Physics astro

Doppler effect

Doppler effect

Doppler Effect: The apparent change in the wavelength of a wave as the source moves relative to an observer. For a source moving away the wavelength increases (red shift), for a source moving towards the observer the wavelength

Arcsecond

Arcsecond: A unit used to measure small angles. An arcsecond is equal to 1/3600th of a degree.

Astronomical unit

Astronomical Unit (AU): The average distance between the centre of the Earth and the centre of the Sun.

Binary star system

Binary Star System: Two stars orbiting a common centre of mass.

Black body radiator

Black Body Radiator: A perfect emitter and absorber of all possible wavelengths of radiation.

Black hole

Black Hole: A region which has an escape velocity greater than the speed of light. Black holes are formed when the core of a giant star collapses.

Ccd

Charge-Coupled Device (CCD): An array of light-sensitive pixels which become charged when they are exposed to light (by the photoelectric effect).

Chromatic aberration

Chromatic Aberration: An effect caused by the different focal lengths of different wavelengths of light that leads to different colours being focused at different points. This can cause a white object to appear as if it has coloured edges.

Collecting power

Collecting Power: A measure of the ability of a lens or mirror to collect incident EM radiation. Collecting power is directly proportional to the area of the objective lens/primary mirror.

Event horizon

Event Horizon: The boundary of a black hole, along which the escape velocity is equal to the speed of light.

Hipparcos scale

Hipparcos Scale: A way of classifying astronomical objects by their apparent magnitude.

The brightest stars have an apparent magnitude of 1 and the faintest visible stars have an apparent magnitude of 6. The intensity of a magnitude 1 star is 100 times greater than a magnitude 6 star so the scale is logarithmic.

Hubbles law

Hubble's Law: The speed of a galaxy moving away from ours is proportional to its distance away from us. The constant of proportionality is Hubble's constant.

Lightyear

Light Year (ly): The distance that an electromagnetic wave travels in a year in a vacuum.

Neutron star

Neutron Star: An incredibly dense star that is formed when the core of a large star collapses. Protons and electrons are forced together under gravity to form neutrons.

Protostar

Protostar: A young star formed when clouds of gas and dust are pulled together under gravity. Protostars are surrounded by a circumstellar disc, and when the centre becomes enough, the star will begin to fuse Elements.

Quantum efficiency

Quantum Efficiency: The percentage of photons incident on a CCD which causes an electron to be released.

Quasar

Quasar: Active galactic nuclei - supermassive black holes surrounded by a disc of matter, which, as it falls into the black hole, causes jets of radiation to be emitted from the poles.

Quasars display very large red shift, indicating they are very far away. The power output of a quasar is around that of an entire galaxy.

Rayleigh criterion

Rayleigh Criterion: This states that two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other.

Red giant

Red Giant: A stage in the life cycle of a star less than 3 solar masses, in which the hydrogen has run out and the temperature of the star increases. Helium nuclei fuse to form heavier elements.

Red shift

Red Shift (z): The shifting of an object's wavelength towards the red end of the spectrum due to the object moving away from the Earth (Doppler effect). Red shift is evidence for the expansion of the universe. The more distant the object, the greater its red shift.

Resolving power

Resolving Power: The ability of a telescope to produce separate images of close-together objects.

Schwarzchild radius

Schwarzschild Radius (Rg): The name given to the radius of a black hole's Event Horizon.

Spherical aberration

Spherical Aberration: An effect caused by the curvature of a lens or mirror that can lead to light rays at the edges to be focused in different places to those from the centre. This can cause an image to be blurry or distorted.

Stefan’s Law

Stefan's Law: A law stating that the power output (luminosity) of a star is directly proportional to its surface area and its absolute temperature to the 4** power.

Supernova

Supernova: When a star greater than 1.4 solar masses dies, the core collapses rapidly inward and becomes rigid. The outer layers then fall inward and rebound off of the core in a shockwave, causing heavy elements to be fused and distributed into space.

Transit method

Transit Method: A method of detecting exoplanets. It involves the monitoring of the host star's light intensity. If a planet crosses in front of it, the intensity will dip. Timings and other data can be combined to determine information about the size and speed of the exoplanet.

Wien’s Displacement law

Wien's Displacement Law: A law stating that the peak wavelength of emitted radiation is inversely proportional to its absolute temperature.