Term Index (O)

Astrophysics (Index)

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Term Index (O)

Astrophysical terms and names of things beginning with the letter O.

O1 O2 O3 O4 O5 OA OB OC OD OE OF OG OH OI OK OL OM ON OO OP OR OS OT OU OV OW OX OZ

O (O-type star): star with spectral characteristics of above 30000 K
O (oxygen): element, O, atomic number 8
O: see Ohio Radio Survey
O star (O-type star): star with spectral characteristics of above 30000 K
OΣ: see Struve Catalogs
OΣΣ: see Struve Catalogs
O-class: see O-type star
O-class star (O-type star): star with spectral characteristics of above 30000 K
O-type star: star with spectral characteristics of above 30000 K
O1: see LIGO
O1/O2 Catalog: see Gravitational Wave Transient Catalog
O2: see LIGO
O3: see LIGO
O3a Catalog: see Gravitational Wave Transient Catalog
O3b Catalog: see Gravitational Wave Transient Catalog
O4: see LIGO
O5 (O5 spectral type): spectral type of star with a temperature of about 45000 K
O5 spectral type: spectral type of star with a temperature of about 45000 K
OAC: see Astrocats
OAO (Okayama Astrophysical Observatory): observatory at Mt. Chikurinji
OAO (Orbiting Astronomical Observatory): series of 1960s-1970s NASA space telescopes
OAO-1: see Orbiting Astronomical Observatory
OAO-2: see Orbiting Astronomical Observatory
OAO-3: see Orbiting Astronomical Observatory
OAO-B: see Orbiting Astronomical Observatory
OASIS: see William Herschel Telescope
OB association: stellar association including O and/or early B class stars
- OB star: a hot star with class O or early B
- stellar association: like a large open cluster but even less tightly bound
OB star: a hot star with class O or early B
- OB association: stellar association including O and/or early B class stars
OB supergiant: see supergiant
Oberon: see Uranus
- moon: a planet's natural satellite, such as Earth's
object (astronomical object): planet, star, galaxy, etc.
- LDN 1527: dark nebula in Taurus
- LDN 1689: dense core within the Rho Ophiuchi Cloud Complex
- moon: a planet's natural satellite, such as Earth's
- Oumuamua: small body that passed through the solar system in 2017
object type

accretion disk: disk around an astronomical body of material that is accreting
asteroid: minor planet without comet characteristics
binary minor planet: minor planet with another body orbiting it
binary SMBH: co-orbiting supermassive black holes
black hole: region of such high mass that light cannot escape
bright nebula: cloud that gives off or reflects light
bulge: denser group of stars within a larger group
candidate companion: possible object apparently orbiting a known object
centaur: asteroid-like body orbiting among the outer planets
circumplanetary disk: disk of dust and gas around a planet
circumstellar disk: disk or ring shaped accumulation of matter around a star
circumstellar envelope: shell-like cloud surrounding a star
cloud: higher-density region of space
comet: Sun-orbiting object with coma and possible tail
compact object: dense astronomical body such as white dwarf or denser
damocloid: minor planet with a comet-like orbit
damped Lyman alpha absorber: object producing broadened lines in a Lyman alpha forest
dark nebula: cloud that obscures stars behind it
decretion disk: disk around an astronomical body of its ejected material
dwarf planet: round like a planet, only smaller
emission nebula: nebula that emits light such as an HII region or PN
emission-line object: source of interest due to an emission line
filament: linear volume of high molecular gas density within a galaxy
galactic worm: high density streak of HI perpendicular to galactic plane
galaxy: gravitationally-bound group of stars
galaxy cloud: substructure of a supercluster
galaxy cluster: large group of galaxies gravitationally-bound to the group
galaxy filament: thread-like formations of galaxies, 50 to 80 megaparsecs
globular cluster: spherical group of stars orbiting a galaxy
Herbig-Haro object: type of glowing nebula near protostars
HI supershell: large shells bordered by HI regions
host: object such as star around which something is orbiting or drawn to
host galaxy: galaxy associated with a particular object or transient
host planet: the planet associated with a moon or ring, etc.
host star: the star associated with a planetary system
infrared source: astronomical object producing observable infrared
integrated flux nebula: fuzzy illuminated nebula within Milky Way halo
intergalactic HI cloud: cloud of HI not within any galaxy
intermediate-mass black hole: black hole between stellar mass and supermassive
jet: stream of matter from an astronomical body
Keplerian disk: disk whose motion follows Kepler's laws
large quasar group: structure consisting of a collection of quasars
long-period comet: comet with an orbital period of more than 200 years
Lyman-alpha blob: Lyman-alpha emitting cloud larger than a galaxy
MACHO: proposed astronomical objects comprising dark matter
Mars crosser: asteroid with an orbit that crosses that of Mars
meteorite: rock found on Earth that is the remnant of a meteor
meteoroid: rocky object in solar system, smaller than an asteroid
microquasar: stellar-sized black hole with quasar-like activity
millisecond pulsar: pulsar spinning with a period in the range of milliseconds
minor planet: lesser solar system body without comet characteristics
moon: a planet's natural satellite, such as Earth's
near-Earth object: object orbiting the Sun that passes close to Earth
nebula: cloud of dust and gas
neutrino hotspot: region of sky from which significant neutrinos have been detected
odd radio circle: extragalactic ring-shaped radio source noticed in 2020
oligarch: term for large planetesimals that dominate
planet: round object orbiting a star that clears out its own orbital path
planetary embryo: object that may grow into a future planet
planetary mass object: has the mass of a planet
planetary nebula: glowing gas shell ejected by a red giant
planetesimal: object held together by gravity that combines with others to form planets
planetoid: an asteroid or physically similar body
plutoid: Pluto-like dwarf planet
point source: celestial object that focuses to a point or single Airy disk
primordial black hole: black holes forming at the most dense volumes when inflation ceased
protoplanet: simplest, least-developed object classified as a planet
protoplanetary disk: disk of dust and gas around a young star or protostar
protoplanetary nebula: short AGB phase before planetary nebula
pseudobulge: like a galaxy bulge only flatter
pulsar: type of astronomical body with EMR in regular cycles of seconds or less
pulsar wind nebula: type of nebula generated by a young pulsar
quasar: distant, bright object that appears something like a star
quasi-satellite: body that orbits near another though they aren't bound
radio source: astronomical object producing observable radio
RAMBO: proposed astronomical objects comprising dark matter
reflection nebula: nebula made visible by light it scatters from nearby stars
ring system: disk or set of rings orbiting a planet
shell: hollow shaped region of some kind of ISM
soft gamma repeater: object that produces repeated gamma-ray bursts
solar system object: the Sun, solar system planets, minor planets, comets, and moons
source: astronomical object producing some observed signal
SSSB: solar system body smaller than a dwarf planet
star: astronomical body shining from fusion energy
stellar stream: long group of co-moving stars
stellar-mass black hole: black hole that is a stellar remnant
subsatellite: object orbiting a moon
supercluster: cluster of galaxy clusters
supermassive black hole: black hole with a mass of millions to billions of solar masses
supernova remnant: nebula of debris resulting from a supernova
supershell: very large ISM shell-shaped region
telluric star: star of known spectrum used for calibration
Tolman VII object: theoretical object adhering to exact GR solution
trans-Neptune object: minor planet beyond Neptune
transitional disk: disk with inner part cleared
very low luminosity object: term sometimes used for protostars with luminosity less than 0.1 solar
vulcanoid: theoretical asteroid orbiting within Mercury's orbit
WIT: "what is it" for an unexplainable object
X-ray burster: neutron star binary producing nova-like bursts
X-ray pulsar: pulsar producing X-rays from accretion
X-ray source: astronomical object producing observable X-ray

objective lens: see refracting telescope
objects

astronomical catalog: list of observed astronomical bodies including characteristics
coeval: the same age
demographics: statistics of characteristics within a group
natural astronomical telescopes: use of an astronomical body as a telescope
phase curve: brightness of an object according to its phase angle
rare designator prefixes: designator prefixes generally used for only a few objects
rotation period: time it takes an astronomical body to revolve
SIMBAD: online database of astronomical objects

oblate spheroid: see spheroid
oblateness: see J₂
obliquity: angle between an object's rotational and orbital axes
obs. (observatory): facility for astronomical observation
obscured fraction: see obscured fraction-luminosity relation
obscured fraction-luminosity relation: relation between AGN luminosity and surrounding dust IR emission
obscured star formation

dusty galaxy: galaxy including substantial dust

observable: any physical phenomenon that you can detect
observable universe: that part of the universe that we can conceivably observe
- particle horizon: furthest point from which light can reach us
observation

band shifting: effect of redshift on photometry
dropout: means of identifying distant galaxies
field: adjective meaning "in this field of view but unrelated"
striae: parallel ridges
surface density: quantity such as mass per unit area
synthetic field method: method of testing galaxy opacity
telluric star: star of known spectrum used for calibration
tracer: chemical whose detection indicates the presence of something else

Observation Run 1: see LIGO
Observation Run 2: see LIGO
Observation Run 3: see LIGO
observational astronomy: term covering observation
Observatoire de la Côte d'Azur (Côte d'Azur Observatory): French astronomical research organization
observatories

observational astronomy: term covering observation

observatorio (observatory): facility for astronomical observation
Observatorio de Calar Alto (Calar Alto Observatory): observatory on mountain in mainland Spain
Observatorio del Roque de los Muchachos (Roque de los Muchachos Observatory): observatory at La Palma in the Canary Islands
Observatorio del Teide (Teide Observatory): observatory on Mount Teide in the Canary Islands
observatory: facility for astronomical observation
- Automated Lunar and Meteor Observatory: ground observatory in Alabama to record Moon-meteoroid impacts
- Baksan Neutrino Observatory: Russian underground particle detector
- Beijing Astronomical Observatory: part of Chinese Academy of Sciences
- Big Bear Solar Observatory: US government sponsored solar observatory
- Bohyunsan Optical Astronomy Observatory: on Bohyeon Mountain, Korea
- Borexino: European underground neutrino observatory
- Burst and All Sky Imaging Survey: late 1990s GRB-observer satellite proposal
- Côte d'Azur Observatory: French astronomical research organization
- Calar Alto Observatory: observatory on mountain in mainland Spain
- CASA-MIA: 1990s cosmic-ray detector in Utah
- Cerro Tololo Inter-American Observatory: NOIRLab observatory in Chile with Blanco telescope
- Cosmic Explorer: concept for a very large LIGO-like GW detector
- DECIGO: proposed Japanese space mission to detect gravitational waves
- Dominion Astrophysical Observatory: long-standing observatory in British Columbia
- Dominion Radio Astrophysical Observatory: radio observatory in British Columbia, Canada
- East Asian Observatory: operator of JCMT
- Einstein Telescope: concept for a larger LIGO-like GW detector
- European Gravitational Observatory: gravitational wave observatory in Italy
- European Southern Observatory: organization operating telescopes in the southern hemisphere
- EUSO-SPB: balloon-borne cosmic ray observatory
- Fenton Hill Observatory: New Mexico GRB-oriented astronomical observatory
- Five College Radio Astronomical Observatory: former observatory near Springfield MA
- Fly's Eye: 1980s cosmic-ray observatory
- Fred Lawrence Whipple Observatory: observatory near Amado, Arizona
- Gemini Observatory: pair of telescopes in Hawaii and Chile
- GEO600: ground-based gravitational-wave detector in Germany
- Green Bank Observatory: radio observatory in West Virginia including GBT
- Haleakala Observatory: observatory on Maui, Hawaii
- Hat Creek Radio Observatory: California observatory with the Allen Telescope Array
- High Altitude Observatory: NCAR unit observing and monitoring the Sun
- High Resolution Fly's Eye: 1990s-2000s cosmic-ray observatory
- IceCube: neutrino detector at the South Pole
- IMB: 1980s neutrino detector
- Interstellar Boundary Explorer: satellite observing the ISM/heliosphere boundary
- Jodrell Bank Observatory: radio observatory in England
- KAGRA: gravitational wave detector in Japan preparing for operation
- Kamioka Observatory: Japanese underground particle detector
- Kitt Peak National Observatory: NOIRLab observatory in Arizona with Mayall telescope
- KM3NeT: neutrino detector under construction in the Mediterranean Sea
- Kumamoto Civil Astronomical Observatory: Japanese amateur observatory
- La Silla Observatory: Atacama-desert observatory run by ESO
- La Silla Paranal Observatory: manages three ESO observatory-sites in Chile
- Las Campanas Observatory: observatory in southern Atacama Desert
- Las Cumbres Observatory: world-wide network of telescopes
- LHAASO: gamma ray/cosmic ray observatory in China
- Lick Observatory: California observatory of long standing
- LIGO: pair of observatories to detect gravitational waves
- LIGO-India: planned gravitational wave detector in India
- LISA: proposed space mission to detect gravitational waves
- LiteBIRD: planned spacecraft to map CMB polarization
- Llano de Chajnantor Observatory: group of observatories on a high plateau of the Atacama Desert
- Long Baseline Array: radio interferometer using Australian telescopes
- Lowell Observatory: Arizona observatory of long standing
- Maui Space Surveillance Site: USAF observatory to track Earth-orbiting objects
- Mauna Kea Observatories: observatories on Mauna Kea in Hawaii
- Mauna Loa Solar Observatory: solar observatory on Mauna Loa
- Mount Wilson Observatory: Los Angeles observatory with Hooker telescope
- Mullard Radio Astronomy Observatory: radio observatory near Cambridge, UK
- National Astronomical Observatory of Japan: Japanese astronomy organization
- National Optical Astronomy Observatory: previous name of US optical astronomy organization
- National Radio Astronomy Observatory: US radio astronomy organization
- National Solar Observatory: US government sponsored solar observatory
- New Gravitational Wave Observatory: past proposal for space mission to detect gravitational waves
- Okayama Astrophysical Observatory: observatory at Mt. Chikurinji
- Owens Valley Radio Observatory: group of radio telescopes in California near the VLBA
- Palomar Observatory: California observatory with Hale telescope
- Paranal Observatory: ESO Atacama observatory with ESO VLT
- Pierre Auger Observatory: cosmic-ray observatory in Argentina
- POEMMA: planned space cosmic-ray neutrino observatory
- Queen's University Radio Observatory: early radio astronomy facility near Kingston, Ontario
- Roque de los Muchachos Observatory: observatory at La Palma in the Canary Islands
- Royal Greenwich Observatory: former UK national observatory
- Siding Spring Observatory: observatory in New South Wales, Australia
- Simons Observatory: CMB observatory on Atacama Desert
- South African Astronomical Observatory: South African observatory with SALT telescope
- Spaceguard Centre: UK private observatory dedicated to NEOs
- Spektr-RG: Russian-German X-ray observatory launched in 2019
- STELLA: two robotic telescopes in Canary Islands
- Sudbury Neutrino Observatory: Canadian underground neutrino detector
- Table Mountain Observatory: NASA observatory near Los Angeles
- TAMA 300: small prototype gravitational wave detector in Japan
- TAMBO: concept for neutrino detector in valley in Peru
- TD-1A: 1970s European observatory-satellite with UV telescopes
- Teide Observatory: observatory on Mount Teide in the Canary Islands
- Telescope Array Project: cosmic-ray observatory aiming for extremely energetic particles
- TianQin: proposed Chinese space mission to detect gravitational waves
- Tonantzintla Surveys: surveys providing early images of quasars and Seyfert galaxies
- Tunka experiment: Russian Cherenkov detector aimed at cosmic rays and gamma rays
- USNO: observatory obtaining astronomical data for Naval use
- Vainu Bappu Observatory: observatory in southern India
- Very Long Baseline Array: radio interferometer stretching 8000 km
- Virgo: ground-based gravitational-wave detector in Italy
- Wendelstein Observatory: German observatory on Mount Wendelstein
- Wilkinson Microwave Anisotropy Probe: spacecraft that measured variations in the cosmic background radiation
- XENON10: dark matter detector in Italy
Observatory of Madrid

Calar Alto Observatory: observatory on mountain in mainland Spain

observed color index: see color index
observed wavelength: see rest wavelength
observing program (astronomical survey): project collecting astronomical data
OC (open cluster): group of stars within a galaxy apparently formed by a single cloud
OCA (Côte d'Azur Observatory): French astronomical research organization
occlusion: see occultation
occultation: one celestial body hiding another from an observer
- occultation observations: gathering astronomical information through measuring occultations
occultation observations: gathering astronomical information through measuring occultations
occultation spectrography: see spectroscopy
occultation spectroscopy: see transit spectroscopy
- earthshine: reflected and emitted electromagnetic radiation from Earth
- exoplanet eclipse light curve: graph of light as an exoplanet transits in front of its star
- secondary eclipse: eclipse of a body that also transits
occultations

International Occultation Timing Association: volunteer organization to observe occultations

occulter: see coronagraph
occulter: see occulting bar
- Exo-S: a study on the concept of a starshade mission
- New Worlds Mission: plan for space occulter to facilitate the view of exoplanets
- starshade: spacecraft to block light from a specific body
occulting bar: telescope feature to block light from a specific body
ocean planet (water world): planet with substantial liquid water
ocean world (water world): planet with substantial liquid water
- hydrology: study of the movement of water
oceanic carbon cycle

carbon dioxide: compound of carbon atom and two oxygen atoms

Ocl: see rare designator prefixes
OCO (Oort Cloud Object): object in Oort Cloud such as a comet
OCPP: see Pioneer Venus Orbiter
octopole: see multipole expansion
octupole: see multipole expansion
- perturbation theory: breaking an equation into a solvable part and approximable part
octupole level: see multipole expansion
octupole term

perturbation theory: breaking an equation into a solvable part and approximable part

odd radio circle: extragalactic ring-shaped radio source noticed in 2020
ODE: see partial differential equation
ODI: see WIYN 3.5m Telescope
Odyssey: research supercomputer at Harvard sometimes used for astrophysics simulations
Oe star: see Be star
OEC (Open Exoplanet Catalogue): independently maintained list of known exoplanets
OEFD: see Pioneer Venus Orbiter
OETP: see Pioneer Venus Orbiter
off the shell: see mass shell
off-axis telescope: telescope
- Herschelian telescope: type of off-axis reflector telescope
OFTI: see orbitize
OGBD: see Pioneer Venus Orbiter
OGC (Ogle Galaxy Catalog): catalog of luminous galaxies
OGLE: survey searching for microlensing transients
- Ogle Galaxy Catalog: catalog of luminous galaxies
Ogle Galaxy Catalog: catalog of luminous galaxies
OGLE-2005-BLG-390L: see OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb: probable rocky planet orbiting red dwarf
OGS: see HEAO-2
OH (hydroxyl): one hydrogen atom and one oxygen atom bound together
- OH/IR source: AGB star with strong OH maser
OH airglow: see airglow
OH maser

OH/IR source: AGB star with strong OH maser

Oh-My-God Particle: extremely energetic particle from space detected in 1991
OH/IR source: AGB star with strong OH maser
OH/IR star (OH/IR source): AGB star with strong OH maser
OHIO: see Ohio Radio Survey
Ohio

IMB: 1980s neutrino detector

Ohio Radio Survey: 1960s radio survey by Ohio State University
Ohio Sky Survey (Ohio Radio Survey): 1960s radio survey by Ohio State University
Ohio State University Radio Observatory

Ohio Radio Survey: 1960s radio survey by Ohio State University

ohm: see siemens
Ohm's law: see telegrapher's equations
ohm-meter: see siemens
Ohmic heating: heating resulting from an electric current through a material
- inflated radii: exceptionally large radii of some hot Jupiters
OHP 1.93 m

51 Pegasi b: first exoplanet found orbiting a main sequence star
HD 189733 b: hot Jupiter orbiting an orange dwarf
HD 209458 b: hot Jupiter discovered in 1999

OHS: see Subaru Telescope
OIG: see Telescopio Nazionale Galileo
OIMS: see Pioneer Venus Orbiter
OIR: "band name" that means both visible light and infrared
OIR: see Pioneer Venus Orbiter
OIR-dark galaxy: see dropout
Okayama Astrophysical Observatory: observatory at Mt. Chikurinji
Oke-Gunn photometric system (griz photometric system): photometric system designed for electronic sensors
OL: see double star designation
Olbers' paradox: argument using the dark of the sky regarding the nature of the universe
oligarch: term for large planetesimals that dominate
- isolation mass: mass of an oligarch after accreting all nearby planetesimals
oligarch theory: see oligarch
oligarch theory: see planet formation
oligarchic accretion: see oligarch
oligarchic growth: see oligarch
olivine

methane: compound of one carbon and 4 hydrogen atoms

OLR: see Lindblad resonance
Olympus Mons: see Mars
OM: see XMM-Newton
OMAG: see Pioneer Venus Orbiter
OMAyA: see Large Millimeter Telescope
OMC: see INTEGRAL
OMC: see Orion Molecular Cloud Complex
OMEGA: see Mars Express
Omega Centauri: large globular cluster
Omega-Prime

Munich Near-Infrared Cluster Survey: 1990s-2000s galaxy cluster survey

OmegaCAM: see VLT Survey Telescope
OMM 1.6m

SIMP: 2000s infrared proper-motion survey

on the shell: see mass shell
On-Orbit Servicing, Assembly and Manufacturing: see OSAM-1
ONC (Orion Nebula Cluster): open cluster in the constellation Orion
ONC: see Hayabusa2
one dimensional climate model: atmospheric model ignoring all horizontal motion
one dimensional model: see three dimensional model
- one dimensional climate model: atmospheric model ignoring all horizontal motion
- subgrid-scale physics: shortcuts handling physics on a smaller scale than a simulation's grid
one meter barrier (meter size barrier): a barrier to planet formation on the order of a meter diameter
ONMS: see Pioneer Venus Orbiter
Onsala: see rare designator prefixes
Ontario

Canadian Institute for Theoretical Astrophysics: national research organization at University of Toronto
Queen's University Radio Observatory: early radio astronomy facility near Kingston, Ontario

Oo: see rare designator prefixes
Oort Cloud: cloud of distant comets surrounding the solar system
- comet: Sun-orbiting object with coma and possible tail
Oort Cloud Object: object in Oort Cloud such as a comet
Oort constants: two constants characterizing local Milky-Way motion
Ooty Radio Telescope: cylindrical radio telescope in India
Ooty Wide Field Array: see Ooty Radio Telescope
OPA: see Pioneer Venus Orbiter
opacity: a measure of opaqueness of a material
- absorption coefficient: measure of how much light a substance absorbs
OPAG (Outer Planets Assessment Group): NASA forum to provide input to long-term planning of outer planets missions
OPE: see Giotto
Open Astronomy Catalog: see Astrocats
open cluster: group of stars within a galaxy apparently formed by a single cloud
- Hyades Cluster: nearest open cluster
- Messier 67: old open cluster in Cancer
- NGC 346: open cluster in SMC
- Orion Nebula Cluster: open cluster in the constellation Orion
- Pleiades: visible open cluster with seven B-stars
- stellar dynamics: stars' gravitational interaction
- Trapezium Cluster: open cluster of stars in Orion Nebula
open Dragone telescope: see Dragone telescope
Open Exoplanet Catalogue: independently maintained list of known exoplanets
Open Kilonova Catalog: see Astrocats
open magnetic field lines

curvature radiation: synchrotron-like radiation when particles follow curved field lines

Open Supernova Catalog: see Astrocats
- MOSFiT: code to match light-curves with light-curve models
Open TDE Catalog: see tidal disruption event
open universe: see critical density
open-Dragone design: see Dragone telescope
OpenSpace: visualization software aimed at astrophysical data
Oph-L 1689 (LDN 1689): dense core within the Rho Ophiuchi Cloud Complex
Ophelia: see shepherd moon
Ophiuchus Molecular Cloud (Rho Ophiuchi Cloud Complex): nebula that is a nearby star-forming region
- Elias 2-27: much-studied YSO
OPI: see double star designation
Opportunity: see Mars Exploration Rover
opposition: positioning of two astronomical objects 180 degrees apart in the sky
optical

visible light: electromagnetic radiation that our eyes can sense

optical and infrared (OIR): "band name" that means both visible light and infrared
optical astronomy: see visible light
optical axis: center line through an optical instrument
- astigmatism: focusing differently according to direction across the focal plane
- off-axis telescope: telescope
optical coma

aberration: types of flaws in telescope images

optical counterpart: see counterpart
optical depth: a measure of opaqueness of a path of light
optical depth to the EOR: see Lambda-CDM model
optical double: double star where the two are not gravitationally bound
- binary star: system of two stars co-orbiting
optical fiber

fiber positioner: machine to set up fibers in multi-object spectrograph

optical filter (filter): optical device to filter out part of the EMR spectrum
Optical Gravitational Lensing Experiment (OGLE): survey searching for microlensing transients
- Ogle Galaxy Catalog: catalog of luminous galaxies
optical interferometer: interferometer of visible light
- CHARA: optical interferometer on Mount Wilson
- COAST: optical interferometer in England
- European Southern Observatory Very Large Telescope: four 8.2-meter optical telescopes in Chile that can act together
- GI2T: optical interferometer in France
- intensity interferometer: interferometer based upon interference from intensity variations
- IOTA: past optical interferometer in Arizona
- Keck Observatory: observatory in Hawaii with two 10-meter reflectors
- Large Binocular Telescope: dual 8.4 meter telescope in Arizona
- MRO Interferometer: optical interferometer in New Mexico
- Narrabri Stellar Intensity Interferometer: 1960s optical interferometer for measuring stellar diameters
- Navy Precision Optical Interferometer: optical interferometer in Arizona
- Palomar Testbed Interferometer: near-infrared interferometer experiment
- SUSI: optical interferometer in Australia
optical interferometry: see optical interferometer
- interferometry: analysis of superimposed EMR waves
optical segmentation: see segmented mirror
optical SFR: see star formation rate
optical star formation rate: see star formation rate
optical telescope assembly: see Hubble Space Telescope
optical thickness (optical depth): a measure of opaqueness of a path of light
optical transient: transient evident from visible light observation
optical transient source: see optical transient
optical violent variable quasar

3C 279: striking variable quasar

optical window: see atmospheric window
optically thick: see optical depth
optically thin: see optical depth
- cooling function: function relating a gas's cooling rate to its temperature
optically violent variable quasar: see blazar
optics: science and technology regarding visible light
- aberration: types of flaws in telescope images
- adaptive optics: optical mechanism to compensate for varying atmospheric distortion
- Airy disk: image-pattern an optical system produces from light from a point
- anastigmat: optics avoiding common aberrations
- angular differential imaging: using telescope rotation to suppress speckles
- angular magnification: a magnification measure for a magnifier with an eyepiece
- angular resolution: measure of a telescope's ability to distinguish spatial detail
- aperture masking interferometry: interferometry by means of a single telescope with a mask
- aperture synthesis: interferometry to simulate a larger aperture telescope
- atmospheric window: frequency bands within which the atmosphere passes EMR
- blended spectra: an observed spectrum comprising light from multiple objects
- cavity-enhanced absorption spectroscopy: passing a beam through a gas multiple times to amplify absorption lines
- circular polarization ratio: ratio of circular polarization handedness in reflected radar signal
- coherent light: beam of light at a single frequency
- collimator: device to align beams of light
- coma: aberration where points of light show a tail
- deformable mirror: mirror that can be flexed for fine control
- differential spectroscopy: analysis of the comparison of two spectrographs
- diffraction: an effect of the edge of an obstacle on a wave
- diffraction limited: telescope limited only by the laws of diffraction
- diffractive interstellar scintillation: diffraction of star light by the ISM
- etalon: two parallel reflective surfaces separating light according to wavelength
- exit pupil: size of the image as presented to the eye
- extreme adaptive optics: high-performance version of AO system
- Fabry-Pérot interferometer: type of chromatic-dispersing interferometer
- Faraday rotation: rotation of optical polarization by a medium in a magnetic field
- field of view: angular measure of how much an optical instrument can view
- focal length: length from optical element to focal plane
- focal plane: plane within optical system at which an image is in focus
- grazing incidence optics: optics that avoid large changes to the angle of rays
- grism: combination of a grating and a prism
- interferometer: device detecting EMR properties using interference between waves
- Kramers opacity law: opacity's relation to density and temperature
- linearly variable filter: filter whose passband varies over one spatial dimension
- lucky imaging: method of combining short-term images to yield better resolution
- Lyot stop: optical element to block unwanted light within instruments
- Mie scattering: scattering of light by large particles
- opacity: a measure of opaqueness of a material
- optical axis: center line through an optical instrument
- optical depth: a measure of opaqueness of a path of light
- phase plate: filter that changes the phase of light passing through
- point-spread function: function describing an optical system's response to a point of light
- pointing error: discrepancy in telescope's aim
- polarization: property of waves that oscillate in a particular direction
- PSF fitting: untangling airy disks to gain effective resolution
- Rayleigh scattering: wave scattering by particles smaller than the wavelength
- real image: image that would show on a screen placed at its location
- reference star differential imaging: identifying speckles by comparing star-images
- refraction: redirection of light passing from one medium to another
- refractive index: measure of how much a material slows down light
- refractive interstellar scintillation: refraction of star light by the ISM
- Rosseland mean opacity: a useful weighted average of opacities at all the frequencies
- Sagnac effect: an effect of the movement of light beams regarding their interference
- scintillometry: measuring of twinkling
- seeing: apparent blurring and twinkling of objects seen through atmosphere
- shift-and-add: common speckle-suppression technique
- Snell's law: optical principle of refraction
- speckle imaging: method of combining short-term images to yield better resolution
- speckle suppression: methods of reducing noise in telescope observations
- speckles: image flaws such as extraneous spots
- stacking: combining images of the same object
- Strehl ratio: measure of optical aberration
- Thomson optical depth: a measure of effects of Thomson scattering
- transit spectroscopy: spectroscopy through an exoplanet's transit
- transmission spectroscopy: spectrum of radiation passing through a substance
- wavefront error: measure of a class of telescope distortion
OPTIMOS: see European Extremely Large Telescope
Optopus

ESO Nearby Abell Cluster Survey: 1990s spectroscopy and photometry survey of galaxy clusters

ORAD: see Pioneer Venus Orbiter
orange dwarf: see K-type star
- red dwarf: small, cool main-sequence star
orbit

conic section: orbit shapes, like the intersection of a plane and cone
Keplerian orbit: orbit following a perfect ellipse or other conic section
orbital element: parameters describing an orbit

orbit plot: graphic showing the path of an orbit as a line
orbital (electron orbital): a nucleus's "locations" where an electron can orbit
orbital decay: gradual reduction in radius of an orbit
orbital eccentricity (eccentricity): degree to which an orbit is non-circular
orbital element: parameters describing an orbit
Orbital Express: see Autonomous Space Transport Robotic Operations
orbital inclination: angle between plane of an orbit and a reference plane
- minimum mass: measured bound on the mass of an exoplanet
orbital period: length of time for an orbiting object to complete one cycle
- mean anomaly: angle describing an object's position within an elliptical orbit
- period derivative: rate of change in the period of an orbit or rotation
- sidereal: relative to the fixed position of the stars
- synodic period: period until a planet returns to the same position relative to the Sun
Orbital perturbation analysis: see J₂
orbital plane: see orbital element
orbital plot (orbit plot): graphic showing the path of an orbit as a line
orbital radius: see semi-major axis
orbital resonance: tendency of orbits of multiple bodies to remain in a simple pattern
- tidal locking: locked rotation of an orbiting body
orbital speed: speed of an orbiting body around the barycenter of a system
Orbiting Astronomical Observatory: series of 1960s-1970s NASA space telescopes
Orbiting Solar Observatory 7 (OSO 7): 1970s observatory-satellite for viewing the Sun
Orbiting Solar Observatory 8 (OSO 8): 1970s observatory-satellite for viewing the Sun
Orbiting Wide-angle Light Collectors: space mission concept to detect cosmic rays through Earth atmosphere light flashes
orbitize: planet orbit-fitting software for direct image observations
orbits

anomalistic year: planet's time between successive periapses
apsis: closest or furthest point in an eccentric orbit
barycenter: center of mass of two orbiting bodies
conic section: orbit shapes, like the intersection of a plane and cone
eccentricity: degree to which an orbit is non-circular
epicycle: small circular motion around a large circular motion
epicyclic frequency: frequency of oscillation in an accretion disk
extreme mass ratio inspiral: inspiraling pair of objects with mass ratio beyond 10,000
halo orbit: type of orbit around an unstable Lagrangian point
hardness: measure of the binding energy of a binary
Hill radius: radius of gravitational influence of a body
Hill stability: planetary orbits which will remain in the same order
innermost stable circular orbit: border of region surrounding a BH in which orbits are unstable
instability region: region around an object where orbits cannot persist
Kepler radius: radius at which a particular velocity would orbit
Kepler's laws: laws of the kinematics of orbits
Keplerian orbit: orbit following a perfect ellipse or other conic section
Kirkwood gap: gaps in asteroid orbital radii corresponding to Jupiter resonances
Lagrange stability: a quality of some stable orbits
Lagrangian point: points around two orbiting bodies where small objects can sit
Laplace radius: a radius for a stable orbit of a satellite around a planet
Laplace-Lagrange secular theory: theory of evolution of planetary orbits
libration: oscillating motion of orbiting bodies relative to each other
Lie transform: type of infinitesimal transform
Lindblad resonance: resonant orbits within a disk due to a high-density portion
Lindblad torque: torque due to Lindblad resonances
low Earth orbit: Earth orbit within 2000 km of the surface
mass ratio: ratio of masses of two objects such as a binary star
mean anomaly: angle describing an object's position within an elliptical orbit
minimum orbit intersection distance: smallest distance between paths of two orbiting bodies
orbit plot: graphic showing the path of an orbit as a line
orbital decay: gradual reduction in radius of an orbit
orbital element: parameters describing an orbit
orbital inclination: angle between plane of an orbit and a reference plane
orbital period: length of time for an orbiting object to complete one cycle
orbital resonance: tendency of orbits of multiple bodies to remain in a simple pattern
orbital speed: speed of an orbiting body around the barycenter of a system
orbitize: planet orbit-fitting software for direct image observations
period derivative: rate of change in the period of an orbit or rotation
perturbation theory: breaking an equation into a solvable part and approximable part
photon sphere: surface surrounding black hole where photons can orbit
Poynting-Robertson effect: force that slows dust grains orbiting a star
precession: periodic change in a body's rotation or orbit
precession of the equinoxes: Earth axial precession with a 26000-year cycle
projected semi-major axis: semi-major axis projected onto line-of-sight or plane of the sky
protected region: an area around a body where an orbit can be stable
Q factor: measure of oscillator's damping
radial mixing: tendency of disk galaxy stars to change their orbital radius
retrograde hot Jupiter: large hot planet orbiting opposite the expected direction
retrograde orbit: orbit opposite the usual direction
Roche limit: nearest a body can orbit another and survive
Roche lobe: region of a body's gravitational influence closest to its competitor
secular: occurring slowly over a long period of time
semi-major axis: maximum radius of an ellipse
synchronous orbit: orbit with angular velocity matching the rotation of the orbited body
tidal locking: locked rotation of an orbiting body
tidal migration: interaction between tide and orbiting body that changes the orbit
tidal Q: measure of orbital energy dissipated per orbit due to tidal forces
two-line element set: data format to record the position and motion of a satellite
Yarkovsky effect: force on spinning, orbiting objects from asymmetric radiation
Yarkovsky-O'Keefe-Radzievskii-Paddack effect: rotational effect on asymmetric asteroids

Orbits for the Impatient: see orbitize
ORC (odd radio circle): extragalactic ring-shaped radio source noticed in 2020
Orcus

moon: a planet's natural satellite, such as Earth's
trans-Neptune object: minor planet beyond Neptune

order: see grating
order: see spherical harmonics
order-of-magnitude (dex): a number or ratio's log base 10
orders-of-magnitude: see dex
ordinary differential equation: see partial differential equation
ORFEUS SPAS

Interstellar Medium Absorption Profile Spectrograph: 1980s/1990s UV spectrometer

ORFEUS Telescope

Berkeley Extreme and Far-UV Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions
Tubingen Ultraviolet Echelle Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions

ORFEUS-SPAS 1

Berkeley Extreme and Far-UV Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions
Interstellar Medium Absorption Profile Spectrograph: 1980s/1990s UV spectrometer
Tubingen Ultraviolet Echelle Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions

ORFEUS-SPAS 2

Berkeley Extreme and Far-UV Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions
Interstellar Medium Absorption Profile Spectrograph: 1980s/1990s UV spectrometer
Tubingen Ultraviolet Echelle Spectrometer: 1990s UV spectrometer used on two Space Shuttle missions

organization

Academia Sinica Institute of Astronomy and Astrophysics: Taiwanese astrophysics research organization
Accelerating Space Commerce, Exploration, and New Discovery: AIAA interest group
ACe consortium: corporations contracted to construct E-ELT
ADEC: NASA unit to coordinate efforts of data archiving and dissemination
Agenzia Spaziale Italiana: includes space-based astronomy research
Air Force Geophysics Laboratory: merged into the Air Force Research Laboratory (AFRL)
Amateur Astronomers Association of New York: American organization
American Association of Variable Star Observers: organization of amateur observers of variable stars
American Astronomical Society: American organization
American Geophysical Union: US organization
American Institute of Aeronautics and Astronautics: society of engineers
American Meteor Society: meteor-science promotion organization
American Museum of Natural History: NYC museum carrying out research including astrophysics
American National Standards Institute: USA standards organization
American Physical Society: American physics organization
AMES Research Center: NASA lab in silicon valley
Arab Union for Astronomy and Space Sciences: has hosted conferences
Armstrong Flight Research Center: NASA aeronautical research center in California
Association Française des Observateurs d'Étoiles Variables: French organization formed in 1920s
Association of Lunar and Planetary Observers: international scientific organization
Association of Universities for Research in Astronomy: collaboration of universities operating observatories
ASTRON: Netherlands radio astronomy research organization
Astronet: European astronomy consortium
Astronomical Data Center: 1980s-1990s NASA/Goddard data archive
Astronomical Society of the Atlantic: interested parties promoting astronomy
Astronomical Society of the Pacific: general astronomy-education organization
Astronomische Gesellschaft: German astronomical society
Astrophysical Research Consortium: owns Apache Point Observatory
Astrophysics Data and Information Services: perhaps one-time name for NASA ADF or ADS
Astrophysics Data Facility: former NASA unit responsible for data from NASA astrophysics missions
Atmospheric Physics of Exoplanets: MPIAA department
Berkeley-Illinois-Maryland Association: operator of 1980s-2000s radio telescope
Bogolyubov Institute for Theoretical Physics: Ukrainian research institute
British Astronomical Association: amateur astronomer organization
British Interplanetary Society: space-exploration/research advocacy organization
British National Space Centre: the older name for UKSA
California Association for Research in Astronomy: nonprofit aimed at operating Keck observatory
Canadian Astronomical Society: professional astronomers
Canadian Astronomy Data Centre: online data distribution
Canadian Institute for Theoretical Astrophysics: national research organization at University of Toronto
Cape Canaveral Air Force Station: Air Force launch facilities adjacent to Kennedy Space Center
Carl Sagan Institute: group researching finding life on other planets and moons
CAS Space: Chinese commericial space-launch corporation
CASPER: initiative to design radio-astronomy instruments
CEA Service d'Astrophysique: French national astrophysics research institute
Center for Advanced Space Studies: unit of Universities Space Research Association
Center for Astrophysical Research in Antarctica: NSF science and technology center
Center for Astrophysics: Harvard and Smithsonian research and education organization
Center for Astrophysics and Space Sciences: UC San Diego academic institute
Center for Earth and Planetary Studies: Smithsonian Institute research unit
Central Bureau for Astronomical Telegrams: organization reporting transients and discoveries
Centre de données astronomiques de Strasbourg: repository of astronomical data
Centre for Astrophysics and Supercomputing: research center at Swinburne University, Australia
Centre for Research in Astrophysics of Quebec: astronomy collaboration of Quebec universities
Centre national d'ètudes spatiales: French space agency
Centre national de la recherche scientifique: French scientific research organization
Centro Astronómico Hispano-Alemán: German-Spanish Astronomical Center at Calar Alto.
Chinese Academy of Sciences: Chinese national scientific organization
CIG: initiative to produce Earth-science software
Committee on Space Research: international collaboration established by International Council for Science
Committee on the Status of Women in Astronomy: AAS committee
Consiglio Nazionale delle Ricerche: Italian national research organization.
Council of German Observatories: German astronomy research council
CSA: includes astronomical research
DLR: German's space and aerotautics research agency
East Asian Core Observatories Association: promotes astronomy
East-Asian Planet Search Network: initiative to find exoplanets
Engineering and Physical Sciences Research Council: UK agency for scientific research, including astronomy
European Association for Astronomy Education: promotes science education
European Astronomical Society: European organization
European Research Infrastructure Consortium: science and technology initiative
European Southern Observatory: organization operating telescopes in the southern hemisphere
European Space Agency: NASA-like international space organization in Europe
European Space Research Organisation: 1960s-1970s European collaboration
Gamma-ray Burst Coordinates Network: system to distribute notifications of GRBs
Global Oscillations Network Group: cooperative organization studying the Sun through helioseismology
Goddard Space Flight Center: NASA space research laboratory near Washington D.C.
Groupe Europáen Observations Stellaires: European organization of variable star observers.
Gruppo Italiano Astrometristi: Italian amateur astronomy organization
HEAD: AAS division supporting X-ray and gamma-ray astrophysics
High Altitude Observatory: NCAR unit observing and monitoring the Sun
Indian Space Research Organisation: national science and technology agency
Institut de radioastronomie millimétrique: organized radio astronomy collaboration between France, Germany, and Spain
Institut National d'Astronomie et de Geophysique: 1960s-1980s French research institute
Institute for Advanced Study: research institution
Institute for Astronomy: U Hawaii astronomy research organization
Institute for research on exoplanets: U Montreéal division researching exoplanets
Institute of Astronomy: Cambridge U astronomy research organization
Institute of Electrical and Electronics Engineers: professional engineering association
Institute of Space and Astronautical Science: Japanese national research organization
Institute of Space and Atmospheric Studies: U Saskatchewan research division
Institute of Theoretical Astronomy: former Cambridge U astronomy research organization
Institute of Theoretical Astrophysics: Heidelberg research institute
Instituto de Astrofísica de Canarias: research institute on Canary Islands
Inter-University Centre for Astronomy and Astrophysics: initiative promoting astrophysics at Indian universities
International Amateur-Professional Photoelectric Photometry: organization active 1980-2000+
International Astronomical Union: association of professional astronomers
International Dark-Sky Association: organization promoting control of light pollution
International Earth Rotation and Reference Systems Service: service monitoring Earth's rotation and precession
International Halley Watch: initiative to coordinate observation of Halley's Comet in 1986
International Meteor Organization: amateur/professional meteor organization
International Occultation Timing Association: volunteer organization to observe occultations
International Organization for Standardization: standards-development organization
International Scientific Optical Network: project to track objects in space
International Small Telescope Cooperative: former cooperative of small research telescopes
International Supernova Network: SN information sharing initiative
International Workshop for Astronomy: organization operating the IAYC
IPAC: Caltech group to collect and analyze infrared observations
Isaac Newton Group of Telescopes: three telescopes in Canary Islands
Istituto di Astrofisica e Planetologia Spaziali: Italian astrophysical research institute within INAF
Istituto di Astrofisica Spaziale: former name of Italian astrophysics research institution
Istituto Nazionale di Astrofisica: Italian astrophysical research institute
ITU: international body coordinating communications technology
Japan Aerospace Exploration Agency: Japan agency includes astronomical research
Japan Amateur Photoelectric Observers Association: past amateur astronomy collaboration
Japan Spaceguard Association: Japanese telescope tracking NEOs and space junk
Jet Propulsion Laboratory: NASA technology research center
JILA: U Colorado/NIST research organization
Joint Astronomy Centre: operated two UK telescopes in Hawaii
Joint Institute for VLBI ERIC: central EVN facility
Kavli Institute: any of a number of science institutes supported by Kavli Foundation
Laboratorio de Astrofisica Espacial y Fisica Fundamental: Spanish astrophysics laboratory
Laboratory for High Energy Astrophysics: former GSFC X-ray/gamma-ray lab of about 1970 to 2004
LAD: AAS division supporting laboratory work relevant to astrophysics
Lawrence Berkeley National Laboratory: US energy-related physics laboratory
Lawrence Livermore National Laboratory: US science and technology laboratory
Lunar and Planetary Institute: scientific research arm of USRA
Lunary and Planetary Laboratory: U Arizona lab
Mars Exploration Program Analysis Group: NASA forum to provide input to long-term planning of Mars missions
Max Planck Institute for Astronomy: Heidelberg research institute
Max Planck Institute for Radio Astronomy: Bonn research institute
Max Planck Society: association of German research institutions
Millimeter Astronomy Legacy Team: Australian initiative producing millimeter surveys
Ministerio de Ciencia e Innovaciónes: Spanish science ministry
Minor Planet Center: organization collecting and organizing minor planet data
NASA: includes space-based astronomy research
NASA Astrobiology Institute: former research funding and coordination organization
NASA Deep Space Network: NASA's world-wide radio communication setup
NASA Space Science Data Coordinated Archive: NASA archive of flight information and astronomy findings
National Acadamy of Sciences: US organization of distinguished scientists
National Academies of Sciences, Engineering, and Medicine: US collective of three national acadamies
National Astronomical Observatory of Japan: Japanese astronomy organization
National Bureau of Standards: name of of NIST before 1988
National Institute of Standards and Technology: US agency defining standards
National Oceanic and Atmospheric Administration: US meteorology and oceanography agency
National Optical Astronomy Observatory: previous name of US optical astronomy organization
National Radio Astronomy Observatory: US radio astronomy organization
National Science Foundation: US government organization funding scientific research
Naval Research Laboratory: US Navy scientific research arm
Nederlandse Organisatie voor Wetenschappelijk Onderzoek: Dutch research council
NOIRLab: US optical/infrared astronomy organization
Norsk Astronomisk Selskap: Norwegian non-profit association of astronomers
North American Meteor Network: former group of amateurs with an interest in meteors
NSF AST: US government astronomical research funding division
Other Worlds Laboratory: initiative researching planets and moons
Outer Planets Assessment Group: NASA forum to provide input to long-term planning of outer planets missions
Particle Physics and Astronomy Research Council: former UK science agency
Planetary Science Institute: US research institution
Radio Camera Initiative: initiative to design radio interferometry correlators
RECONS: consortium for research on stars near the Sun
Red de Infraestructuras de Astronomie: Spanish astronomy research organization
Research Foundation - Flanders: Belgian research council
Research School of Astronomy and Astrophysics: Australian National University astrophysics school
Roscosmos: Russian organization for space technology and research
Royal Astronomy Society: UK society supporting and promoting astronomy
Royal Astronomy Society of Canada: Canadian society supporting and promoting astronomy
Royal Greenwich Observatory: former UK national observatory
Science and Engineering Research Council: old name of UK's EPSRC
Science and Technology Facilities Council, UK: UK research agency
Small Bodies Names Committee: old name of IAU Committee for Small Body Nomenclature
Société astronomique de France: French non-profit astronomical society
Sociedad Astronómica de Valparaíso y Viña del Mar: Chilean amateur astronomy society
Society of Amateur Radio Astronomers: US organization
Solar Influences Data Analysis Center: Royal Observatory of Belgium division on solar weather data
Southeast Asia Astronomy Network: astronomy research/education organization
Southwest Research Institute: Texas research organization
Space Research Organization of the Netherlands: national research institution
Space Studies Board: National Academy of Sciences unit re astronomy an research via spacecraft
Space Studies Institute: organization promoting interest in and use of space technology
Space Telescope Science Institute: science operator of space telescopes
SpaceGuard Foundation: private organization to discover and track NEOs
Students for the Exploration and Development of Space: student-run organization focused on space exploration
Tata Institute of Fundamental Research - India: Indian math and science research institute
Telescopes in Education: program giving grade-school students access to robotic telescopes
UK Space Agency: UK government agency like NASA
United States Geological Survey: US government geological division
United States Space Force: US space-related military organization
Universities Space Research Association: university research collaboration
Variable Star Observers League in Japan: amateur variable star group
Vereinigung der Sternfreunde: German amateur astronomy association
Vetenskapsrådet: Swedish research council
WIYN: Wisconsin-Indiana-Yale-NOAO
Working Group on Accessibility and Disability: AAS working group

Origins Space Telescope: concept for 2030s large space IR telescope
Orion: well-known constellation in the northern hemisphere
Orion (Orion space telescopes): two Russian 1970s UV space telescopes
Orion A: see Orion Molecular Cloud Complex
Orion Arm: see spiral arm
- Scutum-Centaurus Arm: one of two spiral galaxy arms from the Milky-Way bar
Orion B: see Orion Molecular Cloud Complex
Orion Bar: see Orion Nebula
Orion Bridge

Scutum-Centaurus Arm: one of two spiral galaxy arms from the Milky-Way bar

Orion Cluster (Orion Nebula Cluster): open cluster in the constellation Orion
Orion Complex (Orion Molecular Cloud Complex): group of molecular clouds with star-forming regions
Orion Disks: protoplanetary disks visible in front of Orion Nebula
Orion Molecular Cloud Complex: group of molecular clouds with star-forming regions
Orion Molecular Clouds (Orion Molecular Cloud Complex): group of molecular clouds with star-forming regions
Orion Nebula: easily-observed nebula that includes SF regions
- COUP: survey of X-ray sources in the Orion Nebula
- Orion Disks: protoplanetary disks visible in front of Orion Nebula
- Trapezium Cluster: open cluster of stars in Orion Nebula
Orion Nebula Cluster: open cluster in the constellation Orion
Orion space telescopes: two Russian 1970s UV space telescopes
Orion spacecraft: see Space Launch System
Orion Spur: see spiral arm
- Scutum-Centaurus Arm: one of two spiral galaxy arms from the Milky-Way bar
Orion Trapezium Cluster (Trapezium Cluster): open cluster of stars in Orion Nebula
Orion variable: see variable star
Orion-1: see Orion space telescopes
Orion-2: see Orion space telescopes
Orion-Cygnus Arm: see spiral arm
- Scutum-Centaurus Arm: one of two spiral galaxy arms from the Milky-Way bar
ORM (Roque de los Muchachos Observatory): observatory at La Palma in the Canary Islands
- Carlsberg Meridian Catalogue: series of astrometric catalogs from 19th century on
- GOTO: telescope array to follow up gravitational wave signals
- Gran Telescopio Canarias: 10.4 meter telescope in the Canary Islands
- Instituto de Astrofísica de Canarias: research institute on Canary Islands
- Isaac Newton Group of Telescopes: three telescopes in Canary Islands
- Isaac Newton Telescope: 2.5 m telescope in Canary Islands
- MASCARA: distributed array of telescopes to search for exoplanets
- Mercator Telescope: 1.2 m telescope in the Canary Islands
- STELLA: two robotic telescopes in Canary Islands
- Telescopio Nazionale Galileo: 3.58 m telescope in Canary Islands
OROCHI: see MMX
ORP (reduction potential): measure of a chemical species' tendency to acquire electrons
ORPA: see Pioneer Venus Orbiter
orphan afterglow: see gamma-ray burst
orphan GRB afterglow: see gamma-ray burst
Orphan stream: stellar stream wrapping around the Milky Way
- Southern Stellar Stream Spectroscopic Survey: spectroscopic survey of seven southern-hemisphere stellar streams
orrery: mechanical device to simulate the solar system
ORT (Ooty Radio Telescope): cylindrical radio telescope in India
orthogonal transfer CCD: see image stabilization
OSAM

OSAM-1: prototype space robot for servicing satellites

OSAM-1: prototype space robot for servicing satellites
OSAM-2: see OSAM-1
OSC: see Astrocats
- MOSFiT: code to match light-curves with light-curve models
OSCA: see William Herschel Telescope
Oschin Schmidt (Palomar 48 Inch Telescope): 48 inch Schmidt telescope at Palomar Observatory
Oschin Telescope

Palomar Transient Factory: survey designed to find transients
Zwicky Transient Facility: survey designed to find transients

oscillator strength: fraction expressing QM's contribution to the probability of absorption
osculating orbit (Keplerian orbit): orbit following a perfect ellipse or other conic section
- minimum orbit intersection distance: smallest distance between paths of two orbiting bodies
OSIRIS: see Gran Telescopio Canarias
OSIRIS: see Keck Observatory
OSIRIS: see Rosetta
OSIRIS: see SOAR Telescope
- SHARDS: GTC survey in GOODS-N field for quenched galaxies at z = 1 to 2.3
Osiris (HD 209458 b): hot Jupiter discovered in 1999
OSIRIS-APEX: see OSIRIS-REx
OSIRIS-REx: space mission aiming to return a sample of an asteroid
- 101955 Bennu: near-Earth asteroid that is the target of a sample-return mission
- CSA: includes astronomical research
OSMOS: see Hiltner Telescope
OSO 7: 1970s observatory-satellite for viewing the Sun
OSO 8: 1970s observatory-satellite for viewing the Sun
OSO H (OSO 7): 1970s observatory-satellite for viewing the Sun
OSS (Ohio Radio Survey): 1960s radio survey by Ohio State University
OSS: see Origins Space Telescope
OSSE: see Compton Gamma Ray Observatory
OSSOS (Outer Solar System Origins Survey): 2013-2017 survey that searched for trans-Neptune objects
OST (Origins Space Telescope): concept for 2030s large space IR telescope
OSURO

Ohio Radio Survey: 1960s radio survey by Ohio State University

OT (optical transient): transient evident from visible light observation
OT source: see optical transient
OTC: see tidal disruption event
OTCCD: see image stabilization
Other Worlds Laboratory: initiative researching planets and moons
OTL: see rare designator prefixes
OTS: see Hubble Space Telescope
OTS: see optical transient
OTS: see rare designator prefixes
Oukaïmeden Observatory

TRAPPIST: a ground-based automated transiting-planet finding telescope

Oumuamua: small body that passed through the solar system in 2017
outer belt: see Van Allen belts
outer Lindblad resonance: see Lindblad resonance
outer Lindblad resonance: see Lindblad torque
Outer Planets Assessment Group: NASA forum to provide input to long-term planning of outer planets missions
outer solar system

solar system object: the Sun, solar system planets, minor planets, comets, and moons

Outer Solar System Origins Survey: 2013-2017 survey that searched for trans-Neptune objects
outflow

star formation feedback: mechanisms whereby star formation regulates its own rate

outgassing: see atmosphere formation
outrigger: radio receiver some distance from main receiver for direction determination
outside-in growth: see inside-out growth
OUVS: see Pioneer Venus Orbiter
overdensity (stellar overdensity): portion of galactic halo with more stars than normal
- Milky Way subgroup: the Milky Way and its satellites
overfitting: see Akaike information criterion
Overwhelmingly Large Telescope: 100 m telescope plan
OVRO (Owens Valley Radio Observatory): group of radio telescopes in California near the VLBA
- OVRO-LWA: radio telescope in California
OVRO 40 Meter Telescope: see Owens Valley Radio Observatory
OVRO-LWA: radio telescope in California
OVRO-LWA: see Owens Valley Radio Observatory
- Long Wavelength Array: radio telescope in New Mexico
OVSA

Owens Valley Radio Observatory: group of radio telescopes in California near the VLBA

OVV

3C 279: striking variable quasar

OVV quasar: see blazar
Owens Valley Long Wavelength Array (OVRO-LWA): radio telescope in California
Owens Valley Long Wavelength Array: see Owens Valley Radio Observatory
- Long Wavelength Array: radio telescope in New Mexico
Owens Valley Radio Observatory: group of radio telescopes in California near the VLBA
- OVRO-LWA: radio telescope in California
Owens Valley Solar Array

Owens Valley Radio Observatory: group of radio telescopes in California near the VLBA

OWFA: see Ooty Radio Telescope
OWL (Orbiting Wide-angle Light Collectors): space mission concept to detect cosmic rays through Earth atmosphere light flashes
OWL (Other Worlds Laboratory): initiative researching planets and moons
OWL (Overwhelmingly Large Telescope): 100 m telescope plan
OWL: see Hayabusa2
- EAGLE: simulation of galaxy formation and evolution
Oxford SWIFT: see Hale Telescope
oxidation: see reducing atmosphere
- atmosphere formation: capture and modification of a planet's gases
- oxidation state: number of electrons loaned out by an atom in an ionic bond
oxidation number (oxidation state): number of electrons loaned out by an atom in an ionic bond
oxidation state: number of electrons loaned out by an atom in an ionic bond
oxidation-reduction reaction: see reducing atmosphere
oxidation/reduction potential (reduction potential): measure of a chemical species' tendency to acquire electrons
oxide: see geochemistry
oxidizing agent: see reducing atmosphere
oxygen: element, O, atomic number 8
- abiotic oxygen: oxygen from non-life sources
- alpha CO: ratio of CO emissions to H₂
- carbon dioxide: compound of carbon atom and two oxygen atoms
- carbon monoxide: compound of carbon and oxygen, one atom each
- carbonate-silicate cycle: long-term cycle that sequesters Earth carbon
- geochemistry: the chemistry of the Earth, planets, and moons
- hydroxyl: one hydrogen atom and one oxygen atom bound together
- neon burning: fusion reaction starting with neon
- nitrite: ion of a nitrogen atom and two oxygen atoms with extra electron
- oxygen burning: fusion reaction starting with oxygen
- oxygen lines: spectral lines specific to oxygen
- silicate weathering feedback: feedback mechanism stabilizing carbon in Earth atmosphere
- silicon monoxide: compound of silicon and oxygen, one atom each
oxygen burning: fusion reaction starting with oxygen
- magnesium: element, Mg, atomic number 12
- nucleosynthesis: creation of atomic nuclei
- silicon: metal, Si, atomic number 14
- sulfur: metal, S, atomic number 16
oxygen lines: spectral lines specific to oxygen
oxygen-burning shell: see oxygen burning
oxygen-neon-magnesium white dwarf: see white dwarf
oxygen-rich Wolf-Rayet: see Wolf-Rayet star
oxygen-to-iron ratio: see alpha-enhanced
OzDES: see Dark Energy Survey
ozone

earthshine: reflected and emitted electromagnetic radiation from Earth

ozone layer

Upper Atmosphere Research Satellite: 1990s spacecraft to observe Earth ozone layer

Index