Astrophysics Definitions

Real image

An image formed by the convergence of rays of light, which can be captured on a screen.

Virtual image

An image caused by rays that do not converge, meaning it can be seen on the eye but not formed on a screen.

Magnification

the ratio of image size to object size, equal to the ratio of the image distance v to object distance u.

Normal adjustment

The length of the telescope is equal to the focal length of the objective lens plus the focal length of the eyepiece lens, so that the light emerges parallel from the eyepiece lens and the viewer sees the image at infinity.

Minimum angular resolution

The smallest angular separation that an instrument can distinguish between two objects.

Rayleigh criterion

Two sources will be just resolved if the central maximum of the diffraction pattern of one coincides with the first minimum of the other.

Charge coupled device

A silicon chip divided into picture elements (pixels). Each picture element is associated with a potential well in the silicon.

Quantum efficiency

The percentage of photons hitting the CCD which are detected and produce a signal.

1 AU

The average distance between the Earth and the Sun.

Parsec

The distance at which 1 AU subtends an angle of 1 arc second.

Luminosity

The total energy emitted by a star per second.

Intensity / Brightness

The energy received / radiated per second per unit area.

Absolute magnitude

The brightness / apparent magnitude of a star as it would appear at a distance of 10 parsecs.

Black body

A body that absorbs all electromagnetic wavelengths incident upon it, and emits all wavelengths of electromagnetic radiation.

Neutron star

The highly dense remnant of a star after a supernova explosion, composed mainly of neutrons.

White dwarf

A low-mass small star that has exhausted all its nuclear fuel. It is extremely dense and has a high surface temperature.

Black hole

An object whose escape velocity is greater than the speed of light.

Escape velocity

The speed necessary for an object to escape the gravitational pull of another object, such as a planet.

Event horizon

Boundary where escape velocity is equal to the speed of light.

Red shift

An increase in the wavelength of electromagnetic radiation due to relative recessive velocity between observer and source.

Quasars

Extremely strong radio sources, believed to be ancient, supermassive black holes.

Standard candle

An object whose absolute magnitude is known.

Cepheid variable

A variable star that has a brightness with a well-defined period whose frequency is related to its luminosity, allowing its distance from Earth to be estimated.