Astrophysics & Cosmology
Definitions
➢ Luminosity: total power output by radiation of a star
➢ Radiant flux intensity: The observed amount of intensity transmitted normally
through a surface per unit of area, of radiation measured on Earth
➢ Standard candle: An astronomical object which has a known luminosity due to a
characteristic quality possessed by that class of object
➢ Wien’s displacement law: The black body radiation curve for different temperatures
peaks at a wavelength which is inversely proportional to the temperature
➢ Black body: absorbs all light falling upon it and is a good emitter of radiation
➢ Stefan-Boltzmann Law: The total energy emitted by a black body per unit area per
second is proportional to the fourth power of the absolute temperature of the body
➢ Doppler effect: The apparent change in wavelength or frequency of the radiation
from a source due to its relative motion away/toward the observer
➢ Redshift: The fractional increase in wavelength (or decrease in frequency) due to the
source and observer receding from each other
➢ Hubble’s Law: The recession speed of galaxies moving away from Earth is
proportional to their distance from the Earth
Equations
❖ 𝐹=
𝐿
4𝜋𝑑2
➢ F = radiant flux intensity
➢ L = luminosity
➢ d = distance b/w bodies
❖ 𝜆𝑚𝑎𝑥 𝑇 = 2.9 × 10−3
➢ 𝜆𝑚𝑎𝑥 = maximum wavelength emitted by the star at the peak intensity
➢ T = temperature
❖ 𝐿 = 4𝜋𝑟 2 𝜎𝑇 4
L = luminosity of the star (W)
r = radius of the star (m)
σ = the Stefan-Boltzmann constant (5.67 × 10-8 kg s-3 K-4)
T = surface temperature of the star (K)
➢
➢
➢
➢
❖
∆𝜆
𝜆
=
∆𝑓
𝑓
𝑣
=𝑐
❖ 𝑣 = 𝐻0 𝑑
➢ v = recessional velocity
➢ H0 = Hubble's constant / the rate of expansion of the universe
➢ d = distance b/w body and Earth
1
❖ 𝑇0 = 𝐻
0
➢ H0 = Hubble's constant
➢ T0 = age of universe