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Scientists believe the Universe began in a hot ‘big bang’ about 13,600 million years ago. The Universe continues to expand today. The evidence for the Big Bang theory includes the existence of a microwave background radiation, and red-shift.

Stars do not remain the same, but change as they age. The Universe contains extremely dense objects called black holes, and may consist mostly of dark matter that cannot be seen.

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Stars form from massive clouds of dust and gas in space

Gravity pulls the dust and gas together.

As the gas falls together, it gets hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. Our sun is at this stable phase in its life. Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the staR.

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Stars do not stay the same forever. They have a finite life. What happens to them depends on their mass.

Medium-weight stars

Our Sun is a medium-weight star. Stars like our Sun will change to:

  • a red giant star
  • a planetary nebula, and, finally,
  • a white dwarf star.

Heavy-weight stars

Stars that are much heavier than our Sun have a different fate. A heavy-weight star will still become a red giant. But then:

  • it blows apart in a huge explosion called a supernova.
  • the central part left behind forms a neutron star, or even a black hole, if it is heavy enough.
  • black holes have a large mass, and a large gravity. Even light cannot escape them because their gravitational field is so strong.
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Scientists have gathered a lot of evidence and information about the Universe. They have used their observations to develop a theory called the Big Bang. The theory states that about 13.6 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today.

Evidence for the Big Bang includes:

  • All the galaxies are moving away from us.
  • The further away a galaxy is, the faster it is moving away.

These two features are found in explosions - the fastest moving objects end up furthest away from the explosion.

Scientists have also detected a microwave background radiation. This is received from all parts of the Universe and is thought to be the heat left over from the original explosion.

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You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away. This effect, where there is a change in frequency and wavelength, is called the Doppler effect. It happens with any wave source that moves relative to an observer.

This happens with light, too, and is called 'red shift'. Our Sun contains helium. We know this because there are black lines in the spectrum of the light from the Sun where helium has absorbed light. These lines form the absorption spectrum for helium.

This is calledred shift. It is a change in frequency of the position of the lines.

Astronomers have found that the further from us a star is, the more its light is red-shifted. This tells us that distant galaxies are moving away from us, and that the further away a galaxy is, the faster it is moving away.

Since we cannot assume that we have a special place in the Universe, this is evidence for a generally expanding universe. It suggests that everything is moving away from everything else. The Big Bang theory says this expansion started 13.6 billion years ago with an explosion.

Interpreting the evidence

Summary of some of the evidence of the Big Bang and its interpretation

EvidenceInterpretation The light from other galaxies is red-shifted. The other galaxies are moving away from us. The further away the galaxy, the more its light is red-shifted. The most likely explanation is that the whole Universe is expanding. This supports the theory that the start of the Universe could have began with a single explosion.

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