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Copy of G485 5.5.1

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Physics Revision Tools

on 16 January 2014

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Transcript of Copy of G485 5.5.1

Contents of the
Universe
10^11 Galaxies
10^12 Stars
Interstellar Dust and Dark Matter
Electromagnetic Radiation
Star Formation
The Sun's Probable fate
After 5 billion years, hydrogen fusion slows down

Core begins to collapse as radiation pressure decreases
Fate of stars bigger than the Sun
When Stars of mass greater than 3 solar masses reaches end of the red giant phase, they grow into
super
red giants
Astronomical Measurements of Distance
G485 5.5.1
Structure of The Universe
Galaxies
A galaxy is a cluster of many billions of stars rotating slowly around its center of gravity
Region of high star concentration is known as the nucleus
All stars orbit around the nucleus
The distance of a star from the nucleus varies so periods of orbit vary
Stars
Stars are nuclear fusion reactors
that are held together by its own gravity.

Main sequence stars like our Sun are in a state of perfect equilibrium between the force of gravity, which holds it together, and the central outward pressure resulting from the fusion of hydrogen into helium.

This is what's needed in the exam, to read more, visit http://tinyurl.com/px3x8r5
Planets
Cold objects with slightly elliptical orbits

We can't see even the closest planets as they're too far
Our Solar System and Galaxy
The sun makes up 99.8% of our solar system by mass.
Our solar system consists of eight planets excluding Pluto.
It also contains the asteroid belt between Mars and Jupiter

Our galaxy is called the Milky Way. It is know to be a spiral galaxy due to its shape.
It is 13.2 billion years old and contains about 300 million stars
Nebulae
Large gas clouds with high density

Strong gravitational forces pull more matter together forming a
protostar

As matter moves closer, density rises

Ep decreases, Ek rises and temperature rises
As the
temperature rises
, the
pressure rises

Hydrogen nuclei fuse to form helium nuclei

Energy is releases,
temperature rises
even more
A star is
stable

if the gravitational pressure balances the radiation pressure

The radiation pressure is caused by the
photons
released by fusion reactions

Star formation takes 10^4 to 10^6 years

All remaining matter in the nebulae form planets
When the temperature reaches 10^8K,

Helium nuclei begin to fuse

After this, the Sun is unstable again

Radiation pressure < Gravitational pressure

Sun shrinks into white dwarf
As it shrinks, Ep is lost

Ek rises, temperature rises to 20,000K

Sun then gradually cools into a very dense black dwarf
Further nuclear fusion occurs

Core temperature rises to billions of Kelvin

Protons forced to absorb electrons and become neutrons
Final collapse causes intense heat

Explosive blowing of outer shell

Core compression

This is a
supernova
Under certain conditions, the nucleus of the supernova remains forming a
neutron star

Extremely Dense

2x10^30kg but 30km wide
If the core pressure is too large

Neutron star collapses

Infinite density

Nothing escapes the gravitational strength

This is a black hole
Quasars
Quasars are believed to be galaxies in process of formation and only exist at the edge of the universe.
They are the brightest objects in our universe
As bright as 10^12 of our Suns
Summary
Star mass < 3 solar masses ---> Red giant and end as black dwarf

Star mass > 3 solar masses --->
Super red giants and then neutron stars

Stars mass > 10 solar masses eventually end as black holes

One solar mass is the mass of the sun and is used as a unit: 2x10^30kg
Comets
Comets consist of ice, rock and a cloud of gas

Travel around Sun, elliptical orbits

Two trails: Dust tail which curves towards the orbit path

Gas tail which is directed away from sun. It is caused by solar emissions of ions from Sun which ionizes the comet's gases and then they glow
The astronomical unit
This is the
mean
distance from the centre of the Earth to the centre of the Sun.

Earth's orbit is elliptical so the mean distance used

1AU = 1.496x10^11m

Used to measure distances in our solar system
The Light Year
One light year is the distance traveled by light in a vacuum in one year

1ly = (speed of light in vacuum) * (seconds in a year)

1ly = 9.461x10^15m

Light from a star 1 light year away was emitted one year ago.
The Parsec (pc)
Steller Parallax can be observed when you close one eye look at your finger from each eye individually; the image looks different from each eye
Photograph at A and B look different, this is Steller parallax

1 degree is too big,

1 arc second is used
= 1/3600 degrees

1 parsec (pc) is the distance from the baseline of length 1AU when the parallax angle (p) is 1 arc sec
sin(p) = 1AU/x
For very small angles, sin(p) = p (radians)
p= 1AU/x ---> x=1AU/p

1pc = x = (1.496*10^11)/ (4.848*10^-6)

1pc = 3.086*10^16m


Heinrich Olber's Paradox
Olber's paradox states that for an infinite uniform and static universe, the night sky should be bright because of the light received from stars in all directions. (assumes universe is infinite and static)
Volume of sphere is 4/3 pi r^3
Volume of shell is 4/3 pi (ro^3 -ri^3)

Or volume of shell is 4 pi r^2 t where t is the shell thickness

Assuming the universe is uniform, there are n stars per unit volume

Brightness of star is proportional to 1/r^2

Brightness of Earth from shell A is 4pitn
Brightness of Earth from shell B is 4pitn

Every shell produces same brightness.

Infinite number of shells, brightness should be infinite
The Night sky is not bright

Olber concluded that the universe is not infinite and is expanding instead

Sky is dark because universe size is finite

Universe isn't static and is expanding

Light from distant stars/galaxies hasn't reached us yet.
The Doppler Effect
Wavelength of light moving away have stretched wavelengths

When moving towards Earth, compressed wavelength
Receding stars have a line spectra shifted slightly towards the red end of the spectrum,
red shifted

Approaching stars and galaxies have a line spectra shifted slightly towards the blue end of the spectrum,
blue-shifted
Doppler Equation
Hubble's Law
Hubble's Law
states that the recession speed of a galaxy is directly proportional to its distance from Earth
Hubble calculated recession speeds of nearby galaxies of known distances and plotted this graph:
This shows recession speed in kms-1 is directly proportional to distance from Earth Mpc
Gradient = Hubble Constant
1kms-1 = 10^3ms-1
1Mpc = 10^6pc * (3.086x10^16)mpc-1
1Mpc = 3.086x10^22m

Ho = 70kms-1Mpc-1

Ho = 70x10^3/3.086x10^22

Ho = 2.3x10^-18s-1

The cosmological Principle states that on a large enough scale, the universe is uniform and therefore is:

Homogenous
meaning of constant density
Isotropic
meaning the same in all directions
Provided a large enough volume is being referred to.
Cosmic microwave background radiation (CMB)
Radio telescope pointed at different points in sky for months
Noise detected each time, matching frequency, amplitude with max wavelength of 1.1cm
Source temperature of ~3K
This is evidence for The Big Bang Theory of expanding universe that cooled down with CMB radiation
The contraction causes the potential energy to decrease as molecular separation decreases

Kinetic energy rises because the internal energy remains the same

Temperature rises as Ek rises

Pressure rises

This pressure causes the Sun to expand into a
red giant
and engulf Mercury, Venus and Earth
Our Solar System
Scale of the Universe
Earth Time lapse
Neutron Star collision
Background Cosmic Microwave Radiation cont.
During the recombination era of The Big Bang, the temperature had decreased to 2700 degrees Celcius. This was cool enough for electrons and protons to recombine into hydrogen atoms. This process released photons which causes the CMB
Module Summary:
Definitions:

Astronomical Unit(AU)

Is the mean distance from the centre of the Earth to the centre of the sun.
Parsec (pc)
– The distance from a baseline of 1 AU when the angle is one second of arc (1/3600 of a degree)
Light year (ly)
– The distance light will travel through a vacuum in one year.
State the approximate magnitudes in metres, of the parsec and light-year 1pc – 3.086×1016m
1ly – 9.4×1015m
Olbers’ paradox
If the universe was infinite all lines of sight would end on star so night sky should be bright/ not dark

Types of universe

Open
: Universe expands for all time
Flat:
expands to a limit (but never reaches it)
Closed:
Universe contracts / collapses back

Hubble’s law:
The speed of the recession of a galaxy is directly proportional to its distance from the Earth.

State the cosmological principle Universe
is
I
sotropic:
same in all directions
Homogeneous:
evenly distributed

Critical density
The average density of the universe above which the universe will collapse and below which the universe will expand forever.

Questions
1. Explain why stars in the Milky Way show little red-shift.

2. The nearest star, Proxima Centauri, is at a distance of 4.3 ly from Earth.
Calculate the distance to Proxima Centauri in :
(a) metres (m).
(b) astronomical units (AU).
(c) parsecs (pc).

3. State Olber's Paradox.

4. State the cosmological principals.

5. State Hubble's Law.

6. State the probable fate of our Sun

7. State the value of a solar mass and explain the significance.
Answers
1. Stars in our Milky Way are relatively close the change in Wavelength is very small compared with that of distant stars.
2. i) (9.461*10^15) * 4.3 = 4.068*10^16
ii) (4.068*10^16) / (1.496*10^11) = 27,200AU
iii) (4.068*10^16) / (3.086*10^16) = 1.05pc
3. Olber's paradox states that for an infinite uniform and static universe, the night sky should be bright because of the light received from stars in all directions
4. The cosmological principle states that on a large enough scale, the universe is uniform therefore is homogenous meaning of constant density and isotropic meaning the same in all directions.
5. Hubble's law states that the recession speed of a galaxy is directly proportional to its distance from Earth; v/D = Ho
6. Our sun is a
main sequence
star. After five billion years, hydrogen nuclei will start to run out so the rate of fusion decreases meaning the radiation pressure < the the gravitational pressure so the core collapses. Ep decreases, Ek rises, temp rises, pressure rises to the star expands into a
red supergiant
. Helium nuclei start to fuse at this pressure and temperature. After helium runs out, the core collapses and forms a
white dwarf
which eventually cools into a
black dwarf
.
7. (2*10^30 kg), this is the mass of our Sun and is used as a unit for measuring mass.
(a) describe the principal contents of the universe, including stars, galaxies and radiation;

(b) describe the solar system in terms of the Sun, planets, planetary satellites and comets;
(c) describe the formation of a star, such as our Sun, from interstellar dust and gas;

(d) describe the Sun’s probable evolution into a red giant and white dwarf;

(e) describe how a star much more massive than our Sun will evolve into a super red giant and then either a neutron star or black hole;

(f) define distances measured in astronomical units (AU), parsecs (pc) and light-years (ly);

(g) state the approximate magnitudes in metres, of the parsec and light-year;

(h) state Olbers’ paradox;

(i) interpret Olbers’ paradox to explain why it suggests that the model of an infinite, static universe is incorrect;
(j) select and use the equation
v/c=Δλ/λ;
(k) describe and interpret Hubble’s redshift observations;

(l) state and interpret Hubble’s law ;
(m) convert the Hubble constant H0 from its conventional units (km s-1 Mpc-1 ) to SI (s-1);
(n) state the cosmological principle;


(o) describe and explain the significance of the 3K microwave background radiation
Past Paper Questions:
Mark Schemes:
Links:
http://www.sergebrunier.com/gallerie/pleinciel/360.swf

Specification
http://www.ocr.org.uk/download/kd/oc...d_gce_spec.pdf

Past Papers
http://www.ocr.org.uk/qualifications...s_a/documents/

Online Physics Lessons
http://www.freelance-teacher.com/videos.htm#PHYSICS
http://www.khanacademy.org/#physics

Revision Websites
http://www.physicsclassroom.com/Class/
http://en.wikibooks.org/wiki/A-level_Physics http://www.s-cool.co.uk/a-level/physics
http://www.fizzics.org/Pages/default.aspx

Powerpoint Presentations & Worksheets
http://www.mediafire.com/?arg6hhje5r577t8 http://www.mediafire.com/?ypp3g9b1w9yu14z

Flashcards
http://flashcarddb.com/cardset/12324...tes-flashcards
http://flashcarddb.com/cardset/13659...ons-flashcards
3D View of the Universe
Things to Remember:
1AU = 1.496x10^11m
1pc = 3.086*10^16m
1ly = 9.461x10^15m
Ho = 70kms-1Mpc-1
Ho = 2.3x10^-18s-1

Δ𝜆 is the change in wavelength
𝜆 is the original wavelength
v is the recession speed
c is the speed of light in a vacuum
Mathematical Proof
The universe comprises of all energy and matter, the stars, planets, galaxies, and the contents of intergalactic space.
{Not on specification}
(a) describe the principal contents of the universe, including stars, galaxies and radiation;

(b) describe the solar system in terms of the Sun, planets, planetary satellites and comets;

(c) Describe the formation of a star, such as our sun, from interstellar dust and gas
• Dust and gas cloud drawn in together by gravitational forces
• Atoms in cloud lose potential as they move towards each other, so gain kinetic energy
• Fusion of hydrogen nuclei begins when temperature becomes high enough
• Star becomes stable when outward radiation pressure equals inward gravitational pressure; hydrostatic equilibrium
• Main sequence star

(d) Describe the sun’s probable evolution into a red giant and white dwarf
• When all H is used up in core of sun, only fusion products will be left, which begin to fuse
• Outer layers of sun will begin to expand
• Core shrinks
• Red giant formed which eventually becomes a white dwarf

(e) Describe how a star much more massive than our sun will evolve into a super red giant and then either a neutron star or black hole
• After all H is used up inside core, star becomes super red giant
• Fusion of He takes place
• Gravitational collapse of core takes place • Star explodes (supernova) • Neutron star or black hole forms

(h) & (i) State and interpret Olber’s paradox to explain why it suggests that the model of an infinite, static universe is incorrect
• Universe is infinite • Each line of sight ends on star, so sky bright at night
• Not true, so implies that universe is finite

(k) Describe and interpret Hubble’s redshift observations
• Hubble looked at some data from galaxies
• Spectrum from galaxies had been red-shifted
• Waves have been stretched (Doppler shift), meaning the galaxies are receding from earth

(l) State and interpret Hubble’s law
• The speed of recession of a galaxy is directly proportional to its distance from earth
• Galaxies are moving away from us, so universe must be expanding
• All matter and energy must have originated at a single point
• Resolves olbers paradox

(o) Describe and explain the significance of the 2.7K microwave background radiation
• Uniform intensity in all directions, shows ripples
• Produced when matter and radiation decoupled
• Universe becomes transparent
• Was originally gamma radiation
• Radiation red-shifted towards microwave as universe expanded
• Evidence that universe began with a big bang
• Temperature corresponds with that predicted by big bang model
Due to light diffusion and pollution in the atmosphere, the moon only appears in greyscale
The image above is an exaggeration of the true colours
Full transcript