An Etymological Dictionary of Astronomy and Astrophysics
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An observationally determined relationship between the → morphological classification of galaxies and the → environments in which they are located. Specifically, the morphology-density relation indicates that early-type galaxies (→ ETG) are preferentially located in high density environments, whereas late-type galaxies (→ LTG) are preferentially found in low density environments. Hence, spiral galaxies are rare in the high densities of clusters and are common in the lower density group environments. Early-type galaxies, on the other hand, are common in clusters and are rarely found in isolation.

→ morphology; → density; → relation.

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

→ neutral; → density; → filter.

The density of an atomic nucleus (about 10¹⁴ g/cm³).

→ nuclear; → density.

Number of a particular type of object found in each unit volume.

→ number; → density.

The transmittance of a point on a photographic negative equal to the log to the base 10 of the reciprocal of the transmittance through the negative at that point.

→ optical; → density.

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 (C_p/C_v), 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.

→ period; → mean; → density; → relation.

The density corresponding to a → Planck mass in a cubic region of edge length given by the → Planck length: ρ_P = c⁵/(ħG²) ≅ 5.16 x 10⁹³ g cm^-3, where c is the → speed of light, ħ is the → reduced Planck's constant, and G is the → gravitational constant.

→ Planck; → density.

Of a fluid parcel at pressure P, the density that it would acquire if adiabatically brought to a reference pressure.

→ potential; → density.

Same as → spectral density.

→ power; → spectral; → density.

A mathematical function whose integral over any interval gives the probability that a continuous → random variable has values in this interval. Also known as → density function, frequency function, → probability function.

→ probability; → density; → function.

The constant related to the total energy radiated by a → blackbody and defined as: a = 4σ/c, where σ is the → Stefan-Boltzmann constant and c the → speed of light. Its value is a = 7.5657 x 10^-15 erg cm^-3 K^-4. Same as → radiation constant.

→ radiation; → density; → constant.

The flux of → radio waves that falls on a → detector per → unit → surface area of the detector per unit → bandwidth of the radiation per unit → time. The unit is → jansky.

→ radio; → flux; → density.

The ratio of the density of a substance to the density of a given reference material. For a solid or liquid, it is the density (at 20°C) relative to the maximum density of water (at 4°C). For a gas is its density divided by the density of hydrogen (or sometimes dry air) at the same temperature and pressure. Also called → specific density.

→ relative; → density.

A → tensor density of → order 0.

→ scalar; → density.

Same as → relative density.

→ specific; → density.

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

→ spectral; → density.

The amount of a quantity distributed over a surface area divided by the area, such as a surface-charge density.

→ surface; → density.

A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.

→ tensor; → density.

A → tensor density of → order 1.

→ vector; → density.

A constant the value of which is characteristic for any given → tensor density.

→ weight; → tensor; → density.