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Chapter 12 – Our Place in the Universe Overview Section 12.1 Observing the Universe Lesson 1 Outline Much of this Unit can also be followed through using the Powerpoints in the main Chapter 12 folder 12.1 Observing the Universe Learning outcomes astronomical distances in the Solar System can be measured by radar there are many units for astronomical distances including light years and parsecs distances to nearby stars can be found by parallax distances can be found using the inverse-square law for intensity of light some larger distances are estimated by the apparent brightness of ‘standard candles’, e.g. Cepheid variables and Type 1a supernovae distant objects are observed as they once were because it has taken light time to travel the cosmological distance scale is still subject to uncertainty Spectra of distant objects over a wide range of wavelengths provide knowledge of their chemical composition. velocities of astronomical objects can be established by the Doppler shift with d v c for v much less than c Lesson 1: How far away are astronomical objects and how do we know – radar ranging Objectives: an introduction to ‘what is out there’ that trip time of radiation can be used to find out distance that astronomical distances are vast and we need different units to describe them Starter Activity 10E: Experiment: What do you know about cosmology? And discussion Show the ppt – How big is our planet The big idea here is trying to find the distance to remote objects; start nearby and increase the distance, thinking about which methods are suitable and why Radar – use ultrasound detector to introduce idea. Discuss over what range this is useful and why it might not be suitable for space! Either do 20W Using time to measure distance here, or introduce and leave for homework Introduce light second, minute and year as unit of distance Looking into the night sky is like looking back in time because the light takes time to get here. Introduce astronomical unit (AU) (Earth sun distance) as unit of distance, and mention the parsec Try activity 30W Units for distance measurement Lesson 2/3:. How far away are astronomical objects and how do we know – parallax, inverse square law, standard candles and Cepheid variables Objectives that radar ranging is only suitable for certain distances that parallax can be used for nearby stars but then becomes difficult that Standard Candles have been identified (Type II supernova and Cepheids) that allow us to find the distance to far off stars and distant galaxies Starter: Recall units of measurement (put them in order of size) and radar ranging Discuss why radar ranging has limitations and introduce the idea of parallax 20E Range finding and parallax will fit in here, followed by 20S measuring distances within the Solar System and beyond which links this to radar. Inverse square law – look at butter gun example then 40E Brightness and distance can be used as quick demo to demonstrate the idea. With good pupils, you may wish to set and extension to ‘prove’ the inverse square relationship, or move on to 50E Summer Sun remembered which gives a very good answer. 46S Brighter stars aren’t always nearer takes this a stage firther and is worth setting for homework. 52D The brighter stars in the night sky is harder and could be set as extension work for the more able. Now link these ideas to the Universe to see what is out there that can be used as a Standard Candle. Use powerpoint to talk through Cepheid Variables. Simple calculations involving luminosity and brightness are required. 80D Astronomical distances may be used with more able students. Finish this section with the Ladder of Astronomical Distances As an introduction to the next section about Doppler shift, we should have time for the next few slides about spectra and composition. Use the gas discharge tubes to look at spectra. Lesson 4:. How fast are astronomical objects moving and how do we know? Objectives velocities of astronomical objects can be established by the Doppler shift with for v much less than c. d v c Starter: How did we measure speed in Ch 8/9? Summarise methods. Using radar: Display Material 90O: OHT Velocities from radar ranging and discuss quickly. This is straightforward and should not take long. Doppler shift –select from the activities below depending on time. You could lighten this up with 90S the Space Police Use videos in Powerpoint or choose from other Applets in folder or CD. Demo with Doppler ball (110P) and use 95O Non relativistic Doppler shift They need to know d v c Some students will be happy with the derivation, but make it clear that the result is all they need to get. 55S Doppler shifts in astronomy and 60S Binary stars can be attempted now or for homework NB Orbital data to calculate mass needs to be done in Chapter 11. 12.2 Special Relativity will be covered at the end of Chapter 11 12.3 Was there a Big Bang? Learning outcomes red shifts of distant galaxies give evidence of the expansion of the Universe. A red shift z corresponds to an expansion in scale of RNOW 1 z RTHEN evidence that the Universe has evolved from an initial uniform, hot dense state comes from the existence of the cosmic microwave background. further evidence comes from cosmological red-shift Hubble’s Law is v = Hod; Galaxies further away are moving faster 1/Ho gives an estimate of the expansion time-scale of the Universe current estimates of the expansion time-scale of the Universe put it at about 13.7 ± 1 Gy. there are still fundamental problems in explaining the major features of the Universe. Lesson 5: Redshift and the big bang Objectives: the difference between redshift and cosmological redshift z how this provides evidence for the expansion of the universe how to measure expansion further evidence is CMBR After framing the lesson, linking back to ‘what is out there’, discuss ‘how did those things get there?’. This will lead into looking deeper into space, and the ideas of Hubble and red shift. You may wish to use the AN Big Bang timeline to assuage pupils’ interest in the universe. Difference between Doppler shift (things moving towards/away) and cosmological red-shift – the space between galaxies is expanding. You can get Doppler blue shift but NOT cosmological blue shift. There is a simple animation to show this: AN expansion of space Derivation of: And hence z RNOW 1 z RTHEN 95S Redshifts of quasars is a good follow up. Lesson 6: Hubble’s Law and the age of the Universe Objectives: galaxies further away are going faster – Hubble’s Law the age of the universe is approx 1/H it is a very hard thing to measure – values have changed a lot over time. Blow up big balloon with galaxies stuck to it and work out distances and expansion. The P7 worksheet can be successfully used here, or the A Level oriented WS balloon expanding universe Talk about misconception that we are at the centre of the universe using the big balloon. Display Material 160O: OHT Hubble’s law and the age of the Universe Display Material 140O: OHT How the accepted value of the Hubble constant has changed Try 90D Calculating the age of the universe Lesson 7: Further evidence for the expansion of the Universe Objectives: that the cosmic microwave background provides further evidence for the expansion of the universe and its early hot, dense state that there are still problems with our understanding of the nature of the universe Chapter 2 of this TV programme: http://www.pbs.org/wgbh/nova/origins/program-3114.html is a great introduction to the serendipitous discovery made by Penzias and Wilson.