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Our solar system for OCR A-level Physics

Our solar system

This page covers the following topics:

1. Stars and fusion
2. The lifecycle of a star

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

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

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?

3

Can the Sun become a black hole? Explain why or why not.

The Sun is not massive enough to become a black hole. The Sun is currently a main sequence star. It will ltaer become a red giant star and once all fusion reactions cease, it will become a white dwarf.

Can the Sun become a black hole? Explain why or why not.

4

Explain how a protostar becomes a main sequence star.

When the protostar reaches a high enough temperature, the hydrogen nuclei in it 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.

Explain how a protostar becomes a main sequence star.

5

There are two paths which the life cycle of a star could follow. What is the path that a star takes dependent on?

The life path a star has is dependent on its size.

There are two paths which the life cycle of a star could follow. What is the path that a star takes dependent on?

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