Download GRADE 12A: Physics 7

GRADE 12A: Physics 7
UNIT 12AP.7
15 hours
Astrophysics and cosmology
About this unit
Previous learning
Resources
This unit is the seventh of seven units on physics
for Grade 12 advanced.
To meet the expectations of this unit, students should already know that
each element give rise to its own characteristic line spectra, that
electromagnetic radiation travels through space at finite speed, and that
relative motion between an observer and wave source gives rise to the
Doppler effect. They should know that all matter is affected by gravitational
force. They should also know that stars are powered by nuclear fusion
reactions that lead to the production of heavier elements.
The main resources needed for this unit are:
The unit is designed to guide your planning and
teaching of physics lessons. It provides a link
between the standards for science and your
lesson plans.
The teaching and learning activities should help
you to plan the content and pace of lessons.
Adapt the ideas to meet your students’ needs.
For consolidation activities, look at the scheme of
work for Grade 11A.
You can also supplement the activities with
appropriate tasks and exercises from your
school’s textbooks and other resources.
Introduce the unit to students by summarising
what they will learn and how this will build on
earlier work. Review the unit at the end, drawing
out the main learning points, links to other work
and real world applications.
Expectations
By the end of the unit, students know how emission and absorption
spectra yield information about distant stars and galaxies. They explain the
structure of the visible Universe in terms of the gravitational attraction
between objects. They define and use the parsec and the light-year. They
explain the creation and evolution of stars and know how their ultimate fate
depends on their mass. They know how elements are formed in stars and
how planetary systems arise. They know the ‘big bang’ theory of the origin of
the Universe and can adduce evidence for it.
Students who progress further know how a Hertzsprung–Russell diagram
can be used to summarise properties of stars and to represent changes as
they evolve. They are aware of the evidence on which current theories of
star formation and the big bang are based. They understand the importance
of gravity in determining the ultimate fate of the Universe and know how the
Universe can be, at the same time, finite but without boundaries.
569 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
• binoculars
• planisphere or star map
• materials for making scale models (e.g. fruit, marbles, sand)
• tennis ball and table-tennis ball
Key vocabulary and technical terms
Students should understand, use and spell correctly:
• constellation, nebula, star, planet
• luminosity, intensity, flux
• Hertzsprung–Russell (HR) diagram
• light-year, astronomical unit, parsec
• galaxy, Milky Way
• red giant, white dwarf, black hole, neutron star, pulsar
• dense cloud, supernova remnant, planetary nebula
• protostar, main sequence, supernova
• accretion disc, stellar wind, exoplanet, evolutionary track
• redshift, Hubble’s law, Hubble constant, big bang
• open universe, closed universe, critical density
© Education Institute 2005
Standards for the unit
15 hours
3 hours
Measuring the
stars
4 hours
Galaxies
4 hours
Stars and planets
4 hours
SUPPORTING STANDARDS
11A.29.3 Use a diffraction grating to show ...
visible spectra …
Unit 12AP.7
CORE STANDARDS
Grade 12 standards
EXTENSION STANDARDS
12A.30.1 Distinguish between emission and absorption spectra; know how these can
provide information on the elements present in stellar objects and how far
away the objects are.
11A.29.4 Explain the Doppler effect in terms of
wave motion and give examples from
sound and light.
12A.25.4 Recall and use Newton’s law of
universal gravitation in the form
F = G(m1m2)/r2 and relationships
derived from it.
11A.29.6 ... know that all electromagnetic
waves travel with the same velocity
in free space …
Modelling the
Universe
12A.31.1 Describe, and explain in terms of gravitational attraction, the structure of
the visible Universe today and know that our Sun is a star in the Milky Way
galaxy.
12A.31.2 Know why powerful telescopes allow us to look back in time to when the
Universe was much younger than it is now.
12A.31.3 Show an understanding of the size and number of stars and galaxies, the
distances between them, and the size of the Universe. Know and define
the size of the light-year and the parsec.
11A.32.6 Know the source of energy in stars,
including the Sun.
11A.32.7 ... know how heavier elements are
12A.31.4 Know how stars are created, that they are made mainly from the element
hydrogen and that their ultimate fate depends on their size and can lead to
supernovae, white dwarfs, neutron stars (pulsars) or black holes.
formed in older stars by nuclear
fusion.
11A.32.2 ... use the common notation for
representing nuclides and write
equations representing nuclear
transformations.
12A.31.5 Explain the process of element formation in stars and know how this leads
to the generation of energy.
12A.31.6 Describe the process of planet formation by gravitational attraction from the
remains of an older exploded star.
12A.31.7 Know that current thinking favours the ‘big bang’ model of the Universe,
which postulates that all matter, time and space were created in a ‘big
bang’ around 14 billion years ago, and that since then the Universe has
been expanding.
570 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
12A.31.8 Understand how the Universe can at
the same time be finite but have no
boundaries.
© Education Institute 2005
Activities
Unit 12AP.7
Objectives
Possible teaching activities
4 hours
Observing the sky
Measuring the stars
Either arrange an evening for the whole class to meet and observe the night sky, or brief
students so that they can make observations in their own time.
Safety: Ensure that students observe only from
safe locations.
Using a planisphere, or a star-map downloaded from the Internet, identify some easily
recognisable constellations (e.g. Orion, the Plough) that will be visible in the night sky at the
time and location of students’ observations. Students should use binoculars to study the stars in
these constellations, noting their different colours and brightnesses. Introduce the term nebula,
meaning, loosely, an object that appears extended and fuzzy; if possible students should also
observe the Orion and Andromeda nebulae and note how they differ from stars. Students
should also observe any planets that are visible at the time of their observations.
Enquiry skill 12A.1.8
... know how [line spectra]
can provide information on
the elements present in
stellar objects and how far
away the objects are.
Show an understanding of
the size and number of stars
and galaxies, the distances
between them, and the size
of the Universe. Know and
define the size of the lightyear and the parsec.
Notes
School resources
Use this column to note
your own school’s
resources, e.g.
textbooks, worksheets.
Colour and brightness
Discuss students’ observations from the previous activity and ask them to suggest reasons for
the different colours and brightnesses of stars.
Show students a radiant heater warming up: as it gets hotter, its colour changes from a dull red
glow to bright orange. Show a filament lamp connected to a variable power supply: when the
filament is cool it glows faintly red, but when it is hotter it becomes yellow-white and brighter.
Display black-body radiation graphs on the board or OHP and establish that temperature can be
deduced from observations of the relative amounts of radiation in two or three parts of the
electromagnetic spectrum.
Refer to students’ observations of stars and ask them to say which of the stars they observed
are the coolest and which the hottest.
Ask students to suggest reasons for the different observed brightnesses of stars. Introduce the
term luminosity, meaning the total power radiated by a star, and by suitable questioning
establish the following points.
• If two stars are the same size, the hotter one will emit more radiation in all parts of the
spectrum (i.e. it is more luminous).
• If two stars are the same temperature, the larger one will emit more radiation as it has a
larger surface (i.e. it is more luminous).
• If two stars are the same size and temperature, they have the same luminosity, but the closer
one will appear brighter.
Introduce the terms intensity and flux F to mean the received radiant power per unit area.
Demonstrate that the intensity of light from a torch bulb shining onto a screen diminishes with
distance, and use suitable diagrams to show that the intensity of radiation from a point source
obeys an inverse-square law. Provided there is no absorption or scattering of radiation en route,
F = L ⁄ 4d2, where d is distance.
Ask students to say which of the stars they observed are likely to be the closest and which the
most distant.
571 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Let students use the Internet to find information about the sizes, distances and temperatures of
the stars they have observed. Tell them to rank the stars in order of temperature and of
distance, and to compare these rankings with their predictions of relative temperature and
distance. They should note the units used to express distances; even if these are unfamiliar,
they should still be able to produce a rank order for their observed stars.
Notes
School resources
ICT opportunity: Use of the Internet.
In their search for information, students may
come across the term magnitude. Explain that
apparent magnitude is related to observed
brightness, while absolute magnitude is related
to luminosity. Tell students that the magnitude
scale relates to visual perception, so stars that
appear brightest are ranked as ‘first magnitude’
while those that appear fainter are ranked
second, third and so on.
Enquiry skill 12A.1.8
Luminosity of the Sun
Ask students, working in pairs, to use a simple oil-spot photometer to estimate the Sun’s
luminosity as follows.
Safety: Students should not look directly at the
Sun.
• Use a compass needle or similar to place a small drop of cooking oil on a sheet of white
paper (the spot produced should be as small as possible).
Enquiry skills 12A.1.1, 12A.1.3, 12A.1.5
• Arrange the paper so that light from an unshaded filament lamp of known luminosity (e.g.
100 W) shines onto one side, and sunlight onto the other.
• Adjust the position of the paper so that the oil spot appears to merge into the surrounding
paper: the intensities of illumination from the two sources are then equal.
• Measure the distance from lamp to paper and use the known Earth–Sun distance to calculate
the Sun’s luminosity using the inverse-square law.
Encourage students to discuss sources of inaccuracy in this method and to suggest
improvements. They should also consider how results might be affected by atmospheric
absorption of sunlight, and by the difference in temperature of the two light sources.
The HR diagram
Display a large Hertzsprung–Russell (HR) diagram. Ideally this should have luminosity plotted
on the y-axis and temperature on the x-axis. If you use a version with axes showing magnitude
and colour index, explain to students that these quantities are closely related to luminosity and
temperature. Point out the use of logarithmic scales and the convention for labelling the x-axis
so that temperature increases from right to left. Also point out that temperature is expressed in
kelvins, while luminosity can be expressed either in its SI units (watts) or in terms of the Sun’s
luminosity LSun.
Draw attention to the diagonal band known as the main sequence, on which most stars lie. Ask
students what they can deduce about stars lying in the upper right-hand and lower left-hand
regions of the diagram. By suitable questioning, establish that stars in the upper right are cool
luminous stars: tell students these are called red giants. Similarly, establish that stars in the
lower left are hot stars with low luminosity known as white dwarfs.
572 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Distance units
Refer to the previous activities and ask students if they found any unfamiliar non-SI units being
used to express distances: they will probably have come across the light-year and parsec.
Establish that the light-year (ly) is the distance light (and other electromagnetic radiation) travels
through space in 1 year. Ask students to calculate the number of seconds in a year and then
use the speed of light to calculate the size of one light-year.
Use suitable diagrams on the board or OHP to introduce and define the astronomical unit (AU).
Show how the size of the parsec (pc) is related to the AU and explain that a star at 1 pc from
Earth has an annual parallax of 1 arcsec: as the Earth moves once around its orbit, the star
appears to move through an angle of 1 arcsec either side of its central position relative to the
background of more distant ‘fixed stars’. Tell students that the size of the AU is well established
through radar measurements within the Solar System, hence the size of the pc is also well
known. Students should use trigonometry and the small-angle approximation to calculate the
size of the pc using 1 AU = 1.50 × 1011 m.
Mathematics: This activity requires a
knowledge of the trigonometry of right-angled
triangles, angles expressed in arcsec and
radians, and the small-angle approximation.
Provide plenty of examples that allow students to practise calculations involving conventional
non-SI units for astronomical distances.
Point out that parallax measurements can only be used for relatively nearby stars (closer than
about 100 pc). For more distant stars, less direct methods must be used. Explain how the HR
diagram can be used in the following ways to estimate distances of stars.
• Single star. Deduce the star’s temperature from its colour. Assume the star lies on the main
sequence and use the HR diagram to deduce its luminosity. Measure the flux received at
Earth and calculate its distance using the inverse-square law. This method can be refined:
stars of the same temperature but different luminosity can be distinguished by subtle
differences in their line spectra, removing the need for the initial assumption.
• Cluster of many stars. Determine the temperature and flux of each star. Plot an HR diagram
for the cluster of stars using flux instead of luminosity. Superimpose this plot on a standard
HR diagram, aligning the temperature scales, and hence deduce the relationship between
luminosity and flux for the cluster.
Ask students to write a short account of methods for measuring stellar distances.
573 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
4 hours
Observing galaxies
Galaxies
Prepare and present a series of PowerPoint slides showing images of galaxies downloaded
from the Internet and illustrating the following points.
Describe, and explain in
terms of gravitational
attraction, the structure of the
visible Universe today and
know that our Sun is a star in
the Milky Way galaxy.
Know why powerful
telescopes allow us to look
back in time to when the
Universe was much younger
than it is now.
Show an understanding of
the size and number of stars
and galaxies, the distances
between them, and the size
of the Universe. Know and
define the size of the lightyear and the parsec.
Notes
School resources
Prepare a PowerPoint presentation.
• A galaxy is a large collection of stars (typically 108–1012 stars) bound together by mutual
gravitational attraction.
• Many galaxies have the shape of thin discs with bright stars concentrated into spiral arms, but
the majority are featureless ellipsoidal collections of stars and some are irregular in shape.
• The faint band of light across our sky, known as the Milky Way, is the light of many distant
stars lying in a thin disc.
• The Milky Way is a spiral galaxy, and the Sun lies about two-thirds of the way out from the
centre.
• The nearest major galaxy to the Milky Way is the Andromeda galaxy (Andromeda nebula),
which is also a spiral.
• Even the nearest galaxy lies at a distance of a few million light-years. Light reaching us has
been in transit for this time, so it carries information about the galaxy as it was a few million
years in the past.
• Galaxies are found to be grouped into clusters, bound by gravity. A large cluster might
contain thousands of galaxies.
• Powerful telescopes observing at great distances reveal that there are some regions of the
Universe with many clusters of galaxies and others, known as voids, with very few clusters.
• Even the nearest galaxies lie far beyond the reach of the stellar distance measurement
techniques discussed in the previous section.
Measuring distances of galaxies
Divide the class into three groups and set each group the task of using the Internet and library
resources to research one of the following methods for determining distances to galaxies:
ICT opportunity: Use of the Internet.
Enquiry skills 12A.1.4, 12A.1.6, 12A.1.8, 12A.3.4
• Cepheid variable stars;
• planetary nebulae (planetary nebula luminosity function, PNLF);
• Type Ia supernovae.
If the groups are large, subdivide them so that several small groups research the same method.
Ask each small group to prepare an A3 poster containing the following information:
• details about the type of object used and how such objects can be identified from observations;
• a note of the measurements that must be made and how they can be used to deduce
distance;
• some information about the first use of the method (e.g. who was responsible, when and
where they worked);
• the largest distances that can currently be deduced using this method;
• an image of a galaxy whose distance has been determined using this method;
• a note of the information sources consulted.
Display the posters around the lab and allow time for students to read one another’s work.
Students should then each produce their own written summary of all three methods.
574 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Scale mapping and modelling
Divide students into small groups and challenge them to design and produce a map or a scale
model that can be used to help younger students (e.g. Grade 9) appreciate the sizes and
distances involved in astronomy. They should consult the Internet and other resources to obtain
any information about size and distance that they need. Suitable tasks include the following.
ICT opportunity: Use of the Internet.
Enquiry skill 12A.3.4
• Solar System. Make a model that shows the relative sizes of planets using everyday objects
(e.g. fruit, marbles, footballs).
• Stars. Make a map or a model that shows typical sizes and separations of stars in our region
of the Milky Way (e.g. if the Sun and the nearest star are represented by marbles, how far
apart should they be?).
• Milky Way. Devise a way to show the size and shape of the Milky Way and the number of
stars it contains (e.g. if each star is represented by a grain of sand, how much sand would
you need to represent the whole galaxy?).
• Galaxies. Make a model to show the so-called local group of galaxies, which includes the
Milky Way (e.g. if the Milky Way and the Andromeda galaxy are represented by paper discs
10 cm in diameter, how far apart should they be?).
If possible, arrange for students to display and talk about their models to younger students. In
any case, display them where they can be seen by other members of the school (e.g. in a
corridor, hall or playground).
4 hours
Star formation
Stars and planets
Display a star map and an HR diagram and point out that they are static records of stars as we
currently observe them. Tell students that the main sequence was initially thought to represent
an evolutionary sequence (hence the name) showing a gradual change in stellar temperature
with time. When it became possible to deduce the masses of stars (by observing their
gravitational effects on one another) it became clear that hot main-sequence stars had much
greater masses than cool main-sequence stars, and there was no evidence of any processes
that could account for significant changes in stellar masses with age. Discuss with students the
difficulties facing astrophysicists trying to deduce how stars function, how they are formed and
how they change over time: the only information available comes from observations of distant
objects which, with a few exceptions, remain unchanged over human timescales. Discuss the
analogous problem of trying to deduce, from a snapshot of people in a city street, how human
beings develop and change with age.
... know how [line spectra]
can provide information on
the elements present in
stellar objects ...
Know how stars are created,
that they are made mainly
from the element hydrogen
and that their ultimate fate
depends on their size and
can lead to supernovae,
white dwarfs, neutron stars
(pulsars) or black holes.
Explain the process of
element formation in stars
and know how this leads to
the generation of energy.
Describe the process of planet
formation by gravitational
attraction from the remains of
an older exploded star.
Students should be able to appreciate that the HR diagram indicates the existence of certain
stable combinations of stellar luminosity and temperature: many stars are observed to lie on the
main sequence, or in the red giant or white dwarf regions of the diagram, which suggests that
these represent long-lasting phases in a star’s life (just as a photograph in a city street shows
more people of adult size than children). Point out that our current picture of star formation has
been developing since the early twentieth century, and is based on results of stellar observation
and Earth-based laboratory research, which have been brought together to build a picture that
is consistent with the available evidence.
575 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Prepare and present a sequence of PowerPoint slides to establish the following points.
Notes
School resources
Prepare a PowerPoint presentation.
• Stars are powered by nuclear fusion in their cores. This requires high density and temperature.
• Within a galaxy, there is gas and dust (small solid particles) lying between the stars. This
interstellar material (ISM) includes warm glowing clouds of gas and cool denser clouds. Even
the so-called dense clouds are extremely tenuous: on Earth they would be classed as vacuum.
• The temperature, density and composition of interstellar clouds can be deduced from the
radiation they emit and from the absorption they produce in starlight shining through them.
• The surface composition of stars can be deduced from their line spectra.
• Stars and ISM are predominantly made from hydrogen and helium, with traces of other
elements.
• A challenge of astrophysics is to explain how cold tenuous ISM can become hot and dense
enough to sustain nuclear fusion and form a star.
• Another challenge is to account for the predominance of heavier elements making up the
Earth and other planets.
• Theory and observation indicate that star formation takes place in clouds that gradually
collapse under their own gravity. For this to happen, the cloud must be dense and massive
(many times the Sun’s mass) so that internal gravitational forces are strong, and cold so that
random thermal motion does not disperse the cloud.
• During collapse, material falling towards the centre of the cloud gains kinetic energy. The
random thermal motion of particles is increased (i.e. the material gets hotter).
• As a cloud collapses, it fragments into locally collapsing regions, which become mainsequence stars.
• Around each newly formed star there remains a disc of gas and dust, from which planets
form by gravitational attraction.
• Close to the star, where temperatures are high, volatile materials made up of hydrogen and
other light elements are unable to condense and are driven outwards. Planets close to the
star contain a large proportion of heavy elements, while those further out are composed
largely of hydrogen and helium.
Evidence for star and planet formation
Ask students to work in pairs or individually to collect evidence relating to the formation of stars
and planets. As part of this task they should look for the following items in books or on the
Internet and use scientific dictionaries or websites to find the meanings of the key terms in italic.
ICT opportunity: Use of the Internet.
Enquiry skills 12A.1.4, 12A.1.6, 12A.1.8, 12A.3.4
• Images showing infrared emission from warm protostars within the Orion nebula.
• Images of accretion discs around stars.
• Observations of stellar winds.
• Data relating to the composition of Solar System planets.
• Data showing the motion of gas and dust around newly formed stars.
• Graphics or animations showing the evolutionary track of a protostar on an HR diagram.
• Data indicating the existence of exoplanets.
Students should download, or photocopy, relevant images and information in order to compile
their own accounts of star and planet formation.
576 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
End points of stellar evolution
Use a combination of presentation, images, demonstration and questioning to establish the
following points about the end points of stellar evolution.
• A star with mass up to about 8 MSun will reach a point when collapse under its own gravity
does not produce a high enough temperature to initiate the next stage of fusion. The abrupt
halt of fusion causes ejection of the star’s outer layers, which expand to produce a tenuous
shell of glowing gas known as a planetary nebula. The core is less than about 1.4 MSun and
becomes a white dwarf.
• Stars with mass greater than about 8 MSun can produce elements up to iron in their cores.
Further stages of nuclear fusion are endothermic, so there is no release of energy or build-up
of pressure to prevent further gravitational collapse.
• The runaway collapse of a massive star core gives rise to a stellar explosion known as a
supernova as the outer regions go into free fall then bounce outwards.
(Place a table-tennis ball on top of a tennis ball. Drop them both together, taking care to
release them vertically so that they remain in contact as they fall; like the outer layers of a
collapsing star, the table tennis ball rebounds to many times its original height.)
• During a supernova explosion, heavier elements are synthesised, and most of the star is
ejected into space along with a vast output of electromagnetic radiation.
• The ejected material forms a supernova remnant: a cloud of expanding, glowing gas. It
gradually cools and might eventually be able to collapse and form new stars and planets.
• The remaining central part of the star continues to collapse under its own gravity. Electrons
and nuclei are forced very close together and the protons and electrons combine to produce
neutrons.
• If the central remnant is less than about 2.25 MSun, the formation of neutrons prevents further
collapse and it becomes a neutron star. Some neutron stars are observed as pulsars; they
spin rapidly and emit narrow beams of radiation that we detect as short pulses each time they
point towards Earth.
(Mount a torch on a rotating turntable so that it points horizontally. In a darkened room, rotate
the turntable so that students see the torch appear to flash as it points towards them.)
• If the central remnant is more massive than 2.25 MSun, it continues to collapse under its own
gravity to become a black hole. Close to a black hole, the gravitational field is so strong that
not even light can escape.
Ask students to work individually or in pairs to draw a flow chart summarising the stages in a
star’s evolution, starting from the main sequence, showing how its mass influences the outcome
of each stage.
577 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
Enquiry skill 12A.3.4
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Stellar end points
Students should work in pairs or small groups to find more information about the ways stars
come to an end. They should download relevant images and produce about ten PowerPoint
slides on their topic for presentation to the rest of the class. Suitable topics include the following.
ICT opportunity: Use of PowerPoint; use of the
Internet.
Enquiry skills 12A.1.6, 12A.1.8, 12A.2.2, 12A.2.4
• Historical observations of Milky Way supernovae such as those observed in 1006, 1054,
1181, 1572 or 1604: where in the sky each was observed, and from where the observations
were made. Students should suggest why these spectacular events were only noticed in a
few locations.
• Remnants of historical supernovae: modern images and data relating to events such as
those listed above.
• Supernovae observed in other galaxies, including the one observed in the nearby Large
Magellanic Cloud galaxy in 1987.
• The object known as Eta Carinae.
• The discovery of pulsars: when and how they were discovered, and how the observations
were initially interpreted.
• Hubble Space Telescope observations of planetary nebulae.
• Evidence for black holes: if no matter or light can escape, how can such objects be detected?
Compact objects
Produce sets of cards, each of which contains one statement, image or piece of information
relating to a white dwarf, neutron star or black hole. Make at least six cards for each type of
object. Suitable examples include the following:
Prepare suitable sets of cards.
• can be observed as a pulsar;
• the end-point of a Sun-like star;
• the fate of the most massive stars.
Divide students into groups of three to play the following game. Each group should allocate one
type of object to each student, place the cards face down and turn over the top card. In turn,
each student takes either the upturned card or one from the pack. If the card they pick up does
not relate to their object, they should discard it face up. Continue until one person has a full set
of cards for their object.
578 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
4 hours
Universal expansion
Modelling the Universe
Display a graph that plots the redshifts of several galaxies against their distance (either
Hubble’s original results or some more recent data). Introduce and define the term redshift
z =  ⁄ . Remind students about line spectra and tell them that, like starlight, light received
from nearby galaxies includes lines (mainly due to hydrogen) whose wavelengths are precisely
known. Light from distant galaxies is redshifted (i.e. it contains patterns of lines similar to those
of nearby galaxies but at longer wavelengths).
... know how [line spectra]
can provide information on
the elements present in
stellar objects and how far
away the objects are.
Describe, and explain in
terms of gravitational
attraction, the structure of the
visible Universe today ...
Know why powerful
telescopes allow us to look
back in time to when the
Universe was much younger
than it is now.
Notes
School resources
Divide students into pairs or small groups and ask them to brainstorm and suggest how the
redshift–distance data might be interpreted.
In discussion with the whole class, note students’ ideas on the board or OHP, then by suitable
questioning establish the following points.
• A galaxy’s redshift indicates its velocity relative to us. A redshift implies a recession.
• Only a few very nearby galaxies have spectral lines that are blue-shifted. Apart from these
nearby galaxies, all other galaxies are receding from us.
• Redshift is proportional to distance: the more distant a galaxy, the more rapidly it is receding.
• The observations suggest that the entire Universe is expanding.
Know that current thinking
favours the ‘big bang’ model
of the Universe, which
postulates that all matter,
time and space were created
in a ‘big bang’ around 14
billion years ago, and that
since then the Universe has
been expanding.
• Although we see all other galaxies receding, this does not imply that we are at the centre of
the Universe.
Understand how the Universe
can at the same time be finite
but have no boundaries.
• Draw several dots on an acetate sheet. Make an enlarged photocopy onto another acetate
sheet. Choose one dot arbitrarily to be the Milky way and overlay the two sheets so that the
two Milky Way dots coincide. Show that the other dots have receded from the Milky Way at
speeds proportional to their distances.
Use animations and models to demonstrate that, in a uniform expansion, an observer on any
galaxy sees all other galaxies receding with a speed that is proportional to its distance. Suitable
models include the following.
• Stick paper dots representing galaxies onto a wide rubber band. Choose one arbitrarily to be
the Milky Way. Stretching the band causes all other galaxies to recede from our galaxy.
• Stick paper dots representing galaxies onto the surface of a balloon. Inflating the balloon
causes all other galaxies to recede from our galaxy.
Introduce Hubble’s law v = H0d and define the Hubble constant H0.
Tell students how Hubble’s law can be used to deduce distances to galaxies: the relationship
first needs to be established by measuring the distances to some galaxies using methods such
as Type Ia supernovae or Cepheid variables. Then the distances to other galaxies can be
deduced from their redshifts: find v, then calculate d using Hubble’s law. Tell students that, in
practice, astronomers often quote values of redshift as direct indicators of distance.
The big bang
Refer to the previous discussion and establish that the observation of galactic recession implies
that there was a time in the past when all galaxies had zero separation. Tell students that this
can be interpreted in terms of a big bang: an explosion from a state of extremely high density
that marks the beginning of our Universe.
579 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Point out the units of the Hubble constant. Students should appreciate that with speed and
distance expressed in SI units, H0 would have units s–1. Establish that, assuming the expansion
has been proceeding at the same rate since the big bang, the age of the Universe can be
deduced from Hubble’s law: t = d ⁄ v = 1 ⁄ H0.
Tell students that astronomers usually express galaxies’ recession velocities in km s–1 and their
distances in Mpc, giving H0 in non-SI units km s–1 Mpc–1.
Students should be able to use current measurements of H0 (close to 75 km s–1 Mpc–1) to
estimate the age of the Universe.
Discuss the assumptions underlying this simple calculation. Students should be able to
appreciate that gravitational forces between galaxies will decelerate their recession so the
expansion is less rapid now than in the past. They should be able to explain how this will affect
our estimates of the age of the Universe based on Hubble’s law: the time taken for galaxies to
reach their present separations must be less than the simple estimate.
Explain to more advanced students how the finite age of the Universe places a limit on its
observable size: we cannot observe objects whose distance exceeds the distance that light has
travelled since the big bang. Point out that when astronomers talk of ‘the size of the Universe’,
they often mean the size of the observable Universe, which is finite, rather than size of the
entire Universe, which is thought to be infinite.
Evidence for the big bang
Prepare and present a sequence of PowerPoint slides summarising current evidence relating to
the big bang. In addition to the observations of galactic recession, include the two other major
items of evidence.
Prepare a PowerPoint presentation.
• Abundance of elements. The big bang theory predicts that initially the Universe consisted of
fundamental particles and radiation. As the Universe expanded, the particles combined.
Detailed models based on our knowledge of particle reactions predict the proportions of
hydrogen, helium and other elements produced in the first few minutes of the expansion, after
which the density would become too low to allow further reactions. The observed abundances in
interstellar space and the outer regions of stars match the predictions very closely.
• Microwave background radiation. Theory predicts that radiation produced in the first few minutes
after the big bang would still be travelling through space, and that as space expands so does
the wavelength of the radiation. Initially, the wavelength would be very short as the matter
producing it was very hot, but it has now expanded to a few centimetres. Radiation close to the
predicted wavelength has been detected coming from all directions in space.
• Very distant objects. Observations at high redshift involve light that has been in transit for a time
that is comparable to the age of the Universe. Objects known as quasars (extremely luminous
galaxies) are observed only at high redshift, indicating that they were far more common in the
early Universe than now (i.e. there is evidence that the Universe has evolved over time rather
than existing in a steady state).
Let students discuss how current thinking about the big bang relates to their religious beliefs.
580 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
Enquiry skill 12A.2.2
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
The future of the Universe
Ask students to speculate on the future of the Universe: will the expansion continue indefinitely,
or will gravitational attraction eventually halt and reverse it? Display graphs showing how the
separation between two galaxies will change with time in either of these scenarios. Introduce
the terms open universe and closed universe. Ask students to suggest what might determine
the way in which expansion proceeds. By suitable questioning, establish that the expansion will
cease (i.e. the Universe will be closed) only if the average density of matter in the Universe
exceeds a certain critical density.
Collect magazine articles (e.g. from Scientific American) and download webpages relating to current
estimates of the density of matter in the Universe and the ultimate fate of the Universe. Distribute
them to students to read and discuss in small groups. Suitable topics include the following.
• Estimates of the density of visible matter in the Universe (which fall far short of the critical
density).
• Evidence for dark matter adduced from observations of galactic rotation and the motion of
galaxies in clusters.
• The search for dark matter particles.
• Evidence suggesting that the expansion is in fact accelerating because of a hitherto unknown
force.
Light and matter
Using suitable visual aids, introduce more advanced students to some of the basic ideas of
general relativity and how these relate to our current understanding of the Universe. Include the
following points.
• Einstein’s general theory introduced the notion that forces can be understood as distortions in
space-time. In particular, gravity is the distortion of space-time caused by matter and it
causes light to deviate from travelling in a straight line.
• Observational evidence supporting Einstein’s theory was first obtained in 1919 during a solar
eclipse, when stars viewed close to the Sun appeared to shift in position.
• If the density of the Universe exceeds the critical density, then the space-time distortion is
such that it causes light to travel around a closed path.
• The geometry of a closed universe can be represented by analogy with the surface of a
sphere, which is a two-dimensional surface in three-dimensional space and which is finite but
without boundaries (light or anything else travelling in a ‘straight line’ actually follows a closed
path). In a closed universe, our three-dimensional space is finite but without boundaries.
• Similarly, the geometry of an open universe can be represented by analogy with a
hyperboloidal surface, and if the Universe has critical density its geometry is analogous to
that of a flat plane.
581 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Theories of the Universe
Point out to students that current thinking about the Universe differs from earlier ideas. Current
thinking is that we inhabit a planet that orbits an unexceptional star, one of many stars (which
might also have planets) in the outskirts of an unexceptional galaxy that does not occupy any
particularly privileged place in the Universe, whereas in historical times people believed that the
Universe was centred on human civilisation. Divide students into small groups and set each the
task of researching one of the historical developments that preceded our current understanding.
Suitable topics include:
• Ancient Greek geocentric models;
• the heliocentric models of early Islamic philosophers;
• conflicts in Europe involving geocentric and heliocentric models;
• the discovery of ‘island universes’.
Tell students to use the Internet and library resources to research information and to pay
particular attention to the way scientific work is influenced by the social, cultural, moral and
spiritual contexts in which it is undertaken. They should also note the importance of
technological developments, such as the telescope, and the way in which prevailing paradigms
can be overthrown by observational evidence.
ICT opportunity: Use of the Internet; use of
PowerPoint.
Enquiry skills 12A.1.6, 12A.1.8, 12A.2.1,
12A.2.2, 12A.2.4, 12A.2.5, 12A.3.4
Ask each group to prepare a PowerPoint presentation and a handout summarising their findings
for photocopying and distribution to the rest of the class. After students have given their
presentations to the class, allow time for them to continue and extend their discussions from the
previous activities.
582 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
Assessment
Examples of assessment tasks and questions
Assessment
Set up activities that allow
students to demonstrate what
they have learned in this unit.
The activities can be provided
informally or formally during
and at the end of the unit, or
for homework. They can be
selected from the teaching
activities or can be new
experiences. Choose tasks
and questions from the
examples to incorporate in
the activities.
Unit 12AP.7
Notes
School resources
A student who is not studying science says ‘No one has ever travelled to a star. It must be
impossible to know how far away they are or anything else about them.’ Write an explanation
for this student describing how it is possible to deduce the temperatures and distances of stars
using Earth-based observations.
In a scale model, the Milky Way and the Andromeda galaxy are both to be represented by discs
10 cm in diameter. Given that the diameter of the Milky Way is about 30 kpc, and the
Andromeda galaxy lies at a distance of about 2.4 million light-years, how far apart should the
discs be placed in the model?
Draw a sequence of labelled diagrams to show how the collapse of a cold interstellar cloud can
produce a main-sequence star.
Write a short account of how elements heavier than hydrogen are produced in stars and
become the raw material for making planets. Include the following terms: nuclear fusion, star,
supernova, gravity.
Use a simple model (e.g. paper dots stuck onto the surface of a balloon) to explain Hubble’s
observation that the redshifts of galaxies are proportional to their distances.
Using H0 = 75 km s–1 Mpc–1 and assuming a constant rate of expansion since the big bang,
calculate the age of the Universe. Give your answer in seconds and in years.
583 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005
584 | Qatar science scheme of work | Grade 12 advanced | Unit 12AP.7 | Physics 7
© Education Institute 2005