# Physics SL – Astrophysics Luminosity/Spectral classes

Luminosity of a star
A representation of its power output i.e. its “true brightness”
Intensity/Flux of a star
How bright it appears to be i.e. its “apparent brightness”
How is the intensity/flux of a star measured?
By ground based telescopes
Distance to a star using luminosity and flux
D = √(L/4πF), where D is distance in m, L is luminosity in W, F is intensity in W/m^2
What are spectral classes used for?
To classify stars
What is the order of the spectral classes in order of decreasing temperature?
O, B, A, F, G, K, M (Oh Be A Fine Girl, Kiss Me)
What do the different classes represent?
Different temperatures and absorption spectra
How are the absorption spectra produced from the star?
White light is emitted from the star. This white light has to pass through the chromosphere of the star, which is above the surface, and this absorbs specific wavelengths of light, thus creating an absorption spectra
How can the temperature of a star be calculated?
Stars act like “black bodies”, therefore “Wien’s Law” can be used. If the color of a star (λmax) can be determined, then temp can be determined. Red stars (long λmax) are COOLER than blue stars (short λmax)
Briefly map out the types of star on the HR diagram
Main sequence stretching from top left corner to bottom right (where most stars lie). White dwarves are bottom left corner (low luminosity, very hot). [Super] Red giants are top right (super ones are above the normal ones) – they have high luminosity but aren’t hot.
What are the scales on the HR diagram?
Logarithmic
Categories: Astrophysics