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22/05/2017 Further Physics Studying the Universe M Barker Shirebrook Academy P7.1 Naked Eye Astronomy 22/05/2017 The Nine(?) planets of our Solar System 22/05/2017 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Which planets can be seen with the naked eye? 22/05/2017 The Earth orbits the sun… …every year (365 1/4 days) Ellipse The Spin of the Earth The Earth rotates on its axis every 24 hours… What’s wrong with this picture? 22/05/2017 The Spin of the Earth 22/05/2017 Because of this spin the sun and stars appear to “move” across the sky in an east-west direction… The sun takes 24 hours, the stars take slightly less than this and the moon takes longer! Solar days vs Sidereal days 22/05/2017 Basically, a solar day is the normal 24 hours. However, the Earth actually spins in 23 hours 56 minutes. This period of time is called a “Sidereal day”. Here’s why: After one day the Earth is in a different position and so the sun and stars will “appear” in the same place after 23 hours 56 mins, not 24 hours. The Earth and the Moon 22/05/2017 The moon appears to move east-west across the sky in 24 hours 49 minutes. This is because the moon moves around during the course of 24 hours: The moon is directly over this point at the moment. 1) After 24 hours the Earth has rotated once but the moon has “moved on” 2) Therefore the Earth needs to rotate for another 49 minutes for the moon to appear in the same place Phases of the moon Appearance: 22/05/2017 22/05/2017 Appearance: 22/05/2017 Appearance: Eclipses 22/05/2017 Solar eclipses occur when the moon is between the sun and the Earth: Total eclipse Partial eclipse Eclipses 22/05/2017 The moon takes a _____ to orbit the Earth. However, solar eclipses do not occur every month because the moon’s orbit is inclined at 5O to that of the ______. Eclipses only occur when the moon passes through the _____ – the apparent path the ____ traces out across the _____. Words – sky, sun, month, ecliptic, Earth Eclipses 22/05/2017 Lunar eclipses occur when the Earth is between the sun and the moon: During a lunar eclipse the moon might look like this: Viewing stars 22/05/2017 The stars we see in the summer and in the winter should be different: Stars in the summer: Stars in the winter: Viewing Planets 22/05/2017 The position of Venus changes in a different way to the stars behind it (“retrograde motion”). This was the first evidence that planets orbit the sun: Viewing stars Positive angle of declination Negative angle of declination 22/05/2017 22/05/2017 P7.2 Light, Telescopes and Images Refraction 22/05/2017 Refraction through a glass block 22/05/2017 Light slows down and bends towards the normal due to entering a more dense medium Light slows down but is not bent, due to entering along the normal Light speeds up and bends away from the normal due to entering a less dense medium Lenses Lenses use the idea of refraction: When light enters a MORE DENSE medium it slows down… A prism uses this idea to split light. The frequency of light does not change, only the speed and the wavelength do: 22/05/2017 Converging and diverging lenses 22/05/2017 CONVERGING (Convex) Thickest at the centre DIVERGING (Concave) Thinnest at the centre Ray Diagrams for Lenses 22/05/2017 The rays of light are refracted INWARDS and meet at the focus, F. F F The image formed is REAL – in other words, it can be seen on a screen The rays of light are refracted OUTWARDS. A VIRTUAL image is formed – in other words, the image doesn’t actually exist Ray diagrams 22/05/2017 To draw ray diagrams follow these three rules: 1) Draw a ray from the TOP of the object PARALLEL to the axis and then going through F: F F 2) Draw a ray from the TOP of the object going through the CENTRE of the lens (which will be undeviated) 3) Draw a ray from the top of the object through F to the left of the lens and parallel to the axis This image is REAL, INVERTED and DIMINISHED Ray diagrams 2 22/05/2017 If the object is below the axis follow this step: 4) Draw a ray from the bottom of the object parallel to the axis and then up through the focal point: F F 22/05/2017 F F 22/05/2017 F F 22/05/2017 F F 22/05/2017 F F 22/05/2017 F F More about lenses 22/05/2017 Compare thin and thick lenses: Lenses are measured in units called “dioptres”: Power in dioptres = 1 focal length in m …where converging lenses (for long sighted people) have positive values and diverging lenses (for short sighted people) have negative values. Notice that these glasses have got a large curvature. How would you make strong glasses but also make them thinner and with less curvature? Lenses in Telescopes 22/05/2017 Because stars are very far away, the rays of light from them enter a telescope effectively parallel: Objective lens (convex) Focal point Eyepiece lens Notice that this telescope has two converging lenses. The more powerful one is the eyepiece lens. Using Concave Mirrors 22/05/2017 Notice that concave mirrors can be used to focus light: Concave mirror F Mirrors in Telescopes 22/05/2017 Astronomical telescopes tend to use large concave mirrors as well as a convex lens. This allows them to collect more light and the bigger the telescope, the more light they collect: Eyepiece lens to produce a magnified image Objective lens (convex) Flat mirror Concave mirror to collect light from the object Magnification 22/05/2017 Basically, magnification means “how much bigger the object looks”: Magnification = Focal length of objective lens Focal length of eyepiece lens Example questions: 1) What is the magnification of a telescope with an objective lens focal length of 15m and a eyepiece focal length 3m? 2) A telescope has a 100x magnification. If the focal length of the eyepiece is 50cm what is the focal length of the objective lens? Diffraction 22/05/2017 Diffraction is an effect seen when a wave travels around a corner or through a narrow gap: More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased) Diffraction in Telescopes 22/05/2017 The hole at the end of a telescope is called an aperture. To avoid problems caused by diffraction through this aperture, the size of the hole must be much larger than the wavelength of the radiation it is observing. Spectra Recall that spectra can be produced by the refraction of light through a prism: A spectrum can also be produced by the diffraction of light through a diffraction grating (basically a set of very small gaps): 22/05/2017 P7.3 Mapping the Universe 22/05/2017 Distances in space 22/05/2017 The Sun, our closest star, is 1.6x10-5 light years away from us. The next closest star, Proxima Centauri (4.2 light years away) The centre of our galaxy, the Milky Way, is around 26,000 light years away. The Andromeda Galaxy (our closest galaxy) – approximately 2.5 million light years away Parallax Parallax uses the idea that, as the Earth moves around the sun, the apparent position of “nearby” stars changes in relation to distant stars behind it: 22/05/2017 Distant stars Nearby star p Stars that are further away from the Earth will have a smaller parallax angle. 1 A.U Parallax angle p Using parallax 22/05/2017 Astronomers often use the “parsec” to describe galactic distances. A parsec is roughly 3¼ light years. Angles involved in parallax measurements are often very small and are measured in seconds of an arc (arcseconds). A second of an arc is 1/60th of a minute of an arc, which is 1/60th of a degree. In other words, one arcsecond = 1/3600th of a degree. A parsec is defined as the distance to a star with a parallax angle of one arcsecond. The approximate distance to a star can be calculated using this formula: Distance to star (in parsecs, pc) = 1 Parallax angle (arcseconds) 1) Proxima Centauri has a parallax angle of 0.77 arcseconds. How far away is it in parsecs? 2) The nearest galaxy, Andromeda, is 0.77Mpc away. What would the parallax angle be in arcseconds? Luminosity 22/05/2017 “Luminosity” means “how bright the star is”. The luminosity of a star depends on its size and temperature. 22/05/2017 Measuring distance using brightness The sun looks very bright. But, to be fair, it’s very close to us so it should look bright! 22/05/2017 Measuring distance using brightness When I look at these stars some appear brighter than others. This because they are either brighter stars or closer to me. For example, the star Antares is 10,000 times brighter than the sun but it is 500 light years away from me, so it is only the 15th brightest star in the night sky. There are also “Cepheid variable” stars, which vary in brightness. Astronomers use the frequency of its “pulsing” to work out how bright it is and how far away it is. Cepheid Variable Stars 22/05/2017 A “Cepheid Variable” star is one whose magnitude varies over time: Apparent magnitude Time The period is related to their luminosity. You can measure the distance of these stars by: 1) Observe for a few weeks to find the period 2) Use various mathematical relationships to determine the distance Measurements of the distances to Cepheid variable stars shows that they are very far away and must be outside of our galaxy! The Milky Way 22/05/2017 OUR SUN is one of millions of stars that orbit the centre of the Milky Way Scientists realised that the Milky Way contains millions of stars only after the invention of telescopes. Telescopes also helped scientists discover “fuzzy” objects and these were originally called nebulae. 22/05/2017 The Curtis-Shapley debate, 1920 After observing nebulae I said that the universe consisted of many galaxies. Heber Curtis, 1872 - 1942 I disagreed and said that the universe contained only one big galaxy – the Milky Way, and this galaxy contained the nebulae. Harlow Shapley, 1885-1972 I settled the debate by observing Cepheid Variable stars and found that they were much further away than our own galaxy. Edwin Hubble, 1889-1953 Hubble’s Law 22/05/2017 Recession velocity Astronomers have observed Red Shift and Cepheid variable stars in lots of galaxies and deduced the fact that more distant galaxies are moving faster than closer ones: x x x x x x xx x x xx Distance to galaxy Recession velocity Hubble’s Law 22/05/2017 x x x x x x x xx x xx Distance to galaxy Using this evidence I concluded two things: that the universe is expanding AND the recession velocity is proportional to the galaxy’s distance from us, therefore: V = Hd …where H = Hubble’s Constant (2±1 x 10-18 s-1) (or H can be measured in kms-1Mpc-1 and distance is in Mpc) Questions on Hubble’s Law 22/05/2017 1) A galaxy is 5x1020km from Earth. If Hubble’s constant is 2x10-18s-1 calculate how fast that galaxy is moving. 2) Another galaxy is 10Mpc from Earth. Taking H to be 75kms-1Mpc-1 calculate its recessional velocity. 3) Another galaxy has a recessional velocity of 500km/s. Calculate the distance to the galaxy in both megaparsecs and kilometres (take H to be 2x10-18s-1). The Big Bang Theory 22/05/2017 The motion of galaxies, as observed with telescopes, indicates that galaxies are all moving away from each other. In other words, the universe is expanding. What happened to start this off around 14 billion years ago? 22/05/2017 22/05/2017 P7.4 The Sun, the stars and their surroundings Wavelength and Energy Output 22/05/2017 All stars give out lots of different wavelengths of radiation: Luminosity T=6000K T=4500K T=3000K Wavelength When wavelength goes up (and frequency goes down) what happens to the star’s intensity and temperature? Ionisation revised 22/05/2017 Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons: Spectra – introduction 22/05/2017 Spectra Source of light 22/05/2017 “Spectra” Absorption Spectra helium Some wavelengths of light are absorbed by the gas – an “absorption spectrum”. 22/05/2017 Spectra Continuous spectrum Absorption spectrum Emission spectrum 22/05/2017 Emission Spectra Hydrogen Helium Sodium 22/05/2017 Spectra 22/05/2017 Consider a ball in a hole: When the ball is here it has its lowest gravitational potential energy. 5J We can give it potential energy by lifting it up: If it falls down again it will lose this gpe: 5J 30J 20J Spectra 22/05/2017 Absorption spectrum When these wavelengths are absorbed the energy is used to raise the electron up to a higher energy level in the atom: Emission spectrum When the electron drops back down again it gives out these wavelengths depending on the energy level it falls from By looking at the spectra from stars we can work out what gases are inside it. This is how we know the sun is made of hydrogen. An Example Question Here are the emission lines for hydrogen and helium: Hydrogen Helium Which gas is in this star? Absorption spectrum 22/05/2017 Particle Motion in Gases 22/05/2017 Gas pressure is caused by particles hitting the side of a container. Anything we do that increases those collisions will increase the pressure: Particle Motion in Gases 22/05/2017 Consider decreasing the volume: The particles should collide with the sides of the container _____ often, therefore the pressure is ________. Particle Motion in Gases 22/05/2017 Now consider increasing the temperature as well: The particles should collide with the sides of the container _____ often, therefore the pressure is ________. This could cause the container to ______. 22/05/2017 Pressure and Temperature in gases P T 22/05/2017 Pressure and Temperature in gases P “Absolute Zero” -2730C T The Kelvin temperature scale starts at 0K (= -273OC). To convert degrees centigrade into Kelvin simply add 273. Absolute Temperature 22/05/2017 “Absolute Temperature” starts at 0K and represents the temperature at which particles have zero kinetic energy. It goes up in the same steps as OC. For example: 1) The freezing point of water is 273K 2) The boiling point of water is 373K 3) Room temperature is around 293K Lord Kelvin, 1824-1907 22/05/2017 Volume and Temperature in Gases V For a constant mass of gas at fixed pressure, the volume occupied by the gas is proportional to its absolute temperature. Jacques Charles, 1746-1823 0K T 22/05/2017 Volume and Temperature in Gases Provided the pressure of a gas stays the same we can use this relationship to calculate the volume of a gas: V1 = V2T1 T2 Jacques Charles, 1746-1823 1) A gas changes in temperature from 200K to 300K. If its original volume was 2m3 what is the new volume? 3m3 2) Another gas is halved in volume. What will happen to its temperature? It will halve 3) A third gas is kept at constant pressure while being compressed from 20 litres to 15 litres. If its new temperature is 275K what was its original temperature? 367K Boyle’s Law 22/05/2017 “For a fixed mass of gas at constant temperature (“isothermal”), pressure is proportional to 1/volume.” Robert Boyle 1627-1691 Higher temperature? Let’s draw this… P P V 1/V Pressure and Volume in gases 22/05/2017 This can be expressed using the equation: Initial Pressure x Initial Volume = Final Press. x Final Vol. PIVI = PFVF 1) A gas has a volume of 3m3 at a pressure of 20N/m2. What will the pressure be if the volume is reduced to 1.5m3? 40N/m2 2) A gas increases in volume from 10m3 to 50m3. If the initial pressure was 10,000N/m2 what is the new pressure? 20KN/m2 3) A gas decreases in pressure from 100,000 Pascals to 50,000 Pascals. The final volume was 3m3. What was the initial volume? 4) The pressure of a gas changes from 100N/m2 to 20N/m2. What is the ratio for volume change? 1.5m3 1:5 The Gas Equation 22/05/2017 A while ago we said that Pressure x Volume = Constant (Boyle’s Law)… …also, we just said pressure is proportional to temperature (Pressure law)… …and we said volume is proportional to temperature (Charles’ Law)… Combining these three equations gives: Pressure x volume = constant temperature P1V1 = P2V2 T1 T2 Some example questions 1) An ideal gas has a volume of 2m3 and a pressure of 101KPa (101,000 N/m2) at a temperature of 300K. The gas is then increased in temperature to 400K but kept at constant volume. Calculate the new pressure. 2) The same gas is then allowed to cool to 200K while being kept at constant pressure. Calculate the new volume. 3) Another gas at 300K and 101KPa is allowed to halve in volume while being kept at the same pressure. What is the new temperature? 22/05/2017 135KPa 1m3 150K The Life Cycle of a Star 22/05/2017 Stage 1: Nebulae A nebulae is a collection of dust, gas and rock. Some examples of nebulae… 22/05/2017 22/05/2017 Dark nebula 22/05/2017 Emission nebula 22/05/2017 Planetary nebula 22/05/2017 Reflection nebula Stage 2: Protostar Gravity will slowly pull these particles together… As they move inwards their gravitational potential energy is converted into heat and a PROTOSTAR is formed. The temperature increases because the volume of the gas decreases. 22/05/2017 Stage 3: Main Sequence 22/05/2017 In a main sequence star the forces of attraction pulling the particles inwards are _________ by forces acting outwards due to the huge __________ inside the star. Stars are basically ________ reactors that use _______ as a fuel. During its main sequence a star will release energy by converting hydrogen and helium (light elements) into _________ elements. Our sun is an example of a main sequence star – it’s in the middle of a 10 billion year life span Words – heavier, balanced, hydrogen, nuclear, temperatures Nuclear Fusion in stars Proton 22/05/2017 Neutron Nuclear fusion happens in stars but it’s not possible to use it in power stations yet as it needs temperatures of around 10,000,000OC Fusion Reactions 22/05/2017 When the temperature of the star increases nuclear fusion reactions will start. Here’s the equation: 1 41p 4 He 2 + 0 2 1 β+ + 2ν + energy 1) What are β+ particles and why are they emitted? 2) Why is energy released in this reaction? Energy-mass equivalence 22/05/2017 In every fusion (or fission) reaction a little bit of mass is “lost”. This mass turns into energy and you can use my famous equation to work out how much: E=mc2 Einstein (1879-1955) …where E = the energy released, m = the mass converted into energy and c = the speed of light. The Structure of our Sun 22/05/2017 Photosphere – where energy is released into space The core – the hottest part of the sun where nuclear fusion takes place. The hotter the star, the heavier the elements it will make The hotter a star is, the higher peak wavelength it emits (e.g. blue stars are hotter than red stars). The convective zone – where energy is transported to the surface by convection currents. 22/05/2017 The Hertzsprung-Russell Diagram L (relative to our sun) 106 Supergiants 104 102 The sun 1 Main sequence 10-2 10-4 40,000 20,000 10,000 5,000 2,500 T (K) Stage 4: Red Giant 22/05/2017 Eventually the hydrogen and helium will run out and the star will leave the main sequence. When this happens the star will become colder and redder and start to swell… If the star is relatively small (like our sun) the star will become a RED GIANT If the star is big (at least 4 times the size of our sun) it will become a RED SUPERGIANT In red giants and supergiants, nuclei such as oxygen and nitrogen are formed. Stage 5: The Death 22/05/2017 What happens at this point depends on the size of the star… 1) For SMALL stars the red giant will collapse under its own gravity and form a very dense white dwarf, where no fusion reactions will take place: Red giant White dwarf Black dwarf 2) If the star was a RED SUPERGIANT it will shrink and then EXPLODE, releasing massive amounts of energy, dust and gas. Before After 22/05/2017 This explosion is called a SUPERNOVA. Heavier nuclei such as iron could be made in the core prior to the supernova. The dust and gas on the outside of the supernova are thrown away by the explosion and the remaining core turns into a NEUTRON STAR. 22/05/2017 If the star is big enough it could become a BLACK HOLE. Life on Other Planets 22/05/2017 Research task Is it likely that other planets in our solar system could have life? Explain your answer. Extend your inquiry to other solar systems – what criteria must be met in order for a planet to potentially have life on it? Searching for Aliens 22/05/2017 Humans have been searching for me for over 50 years. Do you think I exist? Quite possibly, We know that hundreds of “nearby” stars have planets around them and we also know that there are billions of stars in the universe so surely one of these planets has life… P7.5 The Astronomy Community 22/05/2017 Observing the Universe 22/05/2017 Observing the Universe 22/05/2017 Consider different types of telescope: Ground-based telescopes Space-based telescopes What are the advantages and disadvantages of each? 22/05/2017 Common sites for Ground-Based telescopes Chile Hawaii What advantages do these sites offer? Canary Islands 22/05/2017 Using Computers with Telescopes Telescopes like this one are often controlled by computers. What advantages does this offer? 1) The telescope can be controlled remotely 2) It can track objects continuously 3) It’s more precise 4) The computer can record and process the data Hawaii 22/05/2017 Benefits of observing above the atmosphere Clearly, ground-based telescopes are a problem because of a number of things: • The amount of light absorbed by the atmosphere • Bad weather • Light is refracted, diffracted and scattered by the atmosphere (causing stars to “twinkle”) • Light pollution from nearby cities The solution is to put telescopes in space or build them high up on a mountain. Here are two examples of space-based telescopes: Hubble Space Telescope (HST) 22/05/2017 • Launched in 1990, due to finish operating in 2010 • Takes images in the visible light, ultra-violet and near infra red regions • Orbits the Earth every 97 minutes 22/05/2017 Infra Red Astronomical Satellite (IRAS) • Surveys infra red patterns in space • Launched in 1983 and operated for 11 months • The number of known astronomical bodies was increased by 70% due to infra red observations International Collaboration 22/05/2017 Here’s the International Space Station, a joint project by NASA, the Russian Space Agency, the European Space Agency, the Japan Aerospace Exploration Agency and the Canadian Space Agency: What advantages does international collaboration bring in terms of: 1) Cost? 2) Pooling of expertise? Building Observatories 22/05/2017 What factors are involved in planning, building and closing down a large observatory? • Cost • Environmental impact • Social impact • Working conditions Hawaii