A2 Unit 25 Astronomy & Cosmology

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Define luminosity

The total power output of radiation emitted by a star

Define radiant flux intensity

The observed amount of intensity, or the radiant power transmitted normally through a surface per unit of area, of radiation measured on earth

Give the equation to calculate the radiant flux intensity in terms of luminosity

F = L/4πd²

Define standard candle

An astronomical object which has a known luminosity due to a characteristic quality by that class of object

Give 2 examples of a standard candle

Cephid variable stars

Type 1a supernovae

What is a black body?

Object that absorbs all the radiation that falls on it, and is also a good emitter. Does not reflect or transmit radiation.

What is Wein’s displacement law?

The black body radiation curve for different temperatures peaks at a wavelength which is inversely proportional to the temperature

Give the equation for Wein’s displacement law

λ_max ∝ 1/T

What does the colour of the star tell us about the temperature?

Shorter wavelengths (bluer) means a higher temperature and vice versa

Give the Stefan-Boltzmann law

The total energy emitted by a black body per unit area per second, which is proportional to the fourth power of the absolute temperature of the body

What is redshift?

The fractional increase in wavelength (or decrease in frequency) due to the source and observer receding from each other

Why does redshift explain the universe is expanding?

Light from distant galaxies displays redshift. This means that the galaxies are moving away from us. This leads to the idea that the space between the galaxies must be expanding.

What is Hubble’s Law?

The recession speed of galaxies moving away from Earth is proportional to their distance from the Earth

What is the equation for Hubble’s Law?

v = H₀d

What is the gradient of a graph of recession velocity against distance?

Hubble’s constant

How did Hubble’s law lead to the Big Bang theory?

If galaxies are moving away from each other, they must have started from the same point at sometime in the past. Data can be extrapolated back to the beginning of the universe.

T = 1/H₀