Unit 8 - Astrophysics PDF

Physics Unit 8 - Astrophysics Motion in the universe Objects in space: The universe is a large collection of billions of galaxies. A galaxy is a large collection of billions of stars. Stars are large astronomical objects such as the sun. The solar system In a planetary system, plane...

Physics Unit 8 - Astrophysics Motion in the universe Objects in space: The universe is a large collection of billions of galaxies. A galaxy is a large collection of billions of stars. Stars are large astronomical objects such as the sun. The solar system In a planetary system, planets and other astronomical objects orbit around a star in the centre. Our galaxy is the milky way and the planets orbit around the sun. Our planet, the earth, is the third of eight planets in our solar system. Our solar system consists of eight planets, five officially named dwarf planets, the sun, hundreds of moons and thousands of asteroids and comets. Gravitational field strength Weight is defined as the force acting on an object due to gravitational attraction. Planets have strong gravitational fields and therefore, they attract nearby masses with a strong gravitational force The force of weight causes: Objects to stay firmly on the ground. Objects to fall on the ground. Satellites being kept in orbit. The greater the mass of a planet, the greater its gravitational field strength. A higher gravitational field strength means a larger attractive force at the centre of that planet or moon causing objects to orbit around it. Gravitational force is a force caused by attraction between objects. This attraction is due to the masses of the objects. This size of this force depends on two factors:  Mass of the two objects. Distance between the two objects. The value of gravitational field strength varies for different planets as they have different masses. The gravitational field strength of earth is approximately 10 N/Kg. The gravitational field strength on the surface of the moon is lesser than that on the earth meaning that it would be easier to lift a mass on the surface of the moon than on earth. The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is higher than that on the earth meaning that it would be more difficult to lift a mass on the earth giants than on the earth. Gravitational field strengths of different planets and the sun The mass of an object is always the same, but its weight varies depending on the gravitational field strength of the planet. This means that an object will have the same mass on earth and Jupiter but an object will have higher weight on Jupiter than on earth, so much so that a human would not be able to stand on the surface of jupiter. Orbital motion The solar system is made up of many bodies which orbit other bodies.  The planets orbit the sun, the moons orbit the planets, comets and asteroids orbit the sun and artificial satellites orbit the earth. Smaller bodies orbit around larger bodies. Orbital motion is a result of the gravitational attraction between the two bodies. The gravitational force always acts towards the centre of the larger body causing the orbiting body to move around it in a circular motion. Differences in orbits similarities The orbits of different planets are slightly elliptical (stretched out circles or oval shapes) with the sun at one focus approximately at the centre of orbit. They all orbit in the same plane. They travel in the same direction around the sun. Differences They have different orbital radiuses. They orbit the sun at different distances from it. They orbit at different speeds. They take different amounts of time to orbit the sun. The further away a planet is from the sun,the slower it travels and therefore the longer it will take to orbit. The gravitational force of attraction points towards the centre of the circular path and acts at right angles to the velocity. Orbital motions of moons Moons orbit the planets in circular paths. Some planets are orbited by more than one moon. The closer the moon is to the planet, the greater the speed of the orbit and the shorter the time it takes to complete one orbit. Orbital motions of comets The orbit of comets follows a highly elliptical orbit. This causes the speed of the comet to change as its distance from the sun changes. The closer it is to the sun, the faster it travels and the further away it is from the sun, the slower it travels. Not all comets orbit in the same plane as the planets and some of them orbit in different directions. The sun is not at the centre in elliptical orbits and the orbiting speed increases closer the comet is to the sun and decreases the further away the comet is from the sun. Velocity must decrease as radius increases.  At the furthest point of the comet, the kinetic energy decreases and the gravitational potential energy increases. At the closest point, the kinetic energy increases and the gravitational potential energy decreases. Orbital period Planets orbit the sun and moons orbit the planets in circular motion. This means that the distance travelled by a planet is equal to the circumference of a circle. This is equal to 2πr. Average orbital speed is defined by the equation: v = orbital speed in metres per second r = radius of the orbit in metres t = time taken in seconds Orbital period is the time taken by an object to complete one orbit. Stellar evolution Classification of stars Stars come in a wide range of colours and sizes. From yellow stars to red dwarfs and from blue giants to red supergiants. These are classified according to their colour. Warm objects emit infrared and extremely hot objects emit visible light Therefore the colour they emit depends on how hot they are. A star’s colour is related to its surface temperature. A red star is the coolest at 3000 K. A blue star is the hottest at 30 000 K.  Astronomical objects cool as they expand and heat up as they contract so their colour will also change along with their surface temperature. When a star becomes a red giant, it becomes redder as it expands and cools. When a star becomes a white dwarf, it becomes whiter as it contracts and heats up. The formation of stars All stars are born from a cloud of dust and gas called a nebula. The main components of a nebula are helium and hydrogen gas. Over time, the gravitational attraction pulls the dust and gas together. This grouping of hydrogen and helium nuclei is called a protostar. So the star starts its life as a nebula and becomes a protostar. As more and more particles are attracted to the protostar by strong gravitational forces, the mass of the protostar increases which increases the strength of the gravitational forces which attracts more and more particles until the temperature and pressure at the centre of the star are high enough for nuclear fusion to start. The high temperature created by fusion produces an outward radiation pressure that balances the inward pull of gravity. This stage in the star’s life is called the main sequence. Once this happens, the star enters a long period of stability as it continues to fuse hydrogen and helium in nuclear fusion for billions of years. Low mass stars like our sun are cooler and appear yellow or red. High mass stars are hotter and appear white or blue. Difference between protostar and main sequence star A protostar is much cooler and does not emit as much radiation, heat energy or light because nuclear fusion has not started yet. A main sequence star on the other hand is much hotter and emits more light, heat, energy and radiation pressure because nuclear fusion has started. Once a star has formed and becomes stable as a main sequence star, the remaining stages depend on its initial mass.  In the core of the star, all the hydrogen eventually fuses into helium however, the temperature is not high enough for helium to fuse into other larger elements so without fusion, there will be no radiation pressure to balance out the inward pressure acting on the star. Gravity takes over and the star begins to collapse and as it collapses, it pushes the helium nuclei together causing more collisions to happen. This raises the temperature to the point where nuclear fusion can start again. The new burst of energy from fusion causes the outer layers of the star to cool down, expand and become redder as it becomes a red giant. As helium fuses into heavier elements, the star becomes stable once again for a short period of time. Once the helium supply runs out, fusion stops completely and there is no longer an outward force that keeps the star stable. The gravitational force takes over and compresses the core of the star and meanwhile, the outer layers are cast off and form a planetary nebula. The outer layers of the planetary nebula continue to drift into space. In the centre remains a small, dense, hot core called a white dwarf. Over time, the white dwarf gives off all its leftover energy from fusion and fades into a black dwarf. Stages of the life cycle of a solar mass star (low mass star) 1. Stage 1 - nebula 2. Stage 2 - protostar 3. Stage 3 - main sequence star 4. Stage 4 - red giant 5. Stage 5 - planetary nebula 6. Stage 6 - white dwarf 7. Stage 7 - black dwarf The life cycle of larger stars Once a star becomes stable as a main sequence star, its life cycle differs when it is a high mass star (much larger than our sun). Up to this stage, the life cycle of high mass stars and low mass stars is similar. Red supergiant → supernova → neutron star (or black hole) In the star’s core, hydrogen and helium fuse in nuclear fusion until the hydrogen runs out. The temperature is not high enough for helium fusion so gravity causes the star to collapse. As the core gets denser, the temperature rises, becoming high enough for helium fusion to start which causes the outer layers of the star to cool down, expand and become redder as it becomes a red supergiant. During helium fusion, the star is stable for a short period of time but unlike with low mass stars, once the helium runs out, fusion does not stop. This is because unlike red giants, red supergiant contact and expand many times.  When the core contracts, it becomes hot enough for carbon nuclei to fuse into oxygen which causes the outer layers to expand and contract when carbon runs out. Then the core heats up until it is hot enough to fuse into nitrogen which makes the outer layers expand and contract when oxygen runs out. So after helium fusion: Carbon fuses into oxygen Oxygen fuses into nitrogen Nitrogen fuses into heavier elements Heavier elements fuse into iron Fusion of iron requires an input of energy so fusion cannot continue once an iron core is formed. When a core made from iron is formed, there is no more energy that can be produced from nuclear reactions. Without this energy, the gravitational force causes the star to collapse in a shockwave explosion known as supernova. The outer layers of the star bounce off the iron core. The outer layers of a supernova are ejected into space and the ejected material will form clouds of dust and gas which new stars and planets are created from. The small dense core at the centre could be one of two things depending on the initial mass of the core: Neutron star. Neutron stars are extremely small and extremely dense. (gravity crushes matter into neutrons). They are entirely made of neutrons. Black hole. If the initial star is massive, it becomes a black hole. In a black hole, the matter is compressed into a microscopic unit. Black holes are the densest objects in the universe. They are so dense that light cannot escape from them. The brightness of stars The luminosity of a star is defined as the total amount of light energy emitted by the star. Luminosity is a measure of a star’s brightness or power output. Apparent magnitude The brightness or apparent magnitude depends on two factors: The luminosity of the star. The distance of the star from earth (more distant stars are fainter than nearby stars) Apparent magnitude is defined as the perceived brightness of the star as seen from earth. Apparent magnitude scale The brighter the star, the lower the magnitude. The dimmer the star, the higher the magnitude. Absolute magnitude A bright star that is far away can look the same as a dim star that is nearby. Absolute magnitude is defined as a measure of how bright stars would appear if they were all placed at the same distance away from the earth. The standard distance that astronomers use is 10 parsecs, 32.6 light years or 3.04 x 10^14 km away from the earth. Hertzsprung-russel diagrams The properties of stars can be classified using the hertzsprung-russell diagrams which is a plot of luminosity on the y axis and temperature on the x axis. It is given in solar units where the luminosity of the sun=1. so Stars brighter than the sun, luminosity>1 Stars dimmer than the sun, luminosity