Classification

• Luminosity is the rate of light energy released/the power output 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 pi d^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 10 parsecs 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 perfect emitter 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 = $\mu$ A T^4, where $\mu$ 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 $\lambda_{max}$ 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.