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

1) To remain in one place in the air by rapidly beating the wings.
2) Computers: To place (a pointer) over a particular area of a computer screen (such as a hyperlink on a webpage) so as to cause a pop-up box to appear or other change to occur, without clicking a button on the device.

M.E. hoveren, frequentative of hoven "hover, tarry, linger," of unknown origin.

Parjâ zadan (on the model of darjâ zadan "to march in the same place, moving one's legs up and down without going forward"), from par zadan darjâ "to beat the wings at the same place," from par zadan "to beat the wings," from par "wing, → feather," zadan, → beat, + darjâ "in the same place," from dar, → in, + jâ, → place.

A vehicle capable of travelling over land or water on a cushion of air.

→ space; → craft.

The act of one who hovers.

→ hover; → -ing.

An → excited state in the → triple alpha process leading to the production of the most abundant → isotope of → carbon. The existence of this state is of extreme astrophysical importance concerning the → nucleosynthesis of ¹²C in stellar → cores:
⁴He + ⁴He ↔ ⁸Be,
⁸Be + ⁴He ↔ ¹²C^*,
¹²C^*→ ¹²C + γ.
The process proceeds as follows. First the unstable → ground state of ⁸Be is formed in the collision of two → alpha particles. Since ⁸Be exists roughly 7 x 10^-17 sec, it must fuse with an alpha particle before breaking up. However, the probability of three bodies merging simultaneously is extremely low. Hoyle showed that the ¹²C nucleus needs an excited state or resonance at 7.68 MeV to provide for a high reaction probability. The Hoyle state was soon found at 7.65 MeV with the predicted → spin and → parity.

In honor of the British astrophysicist Fred Hoyle (1915-2001), who predicted this state in 1953 (Hoyle et al. 1953, Physical Review 92, 1095); it was discovered by W. A. Fowler in 1957; → state.

Same as → Hertzsprung-Russell diagram.

Short for → Hertzsprung-Russell diagram.

Edwin Powell Hubble (1889-1953), the American astronomer who provided the observational evidence of the expansion of the Universe in 1929; → Hubble-Lemaitre law.

The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

→ Hubble; → classification.

→ Hubble-Lemaitre constant.

→ Hubble; → constant.

An image of a small region in the constellation → Ursa Major, based on the results of a series of observations by the → Hubble Space Telescope. The image was assembled from 342 separate exposures taken over ten consecutive days between December 18 and December 28, 1995. It covers an area 144 arcseconds across.

→ Hubble; → deep; → field.

A plot of the → redshift of galaxies against their distance or against their → apparent magnitude.

→ Hubble; → diagram.

The distance from the Earth to the → cosmic horizon which marks the edge of the → observable Universe. Same as → Hubble radius, → Hubble length, and → cosmic horizon.

→ Hubble; → distance.

→ Hubble-Lemaitre flow.

→ Hubble; → flow.

→ Hubble-Lemaitre law.

→ Hubble; → law.

The distance traveled by light along a straight → geodesic in one → Hubble time. Also called the → Hubble radius, → Hubble distance, and → cosmic horizon.

→ Hubble; → length.

→ Hubble-Lemaitre parameter.

→ Hubble; → parameter.

The size of the observable Universe as derived from the ratio c/H₀, where H₀ is the → Hubble-Lemaitre constant and c the → speed of light. Same as → Hubble distance, → Hubble length, and → cosmic horizon.

→ Hubble; → radius.

A classification scheme in which galaxies are ordered into a sequence based on their morphology. Same as the → Hubble classification.

→ Hubble; → sequence.

A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that worked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.

→ Hubble; → space; → telescope.

An estimate for the age of the Universe by presuming that the Universe has always expanded at the same rate as it is expanding today. It is the inverse of the → Hubble-Lemaitre constant: t_H = 1/H₀. Also called the Hubble age or the Hubble period.

→ Hubble; → time.

The → Hubble parameter for the → present epoch. It is the constant of proportionality between the → recession velocities of galaxies and their distances from each other. The latest determinations using the → Hubble Space Telescope observations of → Cepheids give H₀ = 72 ± 8 km s^-1 Mpc^-1 (W. L. Freedman et al., 2001, ApJ 553, 47, arXiv:astro-ph/0012376), the → WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1 (N. Jarosik et al., 2011, ApJS 192, 14, arXiv:1001.4744), and the → Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1 (Planck Collaboration, 2014, A&A 571, A16, arXiv:1303.5076). More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy (Adam G. Reiss et al., 2016, arXiv:1604.01424). The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The → Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.

→ Hubble; → Friedmann-Lemaitre Universe; → constant.