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Our solar system for AQA GCSE Physics

Our solar system

This page covers the following topics:

1. The solar system
2. The planets
3. Stars and fusion
4. The lifecycle of a star
5. Orbital motion

A galaxy is a collection of miliions of stars held together by the force of gravity. The galaxy that our planet and the Sun are in is called the Milky Way. The largest object in our Solar System is the Sun. The Sun is a huge star and its gravitational field causes many other objects in the solar system to orbit around it.

Our Solar System has both artificial and natural satellites. These are objects which orbit a planet in the Solar System. Moons are natural satellites of planets. Different planets have different numbers of moons. Artifical satellites are used to obtain information about the planets they are orbitting.

The solar system

Our solar system has 8 planets, each of which orbits the Sun. All of these planets orbit their own axis as well, except for Venus and Uranus. They are ordered in the following way, starting from the closest to the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The planets

A nebula is a cloud of dust and gas mostly made up of hydrogen and it is the first step of formation of any star. The gas and dust in a nebula are pulled together due to gravity and the temperature increases, forming a protostar. When it reaches a high enough temperature, the hydrogen nuclei fuse and release energy. The fusion reaction produces helium, and a main sequence star is formed, whose core is hot due to the energy released. This is the most stable phase in the lifecycle of a star, as the force of gravity is balanced by the high pressure in the star due to the high temperatures. The Sun is currently a main sequence star.

Stars and fusion

There are two paths which the life cycle of a star could follow. Which one of the two it follows depends on the size of the star. A main sequence star is constantly fusing hydrogen. When this process stops due to hydrogen running out, larger nuclei will form and the star will expand and turn into a red giant. At this stage, when all nuclear reactions cease, stars that are approximately equal in size to the Sun will be influenced by the gravitational pull on them. The red giant will contract into a white dwarf. Over time, the white dwarf will cool down further and change colour. More massive stars will continue to have nuclear reactions and thus constantly expanding and increasing their temperatures. When the temperature becomes high enough, the red giant will explode into a supernova and throw gas into space. The initial mass of the star will dictate what happens next. The less massive stars will become neutron stars, which is a tiny region of closely packed neutrons. More massive stars will become black holes.

The lifecycle of a star

To maintain a stable orbit, the planet or satellite requires the force of gravity. A steady, circular orbit is achieved by having a steady speed which has a required value. At speeds greater than this value, the radius of the orbit will increase. At speeds smaller than this value, the radius of the orbit will decrease. An object in orbit has steady speed, however it has changing velocity. This is because the direction of the object is constantly changing, as it is travelling in a circle. The closer to objects are together, the stronger the gravitational force of attraction between them, and thus the faster the speed of the orbit will be. This is because the greater force acting on the object will cause a greater acceleration.

Orbital motion

1

Describe how a red giant is formed.

A main sequence star is constantly fusing hydrogen. When this process stops due to hydrogen running out, larger nuclei will form and the star will expand and turn into a red giant.

Describe how a red giant is formed.

2

Describe how the orbital motion of Venus will compare to that of Uranus.

The closer to objects are together, the stronger the gravitational force of attraction between them, and thus the faster the speed of the orbit will be. This is because the greater force acting on the object will cause a greater acceleration. So, the gravitational attraction of the Sun on Venus will be greater than that on Uranus, therefore the orbital speed of Venus will be greater than that of Uranus.

Describe how the orbital motion of Venus will compare to that of Uranus.

3

What can be said about main sequence star in relation to the life cycle of a star?

A main sequence star is at the most stable phase in its life cycle.

What can be said about main sequence star in relation to the life cycle of a star?

4

Describe what the Sun is.

The Sun is the largest object in the Solar System. The Sun is a huge star and its gravitational field causes many other objects in the Solar System to orbit around it.

largest object and the only star in the Solar System

Describe what the Sun is.

5

Which planet is furthest from the Sun?

Neptune is the planet furthest from the Sun.

Which planet is furthest from the Sun?

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