%. Almost all Gaian Type worlds are found around this type of star.
- Population II - Text by M. Alan Kazlev
Stars composed of nearly pure hydrogen and helium, found in globular clusters (galactic halo) and the center of spiral galaxies, in elliptical galaxies, and to a limited extent in irregular galaxies. They were formed from Population III stars. They almost never have planets.
- Population II, Extreme - Text by M. Alan Kazlev
Very old metal-poor stars, representing the most extreme form of population II. These are the oldest stars with ages of around 10 to 14 billion years. Their metallicity is very low, usually less than 0.8%, and they are found mainly in highly elliptical orbits in the galactic halo, particularly in globular clusters. They are mostly low mass stars that have left the main sequence, although RR Lyrae variables and somewhat more massive red giants are also included here.
- Population III - Text by M. Alan Kazlev
A class of bright and short lived stars that formed previous or synchronous with the actual formation of the Galaxy. Their existence was theorized during the Atomic Age as an attempt to describe the metallicity present in population II stars where nucleosynthesis should not have been able to produce heavy elements. Population III have almost all burned out. During the Interplanetary period long range arrays discovered remnant objects that were found to be Population III.
- Pre-Main-Sequence - Text by M. Alan Kazlev
Evolutionary state of a star prior to arrival on the main sequence, especially just before the main sequence is reached.
- Primary - Text by M. Alan Kazlev
The planet or star around which an orbital, moon, planet, or other permanent satellite revolves.
- Prominent Star - Text by M. Alan Kazlev
One of the brightest stars in the sky, as seen from a particular location, but not necessarily one of the nearest.
- Protoplanet - Text by M. Alan Kazlev and Steve Bowers
Massive object resulting in the colescence of planetesimals in the solar nebula which eventually becomes a planet. Has a thick atmosphere and greater mass than a formed planet. Because of the potential for megascale engineering, protoplanets are often highly sought after by interstellar development corporations and certain clades and hyperturings.
- Protostar - Text by M. Alan Kazlev
A gravitationally stable cloud of interstellar gas and dust of stellar mass contracting in an early pre-main-sequence evolutionary state. Because of the dense and easily harvestable concentration of interstellar dust and gas, protostars are highly sought after by interstellar development corporations and certain clades and hyperturings.
- Pulsar - Text by M. Alan Kazlev
A rapidly rotating neutron star with a strong magnetic field, observed to emit pulses of radiation. Useful as an energy source in some megascale engineering projects, weylforges, etc.
- Red Dwarf - Text by M. Alan Kazlev
A small cool faint main sequence star, spectroscopic class M. Because they are so cool they only burn slowly, and hence live for a very long time. These are the most common stars; there are about two thousand per million cubic light-years of galactic space. They are located at the bottom right of the Hertzsprung-Russell diagram.
- Red Giant - Text by M. Alan Kazlev
An old, post-main-sequence, star, spectroscopic class K or M, whose surface layers have expanded to many solar radii and have relatively low temperatures. However, because they are so large, they are also very luminous. They are located at the middle right of the Hertzsprung-Russell diagram.
- Rift - Text by M. Alan Kazlev
Expanse of interstellar space where are relatively few stars; usually between two spiral arms.
- Roche's Limit - Text by M. Alan Kazlev
The distance from a large body or primary within which tidal forces would disrupt or disintegrate a satellite.
- Rosette Nebula - Text by Steve Bowers
Nebula in Monoceros.
- S-Process Reactions - Text by M. Alan Kazlev
Slow reactions in giant stars in which heavy elements are built up as atomic nuclei capture neutrons. (See also r-process reactions.)
- Sagittarius A* - Text by M. Alan Kazlev
Supermassive black hole in the centre of the Milky Way. It is surrounding by concentric rings of rapidly expanding matter, evidence of a massive explosion.
- Sagittarius Arm - Text by M. Alan Kazlev
The spiral arm next closer (i.e. coreward) to the galactic core than the local arm.
- Sagittarius Dwarf Elliptical Galaxy (SagDEG) - Text by M. Alan Kazlev
This minor galaxy has been spread out by a still on-going collision with the Milky Way. The closest parts of this remnant have been colonised by the Red Star M'Pire.
- Satellite - Text by M. Alan Kazlev
Any small body orbiting a larger body. May be natural (e.g. a moon) or artificial (a ship or orbital).
- Schwarzschild Radius - Text by M. Alan Kazlev
The radius corresponding to the event horizon of a black hole, proportional to the mass of the black hole.
- Scutum-Crux (or Crux-Scutum) Arm - Text by M. Alan Kazlev
The spiral arm next closer to the galactic core (i.e. coreward) than the Sagittarius Arm.
- Shepherd Satellite - Text by M. Alan Kazlev
Satellite that moves near a planetary ring and, through its gravitational field, acts to confine the ring particles onto certain orbits.
- Sidereal - Text by M. Alan Kazlev
Referring to stars, especially as regards observation, measurement, and time-keeping.
- Spectral Class - Text by M. Alan Kazlev
A class to which a star belongs because of its spectrum, which in turn is determined by its temperature. The spectral classes are O, B, A, F, G, K, and M, from hottest to coolest.
- Spiral Arm - Text by M. Alan Kazlev
In spiral galaxies, a long spiral pattern of bright stars (including O and B stars), interstellar matter, open clusters, and nebulae; extending from the center to the galactic disk. In our Milky Way galaxy, the spiral arms are the Norma, Scutum-Crux, Sagittarius Orion-Cygnus (or Local), Perseus, and Outer Arms.
- Star - Text by The Astronomer 2023
Celestial bodies which are sufficiently massive to naturally achieve a significant rate of nuclear fusion reaction of hydrogen to helium in their interior at some point in their lives.
- Starburst - Text by M. Alan Kazlev
A relatively sudden and rapid episode of star formation in a galaxy, probably triggered in some cases by collision with another galaxy.
- Stellar Magnitude - Text by Chris Clowes
The "brightness" of a star, expressed on a logarithmic scale, so that brighter stars have smaller magnitude values. Thus a first magnitude star is very bright, a sixth magnitude star is at the limit of normal baseline hu vision. See also Apparent Magnitude, Absolute Magnitude.
- Superbubble - Text by M. Alan Kazlev
A large volume of hot gas in interstellar space, formed by coalescence of bubbles blown around supernovae.
- Supergiant - Text by M. Alan Kazlev
A very large (10 to 1000 times the diameter of Sol), extremely luminous star in the uppermost part of the H-R Diagram. Supergiants generally result from hot bright O and B class stars exhausting their hydrogen and moving off the main sequence. Supergiants rarely last more than several millions of years. Betelgeuse, Deneb, Rigel A, and Mu Cephei are all supergiants.
- Supernova - Text by M. Alan Kazlev, amended by Steve Bowers
A very energetic stellar explosion expending as much as 1044 joules and blowing off most of the star's mass, leaving a dense core (white dwarf, neutron star or black hole). The explosion fades in a year or two, although an expanding shell of gas speeds outward at about 10,000 to 20,000 km.sec, carrying about a fifth of the mass of the star (proportion varies).. Colliding with the interstellar medium the expanding shell can sweep up even more gas to become a supernova remnant
- Supernova Remnant - Text by M. Alan Kazlev
The expanding and cooling shell of gas and dust that is visible for thousands of years after a supernova. After a few tens of thousands of years supernova remnants mix with the interstellar medium and dissipate.
- T Tauri star - Text by Chris Clowes
Prototype for a class of very young stars, still in the process of gravitational contraction. They constitute a type of variable star, often shedding mass, still forming and contracting; and yet to evolve to reach the main sequence. On a Hertzsprung-Russell Diagram, they plot above the main sequence. T Tauri stars are mostly between 105 and 108 years in age; of low mass (0.5 to 3.0 M sol); surrounded by hot, dense envelopes; and losing mass via stellar winds with typical velocity of around 100 km/s.
- Tidal Force - Text by Stephen Inniss
A secondary effect of the force of gravity, arising from the fact that gravitational effects are not constant across the width of an object: the portions nearer the source of gravity are subject to a stronger pull, and the portions further away are subject to a weaker pull.
- Trojan Asteroids - Text by M. Alan Kazlev
Asteroids caught near the Lagrangian points in a planet's orbit, 60° ahead of and 60° behind the planet. The original Trojans were connected to Jupiter's orbit, but the term is used to designate asteroids or other similar bodies in a relation to any large planet.
- Universe - Text by M. Alan Kazlev
A self-contained cosmos or a part of a cosmos.
- Variable Star - Text by M. Alan Kazlev
Star whose brightness changes regularly. They can have periods ranging from minutes to years. The apparent changes in brightness are caused by different phenomena; some change in size, some eject material, and others are in pairs that periodically obscure and enhance each other.
- W-Virginis Stars - Text by Chris Clowes
Whereas the classical d type Cepheids are fairly closely concentrated in the galactic plane, other Cepheids have been found at high galactic latitudes with considerable velocity components perpendicular to the galactic plane, suggesting they belong to population II. These are the W-Virginis stars. Their light curves and periods - mostly about 18 days - are similar to classical Cepheids, but there are some differences. For a given period, the W-Virginis stars are about 1.5 to 2.0 magnitudes less than classical d cepheids. On the average, W-Virginis stars are of an earlier spectral type, and emission lines sometimes occur in their spectra.
- White Dwarf - Text by The Astronomer 2023
Remnants of stars with insufficient mass to naturally go supernova, held up by electron degeneracy pressure.
- Wolf-Rayet Star - Text by Trolligi
Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. They are characterized by an extraordinary spectrum which is dominated by emission lines of highly ionized elements.
- Year - Text by Trent Shipley
The time it takes for a body in orbit around a star to complete a revolution around the star, variously defined, or a Standard Year, the orbital period of Old Earth, as used in the widespread Tranquility Calendar.
Text by M. Alan Kazlev
from the original by Robert J. Hall
Initially published on 04 October 2001.