Stars SA - CCEA (Higher tier only)

• How stars form.

• The structure of galaxies.

• Measuring huge distances in space.

• The Big Bang and "steady state" models of the Universe

Our Sun was formed around 4.6 billion years ago from a giant cloud called a stellar nebula, mainly made up of hydrogen gas and dust.

The nebula collapsed under its own gravity and, as it did, temperature and pressure increased.

It became denser as gravity pulled the particles of the cloud closer together and rotated more rapidly, spiralling inwards.

The hot core in the centre is called a protostar.

The collapsing and joining together of gas and dust under gravity is called accretion.

Eventually gravity compressed the hydrogen so much that the temperature reached about 15 million °C.

At this temperature and pressure nuclear fusion began and our Sun was born.

Nuclear fusion is the energy source of our Sun and all the stars.

As a result, heat and light energy radiate outwards, and it can be seen shining brightly in the sky.

In fusion reactions:

• Hydrogen nuclei fuse together to form helium nuclei.

• Energy is released and radiates outwards.

Galaxies are huge collections of stars like our Sun.

Our Sun belongs to a spiral galaxy called the Milky Way.

Astronomers estimate that the Milky Way is a collection of about 100 thousand million stars.

That’s 100,000,000,000 stars.

Our Solar System lies in one of the Milky Way’s spiral arms, nearly two thirds of the way from the centre.

Part of the Milky Way is visible on a clear night from Earth, as a thick band of stars stretching across the sky.

The distances between objects in space are huge:

• The distance from one star to another in a galaxy is millions of times more than the distance between the planets in the Solar System.

• The distance from one galaxy to another is millions of times more than the distance between the stars in a galaxy.

This means that the numbers used to describe distances in space become very difficult to understand and to write down.

For example, the distance between the Earth and the Sun is about 150,000,000,000 m but the distance to the next nearest star (Proxima Centauri) is 39,900,000,000,000,000 m.

To get around this problem, scientists use the light year as the unit of astronomical distance.

A light year is defined as the distance light travels in a year.

So, for example:

It takes light from our Sun about 8 minutes to reach the Earth.

Sun to Proxima Centauri distance is about 4.24 light years.

Sun to the centre of the Milky Way is about 27,000 light years.

The Milky Way is about 100,000 light years across.

Milky Way to Andromeda (the next nearest spiral galaxy) distance is about 2.5 million light years.

How far is one light year? (Higher tier only)

Remember that one light year is the distance light travels in one year.

Distance = speed x time.

Speed of light = 300,000,000 m/s.

Speed of light = 3.0 x 10⁸ m/s.

Time = 1 year.

Time = 365 x 24 x 60 x 60.

Time = 31,536,000 s.

Distance = 3.0 x10⁸ x 31,536,000.

Distance = 9.46 x 10¹⁵ m.

1 light year = 9.46 x 10¹⁵ m.

One light year is a distance of 9.46 x 10¹⁵ m.

Summary

• Distances to stars and galaxies are so enormous that they are measured in light years.

• A light year is the distance light travels in one year.

• A light year is a distance not a time.

• One light year is a distance of 9.46 x 10¹⁵ m

Example

Alpha Centauri is 4.4 light years away from us.

How far is that in metres?

Answer

One light year = 9.46 x 10¹⁵ m.

4.4 light years = 4.4 x 9.46 x 10¹⁵.

4.4 light years = 4.2 x 10¹⁶ m.

Alpha Centauri is 4.2 x 10¹⁶ m away from us.

Scientists have gathered a lot of evidence and information about the Universe.

They have used their observations to develop a model called the Big Bang to help explain its formation and evolution.

The theory states that about 13.8 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point.

From this tiny point, the whole Universe expanded outwards to what exists today.

The Big Bang

• The Universe is thought to have originated 13.8 billion years ago from a very small, extremely hot and dense region called a singularity. The Big Bang was a massive expansion that blew space up like a gigantic balloon.

• Initially, the Universe expanded rapidly. Rapid expansion is always accompanied by cooling, so, as the Universe got bigger, it cooled down. This enabled protons and neutrons to form.

• Further expansion and cooling allowed protons and neutrons to combine to form nuclei.

• The Universe continued to expand, although now more slowly. As it cooled even more, electrons combined with neutrons and protons to form atoms of hydrogen.

• The force of gravity acted on the atoms of hydrogen, pulling them together into bigger and bigger clumps.

• Some 400,000 years later the first stars formed from the clumps of hydrogen and begin to light-up the Universe.

What evidence is there for the Big Bang?

Evidence for the Big Bang includes:

• All other 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 cosmic microwave background radiation or CMBR.

This is received from all parts of the Universe and is thought to be the heat left over from the original explosion.

Key points

• During the Big Bang, the Universe expanded and cooled.

• Galaxies are moving away from each other.

• The further away the galaxies are, the faster they are moving apart.

• Space is expanding.

• The Universe is expanding.

You may have noticed that when an ambulance, fire engine or police car goes past, its siren is high-pitched as it comes towards you, and 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 towards or away from an observer.

If the object is moving towards an observer, the waves are shifted to a higher frequency, shorter wavelength.

If the object is moving away from an observer, the waves are shifted to a lower frequency, longer wavelength.

This happens with light as well as sound.

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.

When we look at the spectrum of a star in another galaxy, the absorption spectrum is there, but the pattern of lines has moved, or ‘shifted’, towards the lower frequency, longer wavelength, red end of the spectrum, as you can see below:

This is called redshift.

Light from a star in a distant galaxy is shifted towards the long wavelength, red end of the spectrum.

This tells us that:

• Other distant galaxies are moving away from our galaxy.

• The Universe is expanding.

Astronomers have found that the further from us a star is, the more its light is redshifted.

This tells us 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, it suggests that everything is moving away from everything else as you would expect with an explosion.

Summary

• There is a redshift in light observed from most distant galaxies.

• Redshift shows that a galaxy is moving away from us.

• The further away the galaxies are, the bigger the redshift.

• The further away a galaxy is, the faster it is moving away.

• Redshift gives us evidence that the universe is expanding.

Further evidence for the Big Bang comes from the discovery of cosmic microwave background radiation (CMBR).

Astronomers discovered cosmic microwave background radiation in the 1960s.

The wavelength of this radiation appears to be the same no matter where in space it comes from and corresponds to a temperature of about –270 °C.

It comes from all directions and from all parts of space.

Scientists believe that the continuous microwave background radiation is the remains of the heat energy from the Big Bang, spread thinly across the whole Universe.

The Big Bang is currently the only model that explains the existence of CMBR.

Another theory about the Universe, called the steady state theory, says that the Universe has always existed, and that the Universe is expanding and constantly creating matter as it expands.

This idea is supported by the redshift evidence, but not by CMBR.

After the discovery of CMBR, the steady state theory lost support and is no longer considered likely.