Orion correlation theory
negare-ye hambâzâneš-e Oryon
Fr.: théorie de la corrélation d'Orion
A controversial proposition according to which a coincidence would exist between the mutual positions of the three stars of → Orion's Belt and those of the main Giza pyramids. More specifically, Khufu, Khafre, and Menkaure would be the monumental representation of → Alnitak, → Alnilam, and → Mintaka, respectively.
Fr.: relation période-luminosité
A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud. Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.
period-mean density relation
bâzâneš-e dowré-cagâli-ye miyângin
Fr.: relation période-densité moyenne
A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)1/2, where ρ is the mean density, γ the ratio of → specific heats (Cp/Cv), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs.
Fr.: relation de Phillips
A correlation between the peak brightness of → Type Ia supernovae and the decline rate of their → light curve (15 days after the maximum). The decline rate is also correlated to the width of the peak brightness of the supernova. The brightest events are the broadest in time and brighter SNe Ia decline more slowly than dimmer ones. Applying the Phillips relation reduces the dispersion in the light curves of Type Ia SNe thus making them precise distance indicators which can be observed over large distances.
Named after Mark M. Phillips (1951-), American astronomer (Phillips et al. 1993, ApJ 413, L105); → relation.
Fr.: relation de Pogson
The equation that expresses the → magnitude
→ difference between
two objects in terms of the → logarithm of the
→ flux → ratio:
Named after Norman Robert Pogson (1829-1891), the English astronomer, who introduced the magnitude scale in 1856; → relation.
Fr.: correlation positive
Same as → direct correlation.
Fr.: relation de recurrence
A → sequence based on a → rule that gives the next → term as a → function of the previous term(s). For example, the sequence 3, 9, 21, 45,... can be represented by the recurrence relation un+1 = 2un + 3, where u1 = 3 and n ≥ 1.
Fr.: relation décalage vers le rouge-distance
The correlation, first established by E. Hubble, between the cosmological recession velocities of galaxies and their distances.
General: A connection or association between two or more things.
Verbal noun of → relate.
Fr.: symbole de relation
Same as → predicate symbol.
Of or pertaining to relations.
relational data structure
sâxtâr-e dâdehâ-ye bâzâneši
Fr.: structure de données relationnelle
A type of data structure in which data are represented as tables in which no entry contains more than one value.
Fr.: système relationnel
A database management system in which a relational data structure is used.
Fr.: relation Schmidt-Kennicutt
Same as the → Schmidt law.
Named after the American astrophysicists Maarten Schmidt (1929-), the pioneer of research in this field, and Robert C. Kennicutt, Jr. (1951-), who developed the study; → relation.
Fr.: relation symétrique
A relation between two quantities such that the first is to the second as the second is to the first. In symbols: a R b = b R a. For example, multiplication is an operation with a symmetric relation between the factors: 5 x 3 = 3 x 5.
Fr.: relation Tully-Fisher
An observed correlation between the luminosity of a spiral galaxy and its rate of rotation (measured from its 21 cm hydrogen line). This means that more luminous galaxies have stars that are moving faster. Knowing the rotational velocity of a spiral galaxy, this relation provides its absolute magnitude and then its distance. → Faber-Jackson relation.
Named after R. B. Tully and J. R. Fisher who first derived this relationship (1977, A&A 54, 661); → relation.
Fr.: relation vitesse-distance
The linear relation wherein all galaxies are moving away from one another, with velocities that are greater with increasing distance of the galaxy. Same as → Hubble's law.