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

A member of a rare class of objects found in → globular clusters to lie about one magnitude above and to the blue part of the → horizontal branch. These stars are identified as post → EHB stars on their way from to the → asymptotic giant branch.

→ supra-; → horizontal; → branch; → star.

The physical process that slows the → Earth's rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth's → day and → month are equal, at which point the Moon will never seem to move in Earth's sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

→ tidal; → braking.

A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (M_I = -4.1 ± 0.1).

→ tip; → red giant; → branch; → method.

1) Optics: A dark area, especially the blackest part of a shadow from which all light is cut off. → penumbra.
2) Sun: The central darkest part of a → sunspot, which is also the coolest part (~ 4,200 K).

From L. umbra "shade, shadow."

Sâyé "shadow," from Mid.Pers. sâyak "shadow;" Av. a-saya- "throwing no shadow;" Skt. chāya- "shadow;" Gk. skia "shade;" Rus. sijat' "to shine;" M.H.G. schinen, O.H.G. skinan, Ger. Schein "glow, shine;" PIE base *skai- "bright."

To oscillate with a continuing periodic change relative to a fixed reference point or state of equilibrium. → oscillate.

From L. vibratus, p.p. of vibrare "to move quickly to and fro, shake" (cf. Lith. wyburiu "to wag the tail," Dan. vippe, Du. wippen "to swing," O.E. wipan "to wipe").

Šividan "to vibrate, move to and fro, to tremble," related to šodan, šow- "to go; to become;" Av. šiyav-, š(ii)auu- "to move, go," šiyavati "goes," šyaoθna- "activity; action; doing, working;" O.Pers. šiyav- "to go forth, set," ašiyavam "I set forth;" Skt. cyu- "to move to and fro, shake about; to stir," cyávate "stirs himself, goes;" Gk. kinein "to move;" Goth. haitan "call, be called;" O.E. hatan "command, call;" PIE base *kei- "to move to and fro."

1) The act of vibrating. The state of being vibrated.
2) A continuing → periodic  → oscillation relative to a fixed reference point or → equilibrium state.

Verbal noun from → vibrate.

Of or pertaining to → vibration. → vibrational mode.

→ vibration; → -al.

The energy due to the vibration of the molecules making up atoms (→ molecular vibration). A molecule in space can have energies in various forms: → rotational energy, vibrational energy, or electronic energy. These energies of molecules are → quantized and a particular molecule can exist in different rotational and vibrational → energy levels. The molecules can move from one level to another level only by a jump involving a finite amount of energy. → Quantum mechanics predicts that any molecule can never have zero vibrational energy, that is atoms can never be completely at rest relative to each other. The harmonically oscillating molecules can undergo vibrational changes determined by simple selection rules obtained from → Schrödinger equation.

→ vibrational; → energy.

The frequency at which the atoms in a molecule vibrate. The frequencies of → molecular vibrations in diatomic molecules are in the order of 10^-12 to 10^-14 Hz. In such molecules, the only → vibrational mode available is along the bond. More complicated molecules have many types of vibration and stretching modes.

→ vibrational; → frequency.

Any of the ways in which a → molecule vibrates. Each vibrational mode has a different → frequency frequency. The number of vibrational modes of a molecule is determined by the number of atoms in it. The number of vibrational modes for a non-linear molecule is 3N - 6, where N is the number of atoms making up the molecule. For a linear molecule it is 3N - 5.

→ vibrational; → mode.

A transition between two → quantized  → levels of a → molecule that have different vibrational energies.

→ vibrational; → transition.

A slight change in the → energy level of a → molecule due to → vibrational transition and/or → rotational transition.

→ vibrational; → rotational; → transition.

A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.

→ zero; → age; → horizontal; → branch; → star.