6.1.1 The Earth, Moon & Sun " DOWNLOAD PDF TEST YOURSELF The Earth's Axis 1. MOTION 2. THERMA DOWNLOAD PDF TEST YOURSELF " The Earth is a rocky planet that rotates in a near circular orbit around the Sun It rotates on its axis, which is a line through the north and south poles The axis is tilted at an angle of approximately 23.4° from the vertical The Earth completes one full rotation (revolution) in approximately 24 hours (1 day) The Earth's Axis This rotation creates the apparent daily motion of the Sun rising and setting Rotation of the Earth on its axis is therefore responsible for the periodic cycle of day and night 3. WAVES 4. ELECTR 5. NUCLEA M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The Earth is a rocky planet that rotates in a near circular orbit around th Day and It Night rotates on its axis, which is a line through the north and south poles 6. SPACE The Earth's rotation around its axis creates day and night The axis is tilted at an angle of approximately 23.4° from the vertic Day is experienced by the half of the Earth's surface that is facing the Sun Night is the other half of the Earth's facing away from the Sun The Earth completes onesurface, full rotation (revolution) in approximately 6.1 24Earth ho 6.1.1 The This rotation creates the apparent daily motion of the Sun rising and set 6.1.2 Calcu Rotation of the Earth on its axis is therefore responsible for the pe Day and Night 6.1.3 The S 6.1.4 Orbit The Earth's rotation around its axis creates day and night 6.1.5 Grav Day is experienced by the half of the Earth's surface that is facing Night is the other half of the Earth's surface, facing away from the 6.2 Stars & Day and night are caused by the Earth's rotation Rising and Setting of the Sun The Earth's rotation on its axis makes the Sun looks like it moves from east to west At the equinoxes the Sun rises exactly in the east and sets exactly in the west Equinox (meaning 'equal night') is when day and night are approximately of equal length However, the exact locations of where the Sun rises and sets changes throughout the seasons 1 Rising and Setting of the Sun The Earth's rotation on its axis makes the Sun looks like it moves from east to west At the equinoxes the Sun rises exactly in the east and sets exactly in the west Equinox (meaning 'equal night') is when day and night are approximately of equal length However, the exact locations of where the Sun rises and sets changes throughout the seasons M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N In the northern hemisphere (above the equator): In summer, the sun rises north of east and sets north of west In winter, the sun rises south of east and sets south of west The Sun rises in the east and sets in the west. Its approximate area changes throughout the year The Sun is highest above the horizon at noon (12 pm) In the northern hemisphere, the daylight hours are longest up until roughly the 21st June This day is known as the Summer Solstice and is where the Sun is at its highest point in the sky all year The daylight hours then decrease to their lowest around 21st December This is known the Winter Solstice and is where the Sun is at its lowest point in the sky all year The Earth's Orbit The Earth orbits the Sun once in approximately2365 days This is 1 year The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons The Earth's Orbit The Earth orbits the Sun once in approximately 365 days This is 1 year The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons Seasons in the Northern hemisphere caused by the tilt of the Earth M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Over parts B, C and D of the orbit, the northern hemisphere is tilted towards the Sun This means daylight hours are more than hours of darkness This is spring and summer The southern hemisphere is tilted away from the Sun This means there are shorter days than night This is autumn and winter Over parts F, G and H of the orbit, the northern hemisphere is tilted away from the Sun The situations in both the northern and southern hemisphere are reversed It is autumn and winter in the northern hemisphere, but at the same time it is spring and summer in the southern hemisphere At C: This is the summer solstice The northern hemisphere has the longest day, whilst the southern hemisphere has its shortest day At G: This is the winter solstice The northern hemisphere has its shortest day, whilst the southern hemisphere has its longest day At A and D: Night and day are equal in both hemispheres These are the equinoxes Moon & Earth The Moon is a satellite around the Earth It travels around the Earth in roughly a circular orbit once a month This takes 27-28 days The Moon revolves around its own axis in a month so always has the same side facing the Earth at all times We never see the hemisphere that is always facing away from Earth, although astronauts have orbited the Moon and satellite have photographed it The Moon shines with reflected light from the Sun, it does not produce its own light Phases of the Moon The way the Moon's appearance changes across a month, as seen from Earth, is called its periodic cycle of phases 3 Phases of the Moon M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The way the Moon's appearance changes across a month, as seen from Earth, is called its periodic cycle of phases Phases of the Moon as it orbits around Earth In the image above, the inner circle shows that exactly half of the Moon is illuminated by the Sun at all times The outer circle shows how the Moon looks like from the Earth at its various positions In the New Moon phase: The Moon is between the Earth and the Sun Therefore, the sunlight is only on the opposite face of the Moon to the Earth This means the Moon is unlit as seen from Earth, so it is not visible At the Full Moon phase: The Earth is between the Moon and the Sun The side of the Moon that is facing the Earth is completely lit by the sunlight This means the Moon is fully lit as seen from Earth In between, a crescent can be seen where the Moon is partially illuminated from sunlight TEST YOURSELF NEXT TOPIC Author: Lindsay Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc a many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of commun 4 brought her to Save My Exams where she hopes to help as many students as possible on their next steps. 6.1.2 Calculating Orbital Speeds 6.1.2 Calculating Orbital Speeds " DOWNLOAD PDF TEST YOURSELF Orbital Speed " EXTENDED DOWNLOAD PDF 1. MOTION 2. THERM TEST YOURSELF When planets move around the Sun, or a moon moves around a planet, they orbit in circular motion This means that in one orbit, a planet travels a distance equal to the circumference of a circle (the shape of the orbit) This is equal to 2πr where r is the radius a circle Orbital Speed 3. WAVES 4. ELECTR The relationship between speed, distance and time is: EXTENDED 5. NUCLEA distance time Speed ! When planets move around the Sun, or a moon moves around a planet 6. SPACE This means that in one orbit, a planet travels a distance equal to t 2"r v! (the shape of the orbit) T 6.1 Earth This is equal to 2π r where r is the radius a circle Where: M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N the average orbital speed of an object can be defined by the equation: v = orbital speed in metres per second (m/s) r = average radius of the orbit in metres (m) The relationship between speed, distance and time is: T = orbital period in seconds (s) This orbital period (or time period) is defined as: The time taken for an object to complete one orbit distance Speed ! time The orbital radius r is always taken from the centre of the object being orbited to the object orbiting 6.1.1 The E 6.1.2 Calc 6.1.3 The S the average orbital speed of an object can be defined by the equation: v! 6.1.4 Orbit 2"r T Where: v = orbital speed in metres per second (m/s) r = average radius of the orbit in metres (m) T = orbital period in seconds (s) 6.1.5 Grav 6.2 Stars & This orbital period (or time period) is defined as: The time taken for an object to complete one orbit Orbital radius and orbital speed of a planet moving around a Sun ? The orbital radius r is always taken from the centre of the object being o Worked Example 5 The Hubble Space Telescope moves in a circular orbit. Its distance above the Earth’s surface is 560 km and the radius of the Earth is 6400 km. It completes one orbit in 96 minutes. Orbital radius and orbital speed of a planet moving around a Sun ? Worked Example M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The Hubble Space Telescope moves in a circular orbit. Its distance above the Earth’s surface is 560 km and the radius of the Earth is 6400 km. It completes one orbit in 96 minutes. Calculate its orbital speed in m/s. Step 1: List the known quantities Radius of the Earth, R = 6400 km Distance of the telescope above the Earth's surface, h = 560 km Time period, T = 96 minutes Step 2: Write the relevant equation Step 3: Calculate the orbital radius, r The orbital radius is the distance from the centre of the Earth to the telescope 6 7 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 6.1.3 The Solar System " DOWNLOAD PDF " DOWNLOAD PDF TEST YOURSELF 1. MOTION, FORCES & ENERGY The Solar System THERMAL PHYSICS 1.2.MOTION, FORCES & ENERGY 6.1.3 The Solar System TEST YOURSELF The Solar System consists of: 1. The Sun The Solar System 2. Eight planets Natural and consists artificialof: satellites The3.Solar System Dwarf 1.4. The Sunplanets Asteroids and comets 2.5. Eight planets 3. WAVES 2. THERMAL PHYSICS 4. ELECTRICITY & 3.MAGNETISM WAVES M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 3. Natural and artificial satellites The Sun & the Planets 4. Dwarf planets The Sun lies at thecomets centre of the Solar System 5. Asteroids and The Sun is a star that makes up over 99% of the mass of the solar system The Sun & the Planets There planets anSolar unknown number of dwarf planets which orbit the The Sunare lieseight at the centreand of the System SunThe Sun is a star that makes up over 99% of the mass of the solar system The gravitational field around planets is strong enough to have pulled in all There nearby are eight planets and unknownofnumber dwarf planets which orbit the objects with thean exception naturalof satellites Sun The gravitational field around a dwarf planet is not strong enough to have The gravitational field around planets is strong enough to have pulled in all pulled in nearby objects nearby objects with the exception of natural satellites TheThe 8 planets in our Solar System aindwarf ascending order of the distance from Sun gravitational field around planet is not strong enough to the have are:pulled in nearby objects Mercury The 8 Venus planets in our Solar System in ascending order of the distance from the Sun are: Earth Mercury Mars Venus Jupiter Earth Saturn Mars Uranus Jupiter Neptune Saturn SatellitesUranus There are two types of satellite: Neptune Natural Satellites Artificial There are two types of satellite: Some planets have moons which orbit them Natural Moons are an example of natural satellites Artificial Artificial satellites are man-made andthem can orbit any object in space Some planets have moons which orbit The International Space Station (ISS) orbits the Earth and is an example of an Moons are an example of natural satellites artificial satellite Artificial satellites are man-made and can orbit any object in space Asteroids & Comets The International Space Station (ISS) orbits the Earth and is an example of an artificial satellite Asteroids and comets also orbit the sun An asteroid is a small rocky object which orbits the Sun Asteroids & Comets The asteroid belt lies between Mars and Jupiter Asteroids and comets also orbit the sun An asteroid a small rockyand object which orbits Comets areismade of dust ice and orbit the the SunSun in a different orbit to those of The asteroid belt lies between Mars and Jupiter planets The ice melts when the comet approaches the Sun and forms the comet’s tail Comets are made of dust and ice and orbit the Sun in a different orbit to those of planets The ice melts when the comet approaches the Sun and forms the comet’s tail 8 4.5.ELECTRICITY & NUCLEAR PHYSICS MAGNETISM 6. SPACE PHYSICS 5. NUCLEAR PHYSICS 6.1 Earth & The Solar 6.System SPACE PHYSICS 6.1.1 The Earth, Moon & 6.1 Earth & The Solar Sun System 6.1.2The Calculating Orbital 6.1.1 Earth, Moon & Speeds Sun 6.1.2 6.1.3Calculating The Solar Orbital System Speeds 6.1.4The Orbiting 6.1.3 Solar Bodies System 6.1.5 Gravitational Effects 6.1.4 Orbiting Bodies on Orbits 6.1.5 Gravitational Effects 6.2 Stars & The Universe on Orbits 6.2 Stars & The Universe The ice melts when the comet approaches the Sun and forms the comet’s tail The objects in our solar system ! Exam Tip M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N You need to know the order of the 8 planets in the solar system. The following mnemonic gives the first letter of each of the planets to help you recall them: My Very Excellent Mother Just Served Us Noodles Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune Accretion Model of the Solar System There are 4 rocky and small planets: Mercury, Venus, Earth and Mars These are the nearest to the Sun There are 4 gaseous and large planets: Jupiter, Saturn, Uranus and Neptune There are the furthest from the sun planets-of-our-solar-system, IGCSE & GCSE Physics revision notes The eight planets of our Solar System The differences in the types of planets are defined by the accretion model for Solar System formation 9 The Sun was thought to have formed when gravitational attraction of pulled together clouds of hydrogen dust and gas (called nebulae) 10 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N A rotating accretion disc is formed when the planets emerged The accretion model of the creation of the Solar System As the Sun grew in size it became hotter Where the inner planets were forming near the Sun, the temperature was too high for molecules such as Hydrogen, Helium, water and Methane to exist in a solid state Therefore, the inner planets are made of materials with high melting temperatures such as metals (e.g. iron) Only 1% of the original nebula is composed of heavy elements, so the inner, rocky planets could not grow much and stayed as a small size, solid and rocky The cooler regions were further away from the Sun, and temperature was low enough for the light molecules to exist in a solid state The outer planets therefore could grow to a large size up and include even the lightest element, Hydrogen These planets are large, gaseous and cold 11 These planets are large, gaseous and cold ! Exam Tip When referring to the accretion model for the formation of the Solar System, make sure your answer has the following: (a) the model’s dependence on gravity (b) the presence of many elements in interstellar clouds of gas and dust (c) the rotation of material in the cloud and the formation of an accretion disc NEXT TOPIC M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N TEST YOURSELF Author: Ashika Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, foc on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Phys and Maths tutor, Ashika helps to grow and improve our Physics resources. Resources Members Company Quick Links Home Members Home About Us Join Account Contact Us GCSE Revision Notes Support Logout Jobs Terms Privacy Facebook Twitter IGCSE Revision Notes A Level Revision Notes Biology Chemistry Physics Maths 2022 Advance Information © Copyright 2015-2022 Save My Exams Ltd. All Rights Reserved. IBO was not involved in the production of, and does not endorse, the resources created by Save My Exams. # DOWNLOAD PDF 12 DOWNLOAD PDF 6.1.4 Orbiting Bodies TEST YOURSELF DOWNLOAD DOWNLOAD PDFPDF Light Speed " 1. MOTI ENERGY 6.1.4 Orbiting Bodies " " TEST YOURSELF TEST YOURSELF THER 1. MOTION, 2. FORCES ENERGY The planets and moons of the solar system are visible from Earth when they reflect WAV Light Speed 2. THERMAL3.PHYSIC Light Speed light from the Sun 12 The outer regions of the Solar System are around 5 × 10 m from the Sun, 4. ELEC The planets and moons of the solar system are visible from Earth when they reflect 3. WAVES which means even light takes somesystem time toare travel thesefrom distances The planets and moons of the solar visible Earth when they reflect light from the Sun MAGNE 12 The Sun isouter so far away Earth that are thearound light we leftSun, the Sun eight The regions of from the Solar System 5 ×see 10 actually m from the light from the Sun 4. ELECTRICITY & minutes earlier 12 which meansregions even lightof takes to travel these distances The outer the some Solartime System are around 5 × 10 m from the Sun, MAGNETISM 5. NUCL star to us Earth after that the the Sunlight is so that from it takes four The the Sun nearest is so far away from wefar seeaway actually leftlight the Sun eight which means even light takes some time to travel these distances minutes earlier years to reach us NUCLEAR PHYSIC The the Sunnearest is so star far away from that light actually left the Sun 5.eight us after theEarth Sun is so farthe away thatwe lightsee from it takesaway, four The Milky Way to galaxy contains billions of stars, huge distances with the 6. SPAC minutes yearsearlier to reach us light taking even longer to be seen from Earth M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The Way galaxy billions of stars, huge away, withfrom the it takes theMilky nearest star tocontains us after the Sun is so fardistances away that light 6. four SPACE PHYSICS 8 6.1 Eart light taking even longer to be seen from Earth The speed of light is a constant 3 × 10 m/s years to reach us System 6.1 Earth using the equation: The of Milky Way galaxy the & The Sola The Therefore, speed light is a constant 3 contains × 108 m/s billions of stars, huge distances away, with System Therefore, usingeven the equation: light taking longer to be seen from Earth 6.1.1 Th distance speeddistance ! 6.1.1 The Earth, Sun Moo time 8 speed ! 3 × 10 m/s The speed of light is a constant Sun time 6.1.2 Ca Therefore, using the equation: The time taken to travel a certain distance can be calculated by rearranging The time taken to travel a certain distance can be calculated by rearranging to: distance 6.1.2 Calculating Orb Speeds Speeds distancetime distance timetime ! ! speedspeed 6.1.3 Th 6.1.3 The Solar Syste distance 6.1.5 Gravitational E 6.1.5 Gr on Orbits to: speed ! The time taken to travel a certain distance can be calculated by rearranging 6.1.4 Orbiting Bodie to: 6.1.4 Or Worked Example Example ?? Worked time Calculate the time taken for light from the Sun!to reach Mercury if the radius of Calculate the time taken for light from the Sunspeed to reach Mercury if the radius of 9 Mercury's orbit is 5.8 × 10 m. Mercury's orbit is 5.8 × 109 m. on Orbi 6.2 Stars & The Univ Worked Example ? Step 1: State the equation for the time taken for light to travel a certain distance Step 1: State equation forlight the time taken for to light to travel a certain distance Calculate thethe time taken for from the Sun reach Mercury if the radius of distance time ! 9 speed Mercury's orbit is 5.8 × 10 m. distance time ! Step 2: Substitute in the values speed The distance travelled the radius of the orbit Step 2: Substitute in the is values 9 Distance, d =equation 5.8 × 10 m. Step 1: State the for the time taken for light to travel a certain distance 8 Speed = the speed of light, = 3.0 × 10 of m/s The distance travelled is vthe radius the orbit 9 distance Distance, d = 5.8 × 10 5 "m. 9 8 # 10 time !"8933333 time ! ! 1 speed Speed = the speed of light, v 3 " 0=#3.0 108× 10 m/s 9 5"8 # 10 Step 3:2:Round up the answer and include units Step Substitute in the time values ! ! 1 " 933333 3 " 0 # 108 time = 19.3 s The distance travelled is the radius of the orbit 13 9 Step 3: Round up the answer and include units Distance, d = 5.8 × 10 m. 8 6.2 Star time = 19.3 s ! Exam Tip The speed of light is very fast. This is why in our everyday life, we do not notice that it travels a distance at all. For example, when you switch on a light bulb, the room is flooded with light instantly. However, this is only because it is so fast and our eyes cannot see the difference. In large, astronomical distances which can be millions or even billions of kilometres, the limit of the speed of light starts to have an affect. M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N For example, it takes light 8 minutes to travel from the Sun to the Earth. This means we are technically seeing the Sun 8 minutes ago. If the Sun was to disappear, we would not notice till 8 minutes later. Although, by that time, the time delay is the least of our worries... Elliptical Orbits EXTENDED Orbits of planets, minor planets and comets are elliptical An ellipse is just a 'squashed' circle Planets, minor planets and comets have elliptical orbits However, the Sun is not at the centre of an elliptical orbit This is only the case when the orbit is approximately circular In an elliptical orbit, the Sun is not at the centre of the orbit However, in a circular orbit, the Sun is at the centre Planets and comets travel in elliptical orbits, but the Sun is not at the centre of these orbits ! Exam Tip You will not be asked to do any calculations with elliptical orbits. If you are asked to calculate the time period, orbital speed or radius of an orbit, it can be assumed that it is circular. Analysing Orbits EXTENDED Over many years, data about all the planets, moons and the Sun have been collected This is not just for general interest, but to indicate: 14 Factors that affect conditions on the surface of the planets Environmental problems that a visit (using manned spaceships or robots) Analysing Orbits EXTENDED Over many years, data about all the planets, moons and the Sun have been collected This is not just for general interest, but to indicate: Factors that affect conditions on the surface of the planets Environmental problems that a visit (using manned spaceships or robots) would encounter Table of Data for Planets in our Solar System Planet 57.9 88 days 5427 350 3.7 108.2 225 days 5243 460 8.9 149.6 365 days 5514 20 9.8 227.9 687 days 3933 –23 3.7 778.6 11.9 years 1326 –120 23.1 Saturn 1433.5 29.5 years 687 –180 9.0 Uranus 2872.5 75 years 1271 –210 8.7 Neptune 4495.1 165 years 1638 –220 11.0 Venus Earth Mars Jupiter M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Mercury Orbital Orbital duration / Density / distance / days or years kg/m3 million km Uniform Surface Surface Temperature/ Gravitational Field Strength/ °C N/kg There are some common themes from the data of the planets is: Orbital duration (how long it takes to travel around the Sun) increases with orbital distance (distance from the Sun) The circular path that the planet's travel in has a larger radius Orbital duration increases with orbital distance E.g. Neptune travels much slower than Mercury The planets further away from the Sun experience a weaker gravitational pull, so move slower in their orbit Surface temperature decreases with orbital distance except for Venus Venus has a dense atmosphere of carbon dioxide, trapping in heat through the greenhouse effect The surface gravitational field strength doesn't just depend on a planet's size, but also its mass This is why although Uranus is 4 times larger than Earth, it has a smaller gravitational field strength because it is less dense ! Exam Tip Although you don't need to memorise any of this data, you must be able to confidently analyse and interpret it. Look out for trends such as one variable increasing whilst the the other decreases (or also increases). This carefully about why that may be with what you have already learnt about the planets from this topic. For example, what is the planet made of? What is its distance from the Sun and how does this affect it? 15 6.1.5 Gravitational Effects on Orbits TEST YOURSELF 1. MOTION, FORCES & ENERGY Gravitational Field Strength 2. THERMAL PHYSICS " DOWNLOAD PDF " DOWNLOAD PDF 6.1.5 Gravitational Effects on Orbits TEST YOURSELF The strength of gravity on different planets after an object's weight on that planet Weight is defined as: Gravitational Field Strength The force acting on an object due to gravitational attraction 3. WAVES 4. ELECTRICITY & MAGNETISM Planets have strong gravitational fields The strength of gravity on different planets after an object's weight on that planet Hence, they attract nearby masses with a strong gravitational force 5. NUCLEAR PHYSICS Weight is defined as: Because of weight: The force acting onground an object due to gravitational attraction Objects stay firmly on the Objects will always fall to the ground Satellites arestrong kept in orbit Planets have gravitational fields 6. SPACE PHYSICS 6.1 Earth & The Solar M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Hence, they attract nearby masses with a strong gravitational force System Because of weight: Objects stay firmly on the ground Objects will always fall to the ground Satellites are kept in orbit 6.1.1 The Earth, Moon & Sun 6.1.2 Calculating Orbita Speeds 6.1.3 The Solar System 6.1.4 Orbiting Bodies 6.1.5 Gravitational Effe on Orbits 6.2 Stars & The Univers Objects are attracted towards the centre of the Earth due to its gravitational field strength Both the weight of any body and the value of the gravitational field strength g differs between the surface of the Earth and the surface of other bodies in space, including the Moon because of the planet or moon's mass The greater the mass of the planet then the greater its gravitational field strength A higher gravitational field strength means a larger attractive force towards the centre of that planet or moon g varies with the distance from a planet, but on the surface of the planet, it is roughly the same Objects attracted towards centre of the Earth due tofrom its gravitational field Theare strength of the field around the the planet decreases as the distance the strength planet increases However, the value of g on the surface varies dramatically for different planets and moons 16 Both the weight of any body and the value of the gravitational field strength g The gravitational fieldthe strength (g) on is approximately 10 N/kg differs between surface ofthe theEarth Earth and the surface of other bodies in space, moons The gravitational field strength (g) on the Earth is approximately 10 N/kg The gravitational field strength on the surface of the Moon is less than on the Earth This means it would be easier to lift a mass on the surface of the Moon than on the Earth M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is more than on the Earth This means it would be harder to lift a mass on the gas giants than on the Earth Value for g on the different objects in the Solar System On such planets such as Jupiter, an object’s mass remains the same at all points in space However, their weight will be a lot greater meaning for example, a human will be unable to fully stand up A person’s weight on Jupiter would be so large a human would be unable to fully stand up ! Exam Tip You do not need to remember the value of g on different planets for your exam, the value of g for Earth will be given in the exam question. 17 18 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 19 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 20 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 6.2.1 The Sun as a Star DOWNLOAD PDF TEST YOURSELF 1. MOTION ENERGY The Sun 2. THERM " 6.2.1 The Sun as a Star DOWNLOAD PDF TEST YOURSELF The Sun lies at the centre of the Solar System The Sun is a star which makes up The over 99% Sun of the mass of the solar system The fact that most of the mass of the Solar System is concentrated in the Sun is the reason the smaller planets orbit the Sun pull theSolar Sun on the planets keeps them in orbit The SunThe liesgravitational at the centre ofof the System 3. WAVES 4. ELECTR MAGNETI The Sun is a star which makes up over 99% of the mass of the solar system 5. NUCLEA The Sun is a medium sized star consisting of mainly hydrogen and helium The fact that most of the mass of the Solar System is concentrated in the Sun It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum is the reason the smaller planets orbit the Sun 6. SPACE The gravitational pull of the Sun on the planets keeps them in orbit 6.1 Earth The Sun is a medium sized star consisting of mainly hydrogen and helium System It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum 6.2 Stars M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N " 6.2.1 The 6.2.2 Stars 6.2.3 The Universe Our Sun (Image courtesy of NASA) 6.2.4 The Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs, from blue giants to red supergiants These can be classified according to their colour 6.2.5 Hub the Univer Warm objects emit infrared and extremely hot objects emit visible light as well Therefore, the colour they emit depends on how hot they are A star's colour is related to its surface temperature Our Sun (Image courtesy of NASA) A red star is the coolest (at around 3000 K) A blue star is the hottest (at around 30 000 K) Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs, from blue giants to red supergiants These can be classified according to their colour Warm objects emit infrared and extremely hot objects emit visible light as well Therefore, the colour they emit depends on how hot they are A star's colour is related to its surface temperature A red star the coolest (atcorrelates around 3000 K) Theiscolour of a star to its temperature A blue star is the hottest (at around 30 000 K) 21 Nuclear Fusion in Stars Nuclear Fusion in Stars EXTENDED In the centre of a stable star, hydrogen atoms undergo nuclear fusion to form helium The equation for the reaction is shown here: Deuterium and Tritium are both isotopes of hydrogen. They can be formed through other fusion reactions in the star M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N A huge amount of energy is released in the reaction This provides a pressure that prevents the star from collapsing under its gravity The fusion of deuterium and tritium to form helium with the release of energy ? Worked Example An example of a hydrogen fusion reaction which takes place in stars is shown here. Which of the following is a valid reason as to why hydrogen fusion is not 22 currently possible on Earth? The fusion of deuterium and tritium to form helium with the release of energy ? Worked Example An example of a hydrogen fusion reaction which takes place in stars is shown here. Which of the following is a valid reason as to why hydrogen fusion is not currently possible on Earth? A Hydrogen fusion produces dangerous radioactive waste B Hydrogen nuclei require very high temperature to fuse together C Hydrogen is a rare element that would be difficult to get large amounts of M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N D Hydrogen fusion does not produce enough energy to be commercially viable ANSWER: B Hydrogen nuclei have positive charges So two hydrogen nuclei would have a repulsive force between them High temperatures are required to give the nuclei enough energy to overcome the repulsive force The answer is not A because the products of the hydrogen fusion shown in the reaction is helium Helium is an inert gas The answer is not C because hydrogen is a very abundant element It is the most common element in the universe The answer is not D because hydrogen fusion would produce a huge amount of energy TEST YOURSELF NEXT TOPIC Author: Leander Leander graduated with a first-class degree in science and education from Shef winning the Lord Robert Winston Solomon Lipson Prize for dedication to science excellence. She worked as a science teacher, later going on to become a science educational content creator before joining SME. 23 " DOWNLOAD PDF TEST YOURSELF 1. MOT ENERG " DOWNLOAD PDF TheTEST MilkyYOURSELF Way 2. THER Galaxies are made up of billions of stars Milky Way The Universe is made up of many The different galaxies 3. WAV 6.2.2 Stars The Sun is one of billions of stars in a galaxy called the Milky Way Galaxies made upWay of billions Other starsare in the Milky galaxy of arestars much further away from Earth than the The Universe is made up of many different galaxies Sun is Some of these stars also have planets which orbit them M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The Sun is one of billions of stars in a galaxy called the Milky Way Other stars in the Milky Way galaxy are much further away from Earth than the Sun is Some of these stars also have planets which orbit them Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe Astronomical distances such as the distances between stars and galaxies, are so physicists a special to measure themin called the light-year Ourlarge solarthat system is justuse one out of unit potentially billions our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire One light-year is: universe The distance travelled by light through (the vacuum of) space in one year Astronomical distances such as the distances between stars and galaxies, are so The speed light is theuse universal speed nothing them can travel faster the large thatofphysicists a special unitlimit, to measure called the than light-year speed of light Oneover light-year is: But astronomical distances, light actually travels pretty slowly The diameter of the Milky Way is approximately 100 000 light-years The This distance lighttake through (the vacuum of) across space itin one year meanstravelled that lightby would 100 000 years to travel The speed of light is the universal speed limit, nothing can travel faster than the speed of light 24 12 15 But over astronomical distances, light One light year = 9.5 × 10 km = 9.5 × 10actually m travels pretty slowly The diameter of the Milky Way is approximately 100 000 light-years EXTENDED 4. ELEC MAGNE 5. NUC 6. SPA 6.1 Ear System 6.2 Sta 6.2.1 T 6.2.2 S 6.2.3 T Univers 6.2.4 T 6.2.5 H the Uni EXTENDED One light year = 9.5 × 10 12 km = 9.5 × 10 15 m Life Cycle of Stars EXTENDED 1. Nebula Lifecycle of stars 2, downloadable IGCSE & GCSE Physics revision notes All stars form from a giant interstellar cloud of hydrogen gas and dust called a nebula 2. Protostar M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The force of gravity within a nebula pulls the particles closer together until it forms a hot ball of gas, known as a protostar As the particles are pulled closer together the density of the protostar will increase This will result in more frequent collisions between the particles which causes the temperature to increase 3. Main Sequence Star Once the protostar becomes hot enough, nuclear fusion reactions occur within its core The hydrogen nuclei will fuse to form helium nuclei Every fusion reaction releases heat (and light) energy which keeps the core hot Once a protostar is formed, its life cycle will depend on its mass The different life cycles are shown below: Flow diagram the life cycle of & a GCSE star which is the samenotes size as the Sun Lifecycle of starsshowing 1, downloadable IGCSE Physics revision (solar mass) and the lifecycle of a star which is much bigger than the Sun Once a star is born it is known as a main-sequence star During the main sequence, the star is in equilibrium and said to be stable The inward force due to gravity is equal to the outward pressure force from the fusion reactions 4. Red Giant or Red Super Giant After several billion years the hydrogen causing the fusion reactions in the star will begin to run out Once this happens, the fusion reactions in the core will start to die down This causes the core to shrink and heat up The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down A new series of reactions will then occur25around the core, for example, helium nuclei will undergo fusion to form beryllium 26 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N the fusion reactions 4. Red Giant or Red Super Giant After several billion years the hydrogen causing the fusion reactions in the star will begin to run out Once this happens, the fusion reactions in the core will start to die down This causes the core to shrink and heat up The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form beryllium These reactions will cause the outer part of the star to expand A star the same size as the Sun or smaller will become a red giant A star much larger than the Sun will become a red super giant It is red because the outer surface starts to cool 5. For Red Giant Stars Planetary Nebula Once this second stage of fusion reactions have finished, the star will become unstable and eject the outer layer of dust and gas The layer of dust and gas which is ejected is called a planetary nebula White Dwarf The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease Black Dwarf Once the star has lost a significant amount of energy it becomes a black dwarf It will continue to cool until it eventually disappears from sight Lifecycle of Solar mass stars, downloadable IGCSE & GCSE Physics revision notes 27 28 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 6.2.3 The Expanding Universe DOWNLOAD PDF TEST YOURSELF " 6.2.3 The Expanding Universe " Galaxies & Redshift DOWNLOAD PDF TEST YOURSELF Usually, when an object emits waves, the wavefronts spread out symmetrically If the wave source moves, the waves can become squashed together or stretched out Galaxies & Redshift 1. MOTION, FORCES & ENERGY 2. THERMAL PHYSICS 3. WAVES 4. ELECTRICITY & MAGNETISM 5. NUCLEAR PHYSICS Usually, when an object emits waves, the wavefronts spread out symmetrically If the wave source moves, the waves can become squashed together or stretched 6. SPACE PHYSICS out M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N 6.1 Earth & The Solar System Diagram showing the wavefronts produced from a stationary object and a moving object A moving object will cause the wavelength, λ, (and frequency) of the waves to change: The wavelength of the waves in front of the source decreases and the frequency increases The wavelength behind the source increases and the frequency decreases This effect is known as the Doppler effect 6.2 Stars & The Universe 6.2.1 The Sun as a Star 6.2.2 Stars 6.2.3 The Expanding Universe 6.2.4 The Big Bang Theory 6.2.5 Hubble & The Age of the Universe The Doppler effect also affects light If an object moves away from an observer the wavelength of light increases This is known as redshift as the light moves towards the red end of the spectrum Diagram showing the wavefronts produced from a stationary object and a moving Redshift is: object An increase in the observed wavelength of electromagnet radiation emitted from receding stars and galaxies A moving object will cause the wavelength, λ, (and frequency) of the waves to change: The wavelength of the waves in front of the source decreases and the frequency increases The wavelength behind the source increases and the frequency decreases This effect is known as the Doppler effect The Doppler effect also affects light If an object moves away from an observer the wavelength of light increases This is known as redshift29as the light moves towards the red end of the spectrum Light from a star that is moving towards an observer will be blueshifted and light M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Light from a star that is moving towards an observer will be blueshifted and light from a star moving away from an observer will be redshifted The observer behind observes a red shift The Milky Way is just one of billions of galaxies that make up the Universe Light emitted from distant galaxies appears redshifted when compared with light emitted on Earth The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to us from a distant galaxy 30 Sun, and the light coming to us from a distant galaxy Comparing the light spectrum produced from the Sun and a distant galaxy M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The diagram also shows that the light coming to us from distant galaxies is redshifted The lines on the spectrum are shifted towards the red end This indicates that the galaxies are moving away from us If the galaxies are moving away from us it means that the universe is expanding The observation of redshift from distant galaxies supports the Big Bang theory Another observation from looking at the light spectrums produced from distant galaxies is that the greater the distance to the galaxy, the greater the redshift This means that the further away a galaxy, the faster it is moving away from us Graph showing the greater the distance to a galaxy, the greater the redshift TEST YOURSELF 31 NEXT TOPIC DOWNLOAD PDF " TEST YOURSELF 1. MOT ENERG The Big Bang 2. THE 6.2.4 The Big Bang Theory " DOWNLOAD PDF TEST YOURSELF Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense The Big Bang Then there was a giant explosion, which is known as the Big Bang This caused the universe to expand from a single point, cooling as it does so, to form the universe today Around 14 billion years ago, the Universe began from a very small region that Each point expands away from the others wasThis extremely hotgalaxies and dense is seen from moving away from each other, and the further away they areathe faster they movewhich is known as the Big Bang Then there was giant explosion, This caused the universe to expand from a single point, cooling as it does so, to M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Redshift in the light from distant galaxies is evidence that the Universe is form the and universe today expanding supports the Big Bang Theory Each point expands away from the others As a result of the initial explosion, the Universe continues to expand This is seen from galaxies moving away from each other, and the further away they are the faster they move Redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory As a result of the initial explosion, the Universe continues to expand All galaxies are moving away from each other, indicating that the universe is expanding An analogy of this is points drawn on a balloon where the balloon represents space and the points as galaxies When the balloon is deflated, all the points are close together and an equal distance apart As the balloon expands, all the points become further apart by the same amount This is because the space itself has expanded between the galaxies 32 All galaxies are moving away from each other, indicating that the universe is expanding 3. WA 4. ELE MAGN 5. NUC 6. SPA 6.1 Ea System 6.2 Sta 6.2.1 T 6.2.2 S 6.2.3 T Univer 6.2.4 T Theor 6.2.5 H the Un M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N This is because the space itself has expanded between the galaxies A balloon inflating is similar to the stretching of the space between galaxies Redshift and CMBR Evidence for the Big Bang The Big Bang theory is very well supported by evidence from a range of sources The main pieces of evidence are Galactic red-shift Cosmic Microwave Background Radiation (CMBR) Evidence from Galactic Red-Shift Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy 33 Evidence from Galactic Red-Shift Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy A balloon inflating is similar to the stretching of the space between galaxies Redshift and CMBR Evidence for the Big Bang The Big Bang theory is very well supported by evidence from a range of sources The main pieces of evidence are Galactic red-shift Cosmic Microwave Background Radiation (CMBR) Evidence from Galactic Red-Shift M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy Comparing the light spectrum produced from the Sun and a distant galaxy Red-shift provides evidence that the Universe is expanding because: Red-shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum This is because light waves are stretched by the expansion of the universe so Comparing the light spectrum produced from the Sun and a distant galaxy the wavelength increases (or frequency decreases) Red-shift provides that theare Universe is expanding because:from us This indicates that theevidence galaxies moving away Red-shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum This is because light waves are stretched by the expansion of the universe so the wavelength increases (or frequency decreases) This indicates that the galaxies are moving away from us Light spectrums produced from distant galaxies are red-shifted more than nearby galaxies spectrums produced fromthe distant galaxies are red-shifted than nearby This shows Light that the greater distance to the more galaxy, the greater the galaxies redshift This shows that the greater the distance to the galaxy, the greater the redshift This means that the further away a galaxy is, the faster it is moving away This means that the further away a galaxy is, the faster it is moving away from from the Earth the Earth These observations imply that the universe is expanding and therefore support the Big Bang Theory These observations imply that the universe is expanding and therefore support the Big Bang Theory Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began34 with a “big bang” EXTENDED EXTENDED Evidence from CMB Radiation The discovery of the CMB (Cosmic Microwave Background) radiation led to the Big Bang theory becoming the currently accepted model The CMB is a type of electromagnetic radiation which is a remnant from the early stages of the Universe It has a wavelength of around 1 mm making it a microwave, hence the name Cosmic Microwave Background radiation EXTENDED M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N In 1964, Astronomers discovered radiation in the microwave region of the electromagnetic spectrum coming from all directions and at a generally uniform temperature of 2.73 K They were unable to do this any earlier since microwaves are absorbed by the atmosphere Around this time, space flight was developed which enabled astronomers to send telescopes into orbit above the atmosphere According to the Big Bang theory, the early Universe was an extremely hot and dense environment As a result of this, it must have emitted thermal radiation The radiation is in the microwave region This is because over the past 14 billion years or so, the radiation initially from the Big Bang has become redshifted as the Universe has expanded Initially, this would have been high energy radiation, towards the gamma end of the spectrum As the Universe expanded, the wavelength of the radiation increased Over time, it has increased so much that it is now in the microwave region of the spectrum The CMB is a result of high energy radiation being redshifted over billions of years 35 EXTENDED EXTENDED The CMB radiation is very uniform and has the exact profile expected to be emitted from a hot body that has cooled down over a very long time This phenomenon is something that other theories (such as the Steady State Theory) cannot explain EXTENDED M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N The CMB is represented by the following map: The CMB map with areas of higher and lower temperature. Places with higher temperature have a higher concentration of galaxies, Suns and planets EXTENDED This is the closest image to a map of the observable Universe The different colours represent different temperatures The red / orange / brown regions represent warmer temperature indicating a higher density of galaxies The blue regions represents cooler temperature indicating a lower density of galaxies The temperature of the CMB radiation is mostly uniform, however, there are minuscule temperature fluctuations (on the order of 0.00001 K) This implies that all objects in the Universe are more or less uniformly spread out 36 6.2.5 Hubble & The Age of the Universe " DOWNLOAD PDF TEST YOURSELF Hubble Constant Calculations EXTENDED M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N In 1929, the astronomer Edwin Hubble showed that the universe was expanding He did this by observing that the absorption line spectra produced from the light of distant galaxies was shifted towards the red end of the spectrum This doppler shift in the wavelength of the light is evidence that distant galaxies are moving away from the Earth Hubble also observed that light from more distant galaxies was shifted further towards the red end of the spectrum compared to closer galaxies From this observation he concluded that galaxies or stars which are further away from the Earth are moving faster than galaxies which are closer Hubble’s law states: The recessional velocity v of a galaxy is proportional to its distance from Earth Hubble’s law can be expressed as an equation: H0 ! v d Where: H0 = Hubble constant, this will be provided in your examination along with th -1) 37 correct units (km s-1 Mpc –18 The accepted value is that H0 = 2.2 × 10 per second ENER Hubble Constant Calculations EXTENDED 2. TH 3. WA In 1929, the astronomer Edwin Hubble showed that the universe was expanding He did this by observing that the absorption line spectra produced from the light of distant galaxies was shifted towards the red end of the spectrum This doppler shift in the wavelength of the light is evidence that distant galaxies are moving away from the Earth M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Hubble also observed that light from more distant galaxies was shifted further towards the red end of the spectrum compared to closer galaxies From this observation he concluded that galaxies or stars which are further away from the Earth are moving faster than galaxies which are closer Hubble’s law states: The recessional velocity v of a galaxy is proportional to its distance from Earth Hubble’s law can be expressed as an equation: H0 ! v d Where: H0 = Hubble constant, this will be provided in your examination along with the correct units (km s-1 Mpc-1) –18 The accepted value is that H0 = 2.2 × 10 per second v = recessional velocity of an object, the velocity of an object moving away from an observer (km s-1) d = distance between the object and the Earth (Mpc) As the equation shows, the Hubble Constant, H0 is defined as: The ratio of the speed at which the galaxy is moving away from the Earth, to its distance from the Earth 38 4. ELE MAG 5. NU 6. SP 6.1 E Syste 6.2 S 6.2.1 6.2.2 6.2.3 Unive 6.2.4 6.2.5 the U Age of the Universe EXTENDED Since Hubble's Law states that H0 ! v d It can be rearranged to show that 1 d ! H0 v M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Hubble’s law shows that the further away a star is from the Earth, the faster it is moving away from us A key aspect of Hubble’s law is that the furthest galaxies appear to move away the fastest The gradient of the graph can be used to find the Age of the Universe When the distance equals zero, this represents all the matter in the Universe being at a single point This is the singularity that occurred at the moment of the Big Bang The units of the gradient are per second (the same as the units of the Hubble Constant) 1 By taking the reciprocal, or, H 0 the units will become seconds Therefore the reciprocal of the gradient represents time and gives the amount of time which the Universe has been expanding for Astronomers have used this formula to estimate the age of the Universe at about 39 13.7 billion years 13.7 billion years ? Worked Example A distant galaxy is 20 light-years away from Earth. Use Hubble’s Law to determine the velocity of the galaxy as it moves away from Earth. The Hubble constant is currently agreed to be 2.2 x 10-18 s-1. Step 1: List the known quantities: d = 20 light years Ho = 2.2 x 10-18 s-1 M D PH . M YS AM IC UN S U 01 & R 67 MA RO 48 TH S 98 T HID 29 EA M 7 C IL H O ER N Step 2: Convert 20 light-years to m: From the data booklet: 1 ly ≈ 9.5 x 1015 m 15 17 So, 20 ly = 20 x (9.5 x 10 ) = 1.9 x 10 m Step 3: Substitute values into Hubble's Law: From the data booklet: v ≈ H0d So, v ≈ (2.2 x 10-18 ) x (1.9 x 1017) = 0.418 m s-1 Step 4: Confirm your answer: The velocity of the galaxy as it moves away from Earth 0.42 m s-1 TEST YOURSELF Author: Lindsay Lindsay graduated with First Class Honours from the University of Greenwich and Communication MSc at Imperial College London. Now with many years’ experience and Examiner for A Level and IGCSE Physics (and Biology!), her love of communica Physics has brought her to Save My Exams where she hopes to help as many stude their next steps. Resources Members Company Quick Links Home Members Home About Us Join Account Contact Us GCSE Revision Notes Logout Jobs Support 40 Terms Privacy IGCSE Revision Notes A Level Revision
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