10.12 Concepts

- Aphelion and Perihelion: These words refer to orbits around the Sun.

- Apocynthion and pericynthion: During the Apollo program these terms were used when referring to the moon's orbits.

- Apogee and Perigee, referring to orbits around the Earth.

- Astronomical unit (AU or au or a.u. or sometimes ua): It is a unit of length approximately equal to the distance from the Earth to the Sun. The currently accepted value of the AU is 149,597,870,691 ± 30 metres (nearly 150 million kilometres or 93 million miles). In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). This definition gives a value that is slightly less than the mean Earth-Sun distance.

- BKL (Belinsky-Khalatnikov-Lifshitz) singularity: It is a model of the dynamical evolution of the Universe near the initial singularity, described by a non-symmetric, chaotic, vacuum solution to Einstein's field equations of gravitation. According to this model, the Universe is oscillating (expanding and contracting) around a singular point (singularity) in which time and space become equal to zero. This singularity is physically real in the sense that it is a necessary property of the solution, and will appear also in the exact solution of those equations. The singularity is not artificially created by the assumptions and simplifications made by the other well-known special solutions such as the Friedmann-Lemaître-Robertson-Walker, quasi-isotropic, and Kasner solutions.

- Blazar: It is a very compact and highly variable energy source associated with a supermassive black hole at the centre of a host galaxy. Blazars are among the most violent phenomena in the universe and are an important topic in extragalactic astronomy. Blazars are members of a larger group of active galaxies, also termed active galactic nuclei (AGN). However, blazars are not a homogeneous group and can be divided:
- Highly variable quasars, sometimes called Optically Violent Variable (OVV) quasars
- BL Lacertae objects ("BL Lac objects" or simply "BL Lacs").
- A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BL Lac objects.

- Collapsar: A word for collapsed star is an early word for the end product of stellar gravitational collapse, a stellar-mass black hole. The word, used in this sense, is obsolete; but the term "collapsar" now sometimes refers to a specific model for the collapse of a fast-rotating star.

- Ecliptic: It is the apparent path that the Sun traces out in the sky. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year. More accurately, it is the intersection of a spherical surface, the celestial sphere, with the ecliptic plane.

- Ecliptic plane: It is the geometric plane containing the mean orbit of the Earth around the Sun. The ecliptic plane should be distinguished from the invariable ecliptic plane. The present ecliptic plane is inclined to the invariable ecliptic plane by about 1.5°.

- Ecliptic pole: It is the point on the celestial sphere where the sphere meets the imaginary line perpendicular to the ecliptic plane, the path the Earth travels on its orbit around the Sun. Due to Precession, the celestial pole moves in a circle around the ecliptic pole once every 25,800 years. There are two ecliptic poles:
. The North Ecliptic Pole is in the constellation of Draco.
. The South Ecliptic Pole is in the constellation of Dorado.

- Eddington luminosity (or Eddington limit): It is the largest luminosity that can pass through a layer of gas in hydrostatic equilibrium, supposing spherical symmetry. Using the mass-luminosity relation, it can be used to set limits on the maximum mass of a star. If the luminosity of a star exceeds the Eddington luminosity of a layer on the stellar surface, the gas layer is ejected from the star.

- Einstein ring: It is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational deflection of the source's light by a lens (such as another galaxy, or a black hole). This occurs when the source, lens and observer are all aligned.

- Ergosphere: It is a region located outside a rotating black hole. It received this name because it is theoretically possible to extract energy and mass from the black hole in this region. The ergosphere is ellipsoidal in shape and is situated so that at the poles of rotating black hole it touches the event horizon and stretches out to a distance that is equal to the radius of the event horizon. Within the ergosphere spacetime is dragged along in the direction of the rotation of the black hole at a speed greater than the speed of light in relation to the rest of the universe. This process is known as the Lense-Thirring effect or frame-dragging. Because of this dragging effect objects within the ergosphere are not stationary with respect to the rest of the universe, unless they travel faster than the speed of light, which is impossible based on the laws of physics. Another result of this dragging of space is the existence of negative energies within the ergosphere.

- Frame-Dragging or Lense-Thirring Effect: The theory of general relativity predicts that rotating bodies drag spacetime around themselves. Lense and Thirring predicted that the rotation of an object would alter space and time, dragging a nearby object out of position compared to the predictions of Newtonian physics. The predicted effect is incredibly small -about one part in a few trillion. In order to detect it, it is necessary to look at a very massive object, or build an instrument that is incredibly sensitive.

- Friedmann-Lemaître-Robertson-Walker (FLRW) metric: It is an exact solution of the Einstein field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe.

- Galaxy formation: It is presently believed to proceed directly from structure formation theories, formed as a result of tiny quantum fluctuations in the wake of the Big Bang. It is now widely accepted that galaxy evolution occurs within the frame work of a ? Cold Dark Matter cosmology; that is to say that the clustering and merging is how galaxies gain in mass, and also can determine the shape and structure of a galaxy. However, some properties of galaxies remain unexplained by current galaxy formation theories.

- Gravitational or Einstein lens: It is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer.

- Hertzsprung-Russell diagram (also referred to by the abbreviation H-R diagram or HRD, or as a Colour-Magnitude diagram, or CMD): It shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars.

- Hypernova: It refers to an exceptionally large star that collapses at the end of its lifespan -for example, a collapsar, or a large supernova. Previously it referred to an explosion with an energy of over 100 supernovae (1046 joules). Such explosions were proposed to explain the exceptional brightness of gamma ray bursts. An extensive sky search found several apparent hypernova remnants, but the frequency was too low to support the hypothesis. Today the term is used to describe the supernovae of the most massive stars, the hypergiants, which have masses from 100 to 150 times that of the Sun. Hypernovae can pose serious threats to Earth in terms of radiation output, but no stars capable of creating hypernovae are located near Earth.

- Invariable (Ecliptic?) plane: It is the plane of the solar system passing through its barycentre (centre of mass) which is perpendicular to its angular momentum vector, about 98% of which is contributed by the orbital angular momenta of the four Jovian planets (Jupiter, Saturn, Uranus, and Neptune). It is also called the Laplacian plane. The invariable plane is within 0.5° of the orbital plane of Jupiter, and may be regarded as the weighted average of all planetary orbital planes.

- Kasner metric: It is an exact solution to Einstein's theory of general relativity. It describes an anisotropic universe without matter (i.e., it is a vacuum solution). It can be written in any spacetime dimension D > 3 and has strong connections with the study of gravitational chaos.

- Kerr metric (or Kerr vacuum): It describes the geometry of spacetime around a rotating massive body. According to this metric, such rotating bodies should exhibit frame dragging, an unusual prediction of general relativity. Roughly speaking, this effect predicts that objects coming close to a rotating mass will be entrained to participate in its rotation, not because of any applied force or torque that can be felt, but rather because the curvature of spacetime associated with rotating bodies. At close enough distances, all objects -even light itself- must rotate with the body; the region where this holds is called the ergosphere. The Kerr metric is often used to describe rotating black holes, which exhibit even more exotic phenomena. Such black holes have two event horizons where the metric appears to have a singularity. The outer horizon encloses the ergosphere and has an oblate spheroid shape, a flattened sphere similar to a discus. The inner horizon is spherical and marks the "radius of no return"; objects passing through this radius can never again communicate with the world outside that radius. Objects between these two horizons must co-rotate with the rotating body, as noted above; this feature can be used to extract energy from a rotating black hole, up to its invariant mass energy, Mc2.

- Luminous red novae: They are stellar explosions thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars.

- Mixmaster Universe: It is a solution to Einstein's general relativity studied by Charles Misner in an effort to better understand the dynamics of the early universe. He hoped to solve the horizon problem in a natural way by showing that the early universe underwent an oscillatory, chaotic epoch.

- Nova: It is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. Novae are not to be confused with Type Ia supernovae, or another form of stellar explosion such as, Luminous Red Novae.

- Penrose process (also called Penrose mechanism): It is a process wherein energy can be extracted from a rotating black hole. That extraction is made possible by the existence of a region of the Kerr spacetime called the ergoregion, a region in which a particle is necessarily propelled in locomotive concurrence with the rotating spacetime. In the process, a lump of matter enters into the black hole, and once it enters the ergoregion, is split into two. The momentum of the two pieces of matter can be arranged so that one piece escapes to infinity, whilst the other falls past the outer event horizon into the hole. The escaping piece of matter can possibly have greater mass-energy than the original infalling piece of matter. In summary, the process results in a decrease in the angular momentum of the black hole, and that reduction corresponds to a transference of energy whereby the momentum lost is converted to energy extracted.

The process obeys the laws of black hole mechanics. A consequence of these laws is that if the process is performed repeatedly, the black hole can eventually lose all of its angular momentum, becoming rotationally stationary.

- Supernova: It is a stellar explosion that creates an extremely luminous object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

- Supernova type Ia: Type Ia supernova is a sub-category of cataclysmic variable stars that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion. However, white dwarfs of the common carbon-oxygen variety are capable of further fusion reactions that release a great deal of energy if their temperatures rise high enough. This category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism.

- Supernova type Ib: Supernova type Ib is distinguished from Type Ia due to the lack of an absorption line of singly-ionized silicon at a wavelength of 635.5 nanometres. As a Type Ib supernova ages, it also displays stronger spectral features of helium than Type Ia supernovae. Eventually the Type Ib spectrum contains lines from elements such as oxygen, calcium and magnesium while Type Ia spectra become dominated by lines of iron. Type Ib supernovae are believed to originate in an event nearly identical to a Type II supernova, in which a massive star suffers collapse at the core. However the progenitor star of a Type Ib supernova has expelled its outer shell of hydrogen prior to explosion. Instead the outer shells of these stars consist primarily of helium, resulting in a spectrum more like a Type Ia supernova.

- Supernova type Ic: Type Ic supernovae are distinguished from Type Ib in that the former also lack lines of helium.

- Supernova type II: Type II supernova, or core-collapse supernova, is a sub-category of cataclysmic variable stars that results from the internal collapse and violent explosion of a massive star. Stars must have at least 9 times the mass of the Sun in order to undergo a core-collapse. Massive stars generate energy by the nuclear fusion of elements. Unlike the Sun, these stars possess the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium. The star evolves to accommodate the fusion of these accumulating, higher mass elements, until finally a core of iron is produced. However, the nuclear fusion of iron produces no net energy to sustain the star, so the core becomes an inert mass that is supported only by the degeneracy pressure of electrons. This pressure is created when any further compression of the star would require electrons to occupy the same energy state, a condition that is not possible for this type of particle. (See the Pauli Exclusion Principle.)

- Terraforming: Literally, "Earth-shaping" of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth in order to make it habitable by humans. The term is sometimes used more generally as a synonym for planetary engineering.

- Type Ia supernova: It is a sub-category of cataclysmic variable stars that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion.

- Type Ib and Ic: They are probably massive stars running out of fuel at their centres; however, the progenitors of Types Ib and Ic have lost most of their outer (hydrogen) envelopes due to strong stellar winds or else from interaction with a companion. Type Ib supernovae are thought to be the result of the collapse of a massive Wolf-Rayet star. There is some evidence that a few percent of the Type Ic supernovae may be the progenitors of gamma ray bursts (GRB), though it is also believed that any hydrogen-stripped, Type Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion.

- Type II supernova, or core-collapse supernova: It is a sub-category of cataclysmic variable stars that results from the internal collapse and violent explosion of a massive star. Stars must have at least 9 times the mass of the Sun in order to undergo a core-collapse.

- Vacuum energy: It is an underlying background energy that exists in space even when devoid of matter (known as free space). The vacuum energy results in the existence of most (if not all) of the fundamental forces -and thus too in all effects involving these forces. It is observed in various experiments (relating to the spontaneous emission of light or gamma radiation, the Casimir effect, the Van-Der Waals bonds, the Lamb shift, etc).

- Yerkes spectral classification (MKK): It is a system of stellar spectral classification based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature.