Download Document

P7 21st Century Science OCR revision
P7.1: Naked-eye astronomy
Solar System
• Collection of planets, comets and other
objects which ORBIT the sun.
• The sun is a STAR
Year
• The time it takes the Earth to complete one
orbit of the sun.
• The Earth takes 365.25 days to complete one
orbit
Moon
• The Moon orbits the Earth.
• It takes about 28 days to complete one orbit.
• The Moon’s orbit is
tilted 5 degrees from
the plane of the Earth’s
orbit period.
• 27 days (sidereal)
• 29 days (viewed from Earth)
Axis
• The Earth rotates around an imaginary line called its
axis
Movement of the sun
• Sun appears to move across sky from East  West
• This is because Earth spins on axis
• Sun reappears in same place
once every 24 hours – solar day
• Earth spins on its axis (360o)
every 23h 56 minutes –
sidereal day
Why is a sidereal day different to a solar day?
• During the sidereal day the Earth moves further
along its orbit of the sun.
• Therefore for the same part of the Earth to face
the sun again, the Earth needs to turn for a
further 4 minutes.
• Solar day is 4 minutes
longer than a sidereal day
Movement of the stars
• Long-exposure photographs show the stars to be
moving in circles around the Pole Star.
• They are not actually moving, we are simply
observing from a spinning Earth.
• After 23h 56m (SIDE REAL DAY) they will appear to
be back in the same place.
Constellation
• A group of stars that form a pattern.
• We see different constellations in summer and
winter because we have moved around the sun.
Movement of the Moon
• Moon appears to move across sky from EW
• Moon takes longer to appear in same part of sky –
24h 49m
• This is because:
– As well as Earth’s rotation giving different view of
Moon, Moon is also orbiting the Earth
– Moon orbits from WE so during night the
position of the Moon over 28 days appears to
slip slowly back through the pattern of stars.
Moving planets
• We can see some planets with naked eye.
• They are also orbiting the sun.
• This makes their positions appear to change night
by night when viewed against the background of
fixed stars
Retrograde Motion
• Planets usually appear to move across sky from
E  W like the Sun and Moon.
• Sometimes they appear to go backwards –
retrograde motion
• This is because different planets have different
times to orbit the sun so the place we see a planet
in the sky depends on where both the planet and
the Earth are in their orbits.
Phases of the Moon
• We can only see the part of the Moon that is lit up
by the Sun.
• As the Moon orbits
the Earth we see
different parts of
the Moon lit up.
• ,
What is an eclipse?
• Solar eclipse – Moon
blocks the Sun’s light
• Lunar eclipse – Moon moves into the Earth’s
shadow
Shadows
• Moon and Earth both have shadows
• Region of total darkness – umbra
• Region of partial darkness – penumbra
Why are lunar eclipses more common
than solar eclipses?
• Because the
Earth’s shadow
is bigger than
the Moon’s
shadow
Solar eclipses are also rare because the Moon
does not often line up exactly with the Sun
because the Moon’s orbit is tilted by 5o relative
to the plane of the Earth’s orbit.
Celestial Sphere
• Axis from Pole star
through axis of Earth
• Celestial equator which
is extension of Earth’s
equator.
• Astronomers use two
angles to describe the
positions of astronomical
objects.
• The angles are measured
from a reference point in
the sky.
P7.2: Telescopes and Images
Waves and refraction
• Light travels as waves
• Substance that light travels through is a medium
• Speed of light depends upon medium – as medium
changes speed changes
• Once a vibrating source has made a wave frequency
cannot change. So if speed changes, wavelength
changes.
• If a wave changes direction it is called refraction
Refraction
• If a wave changes direction it is called refraction
Refraction at lenses
• Refracting telescopes use convex lenses
• Convex lenses are thicker in the middle than the
edges
• If parallel rays enter a convex lens they come to a
point called the focus.
• When rays meet at
the focus they have
converged.
Images in lenses
Use these rules to draw ray diagrams
1. Use arrows to show the direction that light is travelling
Images in lenses
Use these rules to draw ray diagrams
1. Use arrows to show the direction that light is travelling
2. A ray through the centre of a lens does not change
direction
Images in lenses
Use these rules to draw ray diagrams
1. Use arrows to show the direction that light is travelling
2. A ray through the centre of a lens does not change direction
3. A ray parallel to the principal axis passes through the focus
Images in lenses
Use these rules to draw ray diagrams
1.
2.
3.
4.
Use arrows to show the direction that light is travelling
A ray through the centre of a lens does not change direction
A ray parallel to the principal axis passes through the focus
A ray through the focus emerges parallel to the principal
axis
principal axis
Stars and light rays
• Stars – very far away
• Rays reaching Earth from stars are parallel
Stars and light rays
• Stars – very far away
• Rays reaching Earth from stars are parallel
• Convex lens refracts rays from star through a single
point
• Point is the
image of
the star.
Inverted images
• Objects in the Solar system are closer than stars.
• Light rays from different parts of the object arrive at
a lens at different angles
• Rays from top of object go to bottom of lens and
vice versa.
• This means the
image is
inverted
Focal length and lens power
• Focal length – distance from lens to image
Focal length and lens power
• Focal length – distance from lens to image
• A fat convex lens has a shorter focal length than a
thin lens – fat lens is more powerful
Measuring the power of a lens
• Units = dioptres
• Power (dioptres) = 1 / focal length (metres-1)
What’s inside a telescope?
• Objective lens (long focal length = low power)
– Forms image inside telescope
• Eyepiece lens (short focal length = high power)
– Magnifies image formed by objective lens
• Distance between lenses = sum of 2 focal lengths
Why do we use telescopes?
• Magnification enables you to see detail you cannot
see with the naked eye.
• They have a greater aperture so they collect more
light. This enables you to see dimmer stars than
with the naked eye.
Magnification
• The angles between stars appear bigger with a
telescope than the naked eye. This is the
angular magnification of the telescope.
• Magnification = focal length of objective lens
focal length of eyepiece lens
Magnification
Example problem
Calculate the magnification of a telescope with an
objective of focal length 1200 mm using two different
eyepieces with focal lengths of: (a) 25 mm (b) 10 mm
Example problem
Calculate the magnification of a telescope with an
objective of focal length 1200 mm using two different
eyepieces with focal lengths of: (a) 25 mm (b) 10 mm
(a) Magnification = 1200/25 = 48x
(b) Magnification = 1200/10 = 120x
Reflecting telescopes
• Most telescopes use a concave mirror rather than a
lens as the objective.
• This brings parallel light to a focus.
• An eyepiece lens then magnifies the image from the
mirror
Advantages of reflecting telescopes
1. Easier to make a big mirror than a big lens
2. Hard to make a glass lens with no imperfections
3. Big convex lenses are fat in the middle, glass
absorbs light on the way through the lens, so faint
objects look even fainter.
X

Advantages of reflecting telescopes
1. Easier to make a big mirror than a big lens
2. Hard to make a glass lens with no imperfections
3. Big convex lenses are fat in the middle, glass
absorbs light on the way through the lens, so faint
objects look even fainter.
4. Mirrors reflect all
colours the same,
lenses refract blue
light more than red
distorting the image.
Dispersion
• White light = mixture of colours
• Violet light = higher frequency & shorter wavelength
than red light
• Violet light slows down more in glass so is refracted
more
• In lenses and prisms, refraction splits white light
into colours = dispersion
Dispersion at a diffraction grating
• Dispersion also occurs at a diffraction grating (narrow
parallel lines on a sheet of glass). When white light shines
on the grating, different colours emerge at different angles.
This forms spectra.
• Astronomers view stars through spectrometers containing
prisms or gratings.
• These show the
frequencies of light
emitted by the star.
Diffraction
• Diffraction is when waves go through a gap, bend
and spread out.
• The effect is greatest when the size of the gap is
similar to or smaller than the wavelength of the
waves.
Diffraction and telescopes
• The light-gathering area of a telescope’s objective
lens or mirror is its aperture
• If diffraction occurs at the aperture, the image will
be blurred.
• Optical telescopes have apertures much bigger than
the wavelength of light to reduce diffraction and
form sharp images.
• Radio waves have long wavelengths. A telescope
that detects radio waves from distant objects
needs a very big aperture.
These pictures show images with a 10 meter
telescope and a 100 meter telescope.
P7.3: Mapping the Universe
Light year
• Distance light travels in one year
• After the sun, the nearest stars are about 4 light
years away
• We see light that left those stars 4 years ago
• Some galaxies are millions of light years away
Parallax
• As the Earth orbits the Sun, the closest starts
appear to change positions relative to the very
distinct ‘fixed stars’.
• This effect is called parallax
• The stars have not actually moved.
It is the Earth that has moved.
Parallax angle
• HALF the angle the star has apparently moved in 6
months (as we travel from one side of the Sun to
the other).
• Parallax angles are tiny. They are measured in
seconds of arc.
• 1 second of arc = 1/3600 of a degree
• The smaller the parallax angle, the further away the star.
•
.
• Distance to star = 1 / parallax angle (in seconds of arc)
(parsecs)
Parsecs (pc)
• A parsec (pc) is the distance to a star whose parallax
angle is 1 second of arc.
• A parsec is similar in magnitude (size) to a light-year
• Distances between stars within a galaxy are usually
a few parsecs
• Distances between galaxies are measured in
megaparsecs (Mpc)
Distances between stars within a galaxy are
usually a few parsecs
Distances between galaxies are measured in
megaparsecs (Mpc)
Star luminosity
• Luminosity = Amount of radiation emitted by a star
every second
• Luminosity depends on:
– size of the star
– temperature of the star
• HOTTER and BIGGER = more energy
radiated per second
Luminosity variables – size of star
• For stars with the same surface temperature
the bigger the star the more energy it gives out.
• A star with double the radius of another one will
have an area four times as great and so have a
luminosity four times greater than the first star
R
Double the radius
R2
Radius
x2
Area
x4
Luminosity x4
Luminosity variables – temp of star
• For stars of the same size the hotter the star the
more energy it gives out.
• A star with a temperature of double another one
will have a luminosity sixteen times greater.
T
Double the temperature
T2
Temperature x2
Luminosity x16
Observed intensity
• How bright a star appears when seen
from Earth
• Brightness depends on:
–luminosity of the star
–distance of the star from the Earth
Brightness
• For two stars of the same luminosity with one star
double the distance of the other from the Earth the
closer star will look four times brighter.
D
D2
Double the distance from earth
Distance
x2
Luminosity
x¼
Light spreads
out, so the more
distant a source is
the less bright it
appears.
Luminosity, brightness and magnitudes
• For stars which give out the same amount of light
Brightness on
Earth
as
Distance from Earth
Cepheid variable stars
• Star whose brightness changes with time
• Variation in brightness thought to be because the star
expands and contracts in size (by 30%) causing a
variation in temp and luminosity.
Calculating the distance to a Cepheid variable star
• Measure the period
• Use the period to work out the luminosity
Calculating the distance to a Cepheid variable star
• Measure the period – 5 days
• Use the period to work out the luminosity
• Measure the observed brightness
• Compare the observed brightness with the luminosity
to work out the distance
The period–luminosity
relation for Cepheid variables
Observing nebulae and galaxies
• We know from telescope observations that our
galaxy (Milky Way) is made up of millions of stars.
• The sun is one of the stars in our galaxy
• In the 1920s astronomers were puzzled by fuzzy
patches of light seen through telescopes.
• They called these fuzzy patches nebulae.
• Nebulae have different shapes including
spirals
Shapeley vs. Curtis
Shapeley
• Thought Milky Way
was entire universe
• Thought nebulae
were clouds of gas
within Milky Way
Curtis
• Thought spiral
nebulae were huge
distant clusters of
stars – other
galaxies outside the
Milky Way
Shapeley vs. Curtis
• Neither astronomer had enough evidence to win
the argument
• Later HUBBLE found a Cepheid
variable in a spiral nebula, Andromeda.
• Hubble measured the distance from Earth  star.
• It was further than any star in the Milky Way galaxy
• He concluded that the star was in a separate galaxy
• Cepheid variable stars have been used to show that
most spiral nebulae are distant galaxies, of which
there are billions in the Universe.
The amazing and expanding Universe!
• Astronomers study absorption
spectra from distant galaxies
• Compared to spectra from
nearby stars, the black
absorption lines for distant
galaxies are shifted towards
the red end of the spectrum.
This is REDSHIFT
• Redshift shows us that galaxies
are moving away from us.
Speed of recession
• Redshift shows us that galaxies are moving away
from us.
• The speed of recession of a galaxy is the speed at
which it is moving away from us. This can be found
from the redshift of the galaxy.
(you can use either set of units)
Hubble, Cepheid Variables and Recession
• Hubble measured the distance
to Cepheid variable stars in
several galaxies.
• He found the further away a
galaxy is, the greater its speed
of recession.
Hubble’s constant
• Since Hubble, many astronomers have
gathered data from Cepheid variable
stars in different galaxies.
• These data have given better values of the Hubble
constant.
• The fact that galaxies are moving away from us
suggests that the Universe began with a big bang
about 14 thousand million years ago.
• Distant galaxies seem to be moving away faster
than nearby galaxies, hence scientists conclude that
space itself is expanding.
P7.4: The Sun, the stars and
their surroundings
Star Spectra and Temperature
• Hot objects (including stars) emit energy across all
wavelengths of the EM radiation spectrum
• Different stars emit different amounts of radiation
and different frequencies depending on their
temperature.
INCREASING TEMPERATURE
Spectrometer
• Used by astronomers to:
– Measure radiation emitted at each frequency
– Identify the peak frequency of a star
• The peak frequency gives an accurate value for the
temperature of a star.
Peak frequency
=
Higher temperature
Identifying elements in stars
• Astronomers use spectra from stars to identify their
elements.
• The surface of the Sun emits white light. As the light
travels through the Sun’s atmosphere, atoms in this
atmosphere absorb light of certain frequencies.
• The light that travels on has these frequencies missing.
• When the light is spread into a spectrum, there are
dark lines across it.
• This is the absorption spectrum of the sun
Identifying elements in stars
• Each element produces a unique pattern of lines in
its absorption spectrum
• Astronomers identify the elements in stars by
comparing star absorption spectra to those of
elements in the lab
How do gases behave?
• Stars are balls of hot gases
• To understand stars, you need to understand gases
• The particles of a gas move very quickly in random
directions
• When they hit the sides of a container they exert a
force as they change direction  this causes gas
pressure
Pressure and volume
• If you decrease the volume of the container,
the particles hit the sides more often and the
pressure increases
Pressure and volume
• Volume and pressure are inversely
proportional
• For a fixed mass of gas at constant temp as
volume decreases pressure increases
pressure x volume = constant
Pressure and temperature
• The hotter the gas, the more energy the particles
have and the faster they move
• The faster the
particles move, the
harder and more
often they hit
the sides of the
container.
Pressure and temperature
• Temperature and pressure are directly
proportional
• For a fixed mass of gas at constant volume as
temperature (K) increases pressure increases
pressure / temperature = constant
Cooling a gas
• As a gas cools, the particles lose energy & they
move more slowly
• At the lowest temperatures particles stop moving
and therefore would never hit the sides of the
container.
• Lowest theoretical temp = absolute zero (-273oC)
X
Kelvin Scale
• Starts at the
lowest
theoretical
temp = absolute
zero (-273oC)
• Zero on the
Kelvin scale is
-273oC =
absolute zero
Volume and temperature
• If you decrease the temperature of a gas at constant
pressure, the volume decreases.
• At absolute zero, the volume would theoretically be zero
• For a fixed mass of gas at a fixed pressure:
o As temp increases, vol increases
o Volume is directly
proportional to temp (K)
volume = constant
temperature
Inside stars
Protostars
•
•
•
•
•
Gravity compresses a cloud of H and He gas
The gas particles get closer and closer
The volume of the gas cloud decreases
As they get closer they move faster
Temperature and pressure increase
• This mass of gas is called a protostar
Protostars and Nuclear Fusion
• When H nuclei get close enough they form He
nuclei – nuclear fusion
• This process releases energy.
• Protostar
star when fusion begins
• Nuclear fusion happens in all stars including our Sun
Protostar
formation
Nuclear fusion in the Sun
0 +
+ e (positron)
1
Nuclear fusion in the Sun
• The product of the
previous reaction
may then fuse with
another hydrogen
nucleus to form an
isotope of helium
Positrons
• Like an electron, but with a positive charge
• Emitted in some nuclear reactions to conserve
charge
Nuclear equations
• You must balance:
– Mass (top number)
– Charge (lower numbers)
• In fusion reactions the total mass of product particles is
slightly less than the total mass of reactant particles.
• The mass that is lost has been released as energy
• You can use Einstein’s equation to calculate energy
released in nuclear fusion / fission reactions
Energy
mass
(speed of light
=
x
released
lost
in a vacuum)2
Inside stars
• Stars which fuse H to form He are main-sequence
stars (e.g. the sun)
Core
Radiative
zone
Convective
zone
Photosphere ~
surface of star
Temperature
and density are
highest. Most
nuclear fusion
happens here
Energy is
transported
outwards from
the core by
radiation
Convection
currents flow
here, carrying
heat energy to
the photosphere
Energy is
radiated
into space
from here
Inside stars
Red giants and supergiant stars
1.
2.
3.
4.
In main sequence stars, H nuclei fuse to form He.
Eventually the H runs out
The pressure decreases
The core collapses
5. Hydrogen containing outer layers of the star fall inwards
6. New fusion reactions happen in the core
7. These reactions make the outer layers of the star expand
8. The photosphere cools and its colour changes from
yellow  red
9. A red giant / supergiant has formed
Red giants and supergiant stars
• While the outer layers of a red giant / supergiant
expand, its core gets smaller
• It becomes hot enough for He nuclei to fuse
together to form heavier nuclei
e
• The more massiv the star, the hotter the core,
the heavier the nuclei it can produce by fusion
– Red giants – fusion reactions produce nuclei of
carbon, then nitrogen and oxygen
– Supergiants (core pressure and temp higher) –
fusion reactions produce elements with nuclei as
heavy as iron!!
Inside stars
White dwarf stars
• The Sun has a relatively low mass
• When it becomes a
it will not be
compressed further once its He has been used up
• The star will shrink to become a
• There is no fusion in a white dwarf.
• It will gradually cool and fade
star
Supernova
• When the core of a supergiant is mainly iron, it
explodes - this is a supernova
• It is so hot that fusion
reactions produce
atoms of elements
as heavy as uranium
After a
supernova
explosion,
a dense
core
remains.
After a supernova
explosion, a dense
core remains.
Smaller core –
neutron star
Bigger core –
black hole (so much
mass concentrated
into a tiny space that
even light cannot
escape from it)
Clouds of dust and
gas blown outwards
by a supernova may
eventually form
new protostars
Hertzsprung-Russell diagram
• H-R diagram plots luminosity against temperature
• For main sequence stars there is a correlation: the
hotter the star, the more radiation emitted and so
the greater its luminosity
Hertzsprung-Russell diagram
• H-R diagram plots luminosity against temperature
• For main sequence stars there is a correlation: the
the star, the more radiation emitted and so
the
Increasing luminosity
Hertzsprung-Russell diagram
• H-R diagram plots luminosity against temperature
• For main sequence stars there is a correlation: the
hotter the star, the more radiation emitted and so
the greater its luminosity
HOT
Increasing temperature
COOL
Hertzsprung-Russell diagram
• H-R diagram plots luminosity against temperature
• For main sequence stars there is a correlation: the
hotter the star, the more radiation emitted and so
the greater its luminosity
You may be asked to
identify regions of the
H-R in which different
types of star are located
Exoplanets
• Astronomers have found evidence of planets
orbiting nearby stars.
• These are exoplanets
• Some may have the right conditions for life
• Because of this scientists think there may be life
elsewhere in the Universe
• No evidence of ET life has yet been discovered
P7.5: The astronomy
community
Choosing your observatory site
• Astronomers use huge telescopes to collect weak
radiation from faint or very distant sources.
• Major optical and infrared telescopes on Earth are
in:
– Chile
– Hawaii
– Australia
– Canary Islands
Choosing your observatory site
• When choosing a site, astronomers consider a
number of factors
• If you have a question in the exam about evaluating
telescope sites
1. Compare the advantages and disadvantages of each site
2. State with reasons which site you believe to be best
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Atmosphere reflects light
Choose a high altitude
location (e.g. a mountain)
to reduce this problem
This distorts images
The Sphinx
Observatory in
the Swiss Alps
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Atmosphere reflects light
Choose a high altitude
location (e.g. a mountain)
to reduce this problem
This distorts images
The Sphinx
Observatory in
the Swiss Alps
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Light is refracted more if Locate your telescope in
the air is damp or polluted an area with dry, clean air
for higher-quality images
Clear skies, dry air
and low pollution
make Arizona a
hotspot for
astronomy.
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Light is refracted more if Locate your telescope in
the air is damp or polluted an area with dry, clean air
for higher-quality images
Clear skies, dry air
and low pollution
make Arizona a
hotspot for
astronomy.
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Astronomical observation Choose an area with
cannot be made in cloudy frequent cloudless nights
conditions
Clear skies, dry air and
low pollution make
Arizona a hotspot for
astronomy.
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Astronomical observation Choose an area with
cannot be made in cloudy frequent cloudless nights
conditions
Clear skies, dry air and
low pollution make
Arizona a hotspot for
astronomy.
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Cities cause light pollution Choose an area far from
cities
Clear skies, dry air and
low pollution make
Arizona a hotspot for
astronomy.
Choosing your observatory site
When choosing a site, astronomers consider a number of factors
Factor
Solution
Cities cause light pollution Choose an area far from
cities
Clear skies, dry air and
low pollution make
Arizona a hotspot for
astronomy.
Choosing your observatory site
• When choosing a site, astronomers consider a
number of factors
• Other factors:
– Cost (inc. travel to and from telescope for
supplies and workers)
– Environmental impact near the observatory
– Impact on local people
– Working conditions for employees
Computer controlled telescopes
• Allows astronomers to use a telescope thousands of
miles away
• Images are recorded digitally and sent electronically
to computers
• Computers can then be used to analyse images and
improve their quality (e.g. adding colour)
• Observations can then
be shared with
other astronomers
Why put telescopes in space???
• Telescopes on Earth are affected by:
– Atmosphere (which absorbs most IR, UV, X-ray
and gamma radiation)
– Atmospheric refraction (distorts images and
makes stars ‘twinkle’)
– Light pollution
– Bad weather
Why put telescopes in space???
• Telescopes on Earth are affected by:
– Atmosphere (which absorbs most IR, UV, X-ray and gamma radiation)
– Atmospheric refraction (distorts images and makes stars ‘twinkle’)
– Light pollution
– Bad weather
• All of these problems are overcome by putting a
telescope in space
• The Hubble Space Telescope has a better resolution
than any telescope on Earth
Problems with telescopes in space???
• High cost of setting up
• High cost of on-going maintenance and repairing a
telescope in space
• Uncertainties of future funding
International collaboration
• Allows the cost of a major telescope to be shared
• Allows expertise to be pooled
• Exam tip: Know two examples showing how
international co-operation is essential for progress
in astronomy
International collaboration –
European Southern Observatory (ESO)
•
•
•
•
Involves 14 European countries + Brazil
Consists of several telescopes in Chile
Chile provides the base and the office staff
1000+ astronomers from all over the world use the
facility each year
International collaboration –
Gran Telescopio Canarias
• In the Canary Islands
• At the top of a high volcanic peak
• Funded mainly by Spain, with contributions from
Mexico and the USA
• Planning involved 1000+ people from 100 companies