List of definitions of terms and concepts commonly used in the study of astronomy
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.
A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. A galaxy hosting an AGN is called an active galaxy.
A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely reflected away from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by a black body) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g. cloud cover and geological or environmental surface features).
A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower its magnitude.
In the orbit of a planetary body, one of the two extreme points of distance between the body and its primary – either the point of minimal distance, called the periapsis, or the point of maximal distance, called the apoapsis. The term may also be used to refer to the value of the distance rather than the point itself. All elliptical orbits have exactly two apsides.
A type of naturally occurring physical entity, association, or structure within the observable universe that is a single, tightly bound, contiguous structure, such as a star, planet, moon, or asteroid. Though the terms astronomical "body" and astronomical "object" are often used interchangeably, there are technical distinctions.
A type of naturally occurring physical entity, association, or structure that exists within the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.
A unit of length used primarily for measuring astronomical distances within the Solar System or between the Earth and distant stars. Originally conceived as the approximate average distance between the centers of the Earth and the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10−6 parsecs; 1.5813×10−5 light-years).
The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. Axial tilt usually does not change considerably during a single orbital period; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from orbital inclination.
An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system.
The common center of mass about which any two or more bodies of a gravitationally bound system orbit. The barycenter is one of the foci of the elliptical orbit of each body participating in the system; its location is strongly influenced by the mass of each body and the distances between them. For example, in a planetary system where the mass of the central star is significantly larger than the mass of an orbiting planet, the barycenter may actually be located within the radius of the star, such that the planet appears to orbit the star itself, though both bodies actually orbit the shared barycenter.
Two bodies of similar mass orbiting a common barycenter that is external to both bodies (a common situation for binary star systems)
The prevailing cosmological model for the origin of the observable universe. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
A star system consisting of exactly two stars orbiting around their common barycenter. The term is often used interchangeably with double star, though the latter can also refer to an optical double star, a type of optical illusion which is entirely distinct from true binary star systems.
The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. At rotational velocities beyond this point, the star begins to eject matter from its surface.
A numeric value that is used to compare the brightness of a star measured from different frequency bands of the electromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.
A relatively small, icy body that displays extended features when it approaches the Sun. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun.
A term used to indicate that two or more stars share the same motion through space, within the margin of observational error. That is, either they have nearly the same proper motion and radial velocity parameters, which may suggest that they are gravitationally bound or share a common origin, or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time).
A region on the celestial sphere surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based in mythology, while the modern demarcation of their borders was established by the International Astronomical Union in 1930. Compare asterism.
An aura of plasma that surrounds cooler stars such as the Sun. It can be observed during a solar eclipse as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
A type of radiation consisting of high-energy protons and atomic nuclei which move through space at nearly the speed of light, and which may originate from the Sun or from outside the Solar System. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
A ring-shaped circumstellar disc of dust and debris orbiting its host star. It is created by collisions between planetesimals. A debris disk can be discerned from an infrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.
The apparent motion of an astronomical object (e.g. the Sun, a planet, or a distant star) around the two celestial poles in the Earth's night sky over the course of one day. Diurnal motion is caused by Earth's rotation about its own axis, such that every object appears to follow a circular path called the diurnal circle.
Any pair of stars which appear near each other on the celestial sphere, either because the two stars coincidentally lie along nearly the same line of sight from the Earth despite being physically distant from each other, or because the two stars are actually located in physical proximity to each other, by which they may form a co-moving pair or a binary star system.
A hotter and more massive star, in contrast to late-type stars that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged. It may be used to refer to the higher-temperature members of any particular population or category of stars, rather than of all stars in general.
A moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or orbital elements of an astronomical object, because such quantities are subject to perturbations and change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin.
The imaginary line on a gravitationally rounded spheroid such as a planet that represents the intersection of the spheroid's surface with a plane perpendicular to its axis of rotation and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the celestial equator.
One of two precise times of year when the imaginary plane of the Earth's equator, extended indefinitely in all directions, passes through the center of the Sun, or, equivalently, when the Sun's apparent geocentric longitude is either 0 degrees or 180 degrees. The equinoxes occur on or near March 20 and September 22 each year. On the day of an equinox, the center of the visible Sun appears to be directly above the equator, and the durations of day and night are approximately equal all over the planet. Contrast solstice.
The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object's kinetic energy and gravitational potential energy is equal to zero. It is a function of the mass of the body and the object's distance to its center of mass. An object which has achieved escape velocity is neither on the surface nor in a closed orbit of any radius.
A curve on the Hertzsprung–Russell diagram that a solitary star of a particular mass and composition is expected to follow during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during part or all of its lifetime.
A bright spot on a star's photosphere formed by concentrations of magnetic field lines. For the Sun in particular, faculae are most readily observed near the solar limb. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.
Any galaxy that does not belong to a larger cluster of galaxies and is gravitationally isolated.
A randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a star cluster. These field stars are important to identify in order to prevent them from contaminating the results of a study.
The region at the center of a galaxy, usually home to a very dense concentration of stars and gas. It almost always includes a supermassive black hole which, when active, can generate a much higher luminosity in a compact region than its surroundings. This excess luminosity is known as an active galactic nucleus, and the brightest such active galaxies are known as quasars.
A large-scale structure consisting of hundreds or thousands of galaxies bound together by gravity. Galaxy clusters are distinct from similarly named galactic clusters and other types of star clusters and from smaller aggregates of galaxies known as galaxy groups. Galaxy groups and galaxy clusters can themselves cluster together to form superclusters.
A cataclysmic event that generates a brief but intense outburst of gamma ray radiation which can be detected from billions of light-years away. The source of most GRBs is theorized to be supernova or hypernova explosions of high-mass stars. Short GRBs may also result from the collision of neutron stars.
The ratio of the brightness of an astronomical body at a phase angle of zero to an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.
A tight, spherical conglomeration of many thousands of stars which are gravitationally bound to each other and which orbit a galactic core as a satellite. They differ from open clusters in having a much higher combined mass, with a typical lifespan extending for billions of years.
Any very large distribution of mass, such as a galactic cluster, which can bend passing light from a distant source by a noticeable degree. The effect, known as gravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.
An ionized nebula powered by young, massive O-type stars. Ultravioletphotons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral lines of hydrogen and other elements. Because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms of spiral galaxies and in star-forming irregular galaxies.
Centered upon the Earth's Sun, e.g. a heliocentric orbit.
The vast, bubble-like cavity in the interstellar medium which surrounds and is created by the plasma emanating from the Earth's Sun. The heliosphere encompasses the entirety of the Solar System and a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.
The approximate region around an astronomical object within which its gravitational attraction dominates the motions of satellites. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or the galactic core. Satellites moving outside this radius tend to be perturbed away from the main body.
An archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that these planets orbit closer to the Sun than the Earth and hence, in the geocentriccosmology of Ptolemy, both appeared to travel with the Sun across the sky. This is in contrast to the so-called superior planets, such as Mars, which appeared to move independently of the Sun.
The matter that exists in the space between the stars in a galaxy. This medium mainly consists of hydrogen and helium, but is enhanced by traces of other elements contributed by matter expelled from stars.
An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known as extinction. This effect causes the more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon of redshift.
A curve on the Hertzsprung–Russell diagram that represents the evolutionary positions of stars having the same age but differing masses. This is in contrast to an evolutionary track, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.
A physical state in which an interstellar cloud of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.
A unit of time defined as exactly 365.25 days of 86,400 SI seconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as the tropical year. It is therefore widely used as the basis for defining the standard astronomical epoch and the light-year.
The motion of one orbiting body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital plane (or sometimes a straight line) in three-dimensional space. Kepler orbits are idealized mathematical constructions which consider only the point-like gravitational attraction of two bodies, neglecting more complex orbital perturbations that may exist in reality.
Also called a Lagrange point, libration point, or L-point.
Any of a set of points near two large bodies in orbit at which a smaller object will maintain a constant position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in certain scenarios) the Coriolis acceleration all align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labels L1 to L5. The phenomenon is the basis for the stable orbits of trojan satellites and is commonly exploited by man-made satellites.
A slight oscillating motion of the Moon as seen from the Earth, a result of the Moon's elliptical orbit. It can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.
A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at the speed of light in a vacuum would travel in one Julian year: approximately 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). Though the light-year is often used to measure galactic-scale distances in non-specialist publications, the unit of length most commonly used in professional astrometry is the parsec.
The angle between a specified reference direction, called the origin of longitude, and the direction of an orbit's ascending node, as measured on a specified plane of reference. The angle is typically measured eastwards from the reference direction to the ascending node (i.e. counterclockwise as seen from the north). It is one of six canonical orbital elements used to characterize an orbit.
A numerical logarithmic scale indicating the brightness of an astronomical object, where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with the naked eye, although this can vary depending on seeing conditions and eyesight.
The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending northward at each zenith passage. It represents the moment at which the North Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. Contrast September equinox.
The fraction of an elliptical orbit's period that has elapsed since the orbiting body passed periapsis, expressed as the angular distance from the pericenter which a fictitious body would have if it moved in a perfectly circular orbit in the same orbital period as the actual body in its elliptical orbit. The mean anomaly does not correspond to a real geometric angle, as does the true anomaly, but is a contrived parameter used to make calculating the position of the orbiting body in the two-body problem mathematically convenient.
The visible passage of a glowing meteoroid, micrometeoroid, comet, or asteroid through the Earth's atmosphere, usually as a long streak of light produced when such an object is heated to incandescence by collisions with air molecules in the upper atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also the shedding of material in its wake.
A series of meteors that seemingly radiate from a single area in the night sky. These are produced by debris left over from a larger body, such as a comet, and hence they follow roughly the same orbit. This makes many meteor showers predictable events, as they recur every year.
A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.
A very small meteorite that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50 µm to 2 mm. Micrometeorites are a major component of cosmic dust.
A very small meteoroid, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed a micrometeorite.
A stellar object such as a variable star that undergoes very small variations in luminosity. Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.
The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
The solid, rocky body that orbits the Earth as its only natural satellite, completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible for tides on Earth; because of tidal locking, only one side of the Moon is ever visible from the Earth. Sunlight reflected from its surface makes the Moon appear very bright in the night sky, though its orbital position with respect to the Earth and the Sun causes its visibility to change in a regular cycle of phases when viewed from the Earth. The adjectival lunar is often used to specifically describe the orbit, gravity, and other properties of the Earth's Moon.
A loose grouping of stars which travel together through space. Although the members were formed together in the same molecular cloud, they have since moved too far apart to be gravitationally bound as a cluster.
A type of compact star that is composed almost entirely of neutrons, which are a type of subatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of the Sun, but with a radius of only 12 km (7.5 mi), making them among the densest known objects in the universe.
The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically in cm−3 or m−3. With stars, cubic parsecs (pc−3) are often used.
A continuous, gravity-induced change in the orientation of an astronomical body's axis of rotation which results from the combined effects of small, short-term variations. Nutation is distinguished from precession, which is a similar but longer-term change in axial orientation.
A group of massive stars which are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of stellar classifications O and B.
A celestial event that occurs when a distant astronomical body or object is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view. Solar and lunar eclipses are specific types of occultations.
A vast theoretical cloud of predominantly icy planetesimals hypothesized to surround the Sun at distances ranging from 2,000 to 200,000 AU. It is thought to be divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of the Solar System.
The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.
The gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately elliptical orbits around a common center of mass positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.
The speed at which an astronomical body or object orbits around a barycenter, or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entire orbital period, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs at periapsis.
The vast, nearly empty expanse that exists beyond the Earth and between all celestial bodies, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the Universe is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.
The parallax shift of a star at a distance of one parsec as seen from the Earth (not to scale)
The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of its primary alone, or any force which complicates the orbital characteristics of the body such that the idealized Kepler orbit of the two-body problem is not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/or atmospheric resistance.
The elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have low phase angles as seen from Earth, so they are often viewed as a crescent; superior planets such as Mars and Jupiter usually have high phase angles, so that little of the shadowed side is visible.
A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a red giant star late in its life. The name derives from their resemblance to a planet. An example is the Ring Nebula.
Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in protoplanetary disks and debris disks during the process of planet formation, but is also sometimes used to refer to various types of small Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and a protoplanet.
An orbit in which the orbiting object passes directly over or nearly over both poles of the body being orbited during each revolution. It therefore has an inclination equal or nearly equal to 90 degrees to the body's equator.
Any slow change in the orientation of an object's axis of rotation. For the Earth in particular, this phenomenon is referred to as the precession of the equinoxes. Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity.
The minimum physical separation between two astronomical objects, as determined from their angular separation and estimated distance. For planets and double stars, this distance is usually given in astronomical units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.
A large planetary embryo that originated within a protoplanetary disk and has since undergone internal melting to produce an interior of non-uniform composition. Protoplanets represent an intermediate step in the formation of a full-sized planet; they are thought to form out of smaller planetesimals as they collide with each other and gradually coalesce into larger bodies.
The velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the gravitational perturbation of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star.
The distance from an astronomical object at which the tidal force matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a ring. Outside this limit, loose material will tend to coalesce.
A broken arc of illuminations seen at the limb of very young or very old lunar crescents. The visual similarity to the moments before and after a total solar eclipse was first noted by American astronomer Stephen Saber.
One half of the longest diameter (the major axis) of an ellipse. It is expressed in units of length and often used to give a physical dimension to a two-body ellipticalKepler orbit, such as for a binary star system or star–planet system. When the distance between the orbiting bodies is unknown, the semi-major axis may be given as an angle.
The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending southward at each zenith passage. It represents the moment at which the North Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere. Contrast March equinox.
A large, bright, transient feature, often in the shape of a loop, consisting of plasma extending outward from the Sun's photosphere into the corona. Prominences may be hundreds of thousands of kilometers long.
A standard unit of distance equal to the radius of the Earth's Sun (typically measured from the Sun's center to the layer in the photosphere at which the optical depth equals 2/3), or approximately 695,700 kilometres (432,300 mi). It is commonly used to express the radii of other stars and astronomical objects relative to the Sun.
One of two precise times of year when the Sun reaches either its most northerly or most southerly point in the sky as seen from Earth. The solstices occur on or near June 20 and December 21 each year. The "Summer Solstice", often used to refer to the June solstice because of its occurrence during the Northern Hemisphere's summer, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime in the Northern Hemisphere. The reverse is true for the "Winter Solstice", which is often used to refer to the December solstice.
A type of binary star system where the individual components have not been resolved with a telescope. Instead, the evidence for the binarity comes from shifts observed in the spectrum. This is caused by the Doppler effect as the radial velocity of the components change over the course of each orbit.
Any small number of stars that orbit each other, bound by gravitational attraction, such as a binary star system. In the broadest sense, very large groups of stars bound by gravitation such as star clusters and galaxies are also star systems. Star systems are distinct from planetary systems, which include planets and other bodies such as comets.
Any galaxy that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as a merger.
One of a class of very large black holes which possess masses ranging from hundreds of thousands to many billions of times the mass of the Sun. These are typically found at a galactic core, where they can have a profound effect upon the evolution of the surrounding galaxy.
The time it takes for an object to rotate once about its own axis (e.g. its rotation period) relative to the primary it is orbiting (rather than to distant fixed stars). The synodic day may be described as the time between two consecutive sunrises (in the case where the primary is a star), which is not necessarily the same as the sidereal day. An object's synodic day may change slightly in duration over the course of the orbital period due to eccentricity and axial tilt. For Earth, the synodic day is often called a solar day.
The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to the background stars visible in the second body's celestial sphere. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in the same location in the night sky as observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from the orbital period, a result of the fact that both the body being studied (e.g. Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (the Sun).
A star with nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to more accurately measure the spectrum.
The layer of the Milky Way galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and centered on the galactic plane. Stars belonging to this population generally follow orbits that lie close to this plane. This is in contrast to members of the thick disk population and halo stars.
The transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will gradually recede from its primary while slowing the rotation rate of both bodies.
The net result of continued tidal braking such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the orbital eccentricity is low, the result is that the satellite orbits with the same face always pointed toward its primary. An example is the Moon, which is tidally locked with the Earth.
A stream of stars and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of a galaxy such as the Milky Way.
An astronomical event during which a body or object passes visibly across the face of a much larger body. An example is the transit of Venus across the face of the Sun, which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the net luminosity from the two objects, the transit method can be used to detect extrasolar planets as they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called an occultation or eclipse.
The angle between the direction of periapsis and the current position of an orbiting body as it moves along an elliptical orbit, as measured from the nearest focus of the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being the eccentric anomaly and the mean anomaly, and also one of six canonical orbital elements used to characterize an orbit.
1. The entirety of space and time and their contents, including galaxies, stars, planets, all other forms of matter and energy, and the physical laws and constants that describe them. When not otherwise qualified, "the Universe" usually refers to the entire Universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from the observable universe, whose size it is possible to measure.
2. One of many hypothetical parallel universes which exist as causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and their contents.
Any star that is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star's radial velocity or its profile of spectral lines.
A reference to the faintness of the spectral lines for a star compared to standard stars with the same stellar classification. Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals"—these are sometimes called metal weak stars.
A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and the absolute visual magnitude of the emitting late-type stars. This linear relation makes it useful for determining the distances of G, K, and M-type stars.
An acronym of X-ray bright optically normal galaxy.
The area of the sky that extends approximately 8 degrees north or south (in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year as observed from Earth. The Sun, Moon, and visible planets appear to travel across a band of twelve Zodiac constellations within this belt as the Earth orbits the Sun.
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