An Etymological Dictionary of Astronomy and Astrophysics
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A dense zone of magnetized → plasma surrounding a → pulsar. The magnetosphere, lying between the surface of the → neutron star and the → light cylinder, corotates with the pulsar like a rigid body under the effect of strong magnetic field. The magnetosphere's thickness is determined by the constraint that the corotation velocity of its upper surface should not exceed the → speed of light.

→ pilsar; → magnetosphere.

A phase transitions that occurs at zero temperature as a function of a non-thermal parameter like → pressure, → magnetic field, or → chemical composition. In contrast to ordinary → phase transitions, which are associated with passage through a critical temperature, quantum phase transitions are associated with → quantum fluctuations, a consequence of → Heisenberg's uncertainty principle. For example, see → Bose-Einstein condensation.

→ quantum; → phase; → transition.

A phase transition, predicted by cosmological models, to have occurred at approximately 10^-5 seconds after the Big Bang to convert a plasma of free quarks and gluons into hadron.

→ quark; → hadron; → phase; → transition.

For a → supernova remnant (SNR), same as the → snowplow phase.

→ radiative; → phase.

The process of producing an image on a sensitive surface by radiation such as X-rays or gamma rays passing through an object.

→ radio + → -graphy.

The brightest star (V = 2.08) in the constellation → Ophiuchus. Rasalhague is a → giant star of type A5 lying 47 light-years from Earth. It has a faint, very close companion only 0''.5 away, 7 A.U., that orbits with a period of 8.7 years.

Rasalhague, from Ar. Ra's al-Hayyah (رأس‌الحیه), literally the "head of the snake," from ra's "head" + Hayyah "snake," referring to the creature in Gk. mythology.

1) An atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidating gasses or vapours. Usually nitrogen or hydrogen gas is used in order to produce specific effects, e.g. on ceramic wares being fired.
2) An atmosphere of a planet or moon which has a high hydrogen content, either in the form of free hydrogen or hydrogen-containing compounds, such as methane or ammonia. The early atmosphere of Earth is thought to be reducing, dominated by carbon dioxide.

Reducing verbal adj. of → reduce; → atmosphere.

A complex region of molecular and dust clouds containing emission and reflection nebulae near the star ρ Oph in the constellation → Ophiuchus. It is one of the closest star forming regions, some 400 light-years distant. Recent studies using the latest X-ray and infrared observations reveal more than 300 young stellar objects within the large central cloud. Their median age is only 300,000 years.

Rho, Gk. letter; → Ophiuchus; → cloud.

In → graph theory, a → graph that has one of its → vertices, called the → root, distinguished from the others.

→ root; → graph.

A position parameter used in → stellar magnetic field studies. Its zero value represents the moment when, during → stellar rotation, the positive → magnetic pole is nearest to the → line of sight.

→ rotation; → phase.

A set of astronomical tables created in 1627 by Johannes Kepler (1571-1630) based on observations by Tycho Brahe (1546-1601). These tables allowed Kepler to derive the three laws of planetary motions bearing his name (→ Kelpler's laws). These are the first tables in which → atmospheric refraction has been taken into account. They overruled the → Prutenic Tables.

From the L. title Tabulae Rudolphinae, in memory of Rudolf II (1552-1612), king of Hungary and Bohemia, and Holy Roman Emperor; → table.

A → supergiant star of visual magnitude 2.06 and → spectral type B0.5 Ia marking the right knee of Orion. It is about 700 light-years away.

Saiph "sword," from Ar. as-saiph al-jabbâr (سیف الجبار) "the Sword of the Giant."

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."

An atmosphere of a planet that forms after primordial gases had been lost or had failed to accumulate. A secondary atmosphere develops from internal volcanic outgassing, or by accumulation of material from comet impacts. It is characteristic of terrestrial planets, such as Earth, Mercury, Venus, and Mars. → primordial atmosphere.

→ secondary; → atmosphere.

The second phase in the evolution of a → supernova remnant (SNR) occurring after the → free expansion phase. After the passage of the → reverse shock, the interior of the SNR is so hot that the energy losses by radiation are very small (all atoms are → ionized, no → recombination). The expansion is driven by the → thermal pressure of the hot gas and can therefore be regarded as → adiabatic; the → cooling of the gas is only due to the → expansion. Pressure forces accelerate the swept-up → interstellar medium (ISM) converting → thermal energy (which came from original explosion) into → kinetic energy of the → shell of swept-up mass. As the mass of the ISM swept up by the shell increases, it eventually reaches densities which start to impede the free expansion. → Rayleigh-Taylor instabilities arise once the mass of the swept-up ISM approaches that of the ejected material. This causes the SNR's ejecta to become mixed with the gas that was just shocked by the initial → shock wave. The Sedov-Taylor phase lasts some 10⁴ years and is followed by the radiative or → snowplow phase. Also called → adiabatic phase.

After Sedov, L. (1959, Similarity and Dimensional Methods in Mechanics, New York, Academic Press) and Taylor, G. I. (1950, Proc. Roy. Soc. London, A, 201, 159 and 175); → phase.

An instrument that detects, magnifies, and records → seismic waves, especially those caused by → earthquakes or → explosions.

→ seismo-; → -graph.

Topographic description and charting of the surface of the Moon.

From seleno- combining form of Gk. selene "moon" + → -graphy.

Mâh-negâri, from mâh, → moon + negâri, → -graphy.

A person who takes care of sheep; a pastor. → shepherd moon.

From M.E. shepherde; O.E. sceaphierde, from sceap "sheep" + hierde "herder," from heord "a herd;" cf. M.L.G., M.Du. schaphirde, M.H.G. schafhirte, Ger. dial. schafhirt.

Cupân "shepherd," variants šobân, šabân; Mid.Pers. šubân, from šu + -bân. The first component from Av. pasu-, fšu- "sheep;" Mid.Pers. pâh, pasvīk "cattle;" Laki and Tâti pas "sheep;" Kurd. pez/paz; Ossetain (Digor.) fus, (Iron.) fys; Zazaki pes "small cattle;" Lâri pah; Qasrâni cu; Sogd. psw "cattle, sheep;" cf. Skt. paśu- "cattle;" L. pecu "flock, farm animals, cattle," pecunia "money, property;" Goth. faihu "money, fortune;" O.E. feoh "cattle, money;" Ger. Vieh "cattle;" Lith. pekus "cattle;" PIE base *peku- "cattle." The second component -pân/-bân a suffix denoting "keeper, guard," sometimes forming agent nouns or indicating relation, → host.

A → natural satellite in orbit near the edge of a → planetary ring, whose → gravitational force on the ring particles strongly controls the distribution of material within the ring, creating ringlets and density waves within the ring and sharp edges at ring boundaries. Examples include → Saturn's → Prometheus and → Pandora, which shepherd the narrow outer → F ring and the → Uranus satellites → Cordelia and → Ophelia and the epsilon ring. The faster-moving inside satellite accelerates the inner ring particles as it passes them, causing them to spiral out to larger orbits. At the same time the slower-moving outer satellite decelerates the outer ring particles as they pass by, causing them to spiral inward. The result is a narrow, well-defined ring.

→ shepherd; → moon.

→ siderophile element.

→ sidero-; → -phile.