Year 8 Study Notes - The Earth in the Universe PDF

Study Notes - Year 8 - The Earth in the Universe The Solar System We are in the Milky Way galaxy. The Milky Way is a galaxy containing billions of stars. The Sun is one of the stars. Our neighbouring galaxy is the Andromeda galaxy. The Sun The Sun is the largest object in the Solar System. The Su...

Study Notes - Year 8 - The Earth in the Universe The Solar System We are in the Milky Way galaxy. The Milky Way is a galaxy containing billions of stars. The Sun is one of the stars. Our neighbouring galaxy is the Andromeda galaxy. The Sun The Sun is the largest object in the Solar System. The Sun's huge gravitational field keeps many other objects in orbit around it. Planets, dwarf planets, asteroids and comets are in orbit around the Sun. Planets The Earth is one of eight planets in the Solar System. The planets orbit the Sun at different distances from it. Earlier, people believed in a geocentric model, with the Earth at the centre, and the Sun moving around it. We now know that this is incorrect - the Solar System is heliocentric, with the Earth and other planets orbiting the Sun. Different planets have different properties and conditions. Generally, as the distance from the Sun increases: ​ Temperature decreases, for example, Mercury (nearest to the Sun) is 430°C whereas Neptune (furthest from the Sun) is -200°C ​ Time taken to orbit the Sun increases, for example, Mercury orbits once every 88 Earth days, but Neptune orbits once every 165 Earth years. When is it big enough to be a planet? For a planet to form, its own gravity needs to be strong enough for it to become a spherical shape. Its gravitational field must also be strong enough to 'clear the neighbourhood', which means it can pull smaller nearby objects into its orbit. Moons Moons are natural satellites which orbit a planet. Many planets have moons, and some planets have many moons. The Earth has just one moon - which we call the Moon! Dwarf planets Pluto is a dwarf planet. The gravitational field of a dwarf planet is not strong enough to clear the neighbourhood, so there could be other objects in its orbit around the Sun. The Solar System has hundreds of dwarf planets, including Ceres (the only dwarf planet in the asteroid belt). This is why Pluto was downgraded in 2006 to be a dwarf planet - it has not cleared its neighbourhood of other objects. Asteroids Our Solar System contains smaller objects called asteroids. These orbit the Sun in elliptical orbits, and may take up to millions of years to go around the Sun. Asteroids are made of metals and rocky material. There are large numbers of asteroids orbiting the Sun in the asteroid belt between Mars and Jupiter. There are also many in a region beyond Neptune called the Kuiper Belt. Comets The Solar System also contains small objects called comets. Comets are similar to asteroids, but are made of rocky material, dust and ice. These orbit the Sun in highly elliptical orbits that stretch beyond Neptune and pass inside the orbit of Mercury. Their orbit periods are regular but vary from a few years to thirty million years. A comet can pass close to the Sun, but then travel a long way out towards the outer Solar System, before returning to travel close to the Sun again. As a comet approaches the Sun, it begins to vaporise, (turns into gas). That is why it produces a distinctive tail. Gravity All objects have gravity. A larger object, with more mass, will have a larger gravitational pull. Gravity is a force, and its effect on an object is measured in Newtons per kilogram N/kg. When you say that you weigh 50 kg, you actually mean that your mass is 50kg. Your weight is measured in Newtons, because weight is a force and if your mass is 50kg, you will weigh around 500 N on planet Earth, because Earth’s gravitational pull is 9.8 N/kg, which we can approximate to 10 N/kg for easier calculation. On the Moon, the same person would still have mass of 50kg, because your mass does not change wherever you go, but their weight will be about 80N, because the gravitational pull from the Moon’s gravity is 1.6 N/kg. In space, with no gravity, you would have a mass of 50kg but weigh 0 N. To calculate weight: Weight = mass x gravity The Earth’s gravity is much larger than that of the Moon, and is enough to keep the Moon in orbit around it, but there is still some effect on Earth from the Moon’s gravity - you can see evidence because there are tides in the oceans. Orbits Gravity provides the force needed to maintain the stable orbit of planets around a star. It also provides the force to keep moons and artificial satellites in orbit around a planet. Explaining orbits For an object to remain in a steady, circular orbit it must be travelling at the right speed. There are three possible outcomes: ​ If the satellite is moving too quickly then the gravitational attraction between the Earth and the satellite is too weak to keep it in orbit. If this is the case, the satellite will move off into space. ​ If the satellite is moving too slowly then the gravitational attraction will be too strong, and the satellite will fall towards Earth. ​ If the satellite's speed is just right - it will not move off into space or spiral into the Earth, but will travel around a fixed path. Orbits and constant speed When an object moves in a circle at a constant speed, its direction constantly changes. The force working on it is centripetal force that acts towards the centre of the circle. Gravitational attraction provides the centripetal force needed to keep planets and all types of satellite in orbit. Orbits and changing speed The gravitational attraction between two objects decreases with distance. This means that the closer the two objects are to each other, the stronger the force of gravity between them. Objects in small orbits travel faster than objects in large orbits. Artificial satellites travel in different orbits, at different distances from Earth, depending on their use. As a comparison, the Moon is much further away - 380 000 km from Earth. It takes about 27 days for the Moon to orbit the Earth. Geostationary orbits - High Earth orbit Geostationary satellites take 24 hours to orbit the Earth, so the satellite appears to stay in the same part of the sky when seen from the ground. These orbits are 36,000 km above the equator and the satellites travel at 3,000 m/s. (You do not need to remember these exact numbers) These satellites are used for communications and weather forecasting. Medium Earth orbit Satellites in medium Earth orbit are positioned about 20,000 km above the Earth. They take about 12 hours to orbit and are used for GPS. Low Earth orbit Satellites in low Earth orbit are positioned between 200 km and 2,000 km above the Earth. They take between 1½ and 2 hours to orbit. Many orbit over the North and South Poles. These polar orbit satellites can observe the whole of the Earth as it spins beneath them. The fastest satellites travel at speeds of 7,600 m/s. How stars are formed Our Solar System started to form about 4.6 billion years ago, from a cloud of gas and dust. The main gases were hydrogen (74%) and helium (24%). This cloud was part of a bigger cloud called a nebula. This nebula started to collapse under the influence of gravity, creating a dense core with a disc of material surrounding it. All stars begin life in the same way. Gravity begins to pull the dust and gas together. As the mass falls together it gets hot. A star is formed when it is hot enough for the hydrogen nuclei to fuse together to make helium. The fusion process releases energy, which keeps the core of the star hot. Increased pressure causes a rise in temperature and eventually nuclear fusion When a gas is compressed, its volume decreases. The speed of the particles in the gas increases, causing the temperature to rise. This is why the temperature in a star increases, leading to nuclear fusion. During this stable phase in the life of a star, the force of gravity holding the star together is balanced by the expansion due to the fusion energy. Our Sun is in this stable phase. The Sun took ten million years to form. Although the Sun has most of the mass, there was material left over. As the disc continued to spin, material bound together because of gravity formed larger objects. Some became large enough to form planets. Smaller objects became asteroids or comets. Scientists have gained a lot of information about the formation of the Solar System by studying asteroids and comets. Stages in the life cycle of a star 1.​ A cloud of dust and gas, also known as a nebula 2.​ Protostar 3.​ Main sequence star. After this, stars develop in different ways depending on their size. Stars that are a similar size to the Sun become: 4.​ Red giant star 5.​ White dwarf 6.​ Black dwarf Stars that are far greater in mass than the Sun become: 4. Red Supergiant star 5. Supernova 6. Neutron star, or a black hole (depending on size) Notes about each stage: Nebula A star forms from massive clouds of dust and gas in space, which we call a nebula. A nebula is mostly hydrogen. Gravity begins to pull the dust and gas together. Protostar As the mass gets closer together it gets hot. A star is formed when it is hot enough for the hydrogen nuclei to fuse together to make helium. The fusion process releases energy, keeping the core of the star hot. Main sequence star During this stable phase, the force of gravity holding the star together is balanced by forces from fusion energy. For most of its lifetime a star is a main sequence star. It is stable, with balanced forces keeping it the same size all the time. -​ gravitational attraction pulls the star in. -​ radiation pressure from the fusion reactions acts outwards -​ forces caused by gravitational attraction and fusion energy are balanced The Sun is expected to be a main sequence star for billions of years. Red giant star When all the hydrogen has been used up in the fusion process, larger nuclei begin to form and the star may expand to become a red giant. White dwarf When all the nuclear reactions are over, a small star like the Sun may begin to contract under the pull of gravity. In this instance, the star becomes a white dwarf which fades and changes colour as it cools. Supernova A large star with more mass will go on making nuclear reactions, getting hotter and expanding until it explodes as a supernova. Neutron star or black hole Depending on the mass at the start of its life, a supernova will leave behind either a neutron star or a black hole. Red-shift and the expansion of the Universe Light from a star does not contain all the wavelengths of the electromagnetic spectrum. Elements in the star absorb some of the emitted wavelengths, so dark lines are present when the spectrum is analysed. Different elements produce different patterns of dark lines. The diagram shows part of the emission spectrum of light from the Sun. Astronomers can observe light from distant galaxies. When they do this, they see it is different to the light from the Sun. The dark lines in the spectra from distant galaxies show an increase in wavelength. The lines are moved or shifted towards the red end of the spectrum. This effect is called red-shift. The diagram shows part of the emission spectrum of light from a distant galaxy. Red-shift and speed Astronomers see red-shift in virtually all galaxies. It is a result of the space between the Earth and the galaxies expanding. This expansion stretches out the light waves during their journey to us, shifting them towards the red end of the spectrum. The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth. Astronomers have found that the further from us a star is, the more its light is red-shifted. This tells us that distant galaxies are moving away from us, and that the further away a galaxy is, the faster it's moving away. There is no evidence to suggest that we have a special place in the Universe, and the red shift is evidence for a generally expanding Universe. It suggests that everything is moving away from everything else. The Big Bang theory Scientists believe that about 13.8 billion years ago the whole Universe came from something very small, extremely hot and dense. From this tiny point, the whole universe expanded outwards to what there is now. Evidence for the Big Bang theory Red-shift Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is. This means that the further away the galaxies are, the faster they are moving. This is similar to an explosion, where the bits moving fastest travel furthest from the explosion. Red-shift data provides evidence that the Universe, including space itself, is expanding. CMBR Astronomers have also discovered a cosmic microwave background radiation (CMBR). This comes from all directions in space and has a temperature of about -270°C. The CMBR is the remains of the thermal energy from the Big Bang, spread thinly across the whole Universe. Does evidence Prediction from Big Bang Evidence observed support the Big theory Bang theory? More distant galaxies should More distant galaxies Yes move away faster have greater red-shift Initial Big Bang heat should CMBR is everywhere at a now be thinly spread across temperature of about Yes the whole Universe -270°C The discovery of red-shift in light from distant galaxies led to the development of the Big Bang theory. The discovery of CMBR, after it had already been predicted by the theory, has provided very strong support for the Big Bang theory. Distances in the Universe and the speed of light ​ ight travels at almost 3,000 billion meters per second. A beam of light from L the sun reaches earth in just over eight minutes. The distance from earth to the sun is eight light minutes. Compare that to the distance to the moon. It's just 1.3 light seconds away from earth. To see something, light has to travel from the object we're looking at to our eyes. If we look at a star that is one light-year away, we don't see that star as it is now, but as it was a year ago. So when you look at the stars at night, you're actually seeing them as they looked thousands of years ago. You're looking into the past.