Term Index (H)
Astrophysical terms and names of things beginning with the letter H.
H2
H3
HA
HB
HC
HD
HE
HF
HG
HH
HI
HJ
HK
HL
HM
HN
HO
HP
HR
HS
HT
HU
HV
HW
HX
HY
HZ
- H (Balmer series): hydrogen lines for electrons settling from n > 2 to n = 2
- H (Herschel Double Star Catalog): series of early double star catalogs
- H (hydrogen): most prominent element, atomic number 1
- H (magnetic field strength): a field associated with magnetism
- H (scale height): measure of exponential falloff
- H: see HEAO-1
- H: see High Energy Transient Explorer
- H: see Hubble constant
- H: see NIRSpec
- H: see rare designator prefixes
- h (Planck constant): quantum of action in quantum mechanics
- h (gravitational wave strain): a measure of the magnitude of a gravitational wave
- h: see Herschel Double Star Catalog
- h: see Hubble constant
- h: see Love number
- H band: see infrared
- H burning (hydrogen burning): any fusion reaction starting with hydrogen
- H field (magnetic field strength): a field associated with magnetism
- H I (neutral atomic hydrogen): state of hydrogen when sufficiently cool
- H II (ionized hydrogen): state of hydrogen when sufficiently hot
- H line
- calcium: element, Ca, atomic number 20
- Hα (H-alpha): hydrogen line for electrons settling from n = 3 to n = 2
- Hα survey: survey looking for the Hα line, typically to find star formation
- Hβ (H-beta): hydrogen line for electrons settling from n = 4 to n = 2
- H-α (H-alpha): hydrogen line for electrons settling from n = 3 to n = 2
- Balmer series: hydrogen lines for electrons settling from n > 2 to n = 2
- H-α Ly-α
- H-β (H-beta): hydrogen line for electrons settling from n = 4 to n = 2
- H-alpha: hydrogen line for electrons settling from n = 3 to n = 2
- Balmer series: hydrogen lines for electrons settling from n > 2 to n = 2
- H-alpha filter
- H-alpha: hydrogen line for electrons settling from n = 3 to n = 2
- H-alpha SFR: see star formation rate
- H-alpha star formation rate: see star formation rate
- H-alpha to UV ratio: see star formation rate
- H-band
- A-LIST: catalog of templates of H-band spectra for SPS
- H-beta: hydrogen line for electrons settling from n = 4 to n = 2
- Balmer series: hydrogen lines for electrons settling from n > 2 to n = 2
- H-beta filter
- H-beta: hydrogen line for electrons settling from n = 4 to n = 2
- H-OH: see Parkes Observatory
- H-R diagram: diagram relating effective temperature to luminosity
- H2: see hydrogen
- H3 (H3 Survey): spectroscopic survey of Milky Way halo stars
- H3 Survey: spectroscopic survey of Milky Way halo stars
- H0 (Hubble constant): current rate of expansion of the universe
- H2: see hydrogen
- H2 dissociation front (molecular hydrogen dissociation front): point in a gas cloud where it is hot enough to dissociate hydrogen
- H2/CO factor
- H2O (water): compound of oxygen atom and two hydrogen atoms
- H2O Southern Galactic Plane Survey: survey targeting water masers
- H2SO4
- Venus: second inner-most planet in the solar system
- hc: see gravitational wave strain
- H++
- H+ (ionized hydrogen): state of hydrogen when sufficiently hot
- H-: see ionized hydrogen
- Ha (H-alpha): hydrogen line for electrons settling from n = 3 to n = 2
- HabEx: proposed space mission to directly image exoplanets
- habitability: ability of a planet to support life
- habitability zone (habitable zone): zone where conditions are favorable for life
- Habitable Exoplanet Imaging Mission (HabEx): proposed space mission to directly image exoplanets
- habitable zone: zone where conditions are favorable for life
- astrobiology: study of extra-terrestrial life
- K2-18b: super-Earth in a habitable zone
- Kepler-1625b: giant exoplanet in a star's habitable zone
- Kepler-186f: Earth-like exoplanet in a star's habitable zone
- Kepler-452b: Earth-like exoplanet discovered in 2015
- Proxima b: exoplanet orbiting Proxima Centauri
- TOI 700 d: Earth-sized exoplanet in a habitable zone
- Had: see rare designator prefixes
- Hadar (Beta Centauri): second brightest star in Centaurus
- Hadean Eon: the first eon in Earth history, up to 4 billion years ago
- Hadley cell: circulation pattern of up at the equator, poleward, down, and back
- Hadley circulation: see Hadley cell
- hadron: see particle
- baryon: protons, neutrons, and other three-quark particles
- hyperon: exotic type of baryon
- neutron: type of baryon that is stable and has no electric charge
- pion: first type of meson discovered
- proton: type of baryon that is stable and has an electric charge of +1
- quasiparticle: disturbance that acts like a particle
- spin: an angular-momentum quantum number
- HAe: see Herbig AeBe star
- HAeBe (Herbig AeBe star): type of pre-main-sequence star more massive than a T-Tauri star
- Haffner: see rare designator prefixes
- HALCA: Japanese space radio telescope operating 1997-2005
- Hale Telescope: 5.1 m reflector telescope in California
- Haleakala
- Haleakala High Altitude Observatory Site (Haleakala Observatory): observatory on Maui, Hawaii
- Haleakala Observatory: observatory on Maui, Hawaii
- half wavefront error: see wavefront error
- half width at half maximum: see full width at half maximum
- half-Earth model: see Moon formation
- half-life: see e-folding time
- half-light radius (effective radius): radius of a galaxy within which 1/2 the light is emitted
- half-wave plate: see waveplate
- half-width: see full width at half maximum
- Hall effect: voltage generated by interacting current and magnetic field
- magnetometer: instrument to measure the strength of a magnetic field
- Hall effect thruster: see ion engine
- Hall MHD: see magnetohydrodynamics
- Hall Telescope: see Lowell Observatory
- Halley's Comet: see comet
- Halley-type comet: see long-period comet
- halo (dark matter halo): hypothetical dark matter throughout and surrounding a galaxy
- halo (galactic halo): spherical region around a galaxy with gas, dark matter, etc.
- halo abundance matching: method of comparing observed galaxies with cosmology models
- halo mass function: mass of distribution of dark matter
- halo model: model of dark matter density presuming it resides in halos
- halo occupation distribution (HOD): model of galaxy distribution
- halo orbit: type of orbit around an unstable Lagrangian point
- halo star: see high-velocity star
- halo star: see stellar halo
- HALOE: see Upper Atmosphere Research Satellite
- HALOGAS (Hydrogen Accretion in Local Galaxies Survey): radio survey looking for HI in galaxy halos
- haloscope: see Axion Dark Matter Experiment
- HAM (halo abundance matching): method of comparing observed galaxies with cosmology models
- Hamburg Schmidt Camera: see Hamburg Schmidt Survey
- Hamburg Schmidt Survey: photographic plates from the Hamburg Schmidt Camera
- Hamburg/ESO Survey: 1990s southern hemisphere survey aimed at finding quasars
- Hamilton Spectrograph: see Shane Telescope
- Hamiltonian: mathematical transform useful in mechanics
- Hamiltonian equations of motion
- Hamiltonian: mathematical transform useful in mechanics
- Hamiltonian function (Hamiltonian): mathematical transform useful in mechanics
- Hamiltonian mechanics
- Hamiltonian: mathematical transform useful in mechanics
- Hanbury Brown and Twiss effect: EMR interference stemming from variation in intensity
- Hanle effect: effect of magnetic field on the polarization of emitted photons
- HAO (High Altitude Observatory): NCAR unit observing and monitoring the Sun
- hard
- Heggie-Hills law: hard binary stars generally get harder when encountering a third star
- hard binary: see hardness
- hard EOS
- hard EoS: see equation of state
- hard X-ray: see X-ray
- Hard X-Ray Modulation Telescope (HXMT): space X-ray observatory launched in 2017
- hard X-ray source: see X-ray source
- harden: see hardness
- hardening rate: see hardness
- hardness: measure of the binding energy of a binary
- hardness ratio: see color-magnitude diagram
- hardness-intensity diagram: see color-magnitude diagram
- Harlan J. Smith Telescope
- HARM²: technique for parallelizing computations of radiation with hydrodynamics
- HARMONI: spectrograph planned for E-ELT
- HARMONI: see European Extremely Large Telescope
- harmonics
- Haro: see rare designator prefixes
- Haro 29: a blue compact galaxy
- HARP: see James Clerk Maxwell Telescope
- HARPS: high-precision spectrograph for exoplanet searches
- HARPS: see ESO 3.6m Telescope
- HARPS M dwarf sample: see HARPS
- HARPS-N: high-precision spectrograph for exoplanet searches in Canary Islands
- HARPS-N: see Telescopio Nazionale Galileo
- HARPS3: see Isaac Newton Telescope
- HARPSpol: see HARPS
- Hartle-Thorne approximation (Hartle-Thorne metric): metric approximating GR effects of slow neutron star rotation
- Hartle-Thorne metric: metric approximating GR effects of slow neutron star rotation
- Hartley 2
- EPOXI: mission using Deep Impact for further exploration
- Hartree: see Rydberg unit
- Harvard: see rare designator prefixes
- Odyssey: research supercomputer at Harvard sometimes used for astrophysics simulations
- Harvard College Observatory: see Center for Astrophysics
- Harvard Observatory Bruce Telescope: see Bruce Proper Motion Survey
- Harvard Revised Photometry Catalog
- Harvard Smithsonian Center for Astrophysics (Center for Astrophysics): Harvard and Smithsonian research and education organization
- Harvard spectral classification (spectral class): overall classification of stars based upon spectral features
- Harvard University
- Harvard-Smithsonian Center for Astrophysics
- HASI: see Cassini
- HAT (HATNet): distributed array of telescopes to search for transients
- Hat Creek Radio Observatory: California observatory with the Allen Telescope Array
- HAT-1: see HATNet
- HAT-7
- HAT-P-7b: hot Jupiter discovered in 2008
- HAT-P-7: see HAT-P-7b
- HAT-P-7b: hot Jupiter discovered in 2008
- HAT-South: see HATNet
- HAT-South telescopes
- HATNet: distributed array of telescopes to search for transients
- HaTr: see rare designator prefixes
- HATS: see HATNet
- Haumea: dwarf planet discovered in 2004
- Hawaii
- Hawaii Deep Survey (Hawaii K-band Galaxy Survey): near-infrared survey of a selection of fields
- Hawaii K-band Galaxy Survey: near-infrared survey of a selection of fields
- Hawaii Survey (Hawaii K-band Galaxy Survey): near-infrared survey of a selection of fields
- HAWC (High-altitude Water Cherenkov Observatory): New Mexico gamma ray telescope using Cherenkov radiation
- HAWC: see SOFIA
- HAWC+: see SOFIA
- HaWe: see rare designator prefixes
- HAWK-I: see European Southern Observatory Very Large Telescope
- Hawking radiation: EMR theorized to be emitted by black holes, powered by their mass
- Hayabusa: see Hayabusa2
- Hayabusa#: see Hayabusa2
- Hayabusa2: JAXA mission that returned a sample of an asteroid
- Hayashi limit: limit on the temperature of a star dependent upon its mass and luminosity
- Hayashi line (Hayashi limit): limit on the temperature of a star dependent upon its mass and luminosity
- Hayashi phase: see Hayashi limit
- Hayashi temperature (Hayashi limit): limit on the temperature of a star dependent upon its mass and luminosity
- Hayashi track: H-R diagram track of evolution of some pre-main-sequence stars
- Hayashi limit: limit on the temperature of a star dependent upon its mass and luminosity
- hazcams: see Mars Science Laboratory
- HB (horizontal branch): a second "giant" stage in the evolution of some main sequence stars
- HB: see rare designator prefixes
- Hb: see rare designator prefixes
- HB star: see horizontal branch
- HB87 (Hewitt & Burbidge Catalog): optical catalog of quasi-stellar objects
- HB89: see Hewitt & Burbidge Catalog
- HB93: see Hewitt & Burbidge Catalog
- HBC: see rare designator prefixes
- HBe: see Herbig AeBe star
- Hbg: see rare designator prefixes
- HBK: study of M31 globular clusters
- HBMM: see brown dwarf
- HBRP (high-B radio pulsar): pulsar with a strong magnetic field
- HBT effect (Hanbury Brown and Twiss effect): EMR interference stemming from variation in intensity
- HBT interferometry: see intensity interferometer
- HBV: see rare designator prefixes
- HC (hydrocarbon): any compound of just carbon and hydrogen
- HCG (Hickson Compact Groups): list of 100 galaxy groups
- HCG 92: see Stephan's Quintet
- HCHII: see HII region
- HCI: imaging of targets very near bright objects
- HCN (hydrogen cyanide): compound of hydrogen, carbon, and nitrogen, one atom each
- HCO: see Center for Astrophysics
- HCO+
- HCRF (Hipparcos Celestial Reference Frame): astronomical reference frame based upon Hipparcos catalog
- HCRO (Hat Creek Radio Observatory): California observatory with the Allen Telescope Array
- HCS: see HCI
- HCS: see current sheet
- HCSS (hypercompact stellar system): SMBH with no galaxy but has a tight stellar cluster
- HD (Henry Draper Catalog): early 20th century spectroscopic catalog of stars
- HD (hydrogen deuteride): hydrogen molecule with one deuterium atom
- HD 114762: star system thought to host the first detected exoplanet
- HD 122563: see Honda-like star
- HD 133131: binary star system with exoplanets
- HD 15082
- WASP-33b: hot Jupiter discovered in 2010
- HD 15082 b (WASP-33b): hot Jupiter discovered in 2010
- HD 163296: YSO with gaps in its protoplanetary disk
- MAPS: ALMA survey researching planet-formation
- HD 163296 b: see HD 163296
- HD 169142: protostar with planet
- HD 189733: see HD 189733 b
- HD 189733 A: see HD 189733 b
- HD 189733 B: see HD 189733 b
- HD 189733 b: hot Jupiter orbiting an orange dwarf
- HD 193793 (WR 140): Wolf-Rayet star that produces much dust
- HD 209458: see HD 209458 b
- HD 209458 b: hot Jupiter discovered in 1999
- HD 36112 (MWC 758): protostar with spiral disk
- HD 44627 (AB Pictoris): K-type star with possible planet
- HD 44627 (RS Ophiuchi): Milky Way recurrent nova
- HD 45348 (Canopus): second brightest star in the sky
- HD 80606 b: hot Jupiter discovered in 2001
- HD 95735 (Lalande 21185): nearby red dwarf
- HD correlation (Hellings and Downs curve): expected correlation in pulsar timing residuals from random waves
- HD1: very distant 2022 galaxy candidate
- HD2: see HD1
- HDE: see Henry Draper Catalog
- HDF (Hubble Deep Field): highly-detailed image of a small region of the sky taken with HST
- HDF-N (Hubble Deep Field): highly-detailed image of a small region of the sky taken with HST
- HDF-S (Hubble Deep Field South): small region of the southern sky explored with HST
- HDFS: see Hubble Deep Field South
- HDM: see dark matter
- HDO: see deuterium
- HDO: see double star designation
- HDS (HRS): high resolution spectrography
- HDS: see Subaru Telescope
- HCI: imaging of targets very near bright objects
- HDS+HCI: see HCI
- HDST (High Definition Space Telescope): proposed 12 m space telescope
- LUVOIR: plan for a space-based telescope
- HDTV: see SELENE
- HDW: see rare designator prefixes
- HE (Hamburg/ESO Survey): 1990s southern hemisphere survey aimed at finding quasars
- He (helium): non-metallic element, He, atomic number 2
- He: see rare designator prefixes
- He II
- HE-II Balmer series: see Lyman series
- HE-II Lyman series: see Lyman series
- HE-II Rydberg constant: see Lyman series
- He-rich: see chemically peculiar star
- He-weak: see chemically peculiar star
- HEA (high-energy astrophysics): astronomy and physics of X-rays and gamma rays
- HEAD: AAS division supporting X-ray and gamma-ray astrophysics
- headlight effect (relativistic beaming): directional relativistic effect on light from relativistic speeds
- HEALPix: type of map projection for spheres
- HEALPix format: see HEALPix
- HEAO A-1 X-ray Source Catalog
- HEAO-1: 1977 X-ray observatory space mission
- HEAO-1: 1977 X-ray observatory space mission
- HEAO-2: 1978 X-ray observatory space mission
- HEAO-2: 1978 X-ray observatory space mission
- HEAO-1: 1977 X-ray observatory space mission
- HEAO-3
- HEAO-1: 1977 X-ray observatory space mission
- HEAO-2: 1978 X-ray observatory space mission
- HEAO-A (HEAO-1): 1977 X-ray observatory space mission
- HEAO-B (HEAO-2): 1978 X-ray observatory space mission
- heartbeat star: type of binary star that produces pulsed light
- HEASARC: NASA archive of data on high energy astronomy
- HEAT (High Elevation Antarctic Terahertz Telescope): 61 cm FIR telescope in Antarctica
- heat death: see Big Crunch
- heat energy: see kinetic energy
- heating: increasing the velocity of particles
- heavy neutrino: see neutrino
- heavy water: see deuterium
- HEBS: see Satech-01
- Hectochelle: see MMT
- H3 Survey: spectroscopic survey of Milky Way halo stars
- Hectochelle in the Halo at High Resolution (H3 Survey): spectroscopic survey of Milky Way halo stars
- Hector: see Anglo-Australian Telescope
- Hectospec: see MMT
- Heggie's law (Heggie-Hills law): hard binary stars generally get harder when encountering a third star
- Heggie's theorem (Heggie-Hills law): hard binary stars generally get harder when encountering a third star
- Heggie-Hills law: hard binary stars generally get harder when encountering a third star
- HEI: see double star designation
- Heinemann: see rare designator prefixes
- Heinrich Hertz Submillimeter Telescope (Submillimeter Telescope): 10-meter far-infrared telescope
- Heisenberg uncertainty principle: limit on observable position and momentum of an object
- Heisenburg uncertainty principle
- HEL1OS: see Aditya-L1
- heliocentric gravitational constant: see standard gravitational parameter
- heliocentric Julian day: see Julian date
- heliocentric system: model of the universe with the planets orbiting the Sun
- heliocentricity: see heliocentric system
- epicycle: small circular motion around a large circular motion
- heliocentrism: see heliocentric system
- heliometer: instrument to precisely measure angles across the sky
- heliopause: boundary between solar wind and the interstellar medium
- heliophysics (solar physics): study of the physics of the Sun
- helioseismology: see asteroseismology
- heliosheath: outer layer of the heliosphere beyond its termination shock
- heliosphere: region of space dominated by the Sun
- heliosheath: outer layer of the heliosphere beyond its termination shock
- Voyager: space probes to observe planets and the edge of the solar system
- heliospheric current sheet: see current sheet
- heliospheric magnetic field: see magnetic field
- helium: non-metallic element, He, atomic number 2
- helium 1080 nm line (helium 1083 nm line): line useful in probing exoplanet atmospheres
- helium 1083 nm line: line useful in probing exoplanet atmospheres
- helium abundance: see helium
- helium burning: fusion reaction starting with helium
- helium burning (triple alpha process): fusion reaction chain turning helium-4 into carbon
- helium capture (alpha process): synthesis of elements through alpha capture
- helium core flash: see helium flash
- helium flash: helium ignition in a star
- helium runaway: helium fusion positive feedback through its generated heat
- thermal pulse: pulse in an AGB star due to helium fusion in a shell
- thermal runaway: situation where heat triggers more heat production
- helium fusion (helium burning): fusion reaction starting with helium
- helium main sequence: see helium star
- helium MS: see helium star
- helium planet: gas giant with helium atmosphere
- helium rain: possible source of heat in giant planet atmospheres
- helium runaway: helium fusion positive feedback through its generated heat
- helium shell flash: see helium flash
- helium star: O or B star with very strong helium lines
- helium valve (kappa mechanism): stellar instability caused by opacity increasing with temperature
- helium white dwarf: see white dwarf
- helium-4: see helium
- helium-burning shell: see helium burning
- Hellings and Downs curve: expected correlation in pulsar timing residuals from random waves
- Hellings Downs correlation (Hellings and Downs curve): expected correlation in pulsar timing residuals from random waves
- helmet streamer: see Sun surface features
- HELOS (EXOSAT): 1980s European space X-ray telescope
- HENA: see IMAGE
- Henry Draper Catalog: early 20th century spectroscopic catalog of stars
- Henry Draper Extension
- Henyey track: H-R diagram track of stellar evolution after the Hayashi track
- HEOSS: see NEOSSat
- Her-CrB GW (Hercules-Corona Borealis Great Wall): large slab of galaxies discovered in 2013
- HERA: low frequency radio telescope
- HERA: see IRAM 30m Telescope
- Herbig Ae star: see Herbig AeBe star
- Herbig Ae/Be star
- Herbig AeBe star: type of pre-main-sequence star more massive than a T-Tauri star
- Herbig Be star: see Herbig AeBe star
- Herbig-Haro object: type of glowing nebula near protostars
- Hercules
- Hercules A (3C 348): giant elliptical radio galaxy
- Hercules Cluster
- galaxy cluster: large group of galaxies gravitationally-bound to the group
- Hercules Supercluster
- Hercules-Corona Borealis Great Wall: large slab of galaxies discovered in 2013
- HERITAGE: see SAGE
- HERMES: four-channel spectrograph on the Anglo-Australian Telescope
- HERMES: see Anglo-Australian Telescope
- HERMES: see Mercator Telescope
- GALAH: survey of Milky Way with an emphasis on old structures
- HerMES J170647.8+584623 (HFLS3): very distant starburst galaxy
- HERMIT: code to carry out N-body simulations
- HERMIT4: see HERMIT
- Hermite interpolation
- HERMIT: code to carry out N-body simulations
- Hermite method
- HERMIT: code to carry out N-body simulations
- HERO: see extremely red object
- HerS (Herschel Stripe 82 Survey): 70 square-degree survey of galaxies redshifted 1.8 to 3.5
- Herschel (Herschel Space Observatory): infrared observatory-satellite with 3.5-meter reflector
- Herschel: see Herschel Double Star Catalog
- Herschel Double Star Catalog: series of early double star catalogs
- Herschel Inventory of the Agents of Galaxy Evolution: see SAGE
- Herschel Orion Protostar Survey: survey of SEDs of YSOs in Orion
- Herschel Redshift Survey (Herschel Stripe 82 Survey): 70 square-degree survey of galaxies redshifted 1.8 to 3.5
- Herschel Space Observatory: infrared observatory-satellite with 3.5-meter reflector
- Herschel Stripe 82 Survey: 70 square-degree survey of galaxies redshifted 1.8 to 3.5
- Herschel Telescope (William Herschel Telescope): 4.2-meter telescope in the Canary Islands
- Herschelian telescope: type of off-axis reflector telescope
- hertz: see frequency
- flux density: measure of the strength of a radio signal
- jansky: unit of measurement of spectral flux density
- Hertzsprung gap: see H-R diagram
- subgiant: slightly larger than normal star
- Hertzsprung gap star: see subgiant
- Hertzsprung-Russell diagram (H-R diagram): diagram relating effective temperature to luminosity
- Herz (Herz catalog): list of 3000 stars in a strip of the southern sky
- Herz catalog: list of 3000 stars in a strip of the southern sky
- Herzog: see rare designator prefixes
- HESE: see IceCube
- HESS (High Energy Stereoscopic System): ground gamma-ray telescope array
- Hess diagram: diagram relating effective temperature to luminosity
- HET (Hobby-Eberly Telescope): 10 meter telescope in Texas
- HET: see ion engine
- HETDEX
- HETE (High Energy Transient Explorer): 2000s space multi-band GRB observatory
- HETE-1: see High Energy Transient Explorer
- HETE-2: see High Energy Transient Explorer
- HETE-S: see Transiting Exoplanet Survey Satellite
- heterodyne: see beat frequency
- heterodyne photodetector: see beat frequency
- heterodyne spectrometer: type of millimeter/submillimeter spectrometer
- heterosphere: see homopause
- HETG: see Chandra X-ray Observatory
- HETGS: see Chandra X-ray Observatory
- heuristic habitable zone: see habitable zone
- Hevelius designation: see Flamsteed designation
- Hewitt & Burbidge Catalog: optical catalog of quasi-stellar objects
- HEXE: see Kvant 1
- HEXPAK: see WIYN 3.5m Telescope
- HEXTE: see RXTE
- HF (high frequency): electromagnetic radiation, frequency 3-30 MHz
- Hf: see rare designator prefixes
- HFI: see Planck
- HFLS: see rare designator prefixes
- HFLS3: very distant starburst galaxy
- HFLS3 (HFLS3): very distant starburst galaxy
- HG (host galaxy): galaxy associated with a particular object or transient
- HG: see H-R diagram
- HG: see Lowell Proper Motion Survey
- HG: see fast radio burst
- HG: see rare designator prefixes
- subgiant: slightly larger than normal star
- HG star: see subgiant
- HGA: see beam
- HgMn: see chemically peculiar star
- HH (Herbig-Haro object): type of glowing nebula near protostars
- HH: see hydrogen
- HH Andromedae (Ross 248): nearby red dwarf
- HHJ: see rare designator prefixes
- HHL: see Herbig-Haro object
- HHO (Herbig-Haro object): type of glowing nebula near protostars
- HHSMT (Submillimeter Telescope): 10-meter far-infrared telescope
- HI (HI region): cloud of neutral atomic hydrogen
- HI (neutral atomic hydrogen): state of hydrogen when sufficiently cool
- HI: see intergalactic HI cloud
- HI 4-Pi Survey: HI map based upon EBHIS and GASS
- HI cloud (HI region): cloud of neutral atomic hydrogen
- HI gas fraction: see HI gas mass
- HI gas mass: mass of an object's atomic hydrogen
- HI gas mass ratio: see HI gas mass
- HI Jodrell All-Sky Survey: northern-hemisphere HI survey carried out in late 90s/early 2000s
- HIPASS: HI survey carried out 1997 to 2002
- HI line (21-cm line): hydrogen line useful for determining redshift
- HI Parkes All Sky Survey (HIPASS): HI survey carried out 1997 to 2002
- HI region: cloud of neutral atomic hydrogen
- HI self-absorption: see self-absorption
- HI supershell: large shells bordered by HI regions
- HI supershell: see shell
- HI supershell: see supershell
- galactic worm: high density streak of HI perpendicular to galactic plane
- HI worm (galactic worm): high density streak of HI perpendicular to galactic plane
- Hi'iaka: see Haumea
- HI-SCALE: see Ulysses
- HI1: see STEREO
- HI2: see STEREO
- HI4PI (HI 4-Pi Survey): HI map based upon EBHIS and GASS
- HIBOU: see Hayabusa2
- HIC: see Galileo
- HIC: see Hipparcos
- HICAT: see HIPASS
- HiCIAO: see Subaru Telescope
- PALMS: survey to find exoplanets for direct imaging
- Hickson: see Hickson Compact Groups
- Hickson Compact Groups: list of 100 galaxy groups
- HID: see color-magnitude diagram
- hierarchical assembly of galaxies: development of large galaxies by combining small ones
- hierarchical clustering: see hierarchical assembly of galaxies
- Hierarchical Equal Area Isolatitude Pixelisation (HEALPix): type of map projection for spheres
- hierarchical merging (hierarchical assembly of galaxies): development of large galaxies by combining small ones
- hierarchical statistical model
- HIFI: see Herschel Space Observatory
- Higgs boson: see particle
- Higgsino: see supersymmetry
- High Accuracy Radial Velocity Planet Searcher (HARPS): high-precision spectrograph for exoplanet searches
- High Accuracy Radial Velocity Planet Searcher: see ESO 3.6m Telescope
- High Accuracy Radial Velocity Planet Searcher for the Northern Hemisphere (HARPS-N): high-precision spectrograph for exoplanet searches in Canary Islands
- High Altitude Observatory: NCAR unit observing and monitoring the Sun
- High Angular Resolution - Monolithic - Optical and Near-infrared - Integral field spectrograph (HARMONI): spectrograph planned for E-ELT
- high angular-resolution imaging (high-resolution imaging): typically, imaging with resolution better than a milliarcsecond
- high contrast imaging (HCI): imaging of targets very near bright objects
- high contrast spectrography: see HCI
- high contrast spectroscopy: see HCI
- High Definition Space Telescope: proposed 12 m space telescope
- LUVOIR: plan for a space-based telescope
- high dispersion spectrograph: see HRS
- high dispersion spectrography (HRS): high resolution spectrography
- high dispersion spectroscope
- HCI: imaging of targets very near bright objects
- High Earth Orbit Space Surveillance: see NEOSSat
- High Efficiency and Resolution Multi-Element spectrograph (HERMES): four-channel spectrograph on the Anglo-Australian Telescope
- High Elevation Antarctic Terahertz Telescope: 61 cm FIR telescope in Antarctica
- High Energy Astronomy Observatory 1 (HEAO-1): 1977 X-ray observatory space mission
- High Energy Astronomy Observatory 2 (HEAO-2): 1978 X-ray observatory space mission
- High Energy Astrophysics Division (HEAD): AAS division supporting X-ray and gamma-ray astrophysics
- High Energy Astrophysics Observatory 2 (HEAO-2): 1978 X-ray observatory space mission
- High Energy Astrophysics Science Archive Research Center (HEASARC): NASA archive of data on high energy astronomy
- high energy event: see IceCube
- high energy physics (particle physics): physics such as carried out in accelerators
- high energy starting event: see IceCube
- High Energy Stereoscopic System: ground gamma-ray telescope array
- High Energy Transient Explorer: 2000s space multi-band GRB observatory
- high frequency: electromagnetic radiation, frequency 3-30 MHz
- high frequency radio astronomy: see high frequency
- high frequency radio astronomy: see radio astronomy
- high gain antenna: see beam
- High Latitude Time Domain Survey: see Roman Space Telescope
- High Latitude Wide Area Survey: see Roman Space Telescope
- High Precision Parallax Collecting Satellite (Hipparcos): ESA astrometry space observatory circa 1990
- high proper motion star: see proper motion
- high redshift: see cosmological redshift
- High Resolution Fly's Eye: 1990s-2000s cosmic-ray observatory
- High Resolution Microwave Survey: 1990s microwave SETI search
- high resolution shock capture: method of modeling shock waves
- high resolution spectrograph: see HRS
- HCI: imaging of targets very near bright objects
- high resolution spectrography (HRS): high resolution spectrography
- high surface gravity: see surface gravity
- High Throughput X-ray Spectroscopy Mission (XMM-Newton): X-ray observatory-satellite operated by ESA
- High Time Resolution Universe Survey: c2010 search for pulsars and radio transients
- high time-resolution astrophysics: second-by-second observation of/for transients
- high-alpha (alpha-enhanced): metallicity with a high percentage of alpha elements
- High-altitude Water Cherenkov Experiment (High-altitude Water Cherenkov Observatory): New Mexico gamma ray telescope using Cherenkov radiation
- High-altitude Water Cherenkov Observatory: New Mexico gamma ray telescope using Cherenkov radiation
- high-B radio pulsar: pulsar with a strong magnetic field
- High-efficiency and High-resolution Mercator Echelle Spectrograph
- HERMES: four-channel spectrograph on the Anglo-Australian Telescope
- high-energy astronomy (high-energy astrophysics): astronomy and physics of X-rays and gamma rays
- high-energy astrophysics: astronomy and physics of X-rays and gamma rays
- High-Energy Celestial X-rays: see OSO 8
- high-mass protostellar object: see protostar
- high-mass X-ray binary: see X-ray binary
- high-resolution imaging: typically, imaging with resolution better than a milliarcsecond
- High-resolution J, H and K Spectrometer (NEWS): planned near-infrared spectrometer on Discovery Channel Telescope
- High-Resolution Ultraviolet Spectrometer Measurements: see OSO 8
- High-Sensitivity Array: see Very Long Baseline Array
- High-Sensitivity Crystal Spectroscopy of Stellar and Solar X-rays: see OSO 8
- high-surface-brightness galaxy: see low-surface-brightness galaxy
- high-velocity cloud: fast-moving clouds in galactic halos
- high-velocity star: star moving noticeably faster than neighbors
- high-z radio galaxy: see radio galaxy
- High-z Supernova Search Team: see supernova survey
- higher order multiple star: see star system
- higher-order method: see finite difference method
- higher-order multiple star system (multiple star system): system of two or more stars co-orbiting
- Highly Advanced Laboratory for Communications and Astronomy (HALCA): Japanese space radio telescope operating 1997-2005
- HII (HII region): cloud of partially-ionized atomic hydrogen
- HII (ionized hydrogen): state of hydrogen when sufficiently hot
- HII: see rare designator prefixes
- HII cloud (HII region): cloud of partially-ionized atomic hydrogen
- HII region: cloud of partially-ionized atomic hydrogen
- HiJak (NEWS): planned near-infrared spectrometer on Discovery Channel Telescope
- HIJASS (HI Jodrell All-Sky Survey): northern-hemisphere HI survey carried out in late 90s/early 2000s
- HIPASS: HI survey carried out 1997 to 2002
- Hilda
- Hill Radius
- Roche lobe: region of a body's gravitational influence closest to its competitor
- Hill radius: radius of gravitational influence of a body
- Hill sphere: see Hill radius
- Roche lobe: region of a body's gravitational influence closest to its competitor
- Hill stability: planetary orbits which will remain in the same order
- Hills Cloud: cloud of comets and minor planets around the solar system
- Hiltner: see rare designator prefixes
- Hiltner Telescope: 2.4-meter reflector telescope at MDM observatory
- HIM: see interstellar medium
- HIP: see Hipparcos
- HIP 45982 b (HD 80606 b): hot Jupiter discovered in 2001
- HIPASS: HI survey carried out 1997 to 2002
- HIPASS BGC: see HIPASS
- Hipparchus: Greek astronomer who developed concept of magnitude
- Hipparcos: ESA astrometry space observatory circa 1990
- Hipparcos Catalog: see Hipparcos
- Hipparcos Celestial Reference Frame: astronomical reference frame based upon Hipparcos catalog
- Hipparcos Input Catalog: see Hipparcos
- Hipparcos Reference Frame (Hipparcos Celestial Reference Frame): astronomical reference frame based upon Hipparcos catalog
- HIRAAS: see ARGOS
- HIRAX: radio telescope array for 21-cm surveying
- HIRES: see European Extremely Large Telescope
- HIRES: see Keck Observatory
- HiRes (High Resolution Fly's Eye): 1990s-2000s cosmic-ray observatory
- HiRes Fly's Eye (High Resolution Fly's Eye): 1990s-2000s cosmic-ray observatory
- HiRes-I: see High Resolution Fly's Eye
- HiRes-II: see High Resolution Fly's Eye
- HiRIC: see Tianwen-1
- HiRISE: see Mars Reconnaissance Orbiter
- HIRMES: see SOFIA
- HISA: see self-absorption
- Hisaki: retired Japanese space ultraviolet telescope
- HIT: see IMAP
- HITEMP: spectroscopic parameters for modeling gas optics
- Hitomi: 2016 Japanese space X-ray telescope
- CSA: includes astronomical research
- XRISM: X-ray space telescope launched in 2023
- HITRAN: spectroscopic parameters for modeling atmospheric optics
- HIZOA (Parkes HI Zone of Avoidance Survey): radio survey to identify galaxies hidden by the zone of avoidance
- HIZSS: see Parkes HI Zone of Avoidance Survey
- HJ (hot Jupiter): Jupiter-sized gas exoplanet orbiting very close to star
- HJ: see Herschel Double Star Catalog
- HJD: see Julian date
- HK (HK Survey): 1985 search for metal poor-stars using calcium lines
- HK Survey: 1985 search for metal poor-stars using calcium lines
- HL Tau: much-studied T-Tauri star
- HL Tauri (HL Tau): much-studied T-Tauri star
- HLD: see double star designation
- HLIRG (hyperluminous infrared galaxy): a galaxy emitting a more far infrared than an ULIRG
- HLM: see double star designation
- HLN: see double star designation
- HLX: see rare designator prefixes
- HM: see rare designator prefixes
- HMC: see Giotto
- HMF (halo mass function): mass of distribution of dark matter
- HMF: see magnetic field
- HMHD: see magnetohydrodynamics
- HMI: see Solar Dynamics Observatory
- HMNS (hypermassive neutron star): neutron star massive enough to collapse
- HMPO: see protostar
- HMS: see rare designator prefixes
- HMXB: see X-ray binary
- HN: see Herschel Double Star Catalog
- HNC
- Ho: see double star designation
- Ho: see rare designator prefixes
- Hoag's Object: peculiar ring galaxy
- HOBART
- hobbit galaxy: see dwarf galaxy
- Hobby-Eberly Telescope: 10 meter telescope in Texas
- NEWS: planned near-infrared spectrometer on Discovery Channel Telescope
- Hobby-Everly Telescope
- Hobby-Everly Telescope Dark Energy Experiment
- HOD: model of galaxy distribution
- Hodge: see rare designator prefixes
- Holm: see rare designator prefixes
- Holm 15A: see Abell 85
- Holm 15A*: see Abell 85
- Holmberg: see rare designator prefixes
- Holmberg 15A: see Abell 85
- Holmberg isophote: see Holmberg radius
- Holmberg radius: a measure of size of galaxy based on surface brightness
- holographic duality: notion that all the physics of three dimensions can be described in two
- holographic principle (holographic duality): notion that all the physics of three dimensions can be described in two
- holographic principle.
- homocentric sphere model: see heliocentric system
- homochirality: see molecular handedness
- homologous collapse: collapse such that the increasing density remains uniform
- homopause: level of atmosphere up to which components are mixed
- homosphere: see homopause
- Homunculus Nebula: see Eta Carinae
- Honda-like star: very-low-metallicity star with r-process elements
- Hooker Telescope: see Mount Wilson Observatory
- Hope Probe: UAE Mars orbiter currently studying Mars
- Hopkins Ultraviolet Telescope: 1990s ultraviolet space telescope used on two Shuttle Shuttle missions
- HOPS (H2O Southern Galactic Plane Survey): survey targeting water masers
- HOPS (Herschel Orion Protostar Survey): survey of SEDs of YSOs in Orion
- HOPS (The H2O Southern Galactic Plane Survey): survey of molecular clouds in the galactic plane
- horizon
- horizon distance: see particle horizon
- horizon problem: see inflation
- horizon size
- decoupling: some type of particle stops being destroyed as often as it is created
- horizontal branch: a second "giant" stage in the evolution of some main sequence stars
- horizontal branch star: see horizontal branch
- horizontal coordinate system: celestial coordinate system based upon horizon and north
- horizontal coordinates: see horizontal coordinate system
- horizontal gradient: see gradient
- horn
- horn antenna: see feedhorn
- horn feed: see phased array
- horsepower: see energy
- horsepower: see watt
- horseshoe orbit
- halo orbit: type of orbit around an unstable Lagrangian point
- HORuS: see Gran Telescopio Canarias
- host: object such as star around which something is orbiting or drawn to
- primary: central body of a system such as the Sun within the solar system
- 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
- primary: central body of a system such as the Sun within the solar system
- hot coronal gas: see interstellar medium
- hot dark matter: see dark matter
- hot disk accretion: see AGN accretion
- hot DOG: high-energy galaxy with a lot of dust
- hot dust-obscured galaxy (hot DOG): high-energy galaxy with a lot of dust
- hot dust-obscured object (hot DOG): high-energy galaxy with a lot of dust
- hot Earth
- Hot Exoplanet Transit Explorer-Survey: see Transiting Exoplanet Survey Satellite
- hot ionized medium: see interstellar medium
- hot Jupiter: Jupiter-sized gas exoplanet orbiting very close to star
- hot mass accretion: see cold mass accretion
- hot Neptune: see Neptune desert
- hot Neptunes: see extra-solar planet
- hot pixel: CCD pixel that shows more brightness than it should
- hot start: see giant planet formation
- hot-Neptune desert (Neptune desert): Neptune-like exoplanets near host star are rare
- Hough transform: algorithm used in image processing
- hour: see arcsecond
- hour: see right ascension
- hour angle: see right ascension
- hour circle: see right ascension
- hour of right ascension: see arcsecond
- HP: see rare designator prefixes
- HP3: see InSight
- HPCA: see Magnetospheric Multiscale Mission
- HPF: see Hobby-Eberly Telescope
- HPGSPC: see BeppoSAX
- HPMS: see proper motion
- HR (Bright Star Catalog): early 20th century catalog of 9000 stars
- HR 4796: see TW Hydrae association
- HR 8799: star with four directly-imaged exoplanets
- HR 8799 b: see HR 8799
- HR 8799 c: see HR 8799
- HR 8799 d: see HR 8799
- HR 8799 e: see HR 8799
- HRC: see Chandra X-ray Observatory
- HRC: see rare designator prefixes
- field of view: angular measure of how much an optical instrument can view
- HRC-I: see Chandra X-ray Observatory
- HRC-S: see Chandra X-ray Observatory
- HRCam: see SOAR Telescope
- HRD (H-R diagram): diagram relating effective temperature to luminosity
- HRD: see Cassini
- Hess diagram: diagram relating effective temperature to luminosity
- HRDI: see Upper Atmosphere Research Satellite
- HRF (Hipparcos Celestial Reference Frame): astronomical reference frame based upon Hipparcos catalog
- HRG: see rare designator prefixes
- Hrg: see double star designation
- HRI: see HEAO-2
- HRI: see ROSAT
- HRMS (High Resolution Microwave Survey): 1990s microwave SETI search
- HRS: high resolution spectrography
- HRS: see Hobby-Eberly Telescope
- HRS: see Hubble Space Telescope
- HRS: see Southern African Large Telescope
- 55 Cancri e: super-Earth orbiting a Sun-like star
- HCI: imaging of targets very near bright objects
- HRS2: see Hobby-Eberly Telescope
- HRSC (high resolution shock capture): method of modeling shock waves
- HRSC: see Mars Express
- HS (Hamburg Schmidt Survey): photographic plates from the Hamburg Schmidt Camera
- HS: see rare designator prefixes
- HSA: see Very Long Baseline Array
- HSB galaxy: see low-surface-brightness galaxy
- HSC: see Subaru Telescope
- HSC-SSP: see Subaru Telescope
- HSH: see rare designator prefixes
- HSP: see Hubble Space Telescope
- HST (Hubble Space Telescope): observatory-satellite with 2.4-meter reflective telescope
- HST MDS (Medium Deep Survey): survey to find galaxies in HST WFPC2 images
- HST-1: see M87
- HSTGC (Guide Star Catalog): catalog of stars to help aim HST
- HT (Hough transform): algorithm used in image processing
- HT metric (Hartle-Thorne metric): metric approximating GR effects of slow neutron star rotation
- HTC: see long-period comet
- HTRA (high time-resolution astrophysics): second-by-second observation of/for transients
- HTRU (High Time Resolution Universe Survey): c2010 search for pulsars and radio transients
- HTRU-N: see High Time Resolution Universe Survey
- HTRU-North: see High Time Resolution Universe Survey
- HTRU-S: see High Time Resolution Universe Survey
- HTRU-S LL: see High Time Resolution Universe Survey
- HTRU-South: see High Time Resolution Universe Survey
- HTRU-South low-latitude: see High Time Resolution Universe Survey
- HTSSE: see ARGOS
- HTT: see POLARBEAR
- HTT: see Simons Array
- HtTr: see rare designator prefixes
- Hu: see double star designation
- Hu: see rare designator prefixes
- Huan Tran Telescope: see POLARBEAR
- Huan Tran Telescope: see Simons Array
- Hubble: see rare designator prefixes
- Hubble Asteroid Hunter: Zooniverse effort to find asteroids in HST images
- Hubble classification (galaxy classification): morphological classification of galaxies
- Hubble constant: current rate of expansion of the universe
- Hubble constant tension (Hubble tension): inconsistency in recent determinations of the Hubble constant
- Hubble Data Archive: see MAST
- Hubble Deep Field: highly-detailed image of a small region of the sky taken with HST
- Hubble Deep Field North (Hubble Deep Field): highly-detailed image of a small region of the sky taken with HST
- Hubble Deep Field South: small region of the southern sky explored with HST
- Hubble diagram: diagram comparing redshift with distance
- Hubble distance: see Hubble constant
- Hubble expansion: ongoing expansion of the universe
- Hubble Extreme Deep Field: see Hubble Ultra-Deep Field
- Hubble flow: see Hubble expansion
- Hubble Heritage Project: effort assembling an archive of HST images
- Hubble Parameter
- scale factor: relative size of the universe as a function of time
- Hubble parameter: see Hubble constant
- Hubble sequence (galaxy classification): morphological classification of galaxies
- Hubble sequence: see galaxy classification
- Hubble Space Telescope: observatory-satellite with 2.4-meter reflective telescope
- GaiaHub: software to extract proper motions form Gaia and HST data
- WFC3: general purpose camera/spectroscope on HST
- Hubble Space Telescope Guide Catalog (Guide Star Catalog): catalog of stars to help aim HST
- Hubble tension: inconsistency in recent determinations of the Hubble constant
- dark energy: energy theorized to accelerate the expansion of the universe
- Hubble constant: current rate of expansion of the universe
- Hubble time: approximation of the age of the universe
- Hubble time: see Hubble constant
- Hubble type: see galaxy classification
- Hubble Ultra-Deep Field: extremely detailed HST image of a small region of space
- GLARE: project to search for distant star-forming galaxies
- Hubble's law: see Hubble constant
- HUDF (Hubble Ultra-Deep Field): extremely detailed HST image of a small region of space
- GLARE: project to search for distant star-forming galaxies
- HUDF09: see Hubble Ultra-Deep Field
- Huge-LQG: possible overly-large structure
- Hulse-Taylor Binary: binary system consisting of a pulsar and neutron star
- Hulse-Taylor Pulsar (Hulse-Taylor Binary): binary system consisting of a pulsar and neutron star
- humidity: amount of water vapor in a portion of the atmosphere
- Humphreys series: hydrogen lines for electrons settling from n > 6 to n = 6
- hydrogen: most prominent element, atomic number 1
- Hungaria
- Hungarian
- HATNet: distributed array of telescopes to search for transients
- Hungarian Automated Telescope Network (HATNet): distributed array of telescopes to search for transients
- Huoxing 1 (Tianwen-1): active Chinese Mars rover/orbiter mission
- HUT (Hopkins Ultraviolet Telescope): 1990s ultraviolet space telescope used on two Shuttle Shuttle missions
- Huya
- Kuiper Belt: further part of solar system beyond Neptune
- Huygens
- Cassini: explorer spacecraft sent to Saturn
- Huygens-Fresnel principle
- diffraction: an effect of the edge of an obstacle on a wave
- HV (hypervelocity star): star moving very much faster than usual
- HV: see rare designator prefixes
- HV 2112
- HVC (high-velocity cloud): fast-moving clouds in galactic halos
- HVS (hypervelocity star): star moving very much faster than usual
- HVS 2
- HW: see rare designator prefixes
- HWE: see double star designation
- HWFE: see wavefront error
- HWHM: see full width at half maximum
- HWP: see waveplate
- HX 1 (Tianwen-1): active Chinese Mars rover/orbiter mission
- HXD: see Suzaku
- HXDF: see Hubble Ultra-Deep Field
- HXI: see Hitomi
- HXI: see Lynx
- HXIS: see Solar Maximum Mission
- HXMT: space X-ray observatory launched in 2017
- HXR: see X-ray source
- HXRBS: see Solar Maximum Mission
- HXT: see Hitomi
- HXT: see Yohkoh
- Hyades Cluster: nearest open cluster
- Hybrid Adaptive Ray-Moment Method (HARM²): technique for parallelizing computations of radiation with hydrodynamics
- hycean planet: see K2-18b
- HYDRA: see WIYN 3.5m Telescope
- Hydra: see Pluto
- Hydra and Centaurus Superclusters (Hydra-Centaurus Supercluster): near neighbor of Local Supercluster
- Hydra-Centaurus Supercluster: near neighbor of Local Supercluster
- hydridocarbon (methylidyne): compound of one carbon and one hydrogen atom
- hydrocarbon: any compound of just carbon and hydrogen
- hydrodynamic equations: equations describing the dynamics of fluids
- hydrodynamic escape: escape of gas particles from atmosphere due to heating
- hydrodynamic gas loss (hydrodynamic escape): escape of gas particles from atmosphere due to heating
- hydrodynamic model: see hydrodynamics
- hydrodynamical model: see hydrodynamics
- hydrodynamics: study of fluids in motion
- hydrogen: most prominent element, atomic number 1
- Hydrogen Accretion in Local Galaxies Survey: radio survey looking for HI in galaxy halos
- hydrogen anion: see ionized hydrogen
- hydrogen bond
- ice: solid state of water
- hydrogen burning: any fusion reaction starting with hydrogen
- hydrogen cyanide: compound of hydrogen, carbon, and nitrogen, one atom each
- hydrogen deuteride: hydrogen molecule with one deuterium atom
- Hydrogen Epoch of Reionization Array (HERA): low frequency radio telescope
- hydrogen fusion (hydrogen burning): any fusion reaction starting with hydrogen
- Hydrogen Intensity and Real-time Analysis Experiment (HIRAX): radio telescope array for 21-cm surveying
- hydrogen intensity mapping: see intensity mapping
- hydrogen line (21-cm line): hydrogen line useful for determining redshift
- hydrogen line series: see hydrogen
- Balmer series: hydrogen lines for electrons settling from n > 2 to n = 2
- Brackett series: hydrogen lines for electrons settling from n > 4 to n = 4
- Humphreys series: hydrogen lines for electrons settling from n > 6 to n = 6
- Lyman series: hydrogen series from electrons settling from n > 1 to n = 1
- Paschen series: hydrogen lines for electrons settling from n > 3 to n = 3
- Pfund series: hydrogen lines for electrons settling from n > 5 to n = 5
- hydrogen nitride (ammonia): particular compound of nitrogen and hydrogen
- hydrogen-alpha filter
- H-alpha: hydrogen line for electrons settling from n = 3 to n = 2
- hydrogen-beta filter
- H-beta: hydrogen line for electrons settling from n = 4 to n = 2
- hydrogen-burning shell: see red-giant branch
- hydrologic cycle: see hydrology
- Titan: well-known Saturn moon
- hydrology: study of the movement of water
- hydrostatic balance (hydrostatic equilibrium): state of a fluid body when forces are such that the fluid is at rest
- hydrostatic equilibrium: state of a fluid body when forces are such that the fluid is at rest
- planet: round object orbiting a star that clears out its own orbital path
- hydrostatics
- hydroxide: see hydroxyl
- hydroxy group: see hydroxyl
- hydroxyl: one hydrogen atom and one oxygen atom bound together
- hydroxyl group: see hydroxyl
- hydroxyl ion: see hydroxyl
- hydroxyl maser: see hydroxyl
- hydroxyl radical: see hydroxyl
- Hygiea: see asteroid
- HyLIRG (hyperluminous infrared galaxy): a galaxy emitting a more far infrared than an ULIRG
- Hynek: see double star designation
- hyper extremely red object: see extremely red object
- Hyper Suprime-Cam: see Subaru Telescope
- Hyper Suprime-Cam Subaru Strategic Program: see Subaru Telescope
- Hyper-K: see Kamioka Observatory
- Hyper-Kamiokande: see Kamioka Observatory
- hyperbola: see conic section
- hyperbolic mirror
- hyperboloid: see paraboloid
- spheroid: sphere-like shape that is flattened or elongated
- hypercompact HII region: see HII region
- hypercompact stellar system: SMBH with no galaxy but has a tight stellar cluster
- hyperfine level: see hyperfine structure
- hyperfine structure: spectral lines from magnetic and spin interactions between electrons and nucleus
- fine structure: separate spectral lines due to electron spin and relativistic effects
- hypergalaxy: sizeable spiral galaxy with satellites
- hypergiant: extremely bright stars
- hypergiant star (hypergiant): extremely bright stars
- Hyperion: Monte Carlo radiative transfer code
- Hyperion: see Saturn
- moon: a planet's natural satellite, such as Earth's
- HyperLEDA: see LEDA
- HyperLEDA: see Principal Galaxies Catalog
- hyperluminous infrared galaxy: a galaxy emitting a more far infrared than an ULIRG
- hypermassive neutron star: neutron star massive enough to collapse
- hypernova (superluminous supernova): stellar explosion larger than a supernova
- collapsar: neutron star or stellar black hole
- hyperon: exotic type of baryon
- hyperon condensate: see hyperon
- hyperonic matter: see hyperon
- hypervelocity star: star moving very much faster than usual
- HZ (habitable zone): zone where conditions are favorable for life
- HZ: see rare designator prefixes
- HZE cosmic ray: see HZE ion
- HZE ion: fast-moving metal atom triply-ionized or more
- HZE ion: see solar energetic particle
- HZE particle (HZE ion): fast-moving metal atom triply-ionized or more
- HZG: see double star designation
- HzRG: see radio galaxy
Index