An Etymological Dictionary of Astronomy and Astrophysics
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The acceleration caused by the force of gravity. At the Earth's surface it is determined by the distance of the object form the center of the Earth: g = GM/R², where G is the → gravitational constant, and M and R are the Earth's mass and radius respectively. It is approximately equal to 9.8 m s^-2. The value varies slightly with latitude and elevation. Also known as the → acceleration of gravity.

→ gravitational; → acceleration.

The force that pulls material bodies toward one another because of → gravitation.

→ gravitational; → attraction.

Collapse of a mass of material as a result of the mutual → gravitational attraction of all its constituents.

→ gravitational; → collapse.

A fundamental constant that appears in → Newton's law of gravitation. It is the force of attraction between two bodies of unit mass separated by unit distance: G = 6.673 x 10^-8 dyn cm² g^-2 or 6.673 x 10^-8 cm³s^-2g^-1, or 6.673 x 10^-11 N m² kg^-2 or 6.673 x 10^-11 m³s^-2kg^-1. It was first measured in 1798 by Henry Cavendish (1731-1810), 71 years after Newton's death. Same as the → Newtonian constant of gravitation.

→ gravitational; → constant.

Decrease in the volume of an astronomical object under the action of a dominant, central gravitational force.

→ gravitational; → contraction.

The dimensionless gravitational constant defined as the gravitational attraction between pair of electrons and normally given by: α_G = (Gm_e²) / (ħc) = (m_e / m_P)² ~ 1.7518 × 10^-45, where ħ is → Planck's reduced constant, c the → speed of light, m_e is the → electron mass, and m_P is the → Planck mass.

→ gravitational; → coupling; → constant.

An encounter in which two moving bodies alter each other's direction and velocity by mutual → gravitational attraction.

→ gravitational; → encounter.

Same as → gravitational potential energy.

→ gravitational; → energy.

The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.

→ gravitational; → equilibrium.

The region of space in which → gravitational attraction exists.

→ gravitational; → field.

The weakest of the four fundamental forces of nature. Described by → Newton's law of gravitation and subsequently by Einstein's → general relativity.

→ gravitational; → force.

The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.

→ gravitational; → instability.

Mutual attraction between any two bodies that have mass.

→ gravitational; → interaction.

A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity.

→ gravitational; → lens.

The main equation of gravitational lens theory that sets a relation between the angular position of the point source and the observable position of its image.

→ gravitational; → lens; → equation.

The act of producing or the state of a → gravitational lens.

→ gravitational; → lensing.

The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing. According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source. A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image. Given a physical model of the gravitational lens, the light travel time for each image can be computed. The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance.

→ gravitational; → lensing; → time; → delay.

The mass of an object measured using the effect of a gravitational field on the object.

→ gravitational; → mass.

1) The energy that an object possesses because of its position in a → gravitational field, especially an object near the surface of the Earth where the → gravitational acceleration can be assumed to be constant, at about 9.8 m s^-2.
2) In a two body system. It is the amount of work done in bringing the mass m to the distance R from M: E_P = -GMm/R, where G is the → gravitational constant.
3) For a uniform sphere. It is E_P = -(3/5)GM²/R, where G is the gravitational constant and M is the mass contained in the sphere of radius R.

→ gravitational; → potential; → energy.

The → energy transported by → gravitational waves. Gravitational radiation is to → gravity what light is to → electromagnetism.

→ gravitational; → radiation.