Topic 8 Space Physics

Our solar system is part of the milky way galaxy. This is a massive collection of billions of stars that are all held together by gravity.

Planets - they are large objects that orbit a star. There are 8 planets in our solar system. The different planets have different properties and conditions. In general, as the distance from the Sun increases. 

For a planet to form its own gravity must be strong enough to make it round or spherical in shape. Its gravitational field must also be strong enough to ‘clear the neighbourhood’, pulling smaller nearby objects into its orbit.

Dwarf planets - Pluto is a dwarf planet. These are planet-like objects that orbit stars but don't meet all the rules for being a planet. 

Moons - They orbit planets. They are a type of natural satellite. Many planets have moons, and some planets have many moons.

Satellite - Body that orbits a planet. For example, the Moon is a natural satellite of the Earth but communication satellites are artificial satellites of the Earth.

Artificial satellite - They are satellites that humans have built. 

Asteroid - A rock in space. Asteroids orbit the Sun but some may cross the Earth's orbit, producing a risk of collision.

Comets - A ball of icy rock that follows an elliptical orbit around the Sun. As a comet approaches the Sun, it begins to vaporise which means that it turns into a gas. It then produces a distinctive tail. 

The Earth's orbit is virtually circular.

The force of gravity acting between the Sun and the Earth holds the Earth in its orbit. 

In circular orbits, the force of gravity leads to a change in velocity. The velocity is constantly changing. The speed is constant. The force of gravity is changing the velocity but not the speed. 

If the speed of the satellite changes then the radius of the orbit must also change. If the speed of the satellite increases then the radius of its orbit decreases. 

This is because at a higher speed the satellite needs a greater force of gravity to prevent a satellite from flying off into space.

By moving the satellite closer to Earth the gravitational force on the satellite is greater and the satellite remains in a stable orbit. 

Stars begin life as a cloud of dust and gas called a nebula.

Gravity causes the cloud of dust and gas to collapse. As the temperature rises to millions of degrees. This early stage is called a protostar.

If the temperature gets high enough then hydrogen nuclei join together to form helium nuclei. This process is called nuclear fusion. At this point, we have a main sequence star. Stars can stay in the main sequence star for a long time.

What happens next depends on the size of the star.

Stars the same size as the sun:

The hydrogen in the star begins to run out. At this point, the outward force due to fusion energy is less than the inward force due to gravity. 

This causes the star to collapse inwards. The collapse of the star causes its temperature to increase. Now helium nuclei fuse together to create heavier elements. The star expands to form a red giant. The red giant stops fusing helium.

At this stage, the star shrinks and forms a white dwarf. The white dwarf is no longer carrying out fusion it cools down. 

Eventually, it stops releasing any energy and it forms a black dwarf. 

Stars that are bigger than the sun:

Once these stars run out of hydrogen they leave the main sequence star stage and expand into a red super giant. Helium nuclei fuse together to produce heavier elements. Nuclear fusion cannot make any element heavier than iron. 

The red super giant stops carrying out nuclear fusion. At this stage the star explodes and this is called a supernova. The temperature of a supernova is high enough to produce elements heavier than iron. 

When the supernova explodes these elements are distributed throughout the universe. 

After the supernova the remains of the star can form one of two objects:

Neutron star is neutrons densely packed together 

OR

Black hole it has such a large gravity that not even light can escape. 

According to the big bang theory, about 13.8 billion years ago the whole Universe was a very small, extremely hot and dense region. From this tiny point, the whole Universe expanded outwards to what exists today.

When we look at wavelengths from different galaxies we find that wavelength has increased. 

The wavelengths are longer than they should be. They have shifted to the red end of the spectrum.

The red shift tells us that the galaxies are moving away from each other. Because the galaxies are moving away the light waves are stretched. 

Galaxies that further away have a bigger red shift so these galaxies are moving faster than the galaxies closer. 

Nearby galaxies have a small red shift showing us that they are moving away.

More distant galaxies have a larger redshift. They are moving away faster

Very distant galaxies have a large red shift. They are moving away very fast.

The fact that distant galaxies are moving faster than nearby galaxies provides evidence that the universe is expanding. 

Scientists have suggested that the universe contains matter and energy that we cannot detect. These are called dark matter and dark energy. It could be possible that these could explain why the expansion of the universe is speeding up.