P1.1 What do we know about the place of the Earth in the Universe?P1.1 What do we know about the place of the Earth in the Universe?

P1.1 What do we know about the place of the Earth in the Universe?

The Earth is one of eight planets orbiting the sun in almost circular paths, together with other smaller objects including asteroids, dwarf planets, comets and moons. The solar system is made.

The smaller masses in our solar system are:

  • Planets – eight large masses that orbit (move around the sun)
  • Moons – small masses that orbit the planets
  • Asteroids – small, rocky masses that orbit the Sun
  • Comets – small, icy masses that orbit the sun
  • Dwarf planets – small spherical objects that have not cleared their orbits of other objects

The solar system was formed over a very long period of time, about 5000 million years ago:

❶ The solar system started as clouds of DUST and GAS, which were pulled together by the force of gravity

❷ This created intense heat. Eventually, NUCLEAR FUSION began to take place and a star was born: the Sun

❸ The remaining dust and gas formed smaller masses, which were attracted to the sun

Our sun is one of thousands of millions of stars which form the Milky Way. There are thousands of millions of galaxies and every galaxy is made up of thousands of millions of stars – all these galaxies make up the UNIVERSE.

  • Diameter of the Earth
  • Diameter of the Sun
  • Diameter of the Earth’s orbit
  • Diameter of the Solar system
  • Distance from the Sun to the nearest star
  • Diameter of the Milky way
  • Distance from the Milky way to the nearest galaxy

Because stars are so far away, all the evidence we have about distant stars and galaxies come from the RADIATION astronomers can detect. A lot is shown when studying the ELECTROMAGNETIC RADIATION it emits – e.g. the colour the star appears shows its surface temperature.

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.

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.

Astronomers work out the distances to different STARS using two different methods:

❶ 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 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 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.

The sun’s energy (heat and light) comes from NUCLEAR FUSION. Hydrogen nuclei fuse (join) together to produce a nucleus with a larger mass, i.e. a new chemical element.

Hydrogen nucleus + Hydrogen nucleus → Helium nuclei

During fusion, some of the energy trapped inside the hydrogen nuclei is released. All the chemical elements with a larger mass than helium were formed by nuclear fusion in earlier stars.

When a galaxy is moving away from us the WAVELENGTH of the light from it changes – the light becomes redder. This is called RED SHIFT:


By seeing how much the light has been red-shifted, you can work out how quickly it’s moving away. The greater the red-shift the faster it’s moving away. From the observations of different red-shifts of different galaxies we know that:

The more DISTANT the galaxy, the FASTER it moves away from us

This provides evidence that the whole universe is EXPANDING.

Different stuff in the universe Age (million years)
 Earth 5000
Sun 5000
Universe 14 000


The Earth and the sun are a similar age. But the universe is about 3x older.

When Scientists trace the paths of galaxies, they all appear to be moving away from the same point. The best theory for this is the BIG BANG THEORY, which says that the universe started with a huge explosion 14,000 million years ago.

The ultimate fate of the Universe this is because the future depends on the amount of MASS in the Universe.

If there is NOT ENOUGH MASS, the universe will keep expanding

If there is TOO MUCH MASS, gravity will be strong enough to pull everything back together and the universe will collapse with a big crunch.

Measuring the amount of mass in the Universe is very difficult, so it’s ultimate fate.