An Etymological Dictionary of Astronomy and Astrophysics
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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.

The general outward motion of → galaxy clusters resulting from the → expansion of the Universe.

→ Hubble-Lemaitre law; → flow.

The speed with which a → galaxy cluster recedes from us is directly proportional to its distance. It can be stated as v = H₀d, where v is the recessional velocity, H₀ the → Hubble-Lamaitre constant, and d the distance. See also → Hubble-Lemaitre flow. It should be underlined that Hubble was not the first to discover the → velocity-distance relation. Two years before Hubble, in 1927, Georges Lemaître (1894-1966) had derived the relation and published it in a paper in French which remained neglected (→ Friedmann-Lemaitre Universe).

The International Astronomical Union (IAU) at its 30th Meeting approved the Resolution B4 proposed by the IAU Executive Committee recommending the use of Hubble-Lemaitre law instead of Hubble's law, after Edwin Hubble (1889-1953), the American astronomer who published his results in 1929 and Georges Lemaître, Belgian priest and astronomer, who published a paper on the expansion of the Universe in 1927; → law.

The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.

→ Hubble-Lemaitre law; → parameter.

The discrepancy between the value of the → Hubble-Lemaitre constant inferred from a ΛCDM fit (→ Lambda cold dark matter model) to the → cosmic microwave background (CMB) and local measurements. The Universe appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents. Based on the → Hubble Space Telescope observations, the Hubble-Lemaitre constant is very recently estimated to be 74.03 km s^-1 Mpc^-1. This value indicates that the Universe is expanding at a rate about 9% faster than that implied by the → Planck satellite's observations of the → early Universe, which give a value for the Hubble constant of 67.4 km s^-1 Mpc^-1. For discussion, see D'Arcy Kenworthy et al. (2019, ApJ 875, 145).

→ Hubble-Lamaitre constant; → tension.

Same as the → Hubble classification.

→ Hubble; → Hubble-Sandage variable; → sequence.