# Glossary (Nuclear, Thermal and Astrophysics) AQA Physics A Unit 5A

Absolute magnitude (M)
The apparent magnitude a star would have if it were placed at a standard distance of 10 pc from Earth.
m – M = 5 log(d/10) where d is the distance in parsecs.
Activity (A)
The number of emissions per second, measured in Becquerels.
Airy disc
The bright central region in an optical diffraction pattern caused by light entering a circular aperture.
Short-range, highly ionising radiation consisting of helium nuclei
Angular magnification
The magnifying power of a refracting telescope, given by the ratio of the objective focal length to the eyepiece focal length.
Apparent magnitude (m)
The apparent brightness of a star expressed on the magnitude scale.
m = -2.5 log(b)
Astronomical unit (AU)
The average distance between the Earth and the Sun.
Atomic mass unit (u)
Unit of mass defined by 1/12 of the mass of a carbon-12 atom.
The number of particles in a mole of a substance.
Law stating that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules.
The number of emissions per second that are detected due to radiation from the environment.
Balmer series
A series of emission or absorption lines in the visible spectrum of hydrogen. Resulting from electrons moving from the first excitation level (n=2) to higher levels.
Ionising radiation in the form of fast-moving electrons.
Binary star system
Two stars revolving around a common centre of mass.
Binding energy
The energy required to separate a nucleus into its constituent protons and neutrons.
Binding energy per nucleon
The average energy required to remove each proton or neutron from a nucleus. The max (and most stable) is Iron (nucleon number 56, binding energy per nucleon 8.7 MeV).
Black body
A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of EM radiation depending only on its temperature.
Black hole
Highly dense matter around which gravity is so strong that the escape velocity exceeds the speed of light.
Blue shift
A decrease in observed wavelength radiation emitted by an object approaching an observer.
Boltzmann constant
The ratio of the molar gas constant to the Avogadro constant.
Boyle’s law
Law stating that for a fixed mass of an ideal gas at constant temperature, the pressure of the gas is inversely proportional to its volume. pV is constant.
Brightness
The amount of electromagnetic energy radiated per second per square metre.
b = L/(4πR^2) where L is luminosity and R is the distance of the object from the observer.
Cassegrain telescope
Type of reflecting telescope in which light rays are reflected by secondary mirror to focus behind the primary mirror.
Charge coupled device (CCD)
An electronic light detector composed of many pixels, each of which builds up charge in proportion to the number of photons detected.
Charles’ law
Law stating that for a fixed mass of an ideal gas at a constant pressure, the volume of the gas is proportional to its absolute temperature. V/T is constant.
Chromatic aberration
An optical defect that causes light of different colours to be focused at different locations.
Continuous spectrum
A spectrum of electromagnetic radiation distributed over a range of wavelengths without any spectral lines.
Control rod
Rod of neutron absorbing material such as boron, which is used to slow the rate of fission in a nuclear reactor.
Isotropic radiation in the microwave region with a black body temperature of 2.7 K, believed to be a remnant of the big bang.
Critical mass
The minimum mass of fissionable material that can sustain a chain reaction.
Decay constant
The probability that a radioactive decay will take place in unit time.
Dioptre
Unit used to measure refractive power of a lens.
Doppler effect
The apparent changing wavelength of radiation due to relative motion of the source and observer.
z = (λapp – λ)/λ = -v/c
Enriched nuclear fuel
Nuclear fuel that has had the proportion of its fissionable isotope artificially increased for example uranium that has had a proportion of U -235 increased
Fission
The decay of some large nuclei by splitting into two smaller nuclei, accompanied by the release of energy.
Fusion
The formation of a larger nucleus by combining two smaller ones, accompanied by a release of energy.
Half life
The time taken for half the nuclei in a sample of a radioisotope to decay, or the time taken for the activity of a radioactive source to drop by half.
Hubble’s law
A law stating that the redshifts of distant galaxies is proportional to their distance from the earth.
Ideal gas
A gas that obeys boyle’s law under all conditions. A gas whose molecules are infinitely small and exert no force on each other except during collisions.
Induced fission
Fission of a nucleus caused by the absorption of an extra neutron.
Ionisation
The process of removing electrons from atoms.
Luminosity
The total energy radiated by a star each second, measured in watts
Mass difference
The difference between the mass of the nucleus and the total mass of its constituent nucleons.
Moderator
Material used in a nuclear reactor to slow down neutrons without absorbing them, for example graphite.
Mole
The amount of substance a system contains in terms of the number of atoms there are in 12 g of carbon-12.
Neutron star
The highly dense remnant of a star after a supernova explosion, composed mainly of neutrons.
Parsec
The distance at which two objects separated by 1 AU make an angle of 1 arcsecond (1/3600 degrees), equal to 3.26 light years or 3.09 x 10^13 km.
Pressure temperature law
Law stating that for a fixed mass of an ideal gas at a constant volume, the pressure of the gas is proportional to its absolute temperature. p/T is constant.
Principal focus
The point at which rays of light which strike a convex lens parallel to the principal axis are brought together.
Pulsar
Rapidly rotating neutron star with strong radio emissions.
Quasar
An astronomical object with a very large red shift and high luminosity, sometimes associated with the radio emission. Thought to be the bright nucleus of a distant active galaxy.
Rayleigh criterion
A way of assessing the resolving power of a telescope, given by θ=λ/d.
Red shift
The increase in wavelength of radiation emitted by an object that is moving away from the observer.
Reflecting telescope
A telescope that uses mirrors to capture and focus the light.
Refracting telescope
The telescope uses lenses to capture and focus the light. An arrangement of an objective lens and an eyepiece lens.
Resolving power
A measure of the ability of a telescope to distinguish between two astronomical features.
The radius of an imaginary sphere from the centre of a black hole at which the escape velocity is equal to the speed of light.
R = 2GM/c^2
Specific heat capacity
The energy required to raise the temperature of a 1 kg mass of a substance by 1 K.
Specific latent heat
The energy required to change the state of a 1 kg mass of a substance with no change in temperature.
Spectral class
The type of star, as classified by its temperature. The sequence of spectral classes is OBAFGKM (Oh Be A Fine Girl Kiss Me).
Spherical aberration
The distortion of an image due to imperfections in the mirror or lens causing differing focal lengths.
The shorter the wavelength, the more precise the mirror has to be.
Standard candle
An astronomical object of known intrinsic brightness, for example a supernova, that is used to determine astronomical distances.
Stefan’s law
Relation that gives the total energy emitted per square metre per second from an object at a given temperature T to be proportional to T^4.
L = σAT^4
Wien’s displacement law
Relationship between temperature of a black body and the peak wavelength. λmaxT = 0.0029 m K
Thermal neutron
A low energy neutron which can be captured by an atomic nucleus to initiate fission. They have lost kinetic energy to heat.
Coolant
This is sent around the reactor to remove heat. It has to be a fluid so it can be pumped around the reactor, and should have a large specific heat capacity. Water or carbon dioxide are often used.
Shielding
The nuclear reactor is surrounded by a thick concrete case, which prevents radiation escaping.
Intensity
The power received from an object per unit area at earth. This is the effective brightness of an object.
I = L/4πd^2
Hertzsprung-Russell diagram
A plot of absolute magnitude of stars against their spectral class (surface temperature).
The vertical axis goes from 15 at the bottom to -10 at the top.
The horizontal axis goes in order of spectral class.
The sun is G2, +4.83.
Cosmological principle
On a large scale the universe is homogeneous (every part is the same as every other part) and isotropic (everything looks the same in every direction).
Categories: Astrophysics