Hubble’s Law and measuring distance
Standard methods, such as using cepheid variables, were originally used to measure distances. The graph below, plotting the velocity against distance from Earth, is a straight line, showing that the two are directly proportional. The gradient of the graph is constant and that value is called Ho - Hubble’s constant, so:
The key observation is that the faster the galaxy is moving away the further it is from us. The red shift was used to calculate velocity.
The current best estimate of Hubble's constant is about:
Ho = 71 kmS-1Mpc -1
Where the velocity away from us is measured in kilometres per second and the distance of the galaxy is measured in megaparsecs. This is the value suggested as a good reflection of recent measurements taking into account their reliability.
The best explanation of “red shift” in the spectra of distant galaxies is that:
★The universe is expanding and has done so since the big bang, this is just like the expansion of a balloon being blown up but imagine that the galaxies are spread through the inside of the balloon, not just on the surface.
★As it expands the distances between the galaxies increases.
★Because of gravitational attraction holds them together, individual galaxies do not expand.
★The radiation traveling between the galaxies stretches and the further it has to travel the longer it takes so the more it stretches.
★The change in the wavelength is directly proportional to the distance the radiation travels, that is, the distance away from us of the galaxy we are looking at.
Other pages of notes and video on astronomy which may be useful are:
Units of distance notes and video Measuring distance by parallax/triangulation notes and video Life cycle of stars Geostationary and polar satellites notes and video Big Bang theory and evidence Development of the Universe after the Big Bang Real and apparent magnitude The age of the universe notes and video Using Hertzsprung Russell diagrams notes and video Cepheid variable stars Type 1A supernova