Lights
Light travels at very high but FINITE (limited) speeds. This means that if the distance to an object is great enough, the time taken for light to get there can be measured.
The speed of light is 300,000km/s in a vacuum
In GCSE physics, vast distances in space are measured in LIGHT YEARS. One light year is the year is the distance light travels in one year (approximately 9500 billion km). The finite speed of light means that very distant objects are observed as they were in the past, when the light we now see left them.
E.g. the nearest galaxy to the Milky Way is 2.2 million light-years away. This means that light from this galaxy has taken 2.2 million years to reach the Earth, and so we are now seeing the galaxy as it was in the past.
In GCSE physics curriculum, astronomers work out the distances to different STARS using two different methods:
- Relative brightness: In general, the dimmer a star is, the further away it is. However, stars can vary in brightness so we can never be 100% certain
- Parallax: Parallax is when something appears to move when you look at it from different places (e.g. if you hold your finger at arm’s length and close each eye in turn the finger appears to move. The closer the finger is to your face, the more it appears to move).
As the Earth orbits the Sun, stars in the near distance appear to move against the background of very distant star. The closer they are, the more they appear to move. The position of a star is measured at six-monthly intervals. These measurements can then be used to calculate its distance from Earth.
However, in GCSE physics curriculum, the further away the star is, the more difficult and less accurate the measurement is.
In the last 200 years, it has become very difficult to make astronomical observations in INDUSTRIALISED COUNTRIES such as the UK. This is due to the bright lights found in cities and towns and on roads. This LIGHT POLLUTION means it is hard to see dim stars.
Also the Earth’s ATMOSPHERE absorbs quite a bit of the light coming from space before it can reach us.This is why scientists put the HUBBLE SPACE TELESCOPE in space – where you don’t get these problems.
End of this part! Lets continue in part III !!
Drafted by Gina (Physics)