An Etymological Dictionary of Astronomy and Astrophysics
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A → thermal escape process by which the atmosphere of a planet loses gases to outer space. This form of thermal escape occurs because some molecules, especially low mass ones, are within the higher-velocity end of the → Maxwell-Boltzmann distribution. The possibility for the gases to escape occurs when the thermal energy of air molecules becomes greater than the → gravitational potential energy of the planet: (3/2)kT = (1/2)mv²  >  GmM/R where v is upward velocity of a molecule of mass m, M is the mass of the planet, and R is the radius of the planet at which thermal escape occurs. The minimum velocity for which this can work is called the → escape velocity is: v_e = (2MG/R)^1/2. Hydrogen molecules (H₂) and helium, or their ions tend to have velocities high enough so that they are not bound by Earth's gravitational field and are lost to space from the top of the atmosphere. This process is important for the loss of hydrogen, a low-mass species that more easily attains escape speed at a given temperature, because v ~ (2kT/m)^1/2. As such, Jeans' escape was likely influential in the atmospheric evolution of all the early terrestrial planets. Jeans' escape currently accounts for a non-negligible fraction of hydrogen escaping from Earth, Mars, and Titan, but it is negligible for Venus because of a cold upper atmosphere combined with relatively high gravity (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ Jeans; → escape.

The process whereby → Lyman continuum photons produced by → massive stars escape from a galaxy without being absorbed by interstellar material. Some observations indicate that the Lyman continuum escape fraction evolves with → redshift.

→ Lyman; → continuum; → escape.

The amount of information which can be retained in a memory, usually expressed as the number of words which can be retained. For comparison of different memories this number is expressed in bits.

→ memory; → capacity.

The → heat capacity of one → mole of substance: C_μ = μ C, where μ is the → molecular weight and C the → specific heat capacity. The molar heat capacity of water is practically 18 cal/mole.C°.

Molar, adj. of → mole; → heat; → capacity.

The → nuclear reaction that occurs when an → atomic nucleus captures a → neutron. Neutron capture is the primary mechanism (principally, the → s-process and → r-process) by which very massive nuclei are formed in stars and during → supernova explosions. Instead of → fusion of similar nuclei, heavy, → neutron-capture elements are created by the addition of more and more neutrons to existing nuclei.

→ neutron; → capture.

A → nucleosynthesis process responsible for the generation of the → chemical elements heavier than the → iron peak elements. There are two possibilities for → neutron capture: the slow neutron-capture process (the → s-process) and the rapid neutron-capture process (the → r-process). The s-process is further divided into two categories: the weak s-component and the main s-component. Massive stars are sites of the weak component of s-process nucleosynthesis, which is mainly responsible for the production of lighter neutron-capture elements (e.g. Sr, Y, and Zr). The s-process contribution to heavier neutron-capture elements (heavier than Ba) is due only to the main s-component. The low- to intermediate-mass stars (about 1.3-8 Msun) in the → asymptotic giant branch (AGB) are usually considered to be sites in which the main s-process occur. There is abundant evidence suggesting that → Type II supernova (SNe II) are sites for the synthesis of the r-process nuclei, although this has not yet been fully confirmed. The observations and analysis on → very metal-poor stars imply that the stars with [Fe/H] ≤ -2.5 might form from gas clouds polluted by a few supernovae (SNe). Therefore, the abundances of → heavy elements in → metal-poor stars have been used to learn about the nature of the nucleosynthetic processes in the early Galaxy (See, e.g., H. Li et al., 2013, arXiv:1301.6097).

→ neutron;→ capture; → element.

Same as → suprathermal escape.

→ non-; → thermal; → escape.

The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10⁴ years for a constant density Sun.

→ photon; → escape; → time.

1) Either of the regions around the poles of the Earth that are permanently covered with ice.
2) Either of the two regions around the poles of the planet Mars, consisting of frozen carbon dioxide and water ice.
3) An area of a → pulsar's surface from where open magnetic field lines emanate.

→ polar; → cap.

Capture of a free electron by an ion with the subsequent emission of photons; also called → radiative recombination.

→ radiative; → capture.

Capture by an atomic nucleus of a particle whose energy is equal to one of the energy levels of the nucleus.

→ resonance; → capture.

A → sundial consisting of an inverted half sphere and a central vertical → gnomon used by ancient Greeks. See also → Eratosthenes experiment.

Gk. skaphe "boat, skiff; a bowl."

A flat triangular bone a pair of which form the back part of the shoulder. Commonly known as → shoulder blade.

L. scapula "shoulder."

Šâné, Mid.Pers. šânag "shoulder-blade."
Ketf, loan from Ar. kataf "shoulder."

An → atmospheric escape mechanism that occurs where individual atoms or molecules in the atmosphere are raised to → escape velocity because of chemical reactions or ionic interactions. Same as → nonthermal escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ supra-; → thermal; → escape.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space. In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent. Thermal escape has two types: → Jeans' escape and → hydrodynamic escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ thermal; → escape.

A process in which two stars remain → bound after their → close encounter, leading to the formation of a → binary system. Tidal capture becomes possible when two stars pass each other so closely (within a few stellar radii) that their → tidal forces are able to absorb the excess energy of → unbound → orbital motion. The process was originally suggested by Fabian et al. (1975) to explain the origin of → low-mass X-ray binary systems observed for the first time in → globular clusters.

→ tidal; → capture.

A parallel of latitude on the Earth, 23°26' south of the equator, where the Sun is directly overhead on the southern → summer solstice (around the 21st December each year), because the Sun reaches its most southerly declination. Some 3,000 years ago, this occurred when Sun was in the → Zodiac constellation → Cancer, hence the name. However, → precession has resulted in a shift of the position of the Sun so that it is now in the constellation → Sagittarius on the northern → winter solstice.

→ tropic; → Capricorn.