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Space Weather: The conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground- based technological systems and can endanger human life or health. Plasma: A state of matter in which some or all of the electrons have been stripped from atoms, creating a mixture of charged particles (ions and electrons). Hydrogen: The lightest and most abundant element in the universe, consisting of a single proton and one electron. Helium: A chemical element with the symbol He and atomic number 2, a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Cecilia Payne-Gaposchkin: An English-American astronomer and astrophysicist who was the first person to suggest that stars are primarily composed of hydrogen and helium. Core: The central region of a celestial object, such as a star or planet, where nuclear fusion reactions occur in the case of stars. Radiative Zone: The layer of a star's interior, just outside the core, where energy is transported primarily by radiation. Convective Zone: The region in the interior of a star where energy is transported by the movement of hot, buoyant material. Photosphere: The visible surface of a star, including the Sun, where most of the light we see originates. Chromosphere: A layer of the Sun's atmosphere located just above the photosphere and below the corona, known for its reddish color and prominence in solar eclipses. Corona: The outermost layer of the Sun's atmosphere, characterized by extremely high temperatures and a halo-like appearance during solar eclipses. Solar Wind: A stream of charged particles, primarily electrons and protons, flowing outward from the Sun's corona into space. Granulation: The cellular pattern on the Sun's surface caused by the convective motion of hot gases. Sunspot: A dark, cooler region on the Sun's surface caused by strong magnetic activity. Coronal Hole: A region in the Sun's corona where the magnetic field is open, allowing solar wind to escape more easily, resulting in reduced density and lower temperatures.

Transition Region: A region in the Sun's atmosphere where the temperature increases rapidly with altitude, transitioning from the cooler chromosphere to the hotter corona. Aurora: A natural light display in the Earth's sky, typically seen in polar regions, caused by the interaction of charged particles from the Sun with the Earth's atmosphere. Umbra: The darkest part of a sunspot, where the magnetic field is strongest. Penumbra: The lighter region surrounding the umbra of a sunspot, where the magnetic field is weaker. Differential Rotation: The phenomenon where different parts of a rotating celestial object, such as the Sun, rotate at different speeds. Sunspot Cycle: The approximately 11-year cycle of solar activity, characterized by changes in the number and distribution of sunspots. Zeeman Effect: The splitting of spectral lines in the presence of a magnetic field, providing information about the strength and direction of the field. Maunder Minimum: A period of reduced sunspot activity, occurring from the late 17th century to the early 18th century, associated with a cooler climate on Earth. Plage: A bright region in the Sun's chromosphere, often associated with strong magnetic fields. Prominence: A large, bright, gaseous feature extending from the Sun's surface into the corona. Flare: A sudden and intense burst of energy and light on the Sun's surface, often accompanied by the ejection of material into space. CME (Coronal Mass Ejection): A massive burst of solar wind and magnetic fields from the Sun's corona into space, often associated with solar flares. Gravitational Contraction: The process by which a cloud of gas and dust contracts under the influence of gravity, leading to the formation of stars and planets. Lord Kelvin: A Scottish physicist known for his work on the laws of thermodynamics and contributions to the understanding of temperature, energy, and heat. Albert Einstein: A German-born physicist who developed the theory of relativity, known for his equation E=mc^2, and made significant contributions to our understanding of the nature of space, time, and gravity.

E=mc^2: Albert Einstein's famous equation, which relates energy (E) to mass (m) and the speed of light (c), expressing the equivalence of mass and energy. Wolfgang Pauli: An Austrian physicist known for the Pauli Exclusion Principle, a fundamental concept in quantum mechanics. Neutrino: A subatomic particle that is electrically neutral and nearly massless, interacting very weakly with matter. Matter: The substance of the physical world, composed of atoms and molecules, with mass and occupying space. Antimatter: Particles that have the same mass as ordinary matter but opposite electrical charges, such as positrons (antielectrons) and antiprotons. Nuclear Fusion: The process by which two or more atomic nuclei combine to form a single, more massive nucleus, releasing a large amount of energy. Nuclear Fission: The process by which the nucleus of an atom splits into two smaller nuclei, releasing a significant amount of energy. Deuterium: A heavy isotope of hydrogen with one proton and one neutron in its nucleus, often used as fuel in nuclear fusion reactions. Positron: A subatomic particle that is the antimatter counterpart of the electron, having a positive charge. Proton-Proton Chain: The sequence of nuclear reactions by which stars like the Sun convert hydrogen into helium, releasing energy in the process. Hydrostatic Equilibrium: The balance between the inward gravitational force and the outward pressure force in a star, ensuring its stability. Conduction: The transfer of heat through a material without the movement of the material itself, typically occurring in solids. Convection: The transfer of heat through the movement of a fluid (liquid or gas) due to density differences, resulting in the rising of warmer fluid and the sinking of cooler fluid. Radiation: The transfer of energy in the form of electromagnetic waves, such as light or heat. Helioseismology: The study of the interior structure and dynamics of the Sun through the observation of solar vibrations and oscillations.

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Raymond Davis: An American chemist and physicist known for his pioneering work in detecting solar neutrinos. Neutrino Oscillation: The phenomenon where neutrinos change from one type (flavor) to another as they travel, implying that neutrinos have mass. Luminosity: The total amount of energy radiated by an astronomical object per unit of time, often expressed in watts. Apparent Brightness: The brightness of a celestial object as it appears to an observer on Earth. Apparent Magnitude: A measure of the brightness of a celestial object as observed from Earth. Color Index: A measure of a star's color, determined by comparing its brightness in different wavelengths. Joseph Fraunhofer: A German physicist and optician known for his work in spectroscopy and the discovery of absorption lines in the solar spectrum. William Huggins: An English astronomer known for his pioneering work in astrophysics, particularly the study of stellar spectra. Margaret Huggins: An English astronomer and wife of William Huggins, who also contributed to astrophysical research. Balmer Lines: A series of spectral lines in the visible region of the electromagnetic spectrum, associated with the hydrogen atom. Spectral Class: A classification system for stars based on their spectral characteristics, including temperature and composition. Annie Jump Cannon: An American astronomer known for her work on the classification of stellar spectra and the development of the Harvard Classification Scheme. Brown Dwarf: A substellar object that is too massive to be a planet but does not have sufficient mass to sustain nuclear fusion in its core. Metals: In astrophysical terms, elements that are heavier than helium, such as carbon, oxygen, and iron. Stellar Giants: Large and luminous stars that have evolved off the main sequence and expanded in size. Doppler Effect: The change in the frequency or wavelength of a wave as observed by an

observer moving relative to the source of the wave. Radial Velocity: The component of an object's velocity along the line of sight of an observer, which can be determined from the Doppler shift of its spectral lines. Proper Motion: The apparent angular motion of a celestial object across the sky, caused by its actual motion through space. Line Broadening: The phenomenon where spectral lines appear wider than expected due to various factors, such as thermal motion or magnetic fields. Edward Pickering: An American astronomer known for his work in stellar spectroscopy and for founding the Harvard College Observatory. Henry Draper: An American physician and amateur astronomer known for his contributions to astrophotography and stellar spectroscopy. Anna Draper: The wife of Henry Draper, who played a significant role in funding and supporting her husband's astronomical work. James Lick: An American land baron and philanthropist known for funding the construction of Lick Observatory. Brown Dwarf: A substellar object that is too massive to be a planet but does not have sufficient mass to sustain nuclear fusion in its core. White Dwarf: The remnant of a star that has exhausted its nuclear fuel and collapsed to a dense, Earth-sized object. Red Dwarf: A small and cool star, often the most common type of star in the universe. Selection Effect: A bias in data collection or analysis that can occur when certain observations are more likely to be included in a study than others. Binary Star: A system of two stars that orbit around a common center of mass. Double Star: A pair of stars that appear close together in the sky but may or may not be physically related. Visual Binary: A binary star system where both components can be observed directly through a telescope. Spectroscopic Binary: A binary star system where the presence of two stars is detected through their spectral lines.

Eclipsing Binary: A binary star system where one star periodically passes in front of the other, causing a temporary decrease in brightness. Radial Velocity Curve: A graphical representation of how the radial velocity of a star in a binary system varies over time. Light Curve: A graph that shows how the brightness of a celestial object changes over time. Mass-Luminosity Relation: The relationship between a star's mass and its luminosity (brightness), indicating that more massive stars are generally more luminous. John Goodricke: An English deaf astronomer known for his discovery of the periodic variations in the brightness of certain variable stars, including Delta Cephei. Ejnar Hertzsprung: A Danish chemist and astronomer known for his work on the Hertzsprung- Russell diagram and the classification of stars. Henry Norris Russell: An American astronomer known for his work on stellar evolution and contributions to the Hertzsprung-Russell diagram. Arthur Eddington: An English astrophysicist known for his work on the structure and evolution of stars and for testing Einstein's theory of relativity. HR Diagram (Hertzsprung-Russell Diagram): A graph that plots stars' luminosity against their surface temperature, providing insight into stellar evolution. Main Sequence: The region on the HR diagram where most stars, including the Sun, spend the majority of their lives, undergoing hydrogen fusion. Supergiant: A massive and highly luminous star that is larger than a giant star. Red Giant: A large and luminous star in the late stages of its evolution, having exhausted its core hydrogen fuel. AU (Astronomical Unit): A unit of distance used in astronomy, approximately equal to the average distance between the Earth and the Sun. Light Year: The distance that light travels in one year in the vacuum of space, approximately 9.46 trillion kilometers (5.88 trillion miles). Light Second: The distance that light travels in one second, approximately 299,792 kilometers (186,282 miles).

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Parallax: The apparent shift in the position of an object when viewed from different angles, used to measure astronomical distances. Parsec: A unit of distance used in astronomy, approximately equal to 3.09 trillion kilometers (1.92 trillion miles). Hipparcos: A European Space Agency mission that accurately measured the positions and distances of over 100,000 stars. Gaia: A European Space Agency mission designed to create a highly accurate three- dimensional map of stars in the Milky Way. Variable Star: A star that exhibits variations in brightness over time. Cepheid Variable: A type of variable star with a well-defined relationship between its pulsation period and luminosity, making it useful for measuring astronomical distances. RR Lyrae Variable: A type of pulsating variable star that is used as a standard candle for measuring distances in the Milky Way and other galaxies. Period-Luminosity Relation: The relationship between the pulsation period of certain variable stars (e.g., Cepheids) and their intrinsic luminosity, used for distance measurements. Henrietta Leavitt: An American astronomer known for her discovery of the period-luminosity relation for Cepheid variable stars. Luminosity Class: A classification of stars based on their luminosity, such as supergiants, giants, and dwarfs. Spectroscopic Distance: The measurement of a star's distance using spectroscopic techniques, often based on the star's spectral characteristics. ISM (Interstellar Medium): The matter and radiation that exists in the space between the stars in a galaxy. Nebula: A cloud of gas and dust in space, often the birthplace of stars. HII Regions: Regions of ionized hydrogen gas in the interstellar medium, often associated with star formation. Molecular Clouds: Dense and cold regions in the interstellar medium where new stars can form. 21-cm Radiation: A wavelength of electromagnetic radiation emitted by neutral hydrogen, used to study the distribution of hydrogen in the Milky Way.

Electron Spin-Flip: A process in which the spin orientation of an electron changes, often associated with transitions in atomic and molecular spectra. Harold Ewen: An American physicist known for his contributions to radio astronomy and the discovery of the 21-cm radiation from hydrogen. Edward Purcell: An American physicist known for his work in nuclear magnetic resonance and his contributions to radio astronomy. Interstellar Dust: Tiny solid particles or grains found in the interstellar medium, which can scatter and absorb light. EE Barnard: An American astronomer known for his pioneering work in astrophotography and the discovery of dark nebulae. Reflection Nebula: A type of nebula that shines by reflecting the light of nearby stars. Emission Nebula: A type of nebula that emits light due to the ionization of its gas by nearby stars, often characterized by colorful and glowing regions. Extinction: The dimming or reduction in brightness of light from a celestial object as it passes through the interstellar medium. Reddening: The shift of light from a celestial object toward longer (redder) wavelengths due to scattering and absorption by interstellar dust. Cosmic Rays: High-energy charged particles, primarily protons and atomic nuclei, originating from various sources in the universe. Baryon Cycle: The cycling of baryonic matter (protons, neutrons, and other particles) through stars, interstellar medium, and galaxies. Local Hot Bubble: A region of the interstellar medium in the Milky Way that is significantly hotter and less dense than its surroundings. Orion Nebula: A well-known emission nebula located in the Orion constellation, often referred to as the "Great Nebula in Orion." Protostar: A young star in the process of formation, still gathering material from its surrounding molecular cloud. Conservation of Angular Momentum: The principle that states that the total angular momentum of a closed system remains constant unless acted upon by an external torque.

Stellar Wind: The flow of charged particles, such as protons and electrons, from the surface of a star into space. Protostar Jets: Narrow streams of gas and dust ejected from a protostar's poles during its formation. Evolutionary Track: A path on an Hertzsprung-Russell diagram that represents the changes in temperature and luminosity of a star over its lifetime. Zero-Age Main Sequence: The stage of a star's evolution when it first arrives on the main sequence, burning hydrogen in its core. Times of Evolutionary Stages: The durations of various stages in a star's life, including the main sequence, red giant phase, and supernova explosion. Protoplanetary Disk: A rotating disk of gas and dust surrounding a young star, from which planets and other objects may form. Accretion Disk: A flattened, rotating disk of gas and dust that forms around a massive object, such as a black hole or young star. Exoplanet: A planet that orbits a star outside our solar system. Doppler Effect: The change in the frequency or wavelength of a wave as observed by an observer moving relative to the source of the wave. Radial Velocity: The component of an object's velocity along the line of sight of an observer, which can be determined from the Doppler shift of its spectral lines. Didier Queloz: A Swiss astronomer known for his discovery of the first exoplanet orbiting a sun- like star. Michel Mayor: A Swiss astrophysicist known for his work on exoplanets and the discovery of the first exoplanet orbiting a sun-like star. 51 Pegasi: A star known for hosting the first discovered exoplanet orbiting a Sun-like star, 51 Pegasi b. Hot Jupiter: A type of exoplanet that is similar in size and mass to Jupiter but orbits very close to its host star. Transit Method: A technique for discovering exoplanets by observing periodic dips in a star's brightness as a planet passes in front of it.

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Kepler Mission: A NASA space telescope mission designed to search for exoplanets using the transit method. Super-Earth: A type of exoplanet with a mass and size larger than Earth but smaller than Neptune. Mini-Neptune: A type of exoplanet that is intermediate in size between Earth and Neptune. Post-Formation Planetary Migration: The movement of exoplanets from their original formation locations to new orbits due to interactions with other objects or forces. Lifetime on Main Sequence: The length of time a star spends on the main sequence, burning hydrogen in its core. Age when Hydrogen Fusion Ends in Core (sometimes called terminal-age main sequence): The age at which a star exhausts its core hydrogen fuel and begins to evolve off the main sequence. Shell Burning (actually shell fusion is a better name): The nuclear fusion that occurs in the outer shell of a star after core hydrogen exhaustion. Red Giant Stage: A later stage in a star's evolution when it has expanded and increased in luminosity after core hydrogen exhaustion. Globular Cluster: A dense and spherical collection of stars that orbits a galaxy's core. Open Cluster: A group of relatively young stars that formed from the same molecular cloud and are loosely bound by gravity. Stellar Association (sometimes called O-B associations): A loose and young group of stars that formed together but are not gravitationally bound. Main Sequence Turnoff Point: The location on an HR diagram where stars leave the main sequence and evolve into other stages. Helium Flash: A brief burst of helium fusion in the core of a red giant star. Triple-Alpha Process: A nuclear reaction in which three helium nuclei (alpha particles) combine to form carbon. Hydrostatic Equilibrium: The balance between the inward gravitational force and the outward pressure force in a star, ensuring its stability. Planetary Nebula: A shell of gas and dust ejected from a dying star during the red giant phase.

Supergiant: A massive and highly luminous star that is larger than a giant star. Stellar Nucleosynthesis: The process by which stars synthesize elements through nuclear reactions in their cores. White Dwarf: The remnant of a star that has exhausted its nuclear fuel and collapsed to a dense, Earth-sized object. Degenerate Matter: A state of matter where particles, such as electrons, cannot occupy the same quantum states, leading to unusual properties. Chandrasekhar Limit: The maximum mass (about 1.4 times the mass of the Sun) beyond which a white dwarf will collapse and explode as a type Ia supernova. Subrahmanyan Chandrasekhar: An Indian-American astrophysicist known for his work on stellar structure and the Chandrasekhar limit. Stellar Nucleosynthesis: The process by which stars synthesize elements through nuclear reactions in their cores. Neutron Star: A dense and compact remnant of a massive star's core after a supernova explosion, primarily composed of neutrons. Type II Supernova: A supernova explosion that occurs when a massive star exhausts its nuclear fuel and collapses under gravity. Cosmic Rays: High-energy charged particles, primarily protons and atomic nuclei, originating from various sources in the universe. Neutrinos: Nearly massless subatomic particles that interact very weakly with matter, often produced in nuclear reactions. Supernova Remnant: The glowing, expanding shell of gas and dust left behind after a supernova explosion. SN 1987A: A well-studied supernova that occurred in the Large Magellanic Cloud in 1987. Core Collapse: The sudden and catastrophic collapse of a massive star's core under the force of gravity, leading to a supernova explosion. Pulsar: A highly magnetized, rotating neutron star that emits beams of electromagnetic radiation, often observed as periodic pulses.

Jocelyn Bell: An astrophysicist from Northern Ireland who co-discovered the first pulsar while a graduate student. Antony Hewish: A British radio astronomer who worked on the discovery of pulsars. Lighthouse Model: A model used to explain the periodicity of pulsar emission, where rotating beams of radiation are observed as pulses. Nova: A sudden and temporary increase in the brightness of a star, caused by the explosive ignition of material on the surface of a white dwarf in a binary system. Type Ia Supernova: A type of supernova explosion that occurs in binary systems when a white dwarf reaches the Chandrasekhar limit. Gamma-Ray Bursts: Extremely energetic and short-lived bursts of gamma-ray radiation, often associated with the collapse of massive stars or other high-energy events. General Theory of Relativity: Albert Einstein's theory of gravitation, which describes gravity as the curvature of spacetime caused by mass and energy. Albert Einstein: A German-born physicist who developed the theory of relativity, known for his equation E=mc^2, and made significant contributions to our understanding of the nature of space, time, and gravity. Principle of Equivalence: A fundamental concept in the general theory of relativity, stating that locally, the effects of gravity are indistinguishable from acceleration. Spacetime: The four-dimensional continuum that combines three dimensions of space and one dimension of time, as described by the theory of relativity. John Wheeler: An American theoretical physicist known for his contributions to general relativity, quantum mechanics, and the study of black holes. Constancy of c: The fundamental postulate of special relativity, stating that the speed of light in a vacuum (c) is the same for all observers, regardless of their motion. Evidences for GR (General Relativity): Observational and experimental evidence that supports the predictions of Albert Einstein's general theory of relativity. Precession of Mercury: The gradual change in the orientation of the elliptical orbit of Mercury's perihelion, predicted and explained by general relativity. Deflection of Light: The bending of light rays as they pass through a gravitational field, a phenomenon confirmed by the 1919 solar eclipse observations.

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Arthur Eddington: An English astrophysicist known for his work on the 1919 solar eclipse expedition that provided experimental evidence for the deflection of light by gravity, confirming Einstein's theory of general relativity. Time Dilation: A consequence of special relativity, where time appears to pass differently for observers in relative motion. Gravitational Redshift: The phenomenon where light emitted from a massive object appears shifted to longer wavelengths due to its escape from the object's gravitational field. Black Hole: A region in spacetime where gravity is so strong that nothing, not even light, can escape from it. Event Horizon: The boundary surrounding a black hole beyond which nothing can escape. Schwarzschild Radius: The critical radius at which the escape velocity from a non-rotating black hole equals the speed of light. Singularity: A point within a black hole where gravitational forces become infinitely strong and spacetime is highly curved. Tidal Forces: The differential gravitational forces experienced by objects in a gravitational field, causing stretching and compression. Accretion Disk: A flattened, rotating disk of gas and dust that forms around a massive object, such as a black hole. Cygnus X-1: A binary star system containing a black hole and a massive companion star, often used in studies of black hole properties. Gravitational Waves: Ripples in spacetime caused by the acceleration of massive objects, such as the collision of black holes. LIGO (Laser Interferometer Gravitational-Wave Observatory): A ground-based observatory designed to detect gravitational waves.