An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The smallest → circular orbit in which a particle can stably orbit a → black hole according to → general relativity without the risk of falling past the → event horizon. In other words, the ISCO is the inner edge of the → accretion disk around a black hole. Therefore, characteristics of accretion disks depend on ISCO. The radius of ISCO is calculated to be three times the → Schwarzschild radius (3 R_Sch). ISCO is closer to event horizon for rotating black hole.

→ inner; → stable; → circular; → orbit.

An excited state in an atom, which is at the origin of the spectral lines called → forbidden lines. The time duration of the excited state being relatively long, under laboratory conditions the atom cannot pass directly to the ground state by emitting radiation. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur.

→ meta-; → stable; → state.

Physics: 1) Having the ability to react to a disturbing force by maintaining or regaining position or condition.
2) Incapable of becoming a different isotope or element by radioactive decay.

M.E., from O.Fr. estable, from L. stabilis "firm, steadfast," literally "able to stand," from stem of stare "to stand;" cognate with Pers. istâdan "to stand" (Mid.Pers. êstâtan; O.Pers./Av. sta- "to stand, stand still; set;" Av. hištaiti; cf. Skt. sthâ- "to stand;" Gk. histemi "put, place, weigh," stasis "a standing still;" L. stare "to stand;" Lith. statau "place;" Goth. standan; PIE base *sta- "to stand").

Pâydâr "stable, firm" literally "having feet," from pâ(y) "foot; step" (Mid.Pers. pâd, pây; Av. pad- "foot;" cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + dâr present stem of dâštan "to have, hold, maintain, possess" (Mid.Pers. dâštan; O.Pers./Av. root dar- "to hold, keep back, maintain, keep in mind;" cf. Skt. dhr-, dharma- "law;" Gk. thronos "elevated seat, throne;" L. firmus "firm, stable;" Lith. daryti "to make;" PIE *dher- "to hold, support").

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will return to its equilibrium state. An example is a pendulum hanging straight down. If it is pushed slightly, it will experience a force back toward the equilibrium position. It may oscillate around the equilibrium position for a while, but it will finally regain its equilibrium position. → unstable equilibrium.

→ stable; → equilibrium.

A nuclide that is not → radioactive and therefore does not spontaneously undergo → radioactive decay.

→ stable; → nuclide.

The quality of a claim, hypothesis, or theory that can be verified by tests or experiments. See also → falsifiable.

→ test; → -able.

Not → stable, as in → unstable atom, → unstable equilibrium.

→ un-; → stable.

An atom whose nuclei → decay by → radioactivity.

→ unstable; → atom.

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will move farther away from equilibrium state. For example, mechanical equilibrium in which the potential energy is a maximum, as a sphere placed on top of a hill. Mathematically, if the second derivative of the energy with respect to the coordinate of interest is negative, the system is in an unstable equilibrium. → stable equilibrium.

→ unstable; → equilibrium.