# Option E Astrophysics definitions

Light year
The distance light travels in a year
luminosity of a star
The power radiated by the star. (energy per unit time)
Apparent brightness
The luminosity per unit area. (the energy per unit time per unit area) b=L/(4πd² ) Wm⁻² (spectroscopic parallax) as seen by an observer on earth
What is the value of 1 astronomical unit? (Distance between the Earth and the Sun)
1.5*10^11
Stefan-Boltzmann law
If we regard stars to be black body radiation, then the luminosity L of a star is given by L=4π(R^2)(σ)(T^4)
R is radius of the star, T is surface temperature, σ is S-B constant
Wein’s displacement law
Relationship between the maximum value of wavelength emitted by a black body and its temperature.
Cepheid variable
A star whose luminosity varies regularly and can be determined from the time period
Stellar cluster
Group of stars that are physically close together rather than just looking as they are, formed by the collapse of a gas cloud
Constellation
Stars that look like they are close together but do not have anything related physically except that they are all bright
Asteroid
A relatively small rocky body between the orbits of Mars and Jupiter
Parsec
1parsec= 3.26 ly
distance at which 1AU subtends an angle of 1 arc sec
p=distance of earth from sun/distance of star from sun
Absolute magnitude
Magnitude of a star viewed from a distance of 10pc
m-M=5 log⁡(d/10)
Comet
A small orbiting body with loose particles of ice and rock that are blown off by the solar wind forming a tail
Categories: Astrophysics