Download Solar System

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