6.1.1 The Earth, Moon & Sun
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The Earth's Axis
1. MOTION
2. THERMA
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" The Earth
is a rocky planet that rotates in a near circular orbit around the Sun
It rotates on its axis, which is a line through the north and south poles
The axis is tilted at an angle of approximately 23.4° from the vertical
The Earth completes one full rotation (revolution) in approximately 24 hours (1 day)
The Earth's Axis
This rotation creates the apparent daily motion of the Sun rising and setting
Rotation of the Earth on its axis is therefore responsible for the periodic cycle of day and night
3. WAVES
4. ELECTR
5. NUCLEA
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The Earth is a rocky planet that rotates in a near circular orbit around th
Day and
It Night
rotates on its axis, which is a line through the north and south poles
6. SPACE
The Earth's rotation around its axis creates day and night
The axis is tilted at an angle of approximately 23.4° from the vertic
Day is experienced by the half of the Earth's surface that is facing the Sun
Night
is the other
half of the Earth's
facing away from
the Sun
The
Earth
completes
onesurface,
full rotation
(revolution)
in approximately 6.1
24Earth
ho
6.1.1 The
This rotation creates the apparent daily motion of the Sun rising and
set
6.1.2 Calcu
Rotation of the Earth on its axis is therefore responsible for the pe
Day and Night
6.1.3 The S
6.1.4 Orbit
The Earth's rotation around its axis creates day and night
6.1.5 Grav
Day is experienced by the half of the Earth's surface that is facing
Night is the other half of the Earth's surface, facing away from the
6.2 Stars &
Day and night are caused by the Earth's rotation
Rising and Setting of the Sun
The Earth's rotation on its axis makes the Sun looks like it moves from east to west
At the equinoxes the Sun rises exactly in the east and sets exactly in the west
Equinox (meaning 'equal night') is when day and night are approximately of equal length
However, the exact locations of where the Sun rises and sets changes throughout the seasons
1
Rising and Setting of the Sun
The Earth's rotation on its axis makes the Sun looks like it moves from east to west
At the equinoxes the Sun rises exactly in the east and sets exactly in the west
Equinox (meaning 'equal night') is when day and night are approximately of equal length
However, the exact locations of where the Sun rises and sets changes throughout the seasons
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In the northern hemisphere (above the equator):
In summer, the sun rises north of east and sets north of west
In winter, the sun rises south of east and sets south of west
The Sun rises in the east and sets in the west. Its approximate area changes throughout the year
The Sun is highest above the horizon at noon (12 pm)
In the northern hemisphere, the daylight hours are longest up until roughly the 21st June
This day is known as the Summer Solstice and is where the Sun is at its highest point in the
sky all year
The daylight hours then decrease to their lowest around 21st December
This is known the Winter Solstice and is where the Sun is at its lowest point in the sky all year
The Earth's Orbit
The Earth orbits the Sun once in approximately2365 days
This is 1 year
The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons
The Earth's Orbit
The Earth orbits the Sun once in approximately 365 days
This is 1 year
The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates the seasons
Seasons in the Northern hemisphere caused by the tilt of the Earth
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Over parts B, C and D of the orbit, the northern hemisphere is tilted towards the Sun
This means daylight hours are more than hours of darkness
This is spring and summer
The southern hemisphere is tilted away from the Sun
This means there are shorter days than night
This is autumn and winter
Over parts F, G and H of the orbit, the northern hemisphere is tilted away from the Sun
The situations in both the northern and southern hemisphere are reversed
It is autumn and winter in the northern hemisphere, but at the same time it is spring and
summer in the southern hemisphere
At C:
This is the summer solstice
The northern hemisphere has the longest day, whilst the southern hemisphere has its shortest
day
At G:
This is the winter solstice
The northern hemisphere has its shortest day, whilst the southern hemisphere has its longest
day
At A and D:
Night and day are equal in both hemispheres
These are the equinoxes
Moon & Earth
The Moon is a satellite around the Earth
It travels around the Earth in roughly a circular orbit once a month
This takes 27-28 days
The Moon revolves around its own axis in a month so always has the same side facing the Earth at
all times
We never see the hemisphere that is always facing away from Earth, although astronauts
have orbited the Moon and satellite have photographed it
The Moon shines with reflected light from the Sun, it does not produce its own light
Phases of the Moon
The way the Moon's appearance changes across a month, as seen from Earth, is called its periodic
cycle of phases
3
Phases of the Moon
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The way the Moon's appearance changes across a month, as seen from Earth, is called its periodic
cycle of phases
Phases of the Moon as it orbits around Earth
In the image above, the inner circle shows that exactly half of the Moon is illuminated by the Sun at
all times
The outer circle shows how the Moon looks like from the Earth at its various positions
In the New Moon phase:
The Moon is between the Earth and the Sun
Therefore, the sunlight is only on the opposite face of the Moon to the Earth
This means the Moon is unlit as seen from Earth, so it is not visible
At the Full Moon phase:
The Earth is between the Moon and the Sun
The side of the Moon that is facing the Earth is completely lit by the sunlight
This means the Moon is fully lit as seen from Earth
In between, a crescent can be seen where the Moon is partially illuminated from sunlight
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Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc a
many years’ experience as a Head of Physics and Examiner
for A Level and IGCSE Physics (and Biology!), her love of commun
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6.1.2 Calculating Orbital Speeds
6.1.2 Calculating Orbital Speeds
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Orbital Speed
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1. MOTION
2. THERM
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When planets move around the Sun, or a moon moves around a planet, they orbit in circular motion
This means that in one orbit, a planet travels a distance equal to the circumference of a circle
(the shape of the orbit)
This is equal to 2πr where r is the radius a circle
Orbital Speed
3. WAVES
4. ELECTR
The relationship between speed, distance and time is:
EXTENDED
5. NUCLEA
distance
time
Speed !
When planets move around the Sun, or a moon moves around a planet
6. SPACE
This means that in one orbit, a planet travels a distance equal to t
2"r
v!
(the shape of the orbit)
T
6.1 Earth
This
is
equal
to
2π
r
where
r
is
the
radius
a
circle
Where:
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the average orbital speed of an object can be defined by the equation:
v = orbital speed in metres per second (m/s)
r = average radius of the orbit in metres (m)
The
relationship between speed, distance and time is:
T = orbital period in seconds (s)
This orbital period (or time period) is defined as:
The time taken for an object to complete one orbit
distance
Speed !
time
The orbital radius r is always taken from the centre of the object being orbited to the object orbiting
6.1.1 The E
6.1.2 Calc
6.1.3 The S
the average orbital speed of an object can be defined by the equation:
v!
6.1.4 Orbit
2"r
T
Where:
v = orbital speed in metres per second (m/s)
r = average radius of the orbit in metres (m)
T = orbital period in seconds (s)
6.1.5 Grav
6.2 Stars &
This orbital period (or time period) is defined as:
The time taken for an object to complete one orbit
Orbital radius and orbital speed of a planet moving around a Sun
?
The orbital radius r is always taken from the centre of the object being o
Worked Example
5
The Hubble Space Telescope moves in a circular orbit. Its distance above the Earth’s surface is
560 km and the radius of the Earth is 6400 km. It completes one orbit in 96 minutes.
Orbital radius and orbital speed of a planet moving around a Sun
? Worked Example
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The Hubble Space Telescope moves in a circular orbit. Its distance above the Earth’s surface is
560 km and the radius of the Earth is 6400 km. It completes one orbit in 96 minutes.
Calculate its orbital speed in m/s.
Step 1: List the known quantities
Radius of the Earth, R = 6400 km
Distance of the telescope above the Earth's surface, h = 560 km
Time period, T = 96 minutes
Step 2: Write the relevant equation
Step 3: Calculate the orbital radius, r
The orbital radius is the distance from the centre of the Earth to the telescope
6
7
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6.1.3 The Solar System
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1. MOTION, FORCES &
ENERGY
The Solar System
THERMAL
PHYSICS
1.2.MOTION,
FORCES
&
ENERGY
6.1.3 The Solar System
TEST YOURSELF
The Solar System consists of:
1. The Sun
The Solar System
2. Eight planets
Natural
and consists
artificialof:
satellites
The3.Solar
System
Dwarf
1.4. The
Sunplanets
Asteroids
and comets
2.5. Eight
planets
3. WAVES
2. THERMAL PHYSICS
4. ELECTRICITY &
3.MAGNETISM
WAVES
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3. Natural and artificial satellites
The Sun & the Planets
4. Dwarf planets
The
Sun lies at
thecomets
centre of the Solar System
5. Asteroids
and
The Sun is a star that makes up over 99% of the mass of the solar system
The Sun & the Planets
There
planets
anSolar
unknown
number of dwarf planets which orbit the
The
Sunare
lieseight
at the
centreand
of the
System
SunThe Sun is a star that makes up over 99% of the mass of the solar system
The gravitational field around planets is strong enough to have pulled in all
There nearby
are eight
planets
and
unknownofnumber
dwarf planets which orbit the
objects
with
thean
exception
naturalof
satellites
Sun The gravitational field around a dwarf planet is not strong enough to have
The
gravitational
field around planets is strong enough to have pulled in all
pulled
in nearby objects
nearby objects with the exception of natural satellites
TheThe
8 planets
in our Solar
System aindwarf
ascending
order
of the
distance
from
Sun
gravitational
field around
planet
is not
strong
enough
to the
have
are:pulled in nearby objects
Mercury
The 8 Venus
planets in our Solar System in ascending order of the distance from the Sun
are: Earth
Mercury
Mars
Venus
Jupiter
Earth
Saturn
Mars
Uranus
Jupiter
Neptune
Saturn
SatellitesUranus
There
are two types of satellite:
Neptune
Natural
Satellites
Artificial
There are two types of satellite:
Some
planets have moons which orbit them
Natural
Moons are an example of natural satellites
Artificial
Artificial
satellites
are man-made
andthem
can orbit any object in space
Some
planets
have moons
which orbit
The
International
Space
Station
(ISS)
orbits the Earth and is an example of an
Moons are an example of natural satellites
artificial satellite
Artificial satellites are man-made and can orbit any object in space
Asteroids
& Comets
The
International Space Station (ISS) orbits the Earth and is an example of an
artificial
satellite
Asteroids and
comets also orbit the sun
An asteroid is a small rocky object which orbits the Sun
Asteroids & Comets
The asteroid belt lies between Mars and Jupiter
Asteroids and comets also orbit the sun
An
asteroid
a small
rockyand
object
which
orbits
Comets
areismade
of dust
ice and
orbit
the the
SunSun
in a different orbit to those of
The asteroid belt lies between Mars and Jupiter
planets
The ice melts when the comet approaches the Sun and forms the comet’s tail
Comets are made of dust and ice and orbit the Sun in a different orbit to those of
planets
The ice melts when the comet approaches the Sun and forms the comet’s tail
8
4.5.ELECTRICITY
&
NUCLEAR PHYSICS
MAGNETISM
6. SPACE PHYSICS
5. NUCLEAR PHYSICS
6.1 Earth & The Solar
6.System
SPACE PHYSICS
6.1.1 The Earth, Moon &
6.1 Earth & The Solar
Sun
System
6.1.2The
Calculating
Orbital
6.1.1
Earth, Moon
&
Speeds
Sun
6.1.2
6.1.3Calculating
The Solar Orbital
System
Speeds
6.1.4The
Orbiting
6.1.3
Solar Bodies
System
6.1.5 Gravitational Effects
6.1.4
Orbiting Bodies
on Orbits
6.1.5
Gravitational
Effects
6.2 Stars
& The Universe
on Orbits
6.2 Stars & The Universe
The ice melts when the comet approaches the Sun and forms the comet’s tail
The objects in our solar system
!
Exam Tip
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You need to know the order of the 8 planets in the solar system. The following
mnemonic gives the first letter of each of the planets to help you recall them:
My Very Excellent Mother Just Served Us Noodles
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Accretion Model of the Solar System
There are 4 rocky and small planets: Mercury, Venus, Earth and Mars
These are the nearest to the Sun
There are 4 gaseous and large planets: Jupiter, Saturn, Uranus and Neptune
There are the furthest from the sun
planets-of-our-solar-system, IGCSE & GCSE Physics revision notes
The eight planets of our Solar System
The differences in the types of planets are defined by the accretion model for Solar
System formation
9
The Sun was thought to have formed when gravitational attraction of pulled
together clouds of hydrogen dust and gas (called nebulae)
10
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A rotating accretion disc is formed when the planets emerged
The accretion model of the creation of the Solar System
As the Sun grew in size it became hotter
Where the inner planets were forming near the Sun, the temperature was too high
for molecules such as Hydrogen, Helium, water and Methane to exist in a solid
state
Therefore, the inner planets are made of materials with high melting
temperatures such as metals (e.g. iron)
Only 1% of the original nebula is composed of heavy elements, so the inner,
rocky planets could not grow much and stayed as a small size, solid and rocky
The cooler regions were further away from the Sun, and temperature was low
enough for the light molecules to exist in a solid state
The outer planets therefore could grow to a large size up and include even the
lightest element, Hydrogen
These planets are large, gaseous and cold
11
These planets are large, gaseous and cold
!
Exam Tip
When referring to the accretion model for the formation of the Solar System,
make sure your answer has the following:
(a) the model’s dependence on gravity
(b) the presence of many elements in interstellar clouds of gas and dust
(c) the rotation of material in the cloud and the formation of an accretion disc
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Author: Ashika
Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, foc
on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Phys
and Maths tutor, Ashika helps to grow and improve our Physics resources.
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6.1.4 Orbiting Bodies
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PDFPDF
Light Speed
"
1. MOTI
ENERGY
6.1.4 Orbiting Bodies
"
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THER
1. MOTION, 2.
FORCES
ENERGY
The planets and moons of the solar system are visible from Earth when they reflect
WAV
Light Speed
2. THERMAL3.PHYSIC
Light Speed
light from the Sun
12
The outer regions of the Solar System are around 5 × 10 m from the Sun,
4. ELEC
The planets and moons of the solar system are visible from Earth when they reflect
3. WAVES
which
means
even
light
takes
somesystem
time toare
travel
thesefrom
distances
The
planets
and
moons
of
the
solar
visible
Earth
when
they
reflect
light from the Sun
MAGNE
12
The
Sun
isouter
so
far
away
Earth
that are
thearound
light we
leftSun,
the Sun eight
The
regions
of from
the Solar
System
5 ×see
10 actually
m from the
light
from
the
Sun
4. ELECTRICITY &
minutes
earlier
12
which
meansregions
even lightof
takes
to travel
these
distances
The outer
the some
Solartime
System
are
around
5 × 10 m from the Sun,
MAGNETISM
5. NUCL
star to
us Earth
after that
the the
Sunlight
is so
that
from
it takes four
The the
Sun nearest
is so far away
from
wefar
seeaway
actually
leftlight
the Sun
eight
which means even light takes some time to travel these distances
minutes
earlier
years
to reach us
NUCLEAR PHYSIC
The the
Sunnearest
is so star
far away
from
that
light
actually
left the Sun 5.eight
us after
theEarth
Sun is
so farthe
away
thatwe
lightsee
from
it takesaway,
four
The Milky Way to
galaxy
contains
billions
of stars,
huge
distances
with the
6. SPAC
minutes
yearsearlier
to reach us
light
taking
even longer to be seen from Earth
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The
Way galaxy
billions
of stars,
huge
away,
withfrom
the it takes
theMilky
nearest
star tocontains
us after
the Sun
is so
fardistances
away that
light
6. four
SPACE PHYSICS
8
6.1 Eart
light
taking
even
longer
to
be
seen
from
Earth
The speed of light is a constant 3 × 10 m/s
years to reach us
System
6.1 Earth
using
the
equation:
The of
Milky
Way
galaxy
the & The Sola
The Therefore,
speed
light
is a constant
3 contains
× 108 m/s billions of stars, huge distances away, with
System
Therefore,
usingeven
the equation:
light taking
longer to be seen from Earth
6.1.1 Th
distance
speeddistance
!
6.1.1 The Earth,
Sun Moo
time
8
speed ! 3 × 10 m/s
The speed of light is a constant
Sun
time
6.1.2 Ca
Therefore,
using
the
equation:
The time taken to travel a certain distance can be calculated by rearranging
The time taken to travel a certain distance can be calculated by rearranging
to:
distance
6.1.2 Calculating Orb
Speeds
Speeds
distancetime
distance
timetime
! !
speedspeed
6.1.3
Th
6.1.3 The Solar
Syste
distance
6.1.5 Gravitational E
6.1.5 Gr
on Orbits
to:
speed !
The time taken to travel a certain distance can be calculated by rearranging
6.1.4 Orbiting
Bodie
to:
6.1.4
Or
Worked Example
Example
?? Worked
time
Calculate the time taken for light from the
Sun!to reach
Mercury if the radius of
Calculate
the
time
taken
for
light
from
the Sunspeed
to reach Mercury if the radius of
9
Mercury's orbit is 5.8 × 10 m.
Mercury's orbit is 5.8 × 109 m.
on Orbi
6.2 Stars & The Univ
Worked Example
? Step
1: State the equation for the time taken for light to travel a certain distance
Step 1: State
equation
forlight
the time
taken
for to
light
to travel
a certain
distance
Calculate
thethe
time
taken for
from
the Sun
reach
Mercury
if the radius
of
distance
time !
9
speed
Mercury's orbit is 5.8 × 10 m.
distance
time !
Step 2: Substitute in the values
speed
The
distance travelled
the radius of the orbit
Step 2:
Substitute
in the is
values
9
Distance,
d =equation
5.8 × 10 m.
Step 1: State
the
for the time
taken for light to travel a certain distance
8
Speed
= the speed
of light,
= 3.0
× 10 of
m/s
The
distance
travelled
is vthe
radius
the orbit
9
distance
Distance, d = 5.8 × 10 5 "m.
9
8 # 10
time
!"8933333
time
!
!
1
speed
Speed = the speed of light, v
3 " 0=#3.0
108× 10 m/s
9
5"8 #
10
Step 3:2:Round
up the answer
and
include
units
Step
Substitute
in the time
values
!
! 1 " 933333
3 " 0 # 108
time = 19.3 s
The distance travelled is the radius
of the orbit
13
9
Step 3: Round
up
the
answer
and
include
units
Distance, d = 5.8 × 10 m.
8
6.2 Star
time = 19.3 s
!
Exam Tip
The speed of light is very fast. This is why in our everyday life, we do not
notice that it travels a distance at all. For example, when you switch on a light
bulb, the room is flooded with light instantly. However, this is only because it is
so fast and our eyes cannot see the difference. In large, astronomical
distances which can be millions or even billions of kilometres, the limit of the
speed of light starts to have an affect.
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For example, it takes light 8 minutes to travel from the Sun to the Earth. This
means we are technically seeing the Sun 8 minutes ago. If the Sun was to
disappear, we would not notice till 8 minutes later. Although, by that time, the
time delay is the least of our worries...
Elliptical Orbits
EXTENDED
Orbits of planets, minor planets and comets are elliptical
An ellipse is just a 'squashed' circle
Planets, minor planets and comets have elliptical orbits
However, the Sun is not at the centre of an elliptical orbit
This is only the case when the orbit is approximately circular
In an elliptical orbit, the Sun is not at the centre of the orbit
However, in a circular orbit, the Sun is at the centre
Planets and comets travel in elliptical orbits, but the Sun is not at the centre of these
orbits
!
Exam Tip
You will not be asked to do any calculations with elliptical orbits. If you are
asked to calculate the time period, orbital speed or radius of an orbit, it can be
assumed that it is circular.
Analysing Orbits
EXTENDED
Over many years, data about all the planets, moons and the Sun have been
collected
This is not just for general interest, but to indicate:
14
Factors that affect conditions on the surface of the planets
Environmental problems that a visit (using manned spaceships or robots)
Analysing Orbits
EXTENDED
Over many years, data about all the planets, moons and the Sun have been
collected
This is not just for general interest, but to indicate:
Factors that affect conditions on the surface of the planets
Environmental problems that a visit (using manned spaceships or robots)
would encounter
Table of Data for Planets in our Solar System
Planet
57.9
88 days
5427
350
3.7
108.2
225 days
5243
460
8.9
149.6
365 days
5514
20
9.8
227.9
687 days
3933
–23
3.7
778.6
11.9 years
1326
–120
23.1
Saturn
1433.5
29.5 years
687
–180
9.0
Uranus
2872.5
75 years
1271
–210
8.7
Neptune
4495.1
165 years
1638
–220
11.0
Venus
Earth
Mars
Jupiter
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Mercury
Orbital
Orbital duration / Density /
distance /
days or years
kg/m3
million km
Uniform
Surface
Surface
Temperature/ Gravitational
Field Strength/
°C
N/kg
There are some common themes from the data of the planets is:
Orbital duration (how long it takes to travel around the Sun) increases with orbital
distance (distance from the Sun)
The circular path that the planet's travel in has a larger radius
Orbital duration increases with orbital distance
E.g. Neptune travels much slower than Mercury
The planets further away from the Sun experience a weaker gravitational pull,
so move slower in their orbit
Surface temperature decreases with orbital distance except for Venus
Venus has a dense atmosphere of carbon dioxide, trapping in heat through
the greenhouse effect
The surface gravitational field strength doesn't just depend on a planet's size, but
also its mass
This is why although Uranus is 4 times larger than Earth, it has a smaller
gravitational field strength because it is less dense
!
Exam Tip
Although you don't need to memorise any of this data, you must be able to
confidently analyse and interpret it. Look out for trends such as one variable
increasing whilst the the other decreases (or also increases). This carefully
about why that may be with what you have already learnt about the planets
from this topic. For example, what is the planet made of? What is its distance
from the Sun and how does this affect it?
15
6.1.5 Gravitational Effects on Orbits
TEST YOURSELF
1. MOTION, FORCES &
ENERGY
Gravitational Field Strength
2. THERMAL PHYSICS
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6.1.5 Gravitational Effects on Orbits
TEST YOURSELF
The strength of gravity on different planets after an object's weight on that planet
Weight is defined as:
Gravitational Field Strength
The force acting on an object due to gravitational attraction
3. WAVES
4. ELECTRICITY &
MAGNETISM
Planets
have strong
gravitational
fields
The strength
of gravity
on different
planets after an object's weight on that planet
Hence, they attract nearby masses with a strong gravitational force
5. NUCLEAR PHYSICS
Weight is defined as:
Because of weight:
The force
acting
onground
an object due to gravitational attraction
Objects
stay firmly
on the
Objects will always fall to the ground
Satellites
arestrong
kept in orbit
Planets
have
gravitational fields
6. SPACE PHYSICS
6.1 Earth & The Solar
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Hence, they attract nearby masses with a strong gravitational force System
Because of weight:
Objects stay firmly on the ground
Objects will always fall to the ground
Satellites are kept in orbit
6.1.1 The Earth, Moon &
Sun
6.1.2 Calculating Orbita
Speeds
6.1.3 The Solar System
6.1.4 Orbiting Bodies
6.1.5 Gravitational Effe
on Orbits
6.2 Stars & The Univers
Objects are attracted towards the centre of the Earth due to its gravitational field
strength
Both the weight of any body and the value of the gravitational field strength g
differs between the surface of the Earth and the surface of other bodies in space,
including the Moon because of the planet or moon's mass
The greater the mass of the planet then the greater its gravitational field
strength
A higher gravitational field strength means a larger attractive force towards
the centre of that planet or moon
g varies with the distance from a planet, but on the surface of the planet, it is
roughly the same
Objects
attracted
towards
centre
of the Earth
due tofrom
its gravitational
field
Theare
strength
of the field
around the
the planet
decreases
as the distance
the
strength
planet increases
However, the value of g on the surface varies dramatically for different planets and
moons
16
Both the weight of any body and the value of the gravitational field strength g
The
gravitational
fieldthe
strength
(g) on
is approximately
10 N/kg
differs
between
surface
ofthe
theEarth
Earth
and the surface
of other bodies in space,
moons
The gravitational field strength (g) on the Earth is approximately 10 N/kg
The gravitational field strength on the surface of the Moon is less than on the Earth
This means it would be easier to lift a mass on the surface of the Moon than
on the Earth
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The gravitational field strength on the surface of the gas giants (eg. Jupiter and
Saturn) is more than on the Earth
This means it would be harder to lift a mass on the gas giants than on the
Earth
Value for g on the different objects in the Solar System
On such planets such as Jupiter, an object’s mass remains the same at all points in
space
However, their weight will be a lot greater meaning for example, a human will be
unable to fully stand up
A person’s weight on Jupiter would be so large a human would be unable to fully
stand up
!
Exam Tip
You do not need to remember the value of g on different planets for your
exam, the value of g for Earth will be given in the exam question.
17
18
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6.2.1 The Sun as a Star
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1. MOTION
ENERGY
The Sun
2. THERM
"
6.2.1
The Sun as a Star
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The Sun lies at the centre of the Solar System
The Sun is a star which makes up The
over 99%
Sun of the mass of the solar system
The fact that most of the mass of the Solar System is concentrated in the Sun
is the reason the smaller planets orbit the Sun
pull
theSolar
Sun on
the planets keeps them in orbit
The SunThe
liesgravitational
at the centre
ofof
the
System
3. WAVES
4. ELECTR
MAGNETI
The Sun is a star which makes up over 99% of the mass of the solar system
5. NUCLEA
The Sun is a medium sized star consisting of mainly hydrogen and helium
The
fact
that
most
of
the
mass
of
the
Solar
System
is
concentrated
in
the
Sun
It radiates most of its energy in the infrared, visible and ultraviolet regions of the
electromagnetic
spectrum
is the reason
the smaller planets orbit the Sun
6. SPACE
The gravitational pull of the Sun on the planets keeps them in orbit
6.1 Earth
The Sun is a medium sized star consisting of mainly hydrogen and helium
System
It radiates most of its energy in the infrared, visible and ultraviolet regions of the
electromagnetic spectrum
6.2 Stars
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"
6.2.1 The
6.2.2 Stars
6.2.3 The
Universe
Our Sun (Image courtesy of NASA)
6.2.4 The
Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs,
from blue giants to red supergiants
These can be classified according to their colour
6.2.5 Hub
the Univer
Warm objects emit infrared and extremely hot objects emit visible light as well
Therefore, the colour they emit depends on how hot they are
A star's colour is related to its surface temperature
Our Sun
(Image
courtesy
of NASA)
A red star is the coolest
(at around
3000
K)
A blue star is the hottest (at around 30 000 K)
Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs,
from blue giants to red supergiants
These can be classified according to their colour
Warm objects emit infrared and extremely hot objects emit visible light as well
Therefore, the colour they emit depends on how hot they are
A star's colour is related to its surface temperature
A red star
the coolest
(atcorrelates
around 3000
K)
Theiscolour
of a star
to its temperature
A blue star is the hottest (at around 30 000 K)
21
Nuclear Fusion in Stars
Nuclear Fusion in Stars
EXTENDED
In the centre of a stable star, hydrogen atoms undergo nuclear fusion to form
helium
The equation for the reaction is shown here:
Deuterium and Tritium are both isotopes of hydrogen. They can be formed through
other fusion reactions in the star
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A huge amount of energy is released in the reaction
This provides a pressure that prevents the star from collapsing under its gravity
The fusion of deuterium and tritium to form helium with the release of energy
? Worked Example
An example of a hydrogen fusion reaction which takes place in stars is shown
here.
Which of the following is a valid reason as to why hydrogen fusion is not
22
currently possible on Earth?
The fusion of deuterium and tritium to form helium with the release of energy
? Worked Example
An example of a hydrogen fusion reaction which takes place in stars is shown
here.
Which of the following is a valid reason as to why hydrogen fusion is not
currently possible on Earth?
A Hydrogen fusion produces dangerous radioactive waste
B Hydrogen nuclei require very high temperature to fuse together
C Hydrogen is a rare element that would be difficult to get large amounts of
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D Hydrogen fusion does not produce enough energy to be commercially viable
ANSWER: B
Hydrogen nuclei have positive charges
So two hydrogen nuclei would have a repulsive force between them
High temperatures are required to give the nuclei enough energy to overcome
the repulsive force
The answer is not A because the products of the hydrogen fusion shown in
the reaction is helium
Helium is an inert gas
The answer is not C because hydrogen is a very abundant element
It is the most common element in the universe
The answer is not D because hydrogen fusion would produce a huge amount
of energy
TEST YOURSELF
NEXT TOPIC
Author: Leander
Leander graduated with a first-class degree in science and education from Shef
winning the Lord Robert Winston Solomon Lipson Prize for dedication to science
excellence. She worked as a science teacher, later going on to become a science
educational content creator before joining SME.
23
"
DOWNLOAD PDF
TEST YOURSELF
1. MOT
ENERG
"
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TheTEST
MilkyYOURSELF
Way
2. THER
Galaxies are made up of billions of stars
Milky
Way
The Universe is made up of many The
different
galaxies
3. WAV
6.2.2 Stars
The Sun is one of billions of stars in a galaxy called the Milky Way
Galaxies
made
upWay
of billions
Other
starsare
in the
Milky
galaxy of
arestars
much further away from Earth than the
The
Universe
is
made
up
of
many
different
galaxies
Sun is
Some of these stars also have planets which orbit them
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The Sun is one of billions of stars in a galaxy called the Milky Way
Other stars in the Milky Way galaxy are much further away from Earth than the
Sun is
Some of these stars also have planets which orbit them
Our solar system is just one out of potentially billions in our galactic neighbourhood,
the Milky Way. There are estimated to be more than 100 billion galaxies in the entire
universe
Astronomical distances such as the distances between stars and galaxies, are so
physicists
a special
to measure
themin
called
the light-year
Ourlarge
solarthat
system
is justuse
one
out of unit
potentially
billions
our galactic
neighbourhood,
the Milky Way. There are estimated to be more than 100 billion galaxies in the entire
One light-year is:
universe
The distance travelled by light through (the vacuum of) space in one year
Astronomical distances such as the distances between stars and galaxies, are so
The
speed
light is theuse
universal
speed
nothing them
can travel
faster
the
large
thatofphysicists
a special
unitlimit,
to measure
called
the than
light-year
speed of light
Oneover
light-year
is:
But
astronomical
distances, light actually travels pretty slowly
The diameter of the Milky Way is approximately 100 000 light-years
The This
distance
lighttake
through
(the
vacuum
of) across
space itin one year
meanstravelled
that lightby
would
100 000
years
to travel
The speed of light is the universal speed limit, nothing can travel faster than the
speed of light
24
12
15
But
over
astronomical
distances,
light
One light year = 9.5 × 10 km = 9.5 × 10actually
m travels pretty slowly
The diameter of the Milky Way is approximately 100 000 light-years
EXTENDED
4. ELEC
MAGNE
5. NUC
6. SPA
6.1 Ear
System
6.2 Sta
6.2.1 T
6.2.2 S
6.2.3 T
Univers
6.2.4 T
6.2.5 H
the Uni
EXTENDED
One light year = 9.5 × 10
12
km = 9.5 × 10
15
m
Life Cycle of Stars
EXTENDED
1. Nebula
Lifecycle of stars 2, downloadable IGCSE & GCSE Physics revision notes
All stars form from a giant interstellar cloud of hydrogen gas and dust called a
nebula
2. Protostar
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The force of gravity within a nebula pulls the particles closer together until it forms
a hot ball of gas, known as a protostar
As the particles are pulled closer together the density of the protostar will increase
This will result in more frequent collisions between the particles which causes
the temperature to increase
3. Main Sequence Star
Once the protostar becomes hot enough, nuclear fusion reactions occur within its
core
The hydrogen nuclei will fuse to form helium nuclei
Every fusion reaction releases heat (and light) energy which keeps the core
hot
Once a protostar is formed, its life cycle will depend on its mass
The different life cycles are shown below:
Flow diagram
the life cycle
of &
a GCSE
star which
is the
samenotes
size as the Sun
Lifecycle
of starsshowing
1, downloadable
IGCSE
Physics
revision
(solar mass) and the lifecycle of a star which is much bigger than the Sun
Once a star is born it is known as a main-sequence star
During the main sequence, the star is in equilibrium and said to be stable
The inward force due to gravity is equal to the outward pressure force from
the fusion reactions
4. Red Giant or Red Super Giant
After several billion years the hydrogen causing the fusion reactions in the star will
begin to run out
Once this happens, the fusion reactions in the core will start to die down
This causes the core to shrink and heat up
The core will shrink because the inward force due to gravity will become
greater than the outward force due to the pressure of the expanding gases as
the fusion dies down
A new series of reactions will then occur25around the core, for example, helium
nuclei will undergo fusion to form beryllium
26
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the fusion reactions
4. Red Giant or Red Super Giant
After several billion years the hydrogen causing the fusion reactions in the star will
begin to run out
Once this happens, the fusion reactions in the core will start to die down
This causes the core to shrink and heat up
The core will shrink because the inward force due to gravity will become
greater than the outward force due to the pressure of the expanding gases as
the fusion dies down
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A new series of reactions will then occur around the core, for example, helium
nuclei will undergo fusion to form beryllium
These reactions will cause the outer part of the star to expand
A star the same size as the Sun or smaller will become a red giant
A star much larger than the Sun will become a red super giant
It is red because the outer surface starts to cool
5. For Red Giant Stars
Planetary Nebula
Once this second stage of fusion reactions have finished, the star will become
unstable and eject the outer layer of dust and gas
The layer of dust and gas which is ejected is called a planetary nebula
White Dwarf
The core which is left behind will collapse completely, due to the pull of gravity,
and the star will become a white dwarf
The white dwarf will be cooling down and as a result, the amount of energy it
emits will decrease
Black Dwarf
Once the star has lost a significant amount of energy it becomes a black dwarf
It will continue to cool until it eventually disappears from sight
Lifecycle of Solar mass stars, downloadable IGCSE & GCSE Physics revision notes
27
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6.2.3 The Expanding Universe
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6.2.3
The Expanding
Universe
"
Galaxies & Redshift
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Usually, when an object emits waves, the wavefronts spread out symmetrically
If the wave source moves, the waves can become squashed together or stretched
out
Galaxies & Redshift
1. MOTION, FORCES &
ENERGY
2. THERMAL PHYSICS
3. WAVES
4. ELECTRICITY &
MAGNETISM
5. NUCLEAR PHYSICS
Usually, when an object emits waves, the wavefronts spread out symmetrically
If the wave source moves, the waves can become squashed together or stretched
6. SPACE PHYSICS
out
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6.1 Earth & The Solar
System
Diagram showing the wavefronts produced from a stationary object and a moving
object
A moving object will cause the wavelength, λ, (and frequency) of the waves to
change:
The wavelength of the waves in front of the source decreases and the
frequency increases
The wavelength behind the source increases and the frequency decreases
This effect is known as the Doppler effect
6.2 Stars & The Universe
6.2.1 The Sun as a Star
6.2.2 Stars
6.2.3 The Expanding
Universe
6.2.4 The Big Bang Theory
6.2.5 Hubble & The Age of
the Universe
The Doppler effect also affects light
If an object moves away from an observer the wavelength of light increases
This is known as redshift as the light moves towards the red end of the
spectrum
Diagram
showing the wavefronts produced from a stationary object and a moving
Redshift is:
object
An increase in the observed wavelength of electromagnet radiation emitted
from receding stars and galaxies
A moving object will cause the wavelength, λ, (and frequency) of the waves to
change:
The wavelength of the waves in front of the source decreases and the
frequency increases
The wavelength behind the source increases and the frequency decreases
This effect is known as the Doppler effect
The Doppler effect also affects light
If an object moves away from an observer the wavelength of light increases
This is known as redshift29as the light moves towards the red end of the
spectrum
Light from a star that is moving towards an observer will be blueshifted and light
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Light from a star that is moving towards an observer will be blueshifted and light
from a star moving away from an observer will be redshifted
The observer behind observes a red shift
The Milky Way is just one of billions of galaxies that make up the Universe
Light emitted from distant galaxies appears redshifted when compared with light
emitted on Earth
The diagram below shows the light coming to us from a close object, such as the
Sun, and the light coming to us from a distant galaxy
30
Sun, and the light coming to us from a distant galaxy
Comparing the light spectrum produced from the Sun and a distant galaxy
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The diagram also shows that the light coming to us from distant galaxies is
redshifted
The lines on the spectrum are shifted towards the red end
This indicates that the galaxies are moving away from us
If the galaxies are moving away from us it means that the universe is expanding
The observation of redshift from distant galaxies supports the Big Bang theory
Another observation from looking at the light spectrums produced from distant
galaxies is that the greater the distance to the galaxy, the greater the redshift
This means that the further away a galaxy, the faster it is moving away from
us
Graph showing the greater the distance to a galaxy, the greater the redshift
TEST YOURSELF
31
NEXT TOPIC
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1. MOT
ENERG
The Big Bang
2. THE
6.2.4 The Big Bang Theory
"
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TEST YOURSELF
Around 14 billion years ago, the Universe began from a very small region that
was extremely hot and dense
The Big
Bang
Then there was a giant explosion, which
is known
as the Big Bang
This caused the universe to expand from a single point, cooling as it does so, to
form the universe today
Around 14 billion years ago, the Universe began from a very small region that
Each point expands away from the others
wasThis
extremely
hotgalaxies
and dense
is seen from
moving away from each other, and the further
away
they
areathe
faster
they movewhich is known as the Big Bang
Then
there
was
giant
explosion,
This caused the universe to expand from a single point, cooling as it does so, to
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Redshift in the light from distant galaxies is evidence that the Universe is
form the and
universe
today
expanding
supports
the Big Bang Theory
Each
point
expands
away
from the
others
As a result of the initial explosion,
the
Universe continues to expand
This is seen from galaxies moving away from each other, and the further
away they are the faster they move
Redshift in the light from distant galaxies is evidence that the Universe is
expanding and supports the Big Bang Theory
As a result of the initial explosion, the Universe continues to expand
All galaxies are moving away from each other, indicating that the universe is
expanding
An analogy of this is points drawn on a balloon where the balloon represents
space and the points as galaxies
When the balloon is deflated, all the points are close together and an equal
distance apart
As the balloon expands, all the points become further apart by the same
amount
This is because the space itself has expanded between the galaxies
32
All galaxies are moving away from each other, indicating that the universe is
expanding
3. WA
4. ELE
MAGN
5. NUC
6. SPA
6.1 Ea
System
6.2 Sta
6.2.1 T
6.2.2 S
6.2.3 T
Univer
6.2.4 T
Theor
6.2.5 H
the Un
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This is because the space itself has expanded between the galaxies
A balloon inflating is similar to the stretching of the space between galaxies
Redshift and CMBR
Evidence for the Big Bang
The Big Bang theory is very well supported by evidence from a range of sources
The main pieces of evidence are
Galactic red-shift
Cosmic Microwave Background Radiation (CMBR)
Evidence from Galactic Red-Shift
Galactic redshift provides evidence for the Big Bang Theory and the expansion of
the universe
The diagram below shows the light coming to us from a close object, such as the
Sun, and the light coming to the Earth from a distant galaxy
33
Evidence from Galactic Red-Shift
Galactic redshift provides evidence for the Big Bang Theory and the expansion of
the universe
The diagram below shows the light coming to us from a close object, such as the
Sun, and the light coming to the Earth from a distant galaxy
A balloon inflating is similar to the stretching of the space between galaxies
Redshift and CMBR
Evidence for the Big Bang
The Big Bang theory is very well supported by evidence from a range of sources
The main pieces of evidence are
Galactic red-shift
Cosmic Microwave Background Radiation (CMBR)
Evidence from Galactic Red-Shift
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Galactic redshift provides evidence for the Big Bang Theory and the expansion of
the universe
The diagram below shows the light coming to us from a close object, such as the
Sun, and the light coming to the Earth from a distant galaxy
Comparing the light spectrum produced from the Sun and a distant galaxy
Red-shift provides evidence that the Universe is expanding because:
Red-shift is observed when the spectral lines from the distant galaxy move closer
to the red end of the spectrum
This is because light waves are stretched by the expansion of the universe so
Comparing the light spectrum produced from the Sun and a distant galaxy
the wavelength
increases (or frequency decreases)
Red-shift
provides
that theare
Universe
is expanding
because:from us
This indicates that
theevidence
galaxies
moving
away
Red-shift is observed when the spectral lines from the distant galaxy move closer
to the red end of the spectrum
This is because light waves are stretched by the expansion of the universe so
the wavelength increases (or frequency decreases)
This indicates that the galaxies are moving away from us
Light spectrums produced from distant galaxies are red-shifted more than nearby
galaxies
spectrums
produced fromthe
distant
galaxies are red-shifted
than nearby
This shows Light
that
the greater
distance
to the more
galaxy,
the greater the
galaxies
redshift
This shows that the greater the distance to the galaxy, the greater the
redshift
This means that
the further away a galaxy is, the faster it is moving away
This means that the further away a galaxy is, the faster it is moving away
from
from the Earth the Earth
These observations imply that the universe is expanding and therefore support the
Big Bang Theory
These observations imply that the universe is expanding and therefore support the
Big Bang Theory
Tracing the expansion of the universe back to the beginning of time leads to the idea
the universe began34
with a “big bang”
EXTENDED
EXTENDED
Evidence from CMB Radiation
The discovery of the CMB (Cosmic Microwave Background) radiation led to the Big
Bang theory becoming the currently accepted model
The CMB is a type of electromagnetic radiation which is a remnant from the
early stages of the Universe
It has a wavelength of around 1 mm making it a microwave, hence the name
Cosmic Microwave Background radiation
EXTENDED
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In 1964, Astronomers discovered radiation in the microwave region of the
electromagnetic spectrum coming from all directions and at a generally uniform
temperature of 2.73 K
They were unable to do this any earlier since microwaves are absorbed by the
atmosphere
Around this time, space flight was developed which enabled astronomers to
send telescopes into orbit above the atmosphere
According to the Big Bang theory, the early Universe was an extremely hot and
dense environment
As a result of this, it must have emitted thermal radiation
The radiation is in the microwave region
This is because over the past 14 billion years or so, the radiation initially from
the Big Bang has become redshifted as the Universe has expanded
Initially, this would have been high energy radiation, towards the gamma end
of the spectrum
As the Universe expanded, the wavelength of the radiation increased
Over time, it has increased so much that it is now in the microwave region of
the spectrum
The CMB is a result of high energy radiation being redshifted over billions of years
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EXTENDED
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The CMB radiation is very uniform and has the exact profile expected to be emitted
from a hot body that has cooled down over a very long time
This phenomenon is something that other theories (such as the Steady State
Theory) cannot explain
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The CMB is represented by the following map:
The CMB map with areas of higher and lower temperature. Places with higher
temperature have a higher concentration of galaxies, Suns and planets
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This is the closest image to a map of the observable Universe
The different colours represent different temperatures
The red / orange / brown regions represent warmer temperature indicating a
higher density of galaxies
The blue regions represents cooler temperature indicating a lower density of
galaxies
The temperature of the CMB radiation is mostly uniform, however, there are
minuscule temperature fluctuations (on the order of 0.00001 K)
This implies that all objects in the Universe are more or less uniformly spread
out
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6.2.5 Hubble & The Age of the Universe
"
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TEST YOURSELF
Hubble Constant Calculations
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In 1929, the astronomer Edwin Hubble showed that the universe was expanding
He did this by observing that the absorption line spectra produced from the
light of distant galaxies was shifted towards the red end of the spectrum
This doppler shift in the wavelength of the light is evidence that distant
galaxies are moving away from the Earth
Hubble also observed that light from more distant galaxies was shifted further
towards the red end of the spectrum compared to closer galaxies
From this observation he concluded that galaxies or stars which are further
away from the Earth are moving faster than galaxies which are closer
Hubble’s law states:
The recessional velocity v of a galaxy is proportional to its distance from Earth
Hubble’s law can be expressed as an equation:
H0 !
v
d
Where:
H0 = Hubble constant, this will be provided in your examination along with th
-1)
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correct units (km s-1 Mpc
–18
The accepted value is that H0 = 2.2 × 10
per second
ENER
Hubble Constant Calculations
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2. TH
3. WA
In 1929, the astronomer Edwin Hubble showed that the universe was expanding
He did this by observing that the absorption line spectra produced from the
light of distant galaxies was shifted towards the red end of the spectrum
This doppler shift in the wavelength of the light is evidence that distant
galaxies are moving away from the Earth
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Hubble also observed that light from more distant galaxies was shifted further
towards the red end of the spectrum compared to closer galaxies
From this observation he concluded that galaxies or stars which are further
away from the Earth are moving faster than galaxies which are closer
Hubble’s law states:
The recessional velocity v of a galaxy is proportional to its distance from Earth
Hubble’s law can be expressed as an equation:
H0 !
v
d
Where:
H0 = Hubble constant, this will be provided in your examination along with the
correct units (km s-1 Mpc-1)
–18
The accepted value is that H0 = 2.2 × 10
per second
v = recessional velocity of an object, the velocity of an object moving away
from an observer (km s-1)
d = distance between the object and the Earth (Mpc)
As the equation shows, the Hubble Constant, H0 is defined as:
The ratio of the speed at which the galaxy is moving away from the Earth, to its
distance from the Earth
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6.2.1
6.2.2
6.2.3
Unive
6.2.4
6.2.5
the U
Age of the Universe
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Since Hubble's Law states that
H0 !
v
d
It can be rearranged to show that
1
d
!
H0
v
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Hubble’s law shows that the further away a star is from the Earth, the faster it is
moving away from us
A key aspect of Hubble’s law is that the furthest galaxies appear to move away the
fastest
The gradient of the graph can be used to find the Age of the Universe
When the distance equals zero, this represents all the matter in the Universe
being at a single point
This is the singularity that occurred at the moment of the Big Bang
The units of the gradient are per second (the same as the units of the Hubble
Constant)
1
By taking the reciprocal, or,
H 0 the units will become seconds
Therefore the reciprocal of the gradient represents time and gives the amount
of time which the Universe has been expanding for
Astronomers have used this formula to estimate the age of the Universe at about
39
13.7 billion years
13.7 billion years
? Worked Example
A distant galaxy is 20 light-years away from Earth.
Use Hubble’s Law to determine the velocity of the galaxy as it moves away from
Earth.
The Hubble constant is currently agreed to be 2.2 x 10-18 s-1.
Step 1: List the known quantities:
d = 20 light years
Ho = 2.2 x 10-18 s-1
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Step 2: Convert 20 light-years to m:
From the data booklet: 1 ly ≈ 9.5 x 1015 m
15
17
So, 20 ly = 20 x (9.5 x 10 ) = 1.9 x 10 m
Step 3: Substitute values into Hubble's Law:
From the data booklet: v ≈ H0d
So, v ≈ (2.2 x 10-18 ) x (1.9 x 1017) = 0.418 m s-1
Step 4: Confirm your answer:
The velocity of the galaxy as it moves away from Earth 0.42 m s-1
TEST YOURSELF
Author: Lindsay
Lindsay graduated with First Class Honours from the University of Greenwich and
Communication MSc at Imperial College London. Now with many years’ experience
and Examiner for A Level and IGCSE Physics (and Biology!), her love of communica
Physics has brought her to Save My Exams where she hopes to help as many stude
their next steps.
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