SPACE PHYSICS

The Earth, Moon & Sun

The Earth's Axis

 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
o 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)
 This rotation creates the apparent daily motion of the Sun rising and setting
o Rotation of the Earth on its axis is therefore responsible for the periodic cycle of
day and night
Day and Night

 The Earth's rotation around its axis creates day and night
o Day is experienced by the half of the Earth's surface that is facing the Sun
o Night is the other half of the Earth's surface, facing away from the Sun

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Rising and Setting of the Sun

 The Earth's rotation on its axis makes the Sun looks like it moves from east to west
o At the equinoxes the Sun rises exactly in the east and sets exactly in the west
o 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
 In the northern hemisphere (above the equator):
o In summer, the sun rises north of east and sets north of west
o In winter, the sun rises south of east and sets south of west

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  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
o 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
o 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 approximately 365 days

o This is 1 year
 The combination of the orbiting of the Earth around the Sun and the Earth's tilt creates
the seasons.

 Over parts B, C and D of the orbit, the northern hemisphere is tilted towards the Sun

o This means daylight hours are more than hours of darkness

o This is spring and summer
 The southern hemisphere is tilted away from the Sun
o This means there are shorter days than night
o This is autumn and winter

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  Over parts F, G and H of the orbit, the northern hemisphere is tilted away from the Sun
o The situations in both the northern and southern hemisphere are reversed
o 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:

o This is the summer solstice

o The northern hemisphere has the longest day, whilst the southern hemisphere has
its shortest day
 At G:
o This is the winter solstice
o The northern hemisphere has its shortest day, whilst the southern hemisphere has
its longest day
 At A and D:
o Night and day are equal in both hemispheres
o 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
o 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
o 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

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  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:
o The Moon is between the Earth and the Sun
o Therefore, the sunlight is only on the opposite face of the Moon to the Earth
o This means the Moon is unlit as seen from Earth, so it is not visible
 At the Full Moon phase:
o The Earth is between the Moon and the Sun
o The side of the Moon that is facing the Earth is completely lit by the sunlight
o 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

Orbital Speed
EXTENDED

 When planets move around the Sun, or a moon moves around a planet, they orbit
in circular motion
o This means that in one orbit, a planet travels a distance equal to the circumference
of a circle (the shape of the orbit)
o This is equal to 2πr where r is the radius a circle

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  The relationship between speed, distance and time is:

 the average orbital speed of an object can be defined by the equation:

 Where:
o v = orbital speed in metres per second (m/s)
o r = average radius of the orbit in metres (m)
o 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

 The orbital radius r is always taken from the centre of the object being orbited to the
object orbiting

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 Worked Example
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

o Radius of the Earth, R = 6400 km

o Distance of the telescope above the Earth's surface, h = 560 km
o Time period, T = 96 minutes

Step 2: Write the relevant equation

Step 3: Calculate the orbital radius, r

o The orbital radius is the distance from the centre of the Earth to the telescope

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Step 4: Convert any units

o The time period needs to be in seconds

o The radius needs to be in metres

Step 5: Substitute values into the orbital speed equation

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The Solar System

 The Solar System consists of:

1. The Sun
2. Eight planets
3. Natural and artificial satellites
4. Dwarf planets
5. Asteroids and comets

The Sun & the Planets

 The Sun lies at the centre of the Solar System

o The Sun is a star that makes up over 99% of the mass of the solar system

 There are eight planets and an unknown number of dwarf planets which orbit the Sun
o The gravitational field around planets is strong enough to have pulled in all
nearby objects with the exception of natural satellites
o The gravitational field around a dwarf planet is not strong enough to have
pulled in nearby objects
 The 8 planets in our Solar System in ascending order of the distance from the Sun are:
o Mercury
o Venus
o Earth
o Mars
o Jupiter
o Saturn
o Uranus
o Neptune

Satellites

 There are two types of satellite:

o Natural
o Artificial

 Some planets have moons which orbit them

o Moons are an example of natural satellites

 Artificial satellites are man-made and can orbit any object in space
o The International Space Station (ISS) orbits the Earth and is an example of an
artificial satellite

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Asteroids & Comets

 Asteroids and comets also orbit the sun

 An asteroid is a small rocky object which orbits the Sun
o The asteroid belt lies between Mars and Jupiter

 Comets are made of dust and ice and orbit the Sun in a different orbit to those of planets
o The ice melts when the comet approaches the Sun and forms the comet’s tail

Accretion Model of the Solar System

 There are 4 rocky and small planets: Mercury, Venus, Earth and Mars
o These are the nearest to the Sun
 There are 4 gaseous and large planets: Jupiter, Saturn, Uranus and Neptune
o There are the furthest from the sun

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 The eight planets of our Solar System

 The differences in the types of planets are defined by the accretion model for Solar
System formation

 The Sun was thought to have formed when gravitational attraction of pulled together
clouds of hydrogen dust and gas (called nebulae)
 The Solar System then formed around 4.5 billion years ago
o The planets were formed from the remnants of the disc cloud of matter left over
from the nebula that formed the Sun
o These interstellar clouds of gas and dust included many elements that were
created during the final stages of a star's lifecycle (a previous supernova)
 Gravity collapsed the matter from the nebula in on itself causing it to spin around the Sun
o The gravitational attraction between all the small particles caused them to join
together and grow in an accretion process
 A rotating accretion disc is formed when the planets emerged

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 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
o Therefore, the inner planets are made of materials with high melting temperatures
such as metals (e.g. iron)
o 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
o The outer planets therefore could grow to a large size up and include even the
lightest element, Hydrogen
o These planets are large, gaseous and cold

Light Speed

 The planets and moons of the solar system are visible from Earth when they reflect light
from the Sun
o The outer regions of the Solar System are around 5 × 1012 m from the Sun, which
means even light takes some time to travel these distances
 The Sun is so far away from Earth that the light we see actually left the Sun eight minutes
earlier
o the nearest star to us after the Sun is so far away that light from it takes four years
to reach us
o The Milky Way galaxy contains billions of stars, huge distances away, with the
light taking even longer to be seen from Earth

 The speed of light is a constant 3 × 108 m/s

o Therefore, using the equation:

o The time taken to travel a certain distance can be calculated by rearranging to:

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 Worked Example
Calculate the time taken for light from the Sun to reach Mercury if the radius of Mercury's orbit
is 5.8 × 109 m.

Step 1: State the equation for the time taken for light to travel a certain distance

Step 2: Substitute in the values

o The distance travelled is the radius of the orbit

 Distance, d = 5.8 × 109 m.
o Speed = the speed of light, v = 3.0 × 108 m/s

Step 3: Round up the answer and include units

time = 19.3 s

Elliptical Orbits
EXTENDED

 Orbits of planets, minor planets and comets are elliptical

o An ellipse is just a 'squashed' circle
 Planets, minor planets and comets have elliptical orbits
o However, the Sun is not at the centre of an elliptical orbit
o 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
o However, in a circular orbit, the Sun is at the centre

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 Planets and comets travel in elliptical orbits, but the Sun is not at the centre of these orbits
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:
o Factors that affect conditions on the surface of the planets
o Environmental problems that a visit (using manned spaceships or robots) would
encounter

Table of Data for Planets in our Solar System

 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)

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 o The circular path that the planet's travel in has a larger radius
 Orbital duration increases with orbital distance
o E.g. Neptune travels much slower than Mercury
o 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
o 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
o This is why although Uranus is 4 times larger than Earth, it has a smaller
gravitational field strength because it is less dense

Gravitational Field Strength

 The strength of gravity on different planets after an object's weight on that planet
 Weight is defined as:

The force acting on an object due to gravitational attraction

 Planets have strong gravitational fields

o Hence, they attract nearby masses with a strong gravitational force
 Because of weight:
o Objects stay firmly on the ground
o Objects will always fall to the ground
o Satellites are kept in orbit

Objects are attracted towards the centre of the Earth due to its gravitational field strength

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 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
o The greater the mass of the planet then the greater its gravitational field strength
o 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
o The strength of the field around the planet decreases as the distance from the
planet increases
 However, the value of g on the surface varies dramatically for different planets and
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
o This means it would be easier to lift a mass on the surface of the Moon than on
the Earth
 The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn)
is more than on the Earth
o 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

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 A person’s weight on Jupiter would be so large a human would be unable to fully stand up

Gravitational Attraction of the Sun

 There are many orbiting objects in our solar system and they each orbit a different type of
planetary body

Orbiting Objects or Bodies in Our Solar System Table

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  A smaller body or object will orbit a larger body
o For example, a planet orbiting the Sun
 In order to orbit a body such as a star or a planet, there has to be a force pulling the
object towards that body
o Gravity provides this force
 Therefore, it is said that the force that keeps a planet in orbit around the Sun is
the gravitational attraction of the Sun
 The gravitational force exerted by the larger body on the orbiting object is always
attractive
o Therefore, the gravitational force always acts towards the centre of the larger
body
 Therefore, the force that keeps an object in orbit around the Sun is the gravitational
attraction of the Sun and is always directed from the orbiting object to the centre of the
Sun

 The gravitational force will cause the body to move and maintain in a circular path

Gravitational attraction causes the Moon to orbit around the Earth

Sun's Gravitational Field & Distance

EXTENDED

 As the distance from the Sun increases:

o The strength of the Sun's gravitational field on the planet decreases

o Their orbital speed of the planet decreases
 To keep an object in a circular path, it must have a centripetal force
o For planets orbiting the Sun, this force is gravity

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 Therefore, the strength of the Sun's gravitational field in the planet affects how much
centripetal force is on the planet
o This strength decreases the further away the planet is from the Sun, and the
weaker the centripetal force
 The centripetal force is proportional to the orbital speed
o Therefore, the planets further away from the Sun have a smaller orbital speed
o This also equates to a longer orbital duration

How the speed of a planet is affected by its distance from the Sun

 This can be seen from data collected for a planet's orbital distance against their orbital
speed
o E.g. Neptune travels much slower than Mercury

Table of Orbital Distance, Speed and Duration

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Orbits & Conservation of Energy
EXTENDED

 An object in an elliptical orbit around the Sun travels at a different speed depending on
its distance from the Sun
 Although these orbits are not circular, they are still stable
o For a stable orbit, the radius must change if the comet's orbital speed changes
 As the comet approaches the Sun:
o The radius of the orbit decreases
o The orbital speed increases due to the Sun's strong gravitational pull
 As the comet travels further away from the Sun:
o The radius of the orbit increases
o The orbital speed decreases due to a weaker gravitational pull from the Sun

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 Comets travel in highly elliptical orbits, speeding up as they approach the Sun
Conservation of Energy

 Although an object in an elliptical orbit, such as a comet, continually changes its speed its
energy must still be conserved
o Throughout the orbit, the gravitational potential energy and kinetic energy of the
comet changes
 As the comet approaches the Sun:
o It loses gravitational potential energy and gains kinetic energy
o This causes the comet to speed up
o This increase in speed causes a slingshot effect, and the body will be flung back
out into space again, having passed around the Sun
 As the comment moves away from the Sun:
o It gains gravitational potential energy and loses kinetic energy
o This causes it to slow down
o Eventually, it falls back towards the Sun once more
 In this way, a stable orbit is formed

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The Sun

 The Sun lies at the centre of the Solar System

o The Sun is a star which makes up over 99% of the mass of the solar system
o 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

 The gravitational pull of the Sun on the planets keeps them in orbit

 The Sun is a medium sized star consisting of mainly hydrogen and helium
 It radiates most of its energy in the infrared, visible and ultraviolet regions of the
electromagnetic spectrum

Our Sun (Image courtesy of NASA)

 Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs, from
blue giants to red supergiants
o These can be classified according to their colour
 Warm objects emit infrared and extremely hot objects emit visible light as well
o Therefore, the colour they emit depends on how hot they are

 A star's colour is related to its surface temperature

o A red star is the coolest (at around 3000 K)
o A blue star is the hottest (at around 30 000 K)

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 The colour of a star correlates to its temperature

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

 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

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 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

D Hydrogen fusion does not produce enough energy to be commercially viable

ANSWER: B


o Hydrogen nuclei have positive charges
o So two hydrogen nuclei would have a repulsive force between them
o High temperatures are required to give the nuclei enough energy to overcome the
repulsive force
o The answer is not A because the products of the hydrogen fusion shown in the
reaction is helium
 Helium is an inert gas
o The answer is not C because hydrogen is a very abundant element
 It is the most common element in the universe
o The answer is not D because hydrogen fusion would produce a huge amount of
energy

Stars

The Milky Way

 Galaxies are made up of billions of stars

 The Universe is made up of many different galaxies

 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

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 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 large that physicists use a special unit to measure them called the light-year

 One light-year is:

The distance travelled by light through (the vacuum of) space in one year

 The speed of light is the universal speed limit, nothing can travel faster than the speed of
light
 But over astronomical distances, light actually travels pretty slowly
 The diameter of the Milky Way is approximately 100 000 light-years
o This means that light would take 100 000 years to travel across it

EXTENDED

 One light year = 9.5 × 1012 km = 9.5 × 1015 m

Life Cycle of Stars

EXTENDED

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1. Nebula

 All stars form from a giant interstellar cloud of hydrogen gas and dust called a nebula

2. Protostar

 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
o 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
o The hydrogen nuclei will fuse to form helium nuclei
o 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:

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28
 Flow diagram showing the life cycle of a star which is the same size as the Sun (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
o 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
o 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 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
o 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
o 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

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 The lifecycle of a solar mass star

6. For Red Super Giants

Supernova

 Once the fusion reactions inside the red supergiant finally finish, the core of the star
will collapse suddenly causing a gigantic explosion
o This is called a supernova
 At the centre of this explosion a dense body, called a neutron star will form
 The outer remnants of the star will be ejected into space during the supernova
explosion, forming a planetary nebula
o The nebula from a supernova may form new stars with orbiting planets

Neutron Star (or Black Hole)

 In the case of the biggest stars, the neutron star that forms at the centre will continue
to collapse under the force of gravity until it forms a black hole
o A black hole is an extremely dense point in space that not even light can escape
from

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 Lifecycle of a star much larger than our Sun

The Expanding Universe

Redshift

 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

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 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:
o The wavelength of the waves in front of the source decreases and
the frequency increases
o The wavelength behind the source increases and the frequency decreases
o This effect is known as the Doppler effect

 The Doppler effect also affects light

o 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

 Redshift is:

An increase in the observed wavelength of electromagnet radiation emitted from receding

stars and galaxies

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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

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 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

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 Comparing the light spectrum produced from the Sun and a distant galaxy

 The diagram also shows that the light coming to us from distant galaxies is redshifted
o 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
o This means that the further away a galaxy, the faster it is moving away from us

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 Graph showing the greater the distance to a galaxy, the greater the redshift

The Big Bang

 Around 14 billion years ago, the Universe began from a very small region that
was extremely hot and dense
 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
 Each point expands away from the others
o 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
o 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
o When the balloon is deflated, all the points are close together and an equal
distance apart

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 o As the balloon expands, all the points become further apart by the same amount
o This is because the space itself has expanded between the galaxies

A balloon inflating is similar to the stretching of the space between galaxies

Evidence for the Big Bang

 The main pieces of evidence for the Big Bang are

o Galactic red-shift
o Cosmic Microwave Background (CMB) radiation

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

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 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
o This is because light waves are stretched by the expansion of the universe so the
wavelength increases (or frequency decreases)
o This indicates that the galaxies are moving away from us

 Light spectrums produced from distant galaxies are red-

shifted more than nearby galaxies
o This shows that the greater the distance to the galaxy, the greater the redshift
o This means that the further away a galaxy is, the faster it is moving away from
the Earth

 These observations imply that the universe is expanding and therefore support the Big
Bang Theory

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 Tracing the expansion of the universe back to the beginning of time leads to the idea the
universe began with a “big bang”

EXTENDED

Using Redshift Observations to Measure the Universe

 The change in wavelength of the galaxy’s starlight due to redshift can be used to find the
velocity, v, with which a galaxy (or any distant object) is moving away from Earth
o Using an equation to compare the ratio of the expected wavelength with the
observed wavelength, the velocity can be found;

This equation will not be directly examined but the idea that the velocity of distant objects
can be found from the redshift seen in easily observed wavelengths is an important one

EXTENDED

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Evidence from CMB Radiation

 The discovery of the CMB (Cosmic Microwave Background) radiation led to the Big
Bang theory becoming the currently accepted model
o The CMB is a type of electromagnetic radiation which is a remnant from the early
stages of the Universe
o It has a wavelength of around 1 mm making it a microwave, hence the name
Cosmic Microwave Background radiation

 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
o They were unable to do this any earlier since microwaves are absorbed by the
atmosphere
o Around this time, space flight was developed which enabled astronomers to send
telescopes into orbit above the atmosphere

EXTENDED

 According to the Big Bang theory, the early Universe was an

extremely hot and dense environment
o As a result of this, it must have emitted thermal radiation

 The radiation is in the microwave region

o 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
o Initially, this would have been high energy radiation, towards the gamma end of
the spectrum
o As the Universe expanded, the wavelength of the radiation increased
o Over time, it has increased so much that it is now in the microwave region of the
spectrum

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 The CMB is a result of high energy radiation being redshifted over billions of years

EXTENDED

 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
o This phenomenon is something that other theories (such as the Steady State
Theory) cannot explain

EXTENDED

 The CMB is represented by the following map:

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 The CMB map with areas of higher and lower temperature. Places with higher temperature
have a higher concentration of galaxies, Suns and planets

EXTENDED

 This is the closest image to a map of the observable Universe

 The different colours represent different temperatures
o The red / orange / brown regions represent warmer temperature indicating
a higher density of galaxies
o 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)
o This implies that all objects in the Universe are more or less uniformly spread
out

Measuring Distance Using Supernovae

 Redshift and CMB radiation allow various measurements of the Universe to be accurately
made

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 o Measuring distance is done using different methods
o A key method is the use of standard candles, including supernovae
 Supernovae are exploding stars
o Certain types have the same peak level of brightness (absolute magnitude),
making them extremely useful in measuring the distance to remote stars and
galaxies
o Type 1a supernovae are so bright that they can be seen clearly even though they
may be deep inside their parent galaxy
o This allows the distance to the galaxy to be calculated

Hubble & The Age of the Universe

Hubble Constant Calculations

EXTENDED

 In 1929, the astronomer Edwin Hubble showed that the universe was expanding
o 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
o 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
o 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

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  Hubble’s law can be expressed as an equation:

 Where:
o H0 = Hubble constant, this will be provided in your examination along with the
correct units (km s-1 Mpc-1)
 The accepted value is that H0 = 2.2 × 10–18 per second
o v = recessional velocity of an object, the velocity of an object moving away from
an observer (km s-1)
o 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

Age of the Universe

EXTENDED

 Since Hubble's Law states that

 It can be rearranged to show that

 Hubble’s law shows that the further away a star is from the Earth, the faster it is moving
away from us

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 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
o When the distance equals zero, this represents all the matter in the Universe being
at a single point
o 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)
o By taking the reciprocal, or, the units will become seconds
o 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 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:


o d = 20 light years

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 o Ho = 2.2 x 10-18 s-1

Step 2: Convert 20 light-years to m:


o From the data booklet: 1 ly ≈ 9.5 x 1015 m
o So, 20 ly = 20 x (9.5 x 1015) = 1.9 x 1017 m

Step 3: Substitute values into Hubble's Law:


o From the data booklet: v ≈ H0d
o So, v ≈ (2.2 x 10-18 ) x (1.9 x 1017) = 0.418 m s-1

Step 4: Confirm your answer:


o The velocity of the galaxy as it moves away from Earth 0.42 m s-1

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