8 Space physics Topic overview Our solar system

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 200 000 million stars, many with planets. • There 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. © Hodder & Stoughton 2016

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. o A cloud of cold dust and hydrogen gas, called a nebula, forms and collapses under the influence of gravity. o The collapsing dust and gas heat to very high temperatures and become a protostar. o At these high temperatures, the hydrogen nuclei can collide fast enough for nuclear fusion to start. o The energy released causes the star to shine for millions of years. o A main sequence star is one that is releasing energy from the fusion of hydrogen to form helium. o In a stable star, the inward force of gravity is balanced by the outward force caused by the nuclear fusion reaction. © Hodder & Stoughton 2016

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: o swell to form a red giant o fuse helium into heavier elements such as carbon and oxygen o collapse when the helium has run out to form a white dwarf o cool down slowly to become a black dwarf. • Stars much heavier than the Sun will then: o collapse and then grow into a super red giant. o fuse helium into even heavier elements up to iron o collapse rapidly when the helium runs out and then explode in a supernova o 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. © Hodder & Stoughton 2016

8 Space physics Topic overview Life cycle of stars © Hodder & Stoughton 2016

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: o this changes the direction of motion without changing the speed o a change of direction is a change of velocity o a change of velocity is acceleration o acceleration always requires a force o this force is provided by gravity. • Gravity is the force required for all orbits; planets, moons and artificial satellites. © Hodder & Stoughton 2016

8 Space physics Topic overview Speed of orbit • © Hodder & Stoughton 2016

8 Space physics Topic overview Red-shift • Red-shift is when the wavelength of light from a distant galaxy is longer than expected. o 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 o galaxies are moving away in all directions o the more distant galaxies are moving faster than closer ones. © Hodder & Stoughton 2016

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 theory: o if space is expanding in all directions it must have started from a point o if space is expanding, there is more expanding space between us and a distant galaxy so it will be moving faster. © Hodder & Stoughton 2016

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. o Scientists have suggested that there must be a lot of matter that we cannot see to account for this. o They have called this additional matter ‘dark matter’. • More recent observations suggest that the rate of expansion of the universe is speeding up. o We do not know why this is happening. o One theory is that ‘dark energy’ is accelerating the expansion of the universe. © Hodder & Stoughton 2016