7.3 - Red Shift

Red-shift in light from distant galaxies

• Every element absorbs its own set of wavelengths of light, producing a fixed pattern of dark lines across a spectrum.
• In light from distant galaxies, these familiar line patterns are shifted towards the red end of the spectrum (longer wavelengths) compared to laboratory measurements.
• This increase in the observed wavelength of light from a galaxy is called red-shift.

What red-shift tells us

• A longer observed wavelength means the source of the light is moving away from us.
• Red-shift is seen in light from galaxies in every direction, showing that distant galaxies are receding from us.
• Red-shift from more distant galaxies is greater than from nearer ones, meaning the more distant a galaxy, the faster it's moving away.

Red-shift is evidence the universe is expanding

• Distant galaxies are receding in every direction, and the further away a galaxy is, the faster it's moving away.
• This is evidence that the whole universe is expanding.

Space itself is stretching

• The expansion happens because space itself is stretching, rather than galaxies flying through fixed space.
• As light travels across expanding space, it's stretched to a longer wavelength.
• Light that has travelled further has crossed more expanding space, which is why more distant galaxies show a greater red-shift.

The balloon model

• As a balloon with spots drawn on it is inflated, every spot moves away from every other spot, and spots that start further apart separate the fastest.
• The surface represents space and each spot represents a galaxy.
• This expansion supports the idea that the universe began from a single hot, dense point.

Identifying elements from a star's spectrum

1. A star emits a continuous spectrum of radiation.
2. As this light passes through the cooler gases of the star's atmosphere, atoms in those gases absorb specific wavelengths.
3. Each absorbed wavelength leaves a dark line in the spectrum, forming an absorption line spectrum.

Matching lines to elements

• Every element absorbs its own unique set of wavelengths, producing a characteristic pattern of lines.
• Comparing the dark lines in a star's spectrum with line patterns measured in the laboratory identifies which elements are present in the star's atmosphere.