Download Chapter 12 - Our Place in the Universe

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.