1. 8 Space physics Topic overview
Our solar system
The Sun, orbited by eight planets and some dwarf planets, makes up our solar system.
Earth and all the outer planets have one or more moons.
Neither planets nor their moons produce light; we see them by light reflected from the Sun.
Our solar system is part of the Milky Way galaxy which contains as many as million stars, many with planets.
There are a similar number of galaxies in the visible universe.
Gravity is the force that keeps galaxies together, the planets orbiting the Sun and keeps the Moon and artificial satellites orbiting the Earth.
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2. 8 Space physics Topic overview
Birth of a star
Our Sun is a main sequence star formed about 4.6 billion years ago. It is about half-way through its lifetime.
A cloud of cold dust and hydrogen gas, called a nebula, forms and collapses under the influence of gravity.
The collapsing dust and gas heat to very high temperatures and become a protostar.
At these high temperatures, the hydrogen nuclei can collide fast enough for nuclear fusion to start.
The energy released causes the star to shine for millions of years.
A main sequence star is one that is releasing energy from the fusion of hydrogen to form helium.
In a stable star, the inward force of gravity is balanced by the outward force caused by the nuclear fusion reaction.
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3. 8 Space physics Topic overview
Death of a star
After a long time, the hydrogen fuel supply starts to run out.
When this happens the star collapses and starts to fuse helium.
Stars about the same size as the Sun will then:
swell to form a red giant
fuse helium into heavier elements such as carbon and oxygen
collapse when the helium has run out to form a white dwarf
cool down slowly to become a black dwarf.
Stars much heavier than the Sun will then:
collapse and then grow into a super red giant.
fuse helium into even heavier elements up to iron
collapse rapidly when the helium runs out and then explode in a supernova
form either a neutron star or possibly a black hole
In a supernova, elements heavier than iron are produced. These are blown out into space to become part of a new solar system like ours.
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4. 8 Space physics Topic overview
Life cycle of stars
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5. 8 Space physics Topic overview
Circular orbits
The force of gravity of the Sun on the Earth keeps it
in a nearly circular orbit.
In a circular orbit, the force of gravity acts at right
angles to the direction of motion:
this changes the direction of motion without
changing the speed
a change of direction is a change of velocity
a change of velocity is acceleration
acceleration always requires a force
this force is provided by gravity.
Gravity is the force required for all orbits; planets, moons and artificial satellites.
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6. 8 Space physics Topic overview
Speed of orbit
The radius of the orbit of a satellite will depend on the speed of the satellite.
If the satellite slows down, the gravitational force will pull it into a lower orbit in order for the orbit to be stable.
If the satellite speeds up, it will need to move outwards to a new, larger, stable orbit.
The average speed of a satellite in a circular orbit can be calculated by using:
average speed= 2𝜋𝑟 𝑇
where r is the radius of orbit in m and T is the time for one orbit in s.
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7. 8 Space physics Topic overview
Red-shift
Red-shift is when the wavelength of light from a distant galaxy is longer than expected.
Red-shift is an effect similar to that noticed when the siren of an ambulance passes: the note of the siren is higher on approach and lower as it recedes.
In 1920 Edwin Hubble noticed that the wavelength of light from distant galaxies was red-shifted.
This means that distant galaxies are moving away from us.
He showed that
galaxies are moving away in all
directions
the more distant galaxies are
moving faster than closer ones.
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8. 8 Space physics Topic overview
Big bang theory
The big bang theory is that the universe began about 13.8 billion years ago when all matter expanded from a single point that was very hot and dense.
Hubble’s work supported the theory:
if space is expanding in all directions it must have started from a point
if space is expanding, there is more expanding space between us and a distant galaxy so it will be moving faster.
© Hodder & Stoughton 2016

9. 8 Space physics Topic overview
Dark energy
The speed of expansion of the universe is too slow to be accounted for just by the gravity from the amount of matter that can be seen.
Scientists have suggested that there must be a lot of matter that we cannot see to account for this.
They have called this additional matter ‘dark matter’.
More recent observations suggest that the rate of expansion of the universe is speeding up.
We do not know why this is happening.
One theory is that ‘dark energy’ is accelerating the expansion of the universe.
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