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Page 1: Module P7 L6

8/9/2019 Module P7 L6

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Module P7 L6

Page 2: Module P7 L6

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Which is Brightest?

 A glow worm 1 m away . . .

They could both appear to be the

same brightness.

. . . or car headlights 1000 m away?

The intrinsic brightness of the glow

worm is 1/1000th of the intrinsic

brightness of the headlights . . .

. . . but the glow worm is 1000 timescloser . . .

. . . so the observed brightness is the

same.

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Analysing the Results

NOTE: The resistance of the LDR gets higher in the dark and lower in

bright light

8. What can you say about the intrinsicbrightness of the bulb during this

experiment?

9. What is the relationship between distance

and the resistance of the LDR?

10. What is the relationship between

distance and observed brightness?

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(4) A has a greater intrinsic

brightness than B and isthe same distance or 

closer but its light is

dimmed by passing

through dust clouds

Two stars A and B could have the same

obser ved brightness because:

(3) B has a greater intrinsicbrightness than A but is

further away

(2) A has a greater intrinsic

brightness than B but is

further away

(1) A and B have the same

intrinsic brightness and 

are the same distance

away from us

(5) A has a greater intrinsic

brightness than B and is

the same distance or closer but its light is

dimmed by passing

through dust clouds

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Colour of a Star 

Stars can appear to be blue, yellow or red . . .

Blue stars are the largest and hottest.

Yellow stars are relatively small and cool (our Sun is classified as a

µyellow dwarf star¶).

Red stars could be either:

red giants (large but cool)

red dwarfs (small and cool)

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Star light, star brightIntensity of 

r adiation at

each frequency

Smaller Frequency

(Longer Wavelength)

Stars emit light at all frequencies.

However, some stars emit more (say) blue light and so they appear blue.

The area under the graph represents

the total energy emitted by the star.

Blue stars give off more energy than

red stars.

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[3]

Real exam 

question

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86 light years

from Earth

62 light years

from Earth

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Nearby stars also show an

annual motion due to the

movement of the Earth

around the Sun.

The effect is only measurable

when nearby stars are

viewed against a background

of more distant stars.

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Par allax

There is no star (other than the Sun) which has

an annual parallax of more than one second.

The star with the largest parallax is Proxima

Centauri: 0.77 seconds of arc. (Note: 1 second

of arc = 1/60th of 1 minute of arc = 1/3600th of 1

degree of arc)

Proxima Centauri is 1/0.77 = 1.295 parsecs

away

1 parsec (pc) is 3.26 light years

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Cepheid Variables

Some stars are variable stars ± their brightness changes over time.

Some stars are Cepheid Variables ± their brightness changes in a regular pattern.

The period of the pattern is fixed. The period of Eta Aquilae is 7.2 days.

From the period or frequency, we can calculate their intrinsic brightness.

If we know their intrinsic brightness, we can work out how far away they are.

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Cepheid Variables 2

In the 1920s, Edwin

Hubble used Cepheid

variables to calculate

the distances to anumber of galaxies.

He used the red shift tocalculate the speed at

which they were

moving.

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Hubble¶s Discover y

He found that . . .

. . . the further a galaxy the f aster 

it moved.

He plotted a graph of:

speed in kilometres per second

( km / s) on the y-axis against

distance in megaparsecs (Mpc) on

the x-axis

The graph showed that the velocity of a galaxy is directly proportional to its

distance from us.

Hubble measured the gradient as 120 kilometres per second per Megaparsec.

120 km / s / Mpc

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The Hubble Constant

 Astronomers have named this gradient theHubble Constant in his honour.

speed of recession = Hubble constant x distance

The gradient of this graph was the very first indication that we live inside an

expanding Universe.

distance

recessionof speed

 ConstantHubble!

Modern measurements mean that the HubbleConstant is about

70 km / s / Mpc.

Finding a more exact value for the HubbleConstant will allow us to find out

whether we live in an open or closed Universe ± we may be able to predict not

 just the future of the Universe, but the future of Time itself . . .

. . . and then, just possibly (in the words of Professor Stephen Hawking) ³we

should know the mind of God.´

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³If we find the answer to

that, it would be the

ultimate triumph of human

reason - for then we should

know the mind of God.´

Stephen Hawking, Lucasian Professor of 

Mathematics,Cambridge University,

writing in  A Brief History of Time (p.193)