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Parsec

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a unit of distance used in astronomy, equal to about 3.26 light years 3.086 × 1013 kilometers

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Star's proper motion

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A proper motion of 1 arcsec per year at a distance of 1 light-year corresponds to a relative transverse speed of 1.45 km/s

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Apparent Brightness

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The Apparent Magnitude (M) of a celestial body is a measure of its Brightness as seen by an observer on Earth

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Magnitude Scale

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Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.

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Energy Flux

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Energy flux is a quantity that measures the rate of transfer of energy per unit area. The word "flux" means change or fluctuation. Heat flux and radiative flux are specific cases of energy flux that involve the rate of transfer of heat and radiation (photons) respectively.

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Apparent Magnitude

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the magnitude of a celestial object as it is actually measured from the earth.

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Absolute Magnitude

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the magnitude (brightness) of a celestial object as it would be seen at a standard distance of 10 parsecs.

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Stellar Classification OBAFGKM

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classification of stars based on their spectral characteristics. Light from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines.

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Spectral Class: O

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An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) star of spectral type O and luminosity class V.

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Spectral Class: B

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A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V.

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Spectral Class: A

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An A-type main-sequence star (A V) or A dwarf star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V. Class A stars are among the more common naked eye stars, and are white or bluish-white.

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Spectral Class: F

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An F-type main-sequence star (F V) is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V.

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Spectral Class: G

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A G-type main-sequence star (G V), often (and imprecisely) called a yellow dwarf, or G dwarf star, is a main-sequence star (luminosity class V) of spectral type G. Class G stars are probably the best known, if only for the reason that the Sun is of this class.

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Spectral Class: K

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A K-type main-sequence star (K V), also referred to as an orange dwarf or K dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. Class K stars are orangish stars that are slightly cooler than the Sun.

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Spectral Class: M

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M-type main-sequence stars are main-sequence stars (luminosity class V) of spectral type M.

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Radius-Luminosity Relationship

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Class M stars are by far the most common.

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Red Giants

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a very large star of high luminosity and low surface temperature. Red giants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion.

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Super Giants

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a very large star that is even brighter than a giant, often despite being relatively cool.

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H-R Diagram

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The Hertzsprung-Russell diagram, abbreviated H-R diagram or HRD, is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral classifications or effective temperatures.

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Main Sequence

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a series of star types to which most stars belong, represented on a Hertzsprung-Russell diagram as a continuous band extending from the upper left (hot, bright stars) to the lower right (cool, dim stars).

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White Dwarfs

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a small very dense star that is typically the size of a planet. A white dwarf is formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula.

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Red Dwarfs

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a small, old, relatively cool star.

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Binary-Star Systems

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A binary star is a star system consisting of two stars orbiting around their common center of mass.

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Visual Binary

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a binary star of which the components are sufficiently far apart to be resolved by an optical telescope.

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Spectroscopic Binary

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Definition of SPECTROSCOPIC BINARY. : a binary star in which shifting of lines in the system's spectrum indicates orbital revolution. To access the complete Unabridged Dictionary, with an additional 300,000 words that aren't in our free dictionary, start a free trial.

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Eclipsing Binary

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a binary star whose brightness varies periodically as the two components pass one in front of the other.

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Proxima-Centauri

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Proxima Centauri is a red dwarf about 4.24 light-years from the Sun, inside the G-cloud, in the constellation of Centaurus

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Stellar evolution

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Stellar evolution is the process by which a star changes during its lifetime. Depending on the mass of the star, this lifetime ranges from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe.

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Hydrostatic Equilibrium

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Hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology

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Sub-giant Branch

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A subgiant star is a star that is slightly brighter than a normal main-sequence (dwarf) star of the same spectral class, but not as bright as true giant stars.

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Red-giant Branch

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a very large star of high luminosity and low surface temperature. Red giants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion.

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CNO Cycle

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Unlike the latter, the CNO cycle is a catalytic cycle. Theoretical models suggest that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. The proton-proton chain is more important in stars the mass of the Sun or less.

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p-p chain reaction

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The proton-proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton-proton chain dominates in stars the size of the Sun or smaller.

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Triple-Alpha Process

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triple alpha process in Science Expand. triple alpha process. (tr?p'?l) A nuclear fusion reaction in which three helium nuclei (alpha particles) fuse to form a carbon nucleus, thereby releasing energy.

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Helium Flash

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A helium flash is the runaway fusion of helium in the core of low mass stars of less than about 2.25 solar masses (M?) and greater than about 0.5 M?, or on the surface of an accreting white dwarf star.

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Electron Degeneracy Pressure

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Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure.

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Horizontal Branch

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The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's.

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Asymptotic Giant Branch

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The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low- to medium-mass stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6-10 solar masses) late in their lives.

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He-Burning Shell

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Fusion of helium in the core of low mass stars. During the red giant phase of stellar evolution, hydrogen burning ceases in the core as hydrogen is depleted, leaving a helium-rich core. Hydrogen burning continues in the star's shell, producing helium ash that falls into the core.

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Planetary Nebula

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a ring-shaped nebula formed by an expanding shell of gas around an aging star.

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White Dwarf

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a small very dense star that is typically the size of a planet. A white dwarf is formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula.

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Black Dwarf

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A black dwarf is a white dwarf that has sufficiently cooled that it no longer emits significant heat or light.

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Blue Stragglers

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Blue stragglers (BSS) are main-sequence stars in open or globular clusters that are more luminous and bluer than stars at the main-sequence turn-off point for the cluster.

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Brown Dwarf

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a celestial object intermediate in size between a giant planet and a small star, believed to emit mainly infrared radiation.

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Zero Age Main Sequence

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Zero Age Main Sequence (ZAMS) is the time when a star first joins the main sequence on the Hertzsprung-Russell diagram (HR diagram) by burning hydrogen in its core through fusion reactions.

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Main-Sequence Turnoff

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The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after the exhaustion of its main fuel. Sun-like stars will enter the red giant branch as subgiants. By plotting the turnoff point of the stars in star clusters, one can estimate the cluster's age.

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Contact Binaries

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a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overcontact binary.

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Roche Lobe

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either of two lobes that form an hourglass-shaped volume of space around a binary star system.

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Lagrangian Point

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one of five points in the plane of orbit of one body around another (e.g., the moon around the earth) at which a small third body can remain stationary with respect to both.

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Nuclear Masses

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Thus, the mass excess is an expression of the nuclear binding energy, relative to the binding energy per nucleon of carbon-12 (which defines the atomic mass unit). If the mass excess is negative, the nucleus has more binding energy than 12C, and vice versa.

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Fission

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Nuclear fission is the process in which a large nucleus splits into two smaller nuclei with the release of energy. In other words, fission the process in which a nucleus is divided into two or more fragments, and neutrons and energy are released.

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Neutrino

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a neutral subatomic particle with a mass close to zero and half-integral spin, rarely reacting with normal matter. Three kinds of neutrinos are known, associated with the electron, muon, and tau particle.

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Photodisintegration

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Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray is absorbed by atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle.

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Chandrasekhar Mass

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the maximum mass at which a star near the end of its life cycle can become a white dwarf and above which the star will collapse to form a neutron star or black hole : a stellar mass equal to about 1.4 solar masses.

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Core-collapse Supernova

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Core collapse supernovae are the spectacular explosions that mark the violent deaths of massive stars. These events are the most energetic explosions in the cosmos, releasing energy of order 10^{53} erg at the staggering rate of 10^{45-46} watts.

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Type I and II Supernovae

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For a star to explode as a Type II supernova, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon.

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Neutron Capture

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Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus.

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Stellar Nucleosynthesis

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Stellar nucleosynthesis is the process by which the natural abundances of the chemical elements within stars vary due to nuclear fusion reactions in the cores and overlying mantles of stars. Stars are said to evolve (age) with changes in the abundances of the elements within.

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Carbon Detonation

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Carbon detonation is the violent reignition of thermonuclear fusion in a white dwarf, which produces a Type Ia supernova.

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SN 1987A

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SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud (a nearby dwarf galaxy). It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye.

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Isotopes

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each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.

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R-Process

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The r-process is a nucleosynthesis process that occurs in core-collapse supernovae (see also supernova nucleosynthesis) and is responsible for the creation of approximately half of the neutron-rich atomic nuclei heavier than iron.

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S-Process

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The s-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions heavier nuclei are created by neutron capture, increasing the atomic weight of the nucleus by one.

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Neutron Degeneracy Pressure

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Similar to electron degeneracy pressure but, because the neutron is much more massive than the electron, neutron degeneracy pressure is much larger and can support stars more massive than the Chandrasekhar mass limit.

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Pulsars

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a celestial object, thought to be a rapidly rotating neutron star, that emits regular pulses of radio waves and other electromagnetic radiation at rates of up to one thousand pulses per second.

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X-ray Bursters

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X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) peaked in the X-ray regime of the electromagnetic spectrum.

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Millisecond pulsars

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A millisecond pulsar (MSP) is a pulsar with a rotational period in the range of about 1-10 milliseconds. Millisecond pulsars have been detected in the radio, X-ray, and gamma ray portions of the electromagnetic spectrum.

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Gravity Wave

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a hypothetical wave carrying gravitational energy, postulated by Einstein to be emitted when a massive body is accelerated.

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Schwarzschild Radius

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a black hole of a kind supposed to result from the complete gravitational collapse of an electrically neutral and nonrotating body, having a physical singularity at the center to which infalling matter inevitably proceeds and at which the curvature of space-time is infinite. A Schwarzschild radius is the radius of the boundary of a hole of this type.

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Event Horizon

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a theoretical boundary around a black hole beyond which no light or other radiation can escape. a point of no return.

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Singularity

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a point at which a function takes an infinite value, especially in space-time when matter is infinitely dense, as at the center of a black hole.

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Time Dilation

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n the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses.

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Hot Jupiter

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Hot Jupiters (also called roaster planets, epistellar jovians, pegasids or pegasean planets) are a class of extrasolar planets whose characteristics are similar to Jupiter, but that have high surface temperatures because they orbit very close—between approximately 0.015 and 0.5 astronomical units

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Tug of Planet

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The tug of war in astronomy is the ratio of planetary and solar attractions on a natural satellite. The term was coined by Isaac Asimov in The Magazine of Fantasy and Science Fiction in 1963.

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Transit

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The tug of war in astronomy is the ratio of planetary and solar attractions on a natural satellite. The term was coined by Isaac Asimov in The Magazine of Fantasy and Science Fiction in 1963.

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Planetary Migration

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the passage of an inferior planet across the face of the sun, or of a moon or its shadow across the face of a planet.

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Galactic Disk

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The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles.

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Galactic Center

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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 approximately in the galactic plane.

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Galactic Plane

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The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles.

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Variable Stars

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a star whose brightness changes, either irregularly or regularly.

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RR Lyrae

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RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances. This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.

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Cepheid

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a variable star having a regular cycle of brightness with a frequency related to its luminosity, so allowing estimation of its distance from the earth.

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Period-Luminosity Relationship

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a law in astronomy: the period of light variation of a Cepheid variable is in direct relation with its absolute magnitude whereby intrinsically fainter stars have the shorter periods.

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Cosmic Distance Ladder

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The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects.

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Rotation Curve

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The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the magnitude of the orbital velocities (i.e., the speeds) of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre, typically rendered graphically as a plot.

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MACHO's Gravitational Lensing

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Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit.

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Hubble Classification Scheme

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The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented.

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SB

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spectroscopic binary: a binary star in which shifting of lines in the system's spectrum indicates orbital revolution. To access the complete Unabridged Dictionary, with an additional 300,000 words that aren't in our free dictionary, start a free trial.

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Spiral Galaxy

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a galaxy in which the stars and gas clouds are concentrated mainly in one or more spiral arms.

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Elliptical Galaxy

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Elliptical galaxies are the most abundant type of galaxies found in the universe

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Irregular galaxy

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Some irregular galaxies are the result of gravitational interactions or collisions between formerly regular galaxies. Many irregular galaxies orbit larger regular ones; the Magellanic Cloud galaxies orbiting the Milky Way are examples. Compare elliptical galaxy, lenticular galaxy, spiral galaxy.

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Magellanic Clouds

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either of the two small galaxies that appear as conspicuous patches of light near the south celestial pole and are companions to the Milky Way galaxy.

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Hubble Sequence

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The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented.

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Hubble Law

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Hubble's law is the name for the observation in physical cosmology that: (1) objects observed in deep space (extragalactic space, ~10 megaparsecs or more) are found to have a Doppler shift interpretable as relative velocity away from the Earth; and (2) that this Doppler-shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.[1][2] This is normally interpreted as a direct, physical observation of the expansion of the spatial volume of the observable universe

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Cosmological Redshift

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In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region.

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Normal Galaxies

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In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region.

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Active Galaxies

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An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum.

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Quasars

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a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies.

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Blazar

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A blazar is a very compact quasar (quasi-stellar radio source) associated with a presumed supermassive black hole at the center of an active, giant elliptical galaxy. Blazars are among the most energetic phenomena in the universe and are an important topic in extragalactic astronomy.

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Galactic Cannibalism

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The most common result of the gravitational merger of two or more galaxies is an irregular galaxy of one form or another, although elliptical galaxies may also result. It has been suggested that galactic cannibalism is currently occurring between the Milky Way and the Large and Small Magellanic Clouds.

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Dark Matter Map

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The largest single high-definition map of mysterious dark matter has been produced. It is the first in a series of maps of the cosmos that will eventually allow a 3D view of dark matter across one eighth of the night sky. And the map should allow astronomers to study how galaxies formed in the universe.

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Gravitational Lensing

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A gravitational lens refers to a distribution of matter (such as a cluster of galaxies) between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer.

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Cosmological Principle

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In modern physical cosmology, the cosmological principle is the notion that the distribution of matter in the Universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throughout the Universe, and should, therefore, produce no observable irregularities in the large scale structuring over the course of evolution of the matter field that was initially laid down by the Big Bang.

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Homogenous Universe

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Isotropy and Homogeneity: The Copernican principle, when applied to cosmology and the structure of the Universe, basically asks the question of whether the Universe is isotropic and homogeneous. These two terms are not equivalent and have a special meaning to cosmology.

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Critical Density

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The critical density is the boundary value between universe models that expand forever (open models) and those that recollapse (closed models). A measurement of the actual density of the universe could be compared to the critical density which would then, in principle, indicate the fate of the cosmos.

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Big Crunch

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a contraction of the universe to a state of extremely high density and temperature, hypothesized as a possible scenario for its demise.

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Closed Universe

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the condition in which there is sufficient matter in the universe to halt the expansion driven by the Big Bang and cause eventual recollapse. The amount of visible matter is only a tenth of the total required for closure, but there may be large quantities of dark matter.

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Flat Universe

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The flat universe is the dividing line between an open universe and a closed universe. Note : Most theorists believe that the universe is flat.

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Dark Energy

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a theoretical repulsive force that counteracts gravity and causes the universe to expand at an accelerating rate.

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