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Space Physics Notes
Fundamental Of Physics (North South University)
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SPACE PHYSICS NOTES CIE
1 Earth and the Solar System
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Define Rotation of Earth:The circular movement of
the Earth about its own axis is called as the rotation
of the Earth.
Define Average orbital speed: It is defined using the
equation: v = 2??r /T where r is the average radius of
the orbit and T is the orbital period
Define Solar system:Solar system is a single term
that includes eight planets and their moons in orbit
round the sun. Also included in the solar system
are the smaller bodies such as the comets.,
asteroids, and the meteoroids.
Define Orbital distance:Orbital distance is the term
that tells us the time taken by one object to revolve
around the other.
Define Orbital duration[ Orbital time}: Orbital duration
is the time taken by one object to completely orbit
around the other.
Define gravitational field strength: Gravitational field
strength is a term that indicates the amount of
gravitational force that is exerted per unit mass at a
particular point.It has a symbol "g"
Define Interstellar clouds of gas and dust: It is a
cloud comprising of dust, plasma and gases present
in all the galaxies, including our own.
Define accretion disc: These are structures that
surround the celestial objects such as the stars and
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the black holes.These structures are made of gas,
plasma and dust
Define Planetary data:It refers to the data that gives
us information on the different planets based on
researches and explorations
6.2 Stars and the Universe
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Define Stars:Stars are massive , self-luminous
celestial objects, made of Hydrogen and Helium
Define Proto stars: A protostar is a young star. .It is
formed as a result of accretion in the nebula.
Define Stable stars:In every star, there is the gas
pressure that exerts an outward force from the
centre and at the same time , there is gravity that is
pulling the atoms of Hydrogen and Helium inwards.
When the outward pushing force balances the
gravitational force, we call the star as a stable
tar.Example of a stable star is the Sun.
Define Sun:The Sun is a very bright, luminous,
celestial and a stable star, situated in the spiral arm
of the Milky Way galaxy and at the centre of the Solar
System.
Define Galaxy:The word galaxy encompasses billions
of stars along with their solar systems,gas and dust
held by their gravity. There are more than a hundred
billion galaxies in the universe.
Define Milky way:Milky way is a spiral galaxy that
includes our solar system. It has billions of stars,
including our Sun.
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Define Universe:Universe refers to the space, time
and matter and all the laws that govern them.
Define Light-year: One light-year is the distance
traveled in (the vacuum of) space by light in one year.
Define Red giants:These are massive , luminous stars
with a very low surface temperature and a high
volume , with a reddish-orange hue, in their final
stage of stellar evolution and that have exhausted
their
supply
of
Hydrogen
in
their core.Examples: Aldebaran (Alpha Tauri) and
Mira (Omicron Ceti).
Define Red supergiants: Red supergiants are aging
giant stars with a minimum of 15 solar masses.
[solar mass refers to the mass of the sun, when used
as a unit of mass. One solar mass is equal to 1.989 x
1030 kg ]
Define Supernova: Supernova is a powerful and
luminous stellar explosion, that happens when a star
has reached the end of its life.Example: Kepler's
Supernova
Define Nebula:Nebula is a Latin word for fog or
cloud.It consists of interstellar clouds consisting
of Helium, Cosmic Dust, ionised gases, hydrogen as
well as molecular clouds. [There are in all 5 different
types of Nebulae, namely;emission nebulae,
reflection nebulae, dark nebulae, planetary nebulae,
and supernova remnants.
Define Black hole:A black hole is a region in the
space, in which the effect of gravity is so strong, that
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it lets nothing escape out f it, not even light.The
region appears to be black and circular.[ There are
approximately 100 billion super massive black holes]
Define Redshift:Redshift is a phenomenon in which
the spectrum of an astronomical object gets
displaced towards the longer( red) wavelength.
Define Cosmic microwave background radiation
(CMBR): Cosmic microwave background radiation
(CMBR) is the leftover of the cosmic radiation after
the big bang.
Define Hubble constant Ho: Hubble constant is
the ratio of the speed at which the galaxy is moving
away from the Earth to its distance from the Earth
NOTES:
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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
There are 8 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
o There are 4 rocky planets: Mercury, Venus,
Earth and Mars
o There are 4 gas planets: Jupiter, Saturn, Uranus
and Neptune
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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
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
Our Place in Space
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Our solar system is just one small part of the Milky
Way galaxy
There are billions of stars in the Milky Way galaxy
Some of these stars also have planets which orbit
them
The Universe is made up of many different galaxies
The Formation of the Sun
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The Sun is formed from massive clouds
of dust and gas in space
o A cloud of dust and gas in space is called
a nebula
Gravity pulled this cloud together into a giant ball
As the nebula collapsed the centre of this ball got
very dense and hot and began to rotate
Eventually nuclear fusion was able to begin and
a dense protostar was formed – our Sun
Equilibrium in Stars
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Stars are held together by a delicate balance of
inwards and outwards forces
One of these forces is the force of gravity
o This is an attractive force which pulls the outer
layers inwards
The other force is the force of pressure
o This is an outward force which is exerted from
the expanding hot gases inside the star
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Equilibrium in Stars
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When the inward pull of gravity and the outward
pressure acting on the star are equal the star will be
in equilibrium
Forces acting within a star. The centre red circle
represents the star's core and the orange circle
represents the stars outer layers
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If the temperature of a star increases, the outward
pressure will also increase
o This will cause the star to expand
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If the temperature drops (because, perhaps, the rate
of fusion has slowed) the outward pressure will
also decrease
o This will cause the star to contract
Fusion in Stars
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All the naturally occurring elements, apart from
hydrogen, have been formed by nuclear fusion in
stars
Nuclear fusion occurs when two light nuclei collide at
high speed and join to create
a larger, heavier nucleus
When the Universe was first formed, 13.8 billion years
ago, the only element present was hydrogen
If two hydrogen nuclei collide with enough energy
they will fuse into a helium nucleus
o For example, the nuclei of two different isotopes
of hydrogen (protium and tritium) can join to form
a helium nucleus by the process of nuclear fusion
An example of the fusion of protium and tritium to
form helium
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The process of nuclear fusion releases energy
o The energy is released in the form
of heat and light
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The Formation of New Elements
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During the majority of a star’s
lifetime, hydrogen nuclei fuse together to
form helium nuclei
As the star runs out of hydrogen, other fusion
reactions take place forming the nuclei of other
elements
For example, two helium nuclei (produced by the
fusion of 2 hydrogen nuclei) could fuse together to
form a beryllium nucleus
Two helium nuclei fusing together to form a beryllium
nucleus
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The beryllium nucleus could then fuse with a helium
nucleus to form a carbon nucleus
A beryllium nucleus fusing with a helium nucleus to
form a carbon nucleus
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Elements lighter than iron are formed in fusion
reactions like the ones above
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Formation of Elements Heavier than Iron
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Elements heavier than iron are produced
in supernovae explosions
o A supernova occurs at the end of a massive
stars life
o When the star explodes it releases very large
amounts of energy and neutrons
All of the elements which have been produced by the
fusion reactions get thrown out and combine with the
neutrons to form heavier elements
The Formation of Stars
1. Nebula
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All stars form from a giant 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
o This will result in more frequent
collisions between the particles which causes
the temperature to increase
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3. Main Sequence Star
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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
Life Cycle of Stars
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Once a protostar is formed, its life cycle will depend
on its mass
The different life cycles are shown below:
Larger Stars
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A large star is one which is bigger than the Sun
Stars that are larger than the Sun have much shorter
lifespans - in the region of hundreds of millions of
years (instead of billions)
o This is because they burn through the fuel in
nuclear fusion much quicker than smaller stars
The life cycle of a star bigger than the Sun starts in
the same way as a solar mass star
1. Nebula
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All stars form from a giant cloud of hydrogen
gas and dust called a nebula
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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
o This will result in more frequent
collisions between the particles which causes
the temperature to increase
3. Main Sequence Star
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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
4. Red Supergiant
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Eventually, the main sequence star will reach a stage
when it starts to run out of hydrogen gas in its core
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 is greater than the outward
force due to the pressure of the expanding
gases
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A new series of fusion reactions will then occur
around the core, for example helium nuclei will
undergo fusion to form beryllium
These fusion reactions will cause the outer part of the
star to expand and it will become a super red giant
o A super red giant is much larger than a red
giant
5. Supernova
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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
6. Neutron Star (or Black Hole)
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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
Supernovae
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A supernova is a bright and powerful
explosion that happens at the end of a massive
star's life
o It occurs when the star is bigger than the Sun
The explosion releases a large amount of energy
During a supernova, all of the elements which were
produced by the fusion reactions are exploded
out along with neutrons
The neutrons combine with the elements to form
even heavier elements
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These elements are ejected into the universe by the
supernova explosion and form new planets and stars
o Since Earth contains many heavy elements up to
Iron, this is proof that it must have once been
made from the remains of a Supernova
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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
Solar Mass Stars
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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
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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
It will become a red giant
o It is red because the outer surface starts to cool
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5. Planetary Nebula
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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
6. White Dwarf
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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
7. Black Dwarf
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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
Orbital Motion
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There are many orbiting objects in our solar system
o 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
In order to orbit a body such as a star or a planet,
there has to be a force pulling things towards that
body
o Gravity provides this force
The gravitational force exerted by the larger body on
the orbiting object is always attractive
Therefore, the gravitational force always
acts towards the centre of the larger body
The gravitational force is the centripetal force as it
will cause the body to move and maintain in
a circular path
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Gravitational attraction causes the Moon to orbit
around the Earth
Circular Motion in an Orbit
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Planets travel around the Sun in orbits that are
(approximately) circular
Objects in circular orbit are travelling at a constant
speed
The orbit is a circular path, therefore the direction in
which the object is travelling will be constantly
changing direction
A change in direction causes a change in velocity
Acceleration is the rate of change of velocity,
therefore if the object is constantly changing direction
then its velocity is constantly changing and so the
object in orbit is accelerating
A resultant force is needed to cause an acceleration
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This resultant force is gravity and it must act at right
angles to the instantaneous velocity of the object to
create a circular orbit
o This is always towards the centre of the orbit
o The instantaneous velocity of the object is the
velocity at a given time
The direction of the instantaneous velocity and the
gravitational force at different points of the Earth’s
orbit around the sun
Circular Orbits
Planets
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There are several similarities in the way different
planets orbit the Sun:
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Their orbits are all slightly elliptical (stretched
circles) with the Sun at one focus (approximately
the centre of the orbit)
o They all orbit in the same plane
o They all travel the same direction around the
Sun
There are also a few differences:
o They orbit at different distances from the Sun
o They orbit at different speeds
o They all take different amounts of time to orbit
the Sun
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Orbit of planets around the Sun
Moons
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Moons will orbit planets in a circular path
Some planets will have more than one moon
The closer the moon is to the planet:
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o
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The shorter the time it will take to orbit
The greater the speed in the orbit
Artificial Satellites
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A satellite needs to travel at a specific speed to
maintain a circular orbit at a particular
distance from the object
If the speed of the satellite is too big:
o The radius of the orbit will increase and the
satellite will spiral into space
o This is because the gravitational attraction
cannot provide enough force to keep it in orbit
If the speed of the satellite is too small:
o The radius of the orbit will decrease and the
satellite will move towards the object it should
be orbiting
o This is because the gravitational attraction is
too strong to maintain a constant orbital radius
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Diagram showing how the speed of an artificial
satellite affects its orbit
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If an artificial satellite is to change the radius at which
it is orbiting then the speed at which it is
travelling must change
To maintain a stable orbit:
o If the speed increases the radius must increase
o If the speed decreases the radius
must decrease
Non-Circular Orbits
Orbit of a Comet
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The orbits of comets are very different to those of
planets:
o The orbits are highly elliptical (very stretched
circles) or hyperbolic
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This causes the speed of the comets to change
significantly as its distance from the Sun
changes
Not all comets orbit in the same plane as the
planets and some don’t even orbit in the same
direction
Orbital path of a comet
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As the comet approaches the Sun, it loses
gravitational potential energy and gains kinetic
energy
This causes the comet to speed up
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 it moves away from the Sun the body will slow
down, eventually finishing its orbit and falling back
into towards the Sun once more
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In this way, a stable orbit is formed
Galactic Red-shift
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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
Diagram showing the wavefronts produced from a
stationary object and a moving object
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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 (λ – Δλ) and
the frequency increases
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o
o
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The wavelength behind the source increases (λ
+ Δλ) and the frequency decreases
This effect is known as the Doppler effect
The Doppler effect also affects light
If an object moves towards an observer
the wavelength of light decreases
o This is known as blueshift as the light moves
towards the blue end of the spectrum
If an object moves away from an observer
the wavelength of light increases
o This is known as redshift as 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 from a star moving away
from an observer will be redshifted
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The observer in front observes a blue shift, the
observer behind observes a red shift
The Expanding Universe
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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
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
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
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o
This means that the further away a galaxy,
the faster it is moving away from us
The Big Bang Theory
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Around 14 billion years ago, the universe began
from a very small region that was extremely
hot and dense
Then there were a series of explosions, which we
call the Big Bang
This caused the universe to expand, cooling as it
does so, to form the universe we currently observe
Each point expands away from the others
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This is seen from galaxies moving away from
each other, and the further away they are the
faster they move
As a result of the initial explosions, the
universe continues to expand
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All galaxies moving away from each other, indicating
that the universe is expanding
Evidence for the Big Bang
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By observing the light
spectrums from supernovae in other galaxies there
is evidence to suggest that distant galaxies
are receding (moving further apart) ever faster
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These observations were first made in 1998
The light spectrums show that light from distant
galaxies is redshifted, which is evidence that
the universe is expanding – the galaxies are moving
away from us - and, indeed, each other
This is what happens in an explosion
o Matter is first densely packed and as it explodes
it, it moves out in all directions getting further and
further from the source of the explosion
o Some matter will be lighter and travel at
a greater speed, further from the source of the
explosion
o Some matter will be heavier and travel at
a slower speed, closer to the source of the
explosion
If you were to travel back in time and compare the
separation distance of the galaxies they would
become closer and closer together until the entire
universe was a single point
If the galaxies were originally all grouped together at
a single point and were then exploded we would see
a similar effect
o The galaxies that are moving fastest would move
the furthest - the distance they move would be
proportional to their speed
o The galaxies that are moving slowest would
move the least
o
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Dark Energy & Dark Matter
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Over the past two or three decades, astronomers
have gathered plenty of evidence helping us to
understand the universe much better
However, there are still many observations that are
still not understood, including:
o Evidence from
the rotation and motion of galaxies suggests
that there is much more matter in the Universe
than we are able to account for
o This unseen matter is given the name dark
matter and its nature is not currently known
Measurements of the expansion of the Universe
using supernovae suggest that the Universe is
starting to expand at a faster rate
o It has been suggested that this accelerated
motion is caused by dark energy pushing
everything in the Universe apart
If gravity is present everywhere in the Universe, it
might be expected that everything will one day
become closer together, but that is the opposite of
what we see
o This is another reason why dark energy is a
possible solution which could be counteracting
gravity and expanding the Universe instead
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