Astronomy

Hermippe: Jupiter's irregular moon (Jupiter XXX) Hermippe (moon) Hermippe (Jupiter XXX) is a small retrograde irregular moon of Jupiter discovered in 2001. It orbits at ~21.18 million km, has a period of ~630 days, and is a member of the Ananke…

Herschel Space Observatory: ESA's Far-Infrared and Submillimetre Telescope Herschel Space Observatory ESA's Herschel Space Observatory (launched 2009) was a large far‑infrared/submillimetre telescope that probed cold dust, star formation, interstellar chemistry and galaxy evolution…

Herse (moon of Jupiter) Herse (moon) Herse is a small irregular, retrograde moon of Jupiter discovered in 2003. It orbits at ~22.13 million km, is about 2 km across, and is considered a member of the Carme group of ca…

Hertzsprung–Russell diagram (H-R diagram) Hertzsprung–Russell diagram A scatter plot that relates stellar luminosity and effective temperature. The H‑R diagram reveals the main sequence, giants, supergiants and white dwarfs and is central to stellar…

Himalia: Jupiter’s largest irregular prograde moon Himalia (moon) Himalia is the largest irregular, non-spherical satellite of Jupiter. Discovered in 1904, it belongs to the Himalia group and is thought to be a fragment of a larger body with a da…

Himalia group — Jupiter's prograde irregular satellite family Himalia group A compact family of prograde irregular moons of Jupiter that share similar orbits and likely a common origin; principal members are Himalia, Leda, Lysithea and Elara.

Hinotori (satellite): Japan's 1981 solar X-ray mission Hinotori (satellite) Hinotori (ASTRO-A) was a Japanese satellite launched in 1981 to study solar flares and high-energy emissions; it carried X-ray and particle detectors and influenced later solar spa…

Hipparcos: ESA's Space Astrometry Mission and Star Catalogue Hipparcos Hipparcos was the European Space Agency's pioneering space astrometry mission, producing a high-precision catalogue of about 118,000 stars and a larger Tycho catalogue, and transfo…

History of Astronomy: From Ancient Skywatching to Modern Astrophysics History of astronomy Survey of astronomy’s development: ancient observations, classical models, medieval advances, the scientific revolution, and modern observational and theoretical astronomy.

Hiten (MUSES-A) — Japan's first lunar probe Hiten (spacecraft) Hiten (MUSES-A) was Japan's first lunar spacecraft, launched by ISAS in 1990. It deployed the subsatellite Hagoromo and demonstrated low‑energy transfer techniques and long‑duratio…

Hoag's Object — a nearly perfect ring galaxy Hoag's object Hoag's object is a rare ring galaxy discovered in 1950 by Arthur Hoag. About 600 million light-years away in Serpens, it has a bright core surrounded by a detached ring of young st…

Hohmann transfer orbit A fuel‑efficient two‑burn orbital maneuver that transfers a spacecraft between two coplanar circular orbits by using an intermediate elliptical trajectory.

Horologium (constellation) — the Clock of the Southern Sky Horologium (constellation) Horologium is a small southern constellation introduced in the 18th century. Named after a clock, it contains faint stars and notable variables and lies among other southern conste…

Horsehead Nebula (Barnard 33): dark cloud in the Orion Molecular Cloud Complex Horsehead nebula The Horsehead Nebula (Barnard 33) is a dark molecular cloud in Orion, silhouetted against emission nebula IC 434. It lies near Alnitak and forms part of the Orion Molecular Cloud C…

HR 4729: a hot blue star in the constellation Crux HR 4729 Overview of HR 4729 — a hot, blue star catalogued in the Bright Star Catalogue; its properties, possible link to Alpha Crucis, observational importance, and notable facts.

HR 5171 (V766 Centauri): a massive yellow hypergiant HR 5171 HR 5171 (V766 Cen) is an extreme yellow hypergiant — one of the largest and most luminous evolved stars known, in a multiple system with at least one close companion and a distant…

HR 6819 (QV Telescopii) — triple star system and disputed black hole claim HR 6819 HR 6819 (QV Telescopii) is a bright triple star system in Telescopium notable for a 2020 claim of a nearby non-accreting black hole that has since been disputed by follow-up studie…

Hubble Deep Field — landmark HST image revealing thousands of distant galaxies Hubble Deep Field Survey of the Hubble Deep Field (1995): its observation toward Ursa Major, discovery of thousands of faint galaxies, scientific importance, methods and lasting legacy for studies o…

Hubble eXtreme Deep Field (XDF) Hubble Extreme Deep Field A composite Hubble Space Telescope image of a tiny patch of sky in Fornax, released in 2012, showing some of the faintest and most distant galaxies known and probing the early univ…

Hubble Space Telescope — overview, design, history and achievements Hubble Space Telescope A concise, neutral overview of the Hubble Space Telescope: its design and instruments, orbit and operations, servicing history, major discoveries, and its role in modern astronomy.

Hubble Ultra Deep Field Hubble Ultra-Deep Field A detailed overview of the Hubble Ultra Deep Field (HUDF): its creation, contents, scientific importance, and key facts about the tiny patch of sky that revealed thousands of early…

Hubble's law (Hubble–Lemaître law): expansion of the universe Hubble's law An overview of Hubble's law, its historical origin, formulation, measurement methods, cosmological interpretation and important caveats about redshift, distances and the Hubble con…

Hyades (Melotte 25): the nearest open star cluster Hyades (star cluster) The Hyades (Melotte 25) is the closest open star cluster to the Solar System, a well-studied group of stars in Taurus used as a key calibrator for distance, stellar evolution, and…

Comet Hyakutake (C/1996 B2) Hyakutake Comet Hyakutake, discovered in 1996, produced an extraordinarily long tail and a close Earth approach that made it widely visible and the subject of extensive scientific study.

Hydra (constellation) Hydra is the largest constellation, a long water‑snake crossing both celestial hemispheres. It is notable for the bright star Alphard and several deep‑sky objects catalogued by Mes…

Hydra — outermost known moon of Pluto Hydra (moon) Hydra is a small outer moon of Pluto discovered in 2005. This article summarizes its discovery, orbit, physical properties, exploration by New Horizons, and the origin of its name.

Hydrus (constellation) Hydrus Hydrus is a small southern constellation representing a little water snake. Introduced in the Age of Discovery, it contains a few notable stars such as Beta Hydri and is distinct f…

Hyperbolic trajectory (astronomy and astrodynamics) Hyperbolic trajectory An unbound orbital path shaped like a hyperbola. Describes objects with eccentricity >1 that escape central gravity, used to characterize cometary flybys, interstellar arrivals, an…

Hypergiant (luminosity class 0): enormous, unstable stars Hypergiant Extremely luminous, massive stars that lose mass rapidly and live only a few million years. Overview of classification, structure, examples (Stephenson 2-18, NML Cygni, UY Scuti),…

Hyperion (moon of Saturn) Hyperion (moon) Hyperion is an irregular, sponge-like moon of Saturn noted for its chaotic tumbling, low-density porous surface, heavy cratering, and detailed imaging by Voyager and Cassini spacec…

Hyrrokkin (Saturn moon) Hyrrokkin (moon) Hyrrokkin is a small irregular retrograde moon of Saturn (~8 km across) discovered in 2006. It orbits at ~18.17 million km with high inclination and eccentricity and is named for a…

Iapetus: Saturn's Two‑Faced Moon Iapetus (moon) Iapetus, a large moon of Saturn discovered by Cassini in 1671, is notable for its stark light‑and‑dark hemispheres, a surprising equatorial ridge, slow rotation, and puzzling origi…

IC 10 — irregular starburst galaxy in Cassiopeia IC 10 IC 10 is a nearby irregular starburst galaxy in Cassiopeia, a member of the Local Group notable for its high density of Wolf–Rayet stars, large hydrogen envelope, and heavy obscura…

IC 2118 — the Witch Head Nebula IC 2118 IC 2118, nicknamed the Witch Head, is a faint blue reflection nebula in Eridanus near Orion. It lies roughly 900 light‑years away and is illuminated by the bright star Rigel and th…

IC 418 — the Spirograph Nebula IC 418 IC 418 is a compact planetary nebula in Lepus, about 1,100 light-years away. Noted for concentric shells and fine filamentary structure, it is a common target for detailed imaging…

Ice giant (planetary class) Ice giant Large outer planets composed mainly of volatile compounds (water, ammonia, methane) and modest hydrogen/helium envelopes; Uranus and Neptune are the Solar System's ice giants.

Icy moon Natural satellites with extensive surface ice and often hidden subsurface oceans; key objects for studying planetary geology, chemistry, and potential habitability.

Ijiraq — an irregular moon of Saturn Ijiraq (moon) Ijiraq is a small, non-spherical, prograde irregular satellite of Saturn (S/2000 S 6), noted for its red color and membership in the Inuit group of moons. Discovered in 2000 by Gla…

Impact event: collisions between space objects and planets Impact event An impact event is a collision between space objects and a planet. This article explains what they are, how craters form, historical examples, frequency, consequences, and efforts…

Indian Space Research Organisation (ISRO) — overview and significance Indian Space Research Organisation Overview of ISRO: history, structure, major programs, launch vehicles, applications and notable achievements including Mars and lunar missions.

Indus (constellation) Indus is a modern southern constellation representing an indigenous person. Introduced in the Age of Discovery, it is faint, lacks bright stars, and is best observed from southern…

Cosmic Inflation (cosmology) Inflation (cosmology) Cosmic inflation is a brief period of extremely rapid expansion in the early universe that explains its large-scale uniformity, flatness, and the origin of structure observed today…

Infrared Telescope: Observing the Heat of the Universe Infrared telescope An infrared telescope detects thermal (infrared) radiation to study cool and obscured cosmic objects. Covers how they work, types, history, applications and observing challenges.

Inner moons of Jupiter The four small moons closest to Jupiter — Metis, Adrastea, Amalthea and Thebe — their characteristics, relationships with Jupiter’s ring system, orbital behavior, discovery history…

Intercalation (timekeeping): adding days, months and seconds to calendars Intercalation (timekeeping) Intercalation is the insertion of extra time units into calendars to keep civil time aligned with astronomical cycles. Methods include leap days, leap months and leap seconds acros…

International Astronomical Union The International Astronomical Union (IAU) is the leading global organization for professional astronomers, setting standards, coordinating research and education, and naming celes…

International Space Station (ISS) International Space Station A modular, multinational laboratory and habitable satellite in low Earth orbit. The ISS supports long-duration human presence, scientific research in microgravity, technology demon…

International Year of Astronomy 2009 — global celebration of sky science International Year of Astronomy A worldwide 2009 campaign led by the IAU and UNESCO marking 400 years since Galileo’s telescope; public outreach, education, observing events and legacy projects in over 140 countr…