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

The parallax of a celestial body when viewed from two points in the Earth's orbit around the Sun. More specifically, the angular difference in a celestial object's position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax.

→ heliocentric; → parallax.

A system in which the Sun is assumed to lie at its central point while the Earth and other bodies revolve around it.

→ heliocentric; → system.

A record made by a → heliograph.

→ helio-; + → -gram.

1) An instrument for photographing the Sun, consisting of a camera and a specially adapted telescope.
2) A simple device using a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send coded messages. The heliograph was a highly effective instrument for instantaneous optical communication over 80km or more in the 19th century.
3) Meteo.: An instrument which records the duration of sunshine and gives a quantitative measure of the amount of sunshine.

→ helio-; + → -graph.

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope.  The instrument was originally designed for measuring the variation of the Sun's diameter at different seasons of the year.

→ helio- + → -meter.

The edge of the solar system where the pressure of the → solar wind balances that of the → interstellar medium plasma. In other words, the surface boundary that separates the → heliosphere from interstellar space. It is estimated to be situated 100 → astronomical units or more from the Sun. A → bow shock likely forms as the interstellar medium wind approaches the heliosphere and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail.

From → helio- + pause "break, cessation, stop," from M.Fr. pause, from L. pausa "a halt, stop, cessation," from Gk. pausis "stopping, ceasing," from pauein "to stop, to cause to cease."

Hurmarz, from hur, → helio-, + marz "frontier, border, boundary," → frontier.

The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. See also → asteroseismology and → stellar pulsation. The surface of the Sun vibrates much like a bell. A piano has 88 keys or musical tones, whereas the Sun has millions of notes. These vibrations are the result of internal pressure waves that reflect off the → photosphere and repeatedly cross the solar interior. They are detected through the → Doppler shift of absorption lines formed in the photosphere. Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun's surface. The mean period of the vibrations is about five minutes, which corresponds to a speed of 0.5 km s^-1 or a frequency of about 3 mHz. See also → p mode.

From → helio- + → seismology.

The region located between the → termination shock and the → heliopause where the turbulent and hot → solar wind is compressed as it passes outward against the interstellar wind.

Heliosheath, from → helio- + sheath, from O.E. sceað, scæð; cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide "scabbard."

Hurniyâm, from hur- "sun," → helio-, + niyâm "sheath," from Proto-Iranian *nigāma-, from ni- "down; into," → ni- (PIE), + gāma- "to go, to come" (Av. gam- "to come; to go," jamaiti "goes;" O.Pers. gam- "to come; to go;" Mod./Mid.Pers. gâm "step, pace," âmadan "to come;" cf. Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE root *gwem- "to go, come"); cf. Skt. nigamá- "insertion, incorporation."

The vast, three-dimensional region of space around the Sun filled with the → solar wind and the remnant of the → solar magnetic field carried in it. It is bounded by the → heliopause, which is estimated to be 100 → astronomical units or more from the Sun. The radius of the heliosphere is expected to vary with the → solar cycle. The heliosphere may be very elongated owing to the presence of an interstellar wind of neutral hydrogen flowing from the direction of the Galactic center.

From → helio- + → sphere.

An instrument consisting of a mirror moved by clockwork for tracking the movement of the Sun and reflecting the sunlight into a stationary solar telescope. A heliostat is similar to a → coelostat.

Heliostat, from → helio- + -stat prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states "one that causes to stand," or statos "standing," from *sta- "to stand."

Hurdâštâr, from hur-, → helio- + dâštâr "holder, maintainer," from dâštan "to hold, maintain; to have; to possess;" Mid.Pers. dâštan; O.Pers./Av. 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 base *dher- "to hold, support."

Chemical element; symbol He; atomic number 2; atomic weight 4.0026; melting point below -272°C at 26 atmospheres pressure; boiling point -268.934°C at 1 atmosphere pressure.

Helium, from Gk. helios "sun;" cognate with Persian hur "sun", variant xor; Mid.Pers. xvar "sun;" Av. hû-, hvar- "sun;" Skt. surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun; PIE *sawel- "sun." The element was discovered by spectroscopy during a solar eclipse in the Sun's chromosphere in 1868 by the French astronomer Pierre-Jules-Cesar Janssen (1824-1907).

The relative amount of helium with respect to another → chemical species, usually → hydrogen, in an astronomical object.

→ helium; → abundance.

The stage in the evolution of a star, after the exhaustion of hydrogen, when the star produces its energy by the fusion of helium into carbon and oxygen.

→ helium; → burning.

The sudden onset of → helium burning in the core of an → intermediate-mass star that has exhausted its hydrogen and has become a → red giant. With a → degenerate core, the temperature increases but the pressure does not. Therefore, the core cannot expand and cool, so the temperature continues to rise. When it approaches 100,000,000 K, helium will begin to fuse into carbon in the → triple alpha process. The helium flash ends the giant star's ascent of the → red giant branch. However, the violent ignition of helium in the core does not increase the star's luminosity. On the contrary, the energy released in the helium flash expands and cools the core and ultimately results in a reduction in the energy output. On the → H-R diagram the star moves down from red giant branch to the → horizontal branch, a stable state with steady helium burning in the core.

→ helium; → flash.

1) The normal component of → liquid helium (⁴He) existing between the superfluid transition point (→ lambda point about 2.17 K) at 1 atmosphere of pressure and its boiling point of 4.2 K.
2) In stellar and interstellar plasma spectroscopy, neutral helium.

→ helium.

1) A → superfluid, colder component of → liquid helium (⁴He), occurring when → helium I is cooled below the → lambda point.
2) 2) In stellar and interstellar plasma spectroscopy, ionized helium.

→ helium.

A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.

→ helium; → shell; → burning.

A violent outburst of energy that occurs periodically in an → asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.

→ helium; → shell; → flash.

An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.

→ helium; → star.

A wavelength calibration of astronomical spectra using a helium-argon light source.

→ helium; → argon; → calibration.