Luminosity is the rate of light energy released/the poweroutput of a star, and is measured in W.
Intensity is the power received from a star (its luminosity) per unit area, and is measured in W/m^2.
Intensity follows an inverse square law with distance, as shown in the equation I = P/4 pid^2.
Apparent magnitude (m) of an object is how bright the object appears from earth.
The Hipparcos scale classifies objects based on their apparent magnitudes on a logarithmic scale, with the intensity of a magnitude 1 star being 100 times greater than a magnitude 6 star.
The absolute magnitude (M) of an object is what the apparent magnitude would be if it was 10parsecs away from earth.
M = m - 5 log (d/10) where d is the distance in parsecs.
Parallax is the apparent change of position of a nearer star in comparison to distant stars in the background, as a result of the orbit of earth around the sun.
The astronomical unit (AU) is the average distance between the centres of the earth and the sun, and equals 1.50 x10 ^11 metres.
The parsec (pc) is the distance at which the angle of parallax is 1 arcsecond, ie the distance at which 1 AU subtends an angle of 1 arcsecond, and equals 3.08 x10 ^16 metres.
A light year is the distance that electromagnetic waves travel in a year in a vacuum, and equals 9.46 x10 ^15 metres.
For small angles, angle of parallax in arcseconds equals 1/distance in parsecs.
A black body radiator is a perfectemitter and absorber of all possible wavelengths of radiation.
Stars can be approximated as black body radiators.
Stefan's law shows how luminosity relates to temperature and surface area in the equation P = μAT^4, where μ is the stefan constant, which equals 5.67 x10 ^-8.
Wien's displacement law shows how peak wavelength of emitted radiation relate to temperature in the equation λmax T = 2.9 x10 ^-3.
The peak wavelength of a black body decreases as temperature increases, and so frequency and energy of the wave increases.
Stars can be classified into spectral classes based on the strength of absorption lines, which are dependant on the temperature of a star.
Hydrogen balmer lines are absorption lines found in the spectra of some stars, and are caused by the excitation of hydrogen atoms from the n = 2 state.
Star spectral classes are named, from hottest to coldest O (25000 - 50000 K), B (11000 - 25000 k), A (7500 - 11000 K), F (6000 - 7000 K), G (5000-6000 K), K (3500 - 5000 K), M (< 3500 K).
Spectral class order can be remembered by the mnemonic Oh Be A Fine Guy, Kiss Me.
The colour of spectral class O is blue, B is blue, A is blue/white, F is white, G is white/yellow, K is orange, M is red.
The prominent absorption lines in spectral class O is He +, He and H, B is He and H, A is H and ionised metals, F is ionised metals, G is ionised and neutral metals, K is neutral metals, M is neutral atoms and titanium oxide.
The prominence of hydrogen balmer lines in spectral class O is weak, B is moderate, A is strong, F is weak, G is none, K is none, M is none.
The sun is a main sequence star with spectral glass G and absolute magnitude of about 5.
The Hertzsprung-Russell (HR) diagram shows the temperature of a star against its absolute magnitude, and can be used to show the evolutionary sequences of stars.
Groups of different types of star tend to cluster together on a HR diagram, with super giants at the top, giants middle right, white dwarfs bottom left and main sequence as a band between the giants and dwarfs.