Chapter 20: Cosmology

Astronomical Unit (AU)

Average distance from Earth to Sun

• 1.5 × 10¹¹ m

Light year (ly)

Distance travelled by light in a vacuum in one year

Arcseconds and arcminutes

60 arcminutes = 1 degree

60 arcseconds = 1 arcminute

1 arcsecond = 1 / 3600 degree

Parsec (pc)

The distance at which a radius of 1 AU subtends an angle of 1 arcsecond

tan (1 arcsecond) = 1AU / 1pc

= 3.1 × 10¹⁶ m = 3.26 ly

Stellar parallax

Used to determine the distance to stars that are relatively close to the Earth

• Apparent shift in the position of a relatively close star against the backdrop of much more distant stars

• distance between Sun and Star = 1 / p

  • p = parallax angle (in arcseconds)

Doppler shift

The change in frequency of a wave that is perceived by an observer when the source of the wave is moving relative to the observer

Doppler Shifts in starlight

If a galaxy is moving:

• Towards the Earth: absorption lines will be blue-shifted

  • Move towards blue end of spectrum, wavelengths appear shorter

• Away from the Earth: absorption lines will be re-shifted

  • Move towards red end of spectrum, wavelengths appear longer