# P7D

GCSE Physics 7.4

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## Lightyears

A light year is the distance light travels in one year.

After the sun the nearest galaxies are about 4 light years away from us. So we see light that left those stars 4 years ago.

Some galaxies are thought to be millions of lightyears away so looking them is like looking back in time.

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## Parallax

As the Earth orbits the Sun, the closest stars appear to change their positions relative to the distant 'fixed stars'

Parallax angle is half the angle moved in 6 months

1 degree of arc = 60 minutes, 1 minute of arc = 60 seconds

A parsec is the distance to a star whose parallax angle is 1 second of arc - same as 3 lightyears

The distance between stars within a galaxy is a few parcsecs.

Distances between galaxies are measured in megaparsecs.

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## Why are some stars brighter than others?

Depends on the star's temperature and surface area - the hotter and larger the star, the more energy radiated per second

Red giants - very large and cool

White dwarfs - dim and small and extremely hot

Obsderved brightness we see from Earth depends on a star's luminosity and distance. Light has been spread over a bigger area.

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## Cepheid variables

Type of star that's brightness varies over a period of a few days.

The time between the peaks of brightness is called the period.

Correlation between luminosity and its period - the greater the luminosity the longer the period.

Scientists work out the distance to all Cepheid variable stars by:

- measuring the period

-measure the observed brightness

- compare brightness with the luminosity to get distance

Distances are too large to measure using parallax. They find a Cepheid variable then use its period and brightness to find the distance.

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## Observing nebulae and galaxies

Nebulae - fuzzy patches of light

Shapley thought that the Milky Way was the centre of the universe and so the nebulae were clouds of gas within the Milky Way

Curtis thought that they were huge, distant clusters of stars- other galaxies outside the Milky Way

Further evidence from Hubble - found a Cepheid variable and measured its distance, it was much further away than any star in the Milky Way - separate galaxy.

Cepheid variable have been used to show that most 'nebulae' are distant galaxies.

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## Redshift and the Hubble law

Redshift - Black absorption lines are all shifted towards the red end of the spectrum

Using Cepheid variable stars to work out distant galaxies that are moving away with greater speed.

speed of recession = Hubble constant x distance

Galaxies are moving away from eachother - the universe is expanding

Supports the big bang theory - that the universe started from one tiny point and has been expanding in size ever since.

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