#stellar

• Alcor — The proper name of a fourth-magnitude star in the constellation Ursa Major (80 Ursae Majoris), located very close to the brighter star Mizar, with which it forms a well-known visual double star pair. Historically, the ability to visually distinguish these two stars with the naked eye was utilized as a classic test for visual acuity across various cultures.
• Aldebaran — The brightest star in the constellation of Taurus (Alpha Tauri) and the fourteenth-brightest star in the night sky, located approximately 65 light-years from Earth. It is a red giant star, much cooler than the Sun, but with a diameter about 44 times larger, easily recognizable due to its distinct reddish-orange hue.
• Alsine — A historical botanical term or archaic designation used in the past to denote the genus Stellaria (specifically Stellaria media, popularly known as chickweed) or related species in the Caryophyllaceae family. In the classical taxonomic lexicon, it described small annual herbaceous plants with fragile stems, opposite leaves, and small white flowers with divided petals, adapted to moist, cultivated soils.
• Ambartsumian — A surname of Armenian origin, globally famous due to the Soviet Armenian astrophysicist Viktor Ambartsumian. He was a pioneer of theoretical astrophysics, founder of the Byurakan Observatory, and renowned for discovering stellar associations and studying galactic evolution.
• Arcturus — The proper name of the brightest star in the constellation Boötes and the fourth-brightest star in the entire night sky, being an orange giant located approximately 36.7 light-years from Earth. In mythology, the name originates from ancient Greek and means 'guardian of the bear', due to its structural proximity to the Ursa Major constellation.
• Arecibo — A proper geographical name designating a coastal municipality in Puerto Rico, globally famous for its iconic Arecibo Observatory, a massive astronomical structure built inside a natural sinkhole depression. Structurally, the observatory served for decades as a cornerstone in astrophysics, ionospheric research, and SETI programs searching for extraterrestrial intelligence, and was also the origin of the famous 'Arecibo Message' beamed into space in 1974.
• Argelander — The surname of the celebrated German astronomer Friedrich Wilhelm Argelander (1799–1875), globally renowned for his fundamental contributions to observational astrophysics and stellar cartography. He coordinated the creation of the 'Bonner Durchmusterung' catalog (1859-1862), a reference structural work recording the position and magnitude of over 324,000 northern hemisphere stars, revolutionizing the study of variable stars and setting modern standards for astronomical photometry.
• Astral — Relating to the stars, celestial bodies, or the starry vault, concerning the stars or originating from them; in esoteric philosophy and mysticism, it describes a non-physical dimension or a subtle form of energy (astral body, astral plane) that mediates the connection between spirit and gross matter, considered the seat of human emotions and desires.
• Astro — Initial combining form derived from the Greek 'astron' used to construct words related to stars, celestial bodies, or outer space. Used independently in modern colloquial language, technology, or hobbyist communities, the term designates astrophotography, stargazing equipment, or software platforms optimized for deep-sky imaging processing.
• Astrophysical — Belonging to astrophysics, relating to astrophysics; referring to the application of the laws, methods, and principles of physics to the study of the structure, chemical composition, evolution, and dynamic phenomena of celestial bodies (stars, planets, galaxies, black holes) and interstellar space. It also defines the instruments, research, or mathematical models used to explain the behavior of matter and energy on a cosmic scale.
• Astrophysics — A branch of astronomy and modern physics that studies the physical properties, chemical composition, structure, dynamics, and evolution of celestial bodies (stars, planets, galaxies, nebulae) as well as the interstellar or intergalactic medium. This discipline applies the laws of thermodynamics, quantum mechanics, relativity, and electromagnetism to explain complex cosmic phenomena, from stellar nuclear fusion processes to cosmic microwave background radiation.
• Astrophysicist — A scientist, researcher, or highly qualified specialist in the field of astrophysics; an expert who applies the methods, theories, and principles of fundamental physics to investigate the structure, dynamic behavior, life cycles, and radiation of celestial bodies. Their activity includes spectral analysis of light emitted by stars, modeling collisions between black holes, or interpreting global gravitational anomalies.
• Astrophysicist — A woman who works as an astrophysicist; a highly qualified female researcher or specialist in the field of astrophysics who applies physical laws and principles to study the chemical composition, internal structure, orbital dynamics, and evolution of celestial bodies. Her activity involves utilizing high-power spectrographs, interpreting gravitational anomalies around black holes, and developing computational mathematical models to explain the expansion of the universe.
• Astrophotometry — A fundamental branch of practical astrophysics that deals with measuring the intensity, flux, and brightness of radiant energy (light) emitted by celestial bodies across different bands of the electromagnetic spectrum. Data obtained through astrophotometry (using photometers or CCD sensors) allow astronomers to determine cosmic distances, surface temperatures, rotational periods of variable stars, and to detect exoplanets via the transit method.
• Astrophotometer — A high-precision astronomical instrument (equipped with photoelectric sensors or calibrated CCD/CMOS cells) used in astrophotometry to quantitatively measure the luminous flux, intensity, and brightness of radiation emitted by celestial bodies. This device allows comparing the magnitude of a target star with a standard artificial light source or reference stars, forming the basis for calculating light curves of variable stars and detecting extrasolar planets.
• Astrognosy — An old descriptive branch of practical astronomy that deals with identifying, naming, and mapping constellations, as well as recognizing the position of the main stars and celestial bodies visible to the naked eye or rudimentary instruments. Unlike modern astrophysics, this classic field focuses strictly on sky orientation, traditional star charts, and celestial nomenclature.
• Astrometric — Belonging to astrometry, relating to astrometry; referring to the precise geometric measurement of positions, angular distances, and proper motions of celestial bodies on the celestial sphere. This term defines the methods, coordinate catalogs, instruments (such as scanning telescopes), or satellite data (such as those provided by the Gaia mission) used to establish fundamental star charts.
• Astrometry — A fundamental branch of positional astronomy that deals with the precise geometric measurement of positions, proper motions, angular distances, and parallaxes of celestial bodies on the celestial sphere. This field establishes space reference systems and fundamental coordinates indispensable for galactic mapping, calculating planetary orbits, or detecting subtle gravitational perturbations generated by exoplanets.
• Astrometer — An astronomical measuring instrument used for the precise determination of angular positions, coordinates, and distances between celestial bodies in the night sky. Although historically the term defined filar micrometers or classic meridian circles used in astrometry, it can currently also designate the human specialists operating these complex mapping systems or automated scanning telescopes linked to sensor arrays.
• Spacecraft — A spacecraft, cosmic vessel, or complex flight craft structurally designed and equipped to navigate in extra-atmospheric space, transport human crews or scientific payloads, and perform interplanetary or interstellar exploration missions. A spacecraft integrates advanced propulsion, regenerative life support systems, thermal and radiation shielding, and automated guidance equipment necessary for survival and operation in the vacuum of space.