Astronomical distances

Astronomical distance (AU) is the average distance from the Earth to the Sun.

A light year is the distance light travels in one year. = 9.46 * 1015 m

A parsec = 3.26 light years.

Is the distance a star would need to be from the Sun if it were to have a stellar parallax of exactly one arcsecond (1/3600 of a degree)

(Stellar parallax is the angle made by the apparent movement of a nearby star against the background of more distance stars as the Earth revolves around the Sun. )

Doppler Effect and red shifts

As waves approach you, the frequency increases, and the wavelength decreases.

As waves move away from you, the frequency decreases and the wavelength increases.

For light, this creates red shift/blue shift galaxies

Red shift: Galaxies moving away from the Milky Way emitted light (to us) which had a smaller frequency than expected i.e. more red.

Hubble made two important findings:

  • Almost all galaxies were moving away from ours;
  • More distance galaxies appeared to be moving faster than closer ones.

Cosmic Background Radiation

  • Evidence for the Big Bang
  • Thermal radiation assumed to be left over from the BB
  • This radiation is uniform background radiation, and not associated with any celestial body

Olber’s paradox: infinitely bright sky?

Models of the Universe

Steady state – universe is infinite in extent

Accounts for how galaxies are spread evenly throughout the sky

Does not account for red shifts and cosmic background radiation

Big Bang Theory

  • If the universe is expanding it is reasonable to assume that at some point in the past, all matter was condensed to a point
  • When time, space and matter expanded from this point, this represents the birth of the universe

As the universe expands, gravitational force continually attempts to put it all back together.

Possible scenarios:

  • Closed Universe (the Big Crunch) – gravity wins, pulling everything back into a super massive black hole
  • Universe undergoes cycles of expansion/contraction (Big Bounce)
  • Open Universe (the Big Rip) – universe continues to expand at an accelerated rate, and does so indefinitely. This may lead to all matter being torn apart into subatomic particles, or
  • heat death, where all energy becomes equally distributed and all matter approaches absolute zero.


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