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.
As the Earth orbits the sun the nearest stars appear to change their positions relative to the very distant fixed stars. This effect is called Parallax.
The stars havn't actually moved, the Earth from which we are obseving from has moved.
The Parallax angle is half the angle apparently moved in 6 months.
- Parallax angles are very small, measured in seconds of arc ( 1 degree of arc = 60 seconds, 1 minute of arc = 60 seconds )
distance to star ( parsecs ) = 1 / parallax angle ( seconds of arc )
A parsec ( pc ) is the distance to a star whose parallax angle is 1 second of arc.
- The distance stars within galaxies are usually a few parsecs.
- The distance between galaxies are measured in megaparsecs ( 1 mpc = 1,000,000 pc )
Why are some stars brighter than others?
The Luminosity of a star describes the total amount of rdiation it emits every second. It depends on the stars temperature and surface area. - the hotter and larger the star, the more energy radiated per second.
- Red giants are bright because they are very large, even though they are relatively cool.
- White dwarfs are quite dim because they are so small even through they are very hot.
The Observed brightness we see from Earth not only depends on the stars lumonosity but its distance from Earth.
Scientists can calculate a stars distance knowing its observed brightness and its luminosity.
Cepheid variables are a type of star whose brightness varies in a regular way over a period of a few days ( due to changes in the stars size ).
The time between peaks of brightness is called the period.
Working out distances using Cepheid variables.
Scientists discovered that there is a correlation between the luminosity of a cephied variable and its period.- the greater the luminosity the longer the period.
Scientists can work out the distance to Cepheid variables like this:
- Measure the period.
- Use the period to work out the Luminosity.
- Measure the observed brightness.
- Compare the observed brightness with the Luminosity to get the distance.
Distance to many galaxies are too far to measure using parallax. Instead astronomers use Cepheid variables within that galaxy and use its period and brightness to find the distance.
Observing Nebulae and Galaxies.
At first scienists were puzzled by some fuzzy patches of light- they called them Nebulae.
- Shapley believed the milky way was the entire universe and that the nebulae were clouds of gas within our milky way.
- Curtis believed that they were huge distant clusters of stars outside the milky way.
- Hubble found further evidence from a Cepheid variable and measured its distance to find that it was too far away to be in ou milky way and had to be another seperate galaxy.
Cephied variables have been used to show that most 'nebulae' are in fact distant galaxies.
Redshift and Hubble law.
When astronomers look at the absorbtion spectra from distant galaxies they find that the black absobtion lines are all shfted towards the left - this is called Redshift.
The speed of recession of a galaxy can be found from its redshift.
By measuring the distance to these galaxies using Cepheid variables, Hubble found that more distant galaxies are moving away with a greater speed.
speed of recession = Hubble constant X Distance
Hubbles results can be explained if all the galaxies are moving away from each other - in other words, the universe is expanding.
This law gave support to the Big bang theory - That the universe started from one tiny point and has been expanding ever since.