module p7 l6
TRANSCRIPT
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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)