144 relations: A Brief History of Time, Accretion disk, Active galactic nucleus, Aerobee, Alnilam, American Astronomical Society, Angular momentum, Apparent magnitude, Astronomer, Astronomical object, Astronomical unit, Astronomische Gesellschaft Katalog, Astrophysical jet, Astrophysical X-ray source, Atmosphere of Earth, Ångström, Binary star, Black hole, Black Holes and Time Warps, Blue supergiant star, Carbon, Carina–Sagittarius Arm, Celestial coordinate system, Celestial sphere, CENBOL, Center of mass, Chandra X-ray Observatory, Compact star, Compton scattering, Conjunction (astronomy), Constellation, Corona, Cygnus (constellation), Cygnus X-1 (song series), David Dunlap Observatory, Declination, Degree (angle), Doppler effect, Durchmusterung, Eclipse, Electromagnetic radiation, Electromagnetic spectrum, Electron, Erg, Eta Cygni, European Space Agency, Event horizon, Extinction (astronomy), Flux, Galactic coordinate system, ..., Gamma ray, Geiger counter, General relativity, Gravitational redshift, Gravity, Gravity darkening, H-alpha, Helium, Henry Draper Catalogue, Herbert Gursky, Hertz, High Speed Photometer, Hipparcos, Hubble Space Telescope, Interstellar medium, Jansky, Kip Thorne, Leiden Observatory, Light, Light-year, Limb darkening, Line-of-sight propagation, List of nearest black holes, Magnetic field, Matter, Metre, Microquasar, Milky Way, Millisecond, Monthly Notices of the Royal Astronomical Society, NASA, National Radio Astronomy Observatory, Nebula, Neutron star, New Mexico, No-hair theorem, Nova (TV series), Orbit, Orbital eccentricity, Orbital inclination, Orion Arm, P Cygni, PBS, Penthouse (magazine), Perpendicular, Photon, Plasma (physics), Potential energy, Precession, Private Eye, Quasar, Quasi-periodic oscillation, Radio astronomy, Riccardo Giacconi, Right ascension, Roche lobe, Royal Observatory, Greenwich, Russia, Schwarzschild radius, Semi-major and semi-minor axes, Signal strength in telecommunications, Smithsonian Astrophysical Observatory, Solar luminosity, Solar mass, Solar radius, Special relativity, Speed of light, Stellar association, Stellar black hole, Stellar classification, Stellar wind, Stephen Hawking, Sub-orbital spaceflight, Sun, Supergiant star, Supernova, The Astrophysical Journal, Thorne–Hawking–Preskill bet, Tidal force, Tom Bolton (astronomer), Turbulence, Uhuru (satellite), Ultraviolet, University of Toronto, Variable star, Visible spectrum, Watt, Wavelength, White Sands Missile Range, Wolf–Rayet star, X-ray, X-ray astronomy, X-ray astronomy detector, X-ray binary. Expand index (94 more) » « Shrink index
A Brief History of Time: From the Big Bang to Black Holes is a popular-science book on cosmology (the study of the universe) by British physicist Stephen Hawking.
An accretion disk is a structure (often a circumstellar disk) formed by diffused material in orbital motion around a massive central body.
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion—and possibly all—of the electromagnetic spectrum, with characteristics indicating that the excess luminosity is not produced by stars.
The Aerobee rocket was a small (8 m) unguided suborbital sounding rocket used for high atmospheric and cosmic radiation research in the United States in the 1950s.
Alnilam, designated Epsilon Orionis (ε Orionis, abbreviated Epsilon Ori, ε Ori) and 46 Orionis (46 Ori), is a large blue supergiant star some 2,000 light-years distant in the constellation of Orion.
The American Astronomical Society (AAS, sometimes spoken as "double-A-S") is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC.
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum.
The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth.
An astronomer is a scientist in the field of astronomy who concentrates their studies on a specific question or field outside the scope of Earth.
An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe.
The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun.
The Astronomische Gesellschaft Katalog (AGK) is an astrometric star catalogue.
An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation.
Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.
The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity.
The ångström or angstrom is a unit of length equal to (one ten-billionth of a metre) or 0.1 nanometre.
A binary star is a star system consisting of two stars orbiting around their common barycenter.
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it.
Black Holes & Time Warps: Einstein's Outrageous Legacy is a 1994 popular science book by physicist Kip Thorne.
Blue supergiant stars are hot luminous stars, referred to scientifically as OB supergiants.
Carbon (from carbo "coal") is a chemical element with symbol C and atomic number 6.
The Carina–Sagittarius Arm (also known as Sagittarius Arm or Sagittarius–Carina Arm, labeled -I) is generally thought to be a minor spiral arm of the Milky Way galaxy.
In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on.
In astronomy and navigation, the celestial sphere is an abstract sphere with an arbitrarily large radius concentric to Earth.
In astronomy, CENBOL (derived from "CENtrifugal pressure supported BOundary Layer) is the region of an accretion flow around a black hole.
In physics, the center of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero, or the point where if a force is applied it moves in the direction of the force without rotating.
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space observatory launched on STS-93 by NASA on July 23, 1999.
In astronomy, the term "compact star" (or "compact object") refers collectively to white dwarfs, neutron stars, and black holes.
Compton scattering, discovered by Arthur Holly Compton, is the scattering of a photon by a charged particle, usually an electron.
In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.
A constellation is a group of stars that are considered to form imaginary outlines or meaningful patterns on the celestial sphere, typically representing animals, mythological people or gods, mythological creatures, or manufactured devices.
A corona (Latin, 'crown') is an aura of plasma that surrounds the Sun and other stars.
Cygnus is a northern constellation lying on the plane of the Milky Way, deriving its name from the Latinized Greek word for swan.
"Cygnus X-1" is a two-part song series by Canadian progressive rock band Rush.
The David Dunlap Observatory (DDO) is a large astronomical observatory site just north of Toronto in Richmond Hill, Ontario, housed on a estate.
In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle.
A degree (in full, a degree of arc, arc degree, or arcdegree), usually denoted by ° (the degree symbol), is a measurement of a plane angle, defined so that a full rotation is 360 degrees.
The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave in relation to observer who is moving relative to the wave source.
In astronomy, Durchmusterung or Bonner Durchmusterung (BD), is the comprehensive astrometric star catalogue of the whole sky, compiled by the Bonn Observatory (Germany) from 1859 to 1903.
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer.
In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space-time, carrying electromagnetic radiant energy.
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electron is a subatomic particle, symbol or, whose electric charge is negative one elementary charge.
The erg is a unit of energy and work equal to 10−7 joules.
Eta Cygni (η Cyg) is a star in the northern constellation of Cygnus.
The European Space Agency (ESA; Agence spatiale européenne, ASE; Europäische Weltraumorganisation) is an intergovernmental organisation of 22 member states dedicated to the exploration of space.
In general relativity, an event horizon is a region in spacetime beyond which events cannot affect an outside observer.
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer.
Flux describes the quantity which passes through a surface or substance.
The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north.
A gamma ray or gamma radiation (symbol γ or \gamma), is penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei.
The Geiger counter is an instrument used for detecting and measuring ionizing radiation used widely in applications such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry.
General relativity (GR, also known as the general theory of relativity or GTR) is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics.
In astrophysics, gravitational redshift or Einstein shift is the process by which electromagnetic radiation originating from a source that is in a gravitational field is reduced in frequency, or redshifted, when observed in a region at a higher gravitational potential.
Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward (or gravitate toward) one another.
Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where a star rotates so rapidly that it has a detectably oblate spheroid shape, such as in Achernar in the constellation Eridanus.
H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level.
Helium (from lit) is a chemical element with symbol He and atomic number 2.
The Henry Draper Catalogue (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension (HDE), published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the Henry Draper Extension Charts (HDEC), published from 1937 to 1949 in the form of charts, which gave classifications for 86,933 more stars.
Herbert Gursky (May 27, 1930, Bronx, New York – December 1, 2006) was the Superintendent of the Naval Research Laboratory's Space Science Division and Chief Scientist of the E.O. Hulburt Center for Space Research.
The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second.
The High Speed Photometer (HSP) is a scientific instrument installed on the Hubble Space Telescope.
Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993.
The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation.
In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy.
The jansky (symbol Jy) is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy.
Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist and Nobel laureate, known for his contributions in gravitational physics and astrophysics.
Leiden Observatory (Sterrewacht Leiden in Dutch) is an astronomical observatory in the city of Leiden, the Netherlands.
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum.
The light-year is a unit of length used to express astronomical distances and measures about 9.5 trillion kilometres or 5.9 trillion miles.
Limb darkening is an optical effect seen in stars (including the Sun), where the center part of the disk appears brighter than the edge or limb of the image.
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver.
This list contains all known black holes relatively near the Solar System (within our Milky Way galaxy).
A magnetic field is a vector field that describes the magnetic influence of electrical currents and magnetized materials.
In the classical physics observed in everyday life, matter is any substance that has mass and takes up space by having volume.
The metre (British spelling and BIPM spelling) or meter (American spelling) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in some metric systems, including the International System of Units (SI).
A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star.
The Milky Way is the galaxy that contains our Solar System.
A millisecond (from milli- and second; symbol: ms) is a thousandth (0.001 or 10−3 or 1/1000) of a second.
Monthly Notices of the Royal Astronomical Society (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics.
The National Aeronautics and Space Administration (NASA) is an independent agency of the executive branch of the United States federal government responsible for the civilian space program, as well as aeronautics and aerospace research.
The National Radio Astronomy Observatory (NRAO) is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy.
A nebula (Latin for "cloud" or "fog"; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases.
A neutron star is the collapsed core of a large star which before collapse had a total of between 10 and 29 solar masses.
New Mexico (Nuevo México, Yootó Hahoodzo) is a state in the Southwestern Region of the United States of America.
The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.
Nova (stylized NOVΛ) is an American popular science television series produced by WGBH Boston.
In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle.
Orbital inclination measures the tilt of an object's orbit around a celestial body.
The Orion Arm is a minor spiral arm of the Milky Way some across and approximately in length, containing the Solar System, including the Earth.
P Cygni (34 Cyg) is a variable star in the constellation Cygnus.
The Public Broadcasting Service (PBS) is an American public broadcaster and television program distributor.
Penthouse is a men's magazine founded by Robert C. "Bob" Guccione.
In elementary geometry, the property of being perpendicular (perpendicularity) is the relationship between two lines which meet at a right angle (90 degrees).
The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles).
Plasma (Henry George Liddell, Robert Scott, A Greek English Lexicon, on Perseus) is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s.
In physics, potential energy is the energy possessed by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Precession is a change in the orientation of the rotational axis of a rotating body.
Private Eye is a British fortnightly satirical and current affairs news magazine, founded in 1961.
A quasar (also known as a QSO or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN).
In X-ray astronomy, quasi-periodic oscillation (QPO) is the manner in which the X-ray light from an astronomical object flickers about certain frequencies.
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies.
Riccardo Giacconi (born October 6, 1931) is an Italian Nobel Prize-winning astrophysicist who laid the foundations of X-ray astronomy.
Right ascension (abbreviated RA; symbol) is the angular distance measured only eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.
The Roche lobe (or Roche limit) is the region around a star in a binary system within which orbiting material is gravitationally bound to that star.
The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, moved from Greenwich to Herstmonceux) is an observatory situated on a hill in Greenwich Park, overlooking the River Thames.
Russia (rɐˈsʲijə), officially the Russian Federation (p), is a country in Eurasia. At, Russia is the largest country in the world by area, covering more than one-eighth of the Earth's inhabited land area, and the ninth most populous, with over 144 million people as of December 2017, excluding Crimea. About 77% of the population live in the western, European part of the country. Russia's capital Moscow is one of the largest cities in the world; other major cities include Saint Petersburg, Novosibirsk, Yekaterinburg and Nizhny Novgorod. Extending across the entirety of Northern Asia and much of Eastern Europe, Russia spans eleven time zones and incorporates a wide range of environments and landforms. From northwest to southeast, Russia shares land borders with Norway, Finland, Estonia, Latvia, Lithuania and Poland (both with Kaliningrad Oblast), Belarus, Ukraine, Georgia, Azerbaijan, Kazakhstan, China, Mongolia and North Korea. It shares maritime borders with Japan by the Sea of Okhotsk and the U.S. state of Alaska across the Bering Strait. The East Slavs emerged as a recognizable group in Europe between the 3rd and 8th centuries AD. Founded and ruled by a Varangian warrior elite and their descendants, the medieval state of Rus arose in the 9th century. In 988 it adopted Orthodox Christianity from the Byzantine Empire, beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium. Rus' ultimately disintegrated into a number of smaller states; most of the Rus' lands were overrun by the Mongol invasion and became tributaries of the nomadic Golden Horde in the 13th century. The Grand Duchy of Moscow gradually reunified the surrounding Russian principalities, achieved independence from the Golden Horde. By the 18th century, the nation had greatly expanded through conquest, annexation, and exploration to become the Russian Empire, which was the third largest empire in history, stretching from Poland on the west to Alaska on the east. Following the Russian Revolution, the Russian Soviet Federative Socialist Republic became the largest and leading constituent of the Union of Soviet Socialist Republics, the world's first constitutionally socialist state. The Soviet Union played a decisive role in the Allied victory in World War II, and emerged as a recognized superpower and rival to the United States during the Cold War. The Soviet era saw some of the most significant technological achievements of the 20th century, including the world's first human-made satellite and the launching of the first humans in space. By the end of 1990, the Soviet Union had the world's second largest economy, largest standing military in the world and the largest stockpile of weapons of mass destruction. Following the dissolution of the Soviet Union in 1991, twelve independent republics emerged from the USSR: Russia, Ukraine, Belarus, Kazakhstan, Uzbekistan, Armenia, Azerbaijan, Georgia, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan and the Baltic states regained independence: Estonia, Latvia, Lithuania; the Russian SFSR reconstituted itself as the Russian Federation and is recognized as the continuing legal personality and a successor of the Soviet Union. It is governed as a federal semi-presidential republic. The Russian economy ranks as the twelfth largest by nominal GDP and sixth largest by purchasing power parity in 2015. Russia's extensive mineral and energy resources are the largest such reserves in the world, making it one of the leading producers of oil and natural gas globally. The country is one of the five recognized nuclear weapons states and possesses the largest stockpile of weapons of mass destruction. Russia is a great power as well as a regional power and has been characterised as a potential superpower. It is a permanent member of the United Nations Security Council and an active global partner of ASEAN, as well as a member of the G20, the Shanghai Cooperation Organisation (SCO), the Council of Europe, the Asia-Pacific Economic Cooperation (APEC), the Organization for Security and Co-operation in Europe (OSCE), and the World Trade Organization (WTO), as well as being the leading member of the Commonwealth of Independent States (CIS), the Collective Security Treaty Organization (CSTO) and one of the five members of the Eurasian Economic Union (EEU), along with Armenia, Belarus, Kazakhstan and Kyrgyzstan.
The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a physical parameter that shows up in the Schwarzschild solution to Einstein's field equations, corresponding to the radius defining the event horizon of a Schwarzschild black hole.
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the widest points of the perimeter.
In telecommunications, particularly in radio frequency, signal strength (also referred to as field strength) refers to the transmitter power output as received by a reference antenna at a distance from the transmitting antenna.
The Smithsonian Astrophysical Observatory (SAO) is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it is joined with the Harvard College Observatory (HCO) to form the Harvard–Smithsonian Center for Astrophysics (CfA).
The solar luminosity,, is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.
The solar mass is a standard unit of mass in astronomy, equal to approximately.
Solar radius is a unit of distance used to express the size of stars in astronomy.
In physics, special relativity (SR, also known as the special theory of relativity or STR) is the generally accepted and experimentally well-confirmed physical theory regarding the relationship between space and time.
The speed of light in vacuum, commonly denoted, is a universal physical constant important in many areas of physics.
A stellar association is a very loose star cluster, looser than both open clusters and globular clusters.
A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a massive star.
In astronomy, stellar classification is the classification of stars based on their spectral characteristics.
A stellar wind is a flow of gas ejected from the upper atmosphere of a star.
Stephen William Hawking (8 January 1942 – 14 March 2018) was an English theoretical physicist, cosmologist, and author, who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge at the time of his death.
A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it will not complete one orbital revolution.
The Sun is the star at the center of the Solar System.
Supergiants are among the most massive and most luminous stars.
A supernova (plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of a star's life, either a massive star or a white dwarf, whose destruction is marked by one final, titanic explosion.
The Astrophysical Journal, often abbreviated ApJ (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler.
The Thorne–Hawking–Preskill bet was a public bet on the outcome of the black hole information paradox made in 1997 by physics theorists Kip Thorne and Stephen Hawking on the one side, and John Preskill on the other, according to the document they signed February 6, 1997, as shown in Hawking's The Universe in a Nutshell.
The tidal force is an apparent force that stretches a body towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for the diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within Roche limit, and in extreme cases, spaghettification of objects.
Charles Thomas Bolton (born 1943) is an American astronomer who was one of the first astronomers to present strong evidence of the existence of a stellar-mass black hole.
In fluid dynamics, turbulence or turbulent flow is any pattern of fluid motion characterized by chaotic changes in pressure and flow velocity.
Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy.
Ultraviolet (UV) is electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays.
The University of Toronto (U of T, UToronto, or Toronto) is a public research university in Toronto, Ontario, Canada on the grounds that surround Queen's Park.
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye.
The watt (symbol: W) is a unit of power.
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
White Sands Missile Range (WSMR) is a United States Army military testing area of almost in parts of five counties in southern New Mexico.
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon.
X-rays make up X-radiation, a form of electromagnetic radiation.
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects.
X-ray astronomy detectors are instruments that detect X-rays for use in the study of X-ray astronomy.
X-ray binaries are a class of binary stars that are luminous in X-rays.