Cosmology

What is the doppler effect?

The change in the frequency and wavelength of waves received from an object moving relative to an observer compared with what would be observed without relative motion

What is pitch?

The frequency of sound waves

Explain red shift

If the galaxy is moving away from the Earth ('receding') the absorption lines will be red-shifted - they all move towards the red end of the spectrum, because the wavelength appears stretched.

Explain blue shift

If the galaxy is moving towards the Earth the absorption lines will be blue-shifted - they move towards the blue end of the spectrum, because the wavelength appears shorter.

State the Doppler equation and the meaning of each symbol and their units

• λ is the source wavelength and Δλ is the change in wavelength recorded by the observer. Units of both are m (metres)

• f is the source frequency and Δf is the change in frequency recorded by the observer. Unit of both is Hz (Hertz)

• v is the magnitude of the relative velocity between the source and observer, and c is the speed of light through a vacuum (3.00 x 10⁸m s^-1). Unit of both is m s^-1 (metres per second)

The Doppler equation can only be used in which situations?

The Doppler equation can only be used for galaxies with speed far less than the speed of light.

What is the Astronomical Unit (definition, symbol, conversion, when the unit is used)?

• The mean distance from the Earth to the Sun

• Symbol AU

• 1 au = 150 million km = 1.50 x 10¹¹m

• Most often used to express the average distance between the Sun and other planets in the Solar System.

What is the light year (definition, symbol, conversion, when the unit is used)?

• The light-year is the distance travelled by light in a vacuum in a time of one year.

• Symbol ly

• 1 ly = 9.46×10¹⁵m

• The light-year is often used when expressing distances to stars or other galaxies.