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

The region of the upper atmosphere in which temperature increases continuously with height, starting at roughly 100 km. The thermosphere includes the exosphere and most of the ionosphere.

→ thermo-; → sphere.

Any of several techniques, such as → Doppler tomography, for constructing a spatial distribution of physical quantity given measurements that are essentially line-integrals ("projections") through the distribution. Most famously, in medical tomography, the absorption of X-rays by a specimen is directly related to the line integral to make detailed images of a predetermined plane section of a solid object while blurring out the images of other planes.

From Gk. tomo- combining form of tomos "a cut, section, slice" tome "cutting" + → -graphy.

Borešnegâri, from boreš "section, slice, cutting," from boridan "to cut" (Mid.Pers. britan, brinitan "to cut off;" Av. brī- "to shave, shear," brin-; cf. Skt. bhrī- "to hurt, injure," bhrinanti "they hurt") + -negâri, → -graphy.

A short-lived change in the brightness of patches on the face of the Moon. The TLPs last from a few seconds to a few hours and can grow from less than a few to a hundred kilometers in size. They have been reported by many observers since the invention of the telescope. However, the physical mechanism responsible for creating a TLP is not well understood. Several theories have been proposed, among which lunar outgassing, that is, gas being released from the surface of the Moon.

→ transient; → lunar; → phenomenon.

A compound → vowel sound resulting from the succession of three simple vowels pronounced in a single syllable (as in power, hour, fire).

Tri-, → three; → diphthong.

A chain of nuclear fusion reactions by which three helium nuclei (→ alpha particles) are transformed into → carbon. First two nuclei of helium collide, fuse, and form a nucleus of → beryllium: ⁴He + ⁴He ↔ ⁸Be, which is unstable and will decay back into two helium nuclei within a few 10^-17 seconds. However, due to sufficiently high density and temperature in the stellar core, during a third collision between beryllium and helium, carbon is formed: ⁸Be + ⁴He → ¹²C + γ. The triple-alpha process is possible owing to the existence of the → Hoyle state. It is the main source of energy production in → red giants and → red supergiants in which the core temperature has reached at least 100 million K. Also called → Salpeter process.

→ triple; → alpha particle; → process.

The lower part of the Earth's atmosphere in which temperature decreases with height, except for local areas of → temperature inversion.

→ tropo- + → sphere.

The simultaneous emission of two photons whose sum of energies is equal to that of a single electron transition. The energy of each individual photon of the pair is not fixed, so that the spectrum of two-photon emission is continuous from the wavelength of that transition to infinity. In practice, there is a peak in wavelength distribution of the emitted photons. Two-photon emission is studied atomic physics with application in astrophysics, as it contributes to the continuum radiation from → planetary nebulae. It was recently observed in condensed matter and specifically in → semiconductors.

→ two; → photon; → emission.

A → Cepheid star of → spectral type A-F with regular pulsation period of 1-3 hours and with small variations in amplitude. This group is also known as δ Scuti stars.

→ ultra-; → short; → period; → Cepheid.

A → paradox encountered in the classical theory of → thermal radiation (→ Rayleigh-Jeans law), whereby a → blackbody should radiate an infinite amount of energy at infinitely short wavelengths, in contradiction with what is observed. The problem was solved by Max Planck in 1900, who suggested that, rather than being continuous, the energy comes in discrete parcels called → quanta. The avoidance of the ultraviolet catastrophe was one of the first great achievements of → quantum mechanics.

This problem was first raised by Lord Rayleigh (1842-1919), whereas the term ultraviolet catastrophe was first used by Paul Ehrenfest (1880-1933); → ultraviolet; → catastrophe.

The general term applied to the atmosphere above the → troposphere.

→ upper; → atmosphere.

A hot star of → apparent visual magnitude V = 5.82 lying in the constellation → Ophiuchus, also known as HR 6684, HD 163472, and HIP 87812. V2052 Oph is of → subgiant  → spectral type B2 IV-V and belongs to the family of magnetic → Beta Cephei → pulsators. It has a luminosity of 620 Lsol, a radius of 6.83 Rsol and lies about 830 → light-years away. Three stellar pulsation modes have been detected and studied from ground-based spectroscopy and multi-color photometry. Its magnetic field was deduced to be → dipolar dipolar with a strength of ~ 400 G, inclined 35° to the rotation axis. The star's → convective overshooting region is small, with no extra mixing in spite of the relatively large rotational velocity. This clearly illustrated the effect of the magnetic field: the inhibition of chemical mixing processes by the magnetic field. This result is in agreement with theoretical criteria, which predict that a surface magnetic field of ~ 70 G is sufficient to limit the overshooting in this star. V2052 Oph is now considered as a prime example of what magneto-asteroseismology can achieve (Neiner et al., 2012, A&A, 537, A148).

→ variable star designation.

A photometric system with five wavelength ranges that does not use filters. Instead it uses prisms and lenses (spectroscopy) to select the bands simultaneously. The wavelengths and the bandwidths are: W, 3250 and 140 Å; U, 3630 and 240 Å; L, 3840 and 230 Å; B, 4320 and 450 Å; and V, 5470 and 720 Å. The Walraven photometer was unique in design and remained literally unique as copies were never built. In addition, during its whole life the photometer was mounted permanently on the same telescope that had been built specifically for this instrument, the 91 cm Lightcollector' reflector, which started in 1958 at the Leiden Southern Station in Broederstroom, South-Africa. After 20 years in South-Africa the telescope and photometer were moved to the European Southern Observatory La Silla observatory in Chile. The photometric observations were resumed in March 1979 and continued for another 12 years until the decommissioning of the photometer in 1991.

After the inventors, the Dutch astronomer Theodore Walraven (1916-) and his wife Johanna Helena Walraven, née Terlinden (1920-89); → photometry.

A photometer in which an → absorbing wedge is inserted in the brighter of two beams until the flux densities of the two light sources are equal.

→ wedge; → photometer.

A blue star, also called HD 149757 and HR 6175, which is the nearest, and probably the most widely studied, → massive star. It is variable in several wavelength bands and has a mean visual magnitude of V = 2.58, B - V = 0.01. It lies ~ 222 pc away and has a formal spectral type of O9.5 Ve and a luminosity of 10⁵ Lsun. ζ Ophiuchi is a very rapid rotator with a v sin i ~ 400 km s^-1, i.e. ~ 85% of the → break-up velocity. It is one of the earliest prototypes of the → Be phenomenon. Moreover, it shows episodes of Hα emission variability, a common feature of Be/Oe stars. It also shows periodic non-radial pulsations and UV → P Cygni profile variability, as is evident in the periodic behavior of → discrete absorption components (DACs). It has a → mass loss rate of 10^-7 Msun yr^-1 and a → terminal velocity of wind v_∞ = 1550 km s^-1. ζ Ophiuchi is a well-known → runaway star with a velocity of 30 km s^-1. The interstellar → CH molecule and → CN molecule were first detected toward ζ Ophiuchi. It has been recognized for some time that this star lies close to the blue edge of the → beta Cephei instability strip.

Zeta (ζ), according to the → variable star designation system; Ophiuchi, → Ophiuchus.