#physics definition

• 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.
• Astronautic — Belonging to astronautics, relating to astronautics; referring to the techniques, technologies, vehicles, or physical laws involved in the navigation and flight of manned or automated craft outside the Earth's atmosphere. The term defines structural engineering elements, propulsion systems, orbital trajectories, or research missions conducted in deep outer space.
• Astrosphere — A bubble-like region of space surrounding a star, continuously generated and maintained by the stream of plasma ejected by that star (the stellar wind) and its magnetic field. This dynamic structure acts as a protective shield against interstellar dust and high-energy cosmic rays, extending up to the astropause, where the stellar wind pressure balances that of the surrounding interstellar medium; the heliosphere is the specific astrosphere of our Sun.
• Astrospectroscopy — An elite branch of practical astrophysics dealing with the photography, digital recording, and structural analysis of the electromagnetic radiation spectra emitted or absorbed by celestial bodies. By decomposing stellar light using prisms or diffraction gratings attached to telescopes, astrospectroscopy enables the determination of chemical composition, temperature, pressure, radial velocity, and magnetic field strength of objects in the universe.
• Astrospectroscopy — A fundamental field of astrophysics dealing with the qualitative and quantitative study of the electromagnetic radiation spectra emitted or absorbed by celestial bodies. This discipline analyzes spectral lines decomposed via diffraction gratings or prisms to deduce the chemical composition, effective temperature, surface gravity, radial velocity, and rotational motions of cosmic objects in the universe.
• Autodynamic — Moving, developing, or operating by its own internal energy, without external impulses or driving forces; a mechanical or physical system endowed with the capacity for self-propulsion or dynamic self-evolution.
• To self-balance — To restore one's own state of stable equilibrium, vertical position, balance of forces, or functional stability through internal mechanisms, sensors, or self-regulation processes, without external corrective command or force.
• Self-organization — A process where a complex structure or system spontaneously increases its internal organization without being guided or managed by an external source or a centralized algorithm; an emergent structural phenomenon where local interactions among a system's components generate a global order. In physics and chemistry, it describes the spontaneous formation of geometric patterns or crystal lattices, while in sociology and management, it reflects the ability of teams to coordinate autonomously to achieve goals.
• Self-oscillation — An unwanted or sustained periodic oscillation that occurs in a physical, mechanical, or electronic system without the action of an external periodic force, being generated and maintained by a continuous energy source through a structural positive feedback mechanism. In electronics, this phenomenon often occurs in unstable amplifiers, turning them into parasitic signal generators, while in mechanics, it can cause destructive vibrations in structures or aircraft wings (flutter phenomenon).
• To self-propel — To propel oneself or move a vehicle or body by its own energetic and motor means, structurally integrated within it, without relying on an external pulling or pushing force. In transportation and aerospace engineering, it describes the ability of a device, vehicle, or projectile to generate the force required for travel by consuming a fuel or energy source stored on board.
• Autoradiography — A biophysical method of investigation and structural imaging consisting of detecting and recording the spatial distribution of radioactive substances or isotope-labeled molecules (tracers), by directly exposing the biological or metallurgical sample onto a photographic emulsion or a digital sensor. This experimental process utilizes the self-emission of radiation (alpha, beta, or gamma) from the sample to expose the contact medium, serving as a fundamental technique in nucleic acid mapping, cellular metabolism studies, and protein localization.
• Auxometer — A special optical instrument or apparatus used to determine and measure the magnifying power of a lens system, microscope, or telescope; a precision device engineered to measure the exit pupil diameter or the ratio between image size and actual object size.
• Audibility — The quality, property, or state of being audible; the degree or level of clarity and intensity with which a sound, voice, or acoustic signal is transmitted and can be distinctly perceived by a receiver in a given environment.
• Azimuth — The horizontal angle measured clockwise (from 0° to 360°) between the North direction of a reference meridian and the direction toward a given point on the Earth's surface or the projection of a celestial body on the horizon.
• Azimuthal — Pertaining to azimuth, belonging to azimuth, or carried out within the horizontal plane of the azimuth; often used to describe telescope mounts, map projections, or measuring instruments that utilize horizontal coordinates based on the angle formed with the North direction.