Topic 7

 

What do we know of the origins of the universe and how it continues to change?

 

In the exam you need to know how:

  • to compare and contrast the particular advantages and disadvantages of using different types of telescope on Earth and in space to make observations on and deductions about the universe.

You need to know about:

  • If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency.

  • There is a red-shift in light observed from most distant galaxies. The further away galaxies are the bigger the red-shift.

  • How the observed red-shift provides evidence that the universe is expanding and supports the ‘big bang’ theory (that the universe began from a very small initial point).

  • Observations of the solar system and the galaxies in the universe can be carried out on the Earth or from space.

  • Observations are made with telescopes that may detect visible light or other electromagnetic radiations such as radio waves or X-rays.

 

Key Words

Telescope

Doppler Effect

Red Shift

Big Bang

 

Exploring the Universe

The exploration of space is called astronomy. 

 

Astronomy has NOTHING to do with astrology which deals with horoscopes (the daily drivel you get in some popular newspapers telling you that Venus will make it a good day for financial speculation).

 

Space is big.  Although we have sent men to the Moon and probes to the planets of the solar system, the distances are so massive that the probes have hardly got anywhere!  So most of our observations in space have come from using telescopes of different kinds.

 

The simplest telescope is the refracting or lens telescope.  Here is a picture of one.  You will learn more about how lenses work later on.

 

 

The lens telescope has the advantage that it is quite inexpensive to buy, and many beginners use it.  Its disadvantage is the lenses can cause optical distortion.  Also the magnification is limited.

 

More advanced amateurs and professional astronomers use a reflecting telescope, either a Newtonian or a Cassegrain instrument.

 

Here is a picture of a Newtonian telescope and how it works:

 

   

 

 

The Cassegrain instrument is similar, except that the eyepiece is at the end, rather than the side.

 

  

 

 

 

Both are more expensive than the lens telescope.  However they have a bigger aperture (width of tube) which means they have a better resolution.  This means that the astronomer can see finer detail.  There is a maximum size above which lenses become impractical.  Since these instruments only have a lens in the eyepiece, the aperture can be very big, 3 to 4 metres.  The optical quality is determined by the curved mirror at the back of the telescope.  This is made very accurately.

 

Observing the universe from the ground has problems:

Some big telescopes are built high up on mountain tops.

 

An alternative is to fly your telescope in an aeroplane (a flying observatory) as shown in this picture:

 

 

You can see the big hole in the side of the plane where the telescope is.  In a plane the astronomers are far higher than on a mountain top and the atmosphere is thinner.  Therefore the pictures should be better.  However running a plane like this is very expensive and the quality of the images depends on the flying conditions and the skill of the pilot.

 

The best images come from space telescopes which orbit the Earth.  The pictures are transmitted back to the ground by microwave links, and are enhanced by computers.  The Hubble Space Telescope has produced exceptional images of superb clarity of some of the most distant objects in the Universe.

 

 

 

Now answer Question 1

 

 

Other Kinds of Telescope

All these telescopes rely on visible light.  However astronomers use radiation from other parts of the electromagnetic spectrum. 

 

Radio telescopes pick up radio frequency radiation.  They have large dishes that focus radio waves onto a receiving antenna (aerial).  Electronic equipment helps the astronomers to interpret the signals.

 

 

 

Other telescopes pick up:

Obviously we can't see these radiations so astronomers rely on computers to generate the images.

 

Here is a picture of the same piece of the Universe seen through different areas of the EM spectrum.

 

 

 

Now answer Question 2

 

 

Doppler Shift

You may well have been on a street when an ambulance has gone past with its siren blaring.  The note of the siren has a high pitch as it approaches, and the pitch goes down as it moves away.  This change in frequency is called the Doppler Effect.  The picture shows the idea with a train.

 

 

The Doppler effect also works with light as we will see next.

 

About a hundred years ago, astronomers observed an interesting result.  If you analyse elements with an instrument called a spectroscope, you find that each element has a characteristic pattern of coloured lines.  If you put the coloured lines against a spectrum of visible light, the coloured lines show up as black lines, but the pattern is the same.  Astronomers found this to be a good way of working out what chemicals there were in stars.

 

They found that when they analysed elements in distant stars, the patterns were there, but moved over towards the red end of the spectrum (longer wavelength). 

 

 

You can see how the pattern is the same, but the colours of the lines have changed.  For example the blue line has become green; its wavelength has increased.

 

This could be explained by the Doppler Effect.  The longer wavelength (lower frequency) is called Red-Shift, and shows that the star is moving away from us.

 

The further the star is away from us, the more the red-shift, which means the faster the star is travelling away from us.

 

Question 3  What would you see if a star or galaxy was coming towards us?  ANSWER

 

 

How the Universe Began

Most astronomers believe that the Universe started from a single tiny point in a titanic explosion called the Big Bang.  (This term was first used in a radio broadcast about fifty years ago by the then Astronomer Royal, Sir Fred Hoyle, who didn't believe a word of it and was pretty sarcastic about the theory.) 

 

The Big Bang is thought to have occurred about 18 000 million years ago.

 

What is the evidence for this?

 

Material was thrown out as the Universe expanded, and in places came together under the influence of gravity to form galaxies, stars, and planets.  It is a mind-boggling thought that the light reaching us from the most distant galaxies left those galaxies not long after the big bang.  There are many thousands of millions of stars.  Latest evidence suggests that many have planets.  Who knows; there may be life.

 

Many astronomers believe in other theories, e.g. the Steady State theory.

 

For more about the Big Bang look at:

http://liftoff.msfc.nasa.gov/academy/universe/b_bang.html

 

Summary

  • Telescopes are used to observe the universe;

  • Satellites and probes are also used;

  • Ground based telescopes are limited in use;

  • Space telescopes give very clear images;

  • Telescopes can be made to use any part of the electromagnetic spectrum;

  • Most astronomers believe that the Universe was formed by the Big Bang.

 

 

Now try the Topic Quiz

 

 

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