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

A dark lane that appears behind the coma in some comets. It is not the shadow of the true nucleus but sometimes may be a region of the near-tail that lies behind the densest part of the inner coma and therefore receives less sunlight.

→ shadow; → nucleus.

A central region of an → active galaxy, which is a → light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (→ gamma rays, → X-rays, → ultraviolet) and shows → variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 10⁴ km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure. See also → broad-line region, → narrow-line region, → quasar.

→ active; → galactic; → nucleus.

The central part of the → atom. It is made up of → protons and, in most cases, → neutrons. The nucleus is surrounded by a swarm of fast-moving → electrons. Almost all of the mass (more than 99%) of an atom is contained in the dense nucleus. The number of protons in the nucleus (called → atomic number) determines the type of → chemical element. Atoms that differ only in the number of neutrons in their nuclei are called → isotopes.

→ atomic; → nucleus.

The solid, centrally located part of a → comet. The nucleus is a mass of dust and frozen gases. When heated by the → Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the → coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of → Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant → volatile is → water, followed by → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water) and many other molecules at a lower level.

→ comet; → nucleus.

→ comet nucleus.

→ cometary; → nucleus.

A temporary excited state of the atomic nucleus which results from the collision of an incident energetic particle with a target nucleus. Subsequently the nucleus decays by emitting particle(s).

→ compound; → nucleus.

A tiny suspended particle in the → atmosphere around which → water vapor condenses to form → droplets. Condensation nuclei are usually less than about 0.001 mm in diameter and can be made of → ice, → salt, → dust, and other materials. The droplets that form can be liquid water or ice, depending on the surrounding temperature. When the number of these droplets is large enough, a cloud is visible.

→ condensation; → nucleus.

An especially concentrated region in the → coma of some → comets, representing the dense cloud of inner coma rather than the much smaller true nucleus. Also called apparent nucleus and → pseudo-nucleus.

→ false; → nucleus.

A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.

→ galactic; → nucleus.

1) Core of an atom, where most mass and all positive charge is concentrated. It consists of protons and neutrons.
2) Of a comet, the stellar-appearing frozen core, containing most of the cometary mass, in the head of a comet.

From L. nucleus "kernel," from nucula "little nut," diminutive of nux "nut," from PIE *knu(k) "lump" (cf. M.Ir. cnu, Welsh cneuen, M.Bret. knoen "nut," O.N. hnot, O.E. hnutu "nut").

Hasté, variants asté "kernel, fruit stone," ostoxân "bone," from Mid.Pers. astak "fruit stone, bone," ast "bone;" Av. ast- "bone;" cf. Skt. asthi- "bone;" Gk. osteon; L. os; Hittite hashtai-; PIE base *os-.

Nucleus which contains an odd number of protons and an even number of neutrons.

→ odd; → even; → nucleus.

Nucleus which contains an odd number of both protons and neutrons.

→ odd; → nucleus.

Same → false nucleus.

→ pseudo-; → nucleus.

A nucleus from which a variety of → fusion  → chain reactions derive in → stellar nucleosynthesis.

→ seed; → nucleus.