IB Physics II
Option D: Astrophysics
ASTROPHYSICS OPTION D
NOTES AND STUDY GUIDE
PART ONE AND TWO – SL AND HL
PART THREE- HL ONLY
NOTE ON TEXTBOOKS: These notes are based on Tsokos
Textbook . It will also be helpful to read the new textbook by
Oxford Option D pp. 641-683 for further explanations and extra
problems. This section has been scanned and is on Moodle.
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IB Physics II
Option D: Astrophysics
ASTROPHYSICS OPTION E – PART 1 : SL AND HL
INTRODUCTION AND STELLAR RADIATION
Introduction – Read Tsokos pp. 487-493
Modern popular astrology runs back to the time of Ptolemy (2nd century). Despite believing the Earth
was the center of the universe, Ptolemy named the stars, listed their brightness, gave good reason that the
Earth was a sphere, and tried to explain the motion of the wandering stars in the sky (today’s planets).
E.1.1:
Outline the general structure of the solar system.
S
Student note: tudents should know that the planets orbit the Sun in
ellipses and moons orbit the planets (details of Kepler’s laws are not
required). Student should also know the names of the planets, their
approximate comparative sizes and the comparative distances from the
Sun, the nature of comets, and the nature and position of the asteroid
belt.
My:
Very:
Educated:
Mother:
Just:
Served:
Us:
Nine:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pizzas:
Pluto
(Note that Pluto is no
longer classified as a
planet)
Asteroids:
Asteroids are bits of space debris (rock, metal, ice)
anywhere from dust size to several hundred kilometers in radius. There
is a large asteroid belt between the orbits of Mars and Jupiter.
Comets:
Comets are large asteroids made of loose rock and ice. They
leave a tail as they travel.
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E.1.2:
Distinguish between a stellar cluster and a constellation.
Stellar Cluster:
A grouping of stars formed
from collapsing gas clouds.
E.1.3:
Constellations:
An artificial (man made) “connect the
dots” relationship between various stars, i.e. big dipper.
Define the light year
Light Year:
A light year (ly) is a measure of
distance. It is the distance that light travels in
one year.
1 ly = 9.46 ´1015 m
E.1.4:
Compare the relative distances between stars within a galaxy and between galaxies, in terms of
order of magnitude.
The average distance between stars
in a galaxy is:
3.26 ly = 3.09 ´10 m
16
X 100,000 =
The average distance between
galaxies is:
326,000 ly = 3.09 ´1021 m
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IB Physics II
Option D: Astrophysics
E.1.5:
Describe the apparent motion of the stars/constellations over a period of a night and over a period of
a year, and explain these observations in terms of the rotation and revolution of the Earth.
The movement of the stars through the sky (from
east to west) is due to the rotation of the Earth.
The stars appear to rotate in a circle with the pole
star (the North Star) at its center.
The earth rotates on its N-S axis.
So the stars appear
to rotate differently depending on where you are on
the planet.
At the equator the stars rise straight up.
At the poles, they circle the horizon.
In Northern (or southern) latitudes, they rise at an angle.
In addition to the stars changing position throughout a
day, the position of the star will change position
throughout the year. This is due to the earths rotation
around the sun. Depending on the month, the night sky
will point in different directions, therefore different
stars.
The change from one night to the next is less than 1 so
it is hard to notice unless observed over several nights.
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Option D: Astrophysics
E.2.1:
State that fusion is the main source of energy for the stars.
The Sun radiates about 1026 Js-1 of
energy. The source of this energy is from
fusion reactions in the core of the Sun. A
fusion reaction involves hydrogen fusing
into helium and releasing energy in the
process.
As discussed in previous topics, the high temperature
and high pressures in the Sun make fusion possible,
whereby, overcoming the electrostatic repulsive
forces between the atoms.
This process releases energy
because the product is less massive
than its constituents. You have the
skills to calculate this energy
release, which turns out to be
» 26.7MeV = 3.89 ´10 -12 J
Energy Source of Stars – pp. 494-495
E.2.2:
Explain that, in a stable start (for example, our Sun), there is an equilibrium between radiation
pressure and gravitational pressure.
The massive amount of kinetic energy released
from fusion reactions is absorbed by atoms
throughout the Sun (high KE equals high
temperature). This high temperature due to the
energetic motion of the atoms of the Sun is able to
support it from collapsing due to its massive
gravitational.
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Option D: Astrophysics
Luminosity p. 495
E.2.3:
Define the luminosity of a star.
Luminosity (L):
The amount of energy emitted by a
star per second (power output). Luminosity depends on
the temperature and surface area of the star.
Unit: Watt (W)
E.2.4:
Define apparent brightness and state how it is measured.
Apparent brightness (b): If we assume that a star emits
its power uniformly across its surface area then the
fraction of power received by a receiver of area a will
be:
L = luminosity
L
where d = ditance to star
b=
4p d 2
unit: Wm -2
Apparent brightness can be measured by capturing (using a
telescope) the received light of the star into a CCD (charged couple
device). The light intensity is then determined by the charge
developed on the device due to the incoming photons of light (see
Topic 12).
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E.2.5:
Apply the Stefan-Boltzmann law to compare the luminosities of different stars.
Luminosity is dependent on the temperature and the
surface area of a “glowing” object:
L = s AT 4
with
Given
s = 5.67 ´10-8 Wm-2 K -4
Examples: Stephan-Boltzmann Law
1.
L = s AT 4
2.
b=
L
4p d 2
3.
b=
L
4p d 2
1 ly = 9.46 ´1015 m
4.
L = s AT 4
A = area = 4πr2 Find ratio of the radius of star/ sun
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Option D: Astrophysics
Black Body and Wien’s Displacement Law pp. 496-497
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E.2.6:
State Wien’s (displacement) law and apply it to explain the connection between color and
temperature of stars.
Recall our discussions on black-body radiation.
When hot objects radiate energy they do so at a
continuous spectrum. The color that is seen, however,
depends on which of the wavelengths of that spectrum
is the most intense. The intensity can be seen to “peak”
at different wavelengths for different temperatures.
The area under the curve of this graph will give the
total power radiated.
Wein’s Law:
2.90 ´ 10 -3
T
where
T = temp (K )
lo =
Given
lo = peak intensity (m)
Examples: Wien’s Law
5.
6.
L = s AT 4
A = area = 4πr2
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Option D: Astrophysics
Stellar Spectrum p. 498
E.2.7:
Explain how atomic spectra may be used to deduce chemical and physical data for stars.
The absorption spectrum of a star can tell us both the chemical composition and the temperature of a star.
Chemical Composition:
When a star emits light the atoms in the star absorb the light energy, and reemit the light that they don’t absorb. In
this way, we can look at the absorption spectrum of a star. The “missing” spectral lines will indicate the kinds of
atoms doing the absorbing, or the chemical composition.
The absence of this spectral line
may indicate the presence of
hydrogen.
Temperature:
Absorption spectrums can also tell us the temperature of a star. For instance, hot hydrogen will absorb light
differently than cold hydrogen because hot hydrogen is too hot to hold onto is electrons, therefore it can’t absorb the
energy required to bump its electrons into higher orbitals because it has none. Cold hydrogen can absorb energy.
Therefore, even if the emission spectrum has a dark line at hydrogen it could mean that there is hydrogen; it was just
to “hot” to absorb energy. In this way, a VERY carful analysis of the spectral lines can tell us both temp and
composition.
Recall the Doppler Effect
Also called Red shift
The apparent wavelength of a light source depends on the relative motion between source and observer. When
relative motion is towards each other the wavelength will be shorter, when relative motion is moving away from
each other the wavelength will be longer.
Using this we can examine the shift in emission lines (short shifted or long shifted) to determine the relative motion
between us and the stars!
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Option D: Astrophysics
E.2.8:
Describe the overall classification system of spectral classes.
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Hertzsprung-Russell (HR) diagram – p. 499-501
E.2.11:
Identify the general regions of star types on a Hertzsprung-Russell (HR) diagram.
When astronomers Ejnar Hertzsprung and Henry Russell plotted the luminosity of the stars versus their temperatures
(or absolute magnitude versus spectral class) a pattern emerged. This pattern allows us to classify stars.
This says:
1. Most stars fall on the main diagonal
line. These are main sequence stars.
2. Large stars are red and cool (red
giants)
3. Small stars are white and hot (white
dwarfs)
E.2.9:
Describe the different types of stars.
You should be able to compare and contrast the following stars:






Main sequence star
Red Giant
White Dwarf
Variable Stars
Cepheids
Binary Stars
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Option D: Astrophysics
More Examples
Data Booklet
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Option D: Astrophysics
1.
This question is about the Hertzsprung–Russell (HR) diagram and using it to determine some
properties of stars.
The diagram below shows the grid of an HR diagram, on which the positions of selected stars are
shown. (LS = luminosity of the Sun.)
(a)
(i)
Draw a circle around the stars that are red giants. Label this circle R.
(1)
(ii)
Draw a circle around the stars that are white dwarfs. Label this circle W.
(1)
(iii)
Draw a line through the stars that are main sequence stars.
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(b) Explain, without doing any calculation, how astronomers can deduce that star B has a
larger diameter than star A.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
(c)
Using the following data and information from the HR diagram, show that star A is at a
distance of about 800 pc from Earth.
Apparent brightness of the Sun
Apparent brightness of star A
Mean distance of Sun from Earth
1 pc
Use ratios ( see # 2 p. 7 of packet) b =
= 1.4 × 103 W m–2
= 4.9 × 10–9 W m–2
= 1.0 AU
= 2.1 × 105 AU
L
4p d 2
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
(4)
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Option D: Astrophysics
2- 5 : b =
L
4p d 2
1 ly = 9.46 ´1015 m
2.
3.
4.
5.
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Option D: Astrophysics
6.
7.
8.
p. 10 of packet
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Option D: Astrophysics
9.
Hint: Rotating  stars in motion toward and away  Doppler Shift
(1)
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Option D: Astrophysics
ASTROPHYSICS OPTION E – PART 2 : SL AND HL
STELLAR OBJECTS
The Parallax Method – pp. 506-507
E.3.1:
Define the parsec.
Parsec:
The parsec is a convenient unit of measurement for
measuring long distances.
1AU = 1.5 ´1011 m
1 parsec (pc) = 3.26 light years (ly) = 3.09 ´1016 m
E.3.2:
Describe the stellar parallax method of determining the distance to a star.
Parallax method takes advantage of the fact that an object appears to be in two different positions when viewed
from two different locations.
This is noticed directly when you view something with just your right eye then just your left eye.
The distance between the earth at the two different times is:
= 2R (which is the diameter of the earth's orbit around the sun)
We want to find the distance (d) to the star:
d=
R
, but with very small angle tan p » p
tan p
then
d=
R
, where p is measured in radians
p
The radius of the Earth’s orbit around the Sun is defined as the
astronomical unit:
1AU = 1.5 ´1011 m
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Option D: Astrophysics
Parallax (p) and distance ( d)
This enables us to define a parsec in terms of an arcsec:
d ( parsec) = 1
p (arc-second)
in data booklet
d in parsecs ( pc)
Note : p is called the parallax or the parallax angle and uses units of “ :
1 “ = 1/3600 of a degree = 1 arc-second
Example 0.26” means it has a parallax of 0.26. Same symbol used for seconds ( hence arc-seconds)
Remember :
1 pc = 3.26 ly
( In data booklet)
15
1 ly = 9.46 x 10 m ( In data booklet)
1 pc = 3.08 x 1016 m ( Not in data booklet)
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Option D: Astrophysics
E.3.4:
Solve problems involving stellar parallax.
1.
2.
1 parsec (pc) = 3.26 light years (ly) = 3.09 ´1016 m
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Option D: Astrophysics
E.3.3:
Explain why the method of stellar parallax is limited to measuring stellar distances less than several
hundred parsecs.
pp. 506-507
Basically, when the start gets too far away the parallax angle will be too small to measure:
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Option D: Astrophysics
Data Booklet Review
1 parsec (pc) = 3.26 light years (ly) = 3.09 ´1016 m
Review Problem
1. Some data for the variable star Betelgeuse are given below.
Average apparent brightness
Radius
= 1.6 × 10–7 W m–2
= 790 solar radii
The luminosity of the Sun is 3.8 × 1026 W and it has a surface temperature of 5700 K.
a) Determine, in terms of the luminosity of the Sun, the luminosity of Betelgeuse.
HINT : Find Ratio of LB / Lsun
L = b 4 π d2
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
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b)
Calculate the surface temperature of Betelgeuse.
L = s AT 4
T α 4√L/A
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
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Option D: Astrophysics
Spectroscopic Parallax and the Cepheids pp. 510-512
E.3.8:
E.3.10:
E.3.12:
Solve problems involving apparent brightness and apparent magnitude.
Explain how stellar distance may be determined using apparent brightness and luminosity.
Solve problems involving stellar distances, apparent brightness and luminosity.
Recall:
Apparent brightness (b): If we assume that a star emits its
power uniformly across its surface area then the fraction of power
received by a receiver of area a will be:
L
b=
4p d 2
L = luminosity
d = ditance to star
unit: Wm -2
1.
Given L sun = 3.8 x 1026 W
b=
L
4p d 2
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Option D: Astrophysics
E.3.11:
State that the method of spectroscopic parallax is limited to measuring stellar distances less than
about 10 pc.
The method of spectroscopic parallax uses the stars spectrum to determine
its luminosity before finally finding its stellar distance. This method only
works for distances of less than about 10 pc.
E.3.13:
Outline the nature of a Cepheid variable.
A Cepheid variable is a star in which the outer layers undergo a periodic
expansion and contraction, which produces a periodic variation in its
luminosity.
E.3.14:
State the relationship between period and absolute magnitude for Cepheid variables.
It was determined through observations, that the larger the period of a Cepheid
the larger the luminosity (and larger the absolute magnitude). A graph of this
relationship is shown, and can be used to determine the luminosity of any
Cepheid variable once you know its period.
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Option D: Astrophysics
E.3.15:
Explain how Cepheid variables can be used to find distance to a galaxy ( called “standard candles”).
By determining the period of a Cepheid star, the luminosity and stellar
distance can be calculated. Therefore, by finding this distance we can
determine the distance to the galaxy in which that the Cepheid is located.
In Summary: the following stellar distances can be determined with these methods.
E.3.16:
Determine the distance to a Cepheid variable using the luminosity-period relationship.
2.
b=
L
4p d 2
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Option D: Astrophysics
Cosmology: The Expanding Universe , the Big Bang and Cosmic
Background Radiation ( CMB) pp. 516-520
E.4.3:
Suggest that the red-shift of light from galaxies indicates that the universe is expanding.
As explained by the Doppler Effect, as a moving
source of light moves away from the receiver the
light will “appear” to have a longer wavelength its
actual. This will cause the received wavelengths to
be shifted towards the side of red, or, red shifted.
Because the light received from distant galaxies is
red shifted the Universe is theorized to be an
expanding one.
E.4.4:
Describe both space and time as originating with the Big Bang.
Before the Big Bang there was no such thing as time or space – there was “no-thing”.
In this sense, it is impossible
to describe a time or space before the Big Bang.
As the Universe expands it does not expand into something (like expanding into a void) because if space came into
existence at the Big Bang then there must be “no-thing” before it that it could expand “into”.
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Option D: Astrophysics
E.4.5:
Describe the discovery of cosmic background (CMB) radiation by Penzias and Wilson.
In the 1960’s, two young physicists at Bell Laboratories in California, USA, found that no
matter where in the sky they pointed their radio antenna they picked up a subtle signal.
The wavelength of the signal turned out to correspond to a temperature of 2.7 K ( very low
= -270 0C) .
E.4.6:
Explain how cosmic radiation in the microwave region is consistent with the Big Bang
model.
The original energy radiated by the Big Bang was obviously very low wavelength (high energy =
high frequency) photons. As the Universe has expanded it has caused these wavelengths
to increase (cool down). This radiation has now cooled down to the microwave region of
the electromagnetic spectrum .
The Big Bang suggests the following:


The Universe is not infinite – it began and grew, and is still growing from a particular point.
Time is not infinite – this implies some light from distance galaxies has not even reached us yet.
Three Pieces of Evidence that Support the Big Bang Theory:
1. Expansion of the Universe- red shift – Doppler effect, accurate data describing a Big Bang
explosion moving outward.
2. Cosmic Background radiation - today there is background radiation with low temperatures of 2.7
K ( about – 2700C) . This is consistent with a small , hot universe in the past, which began to cool
down as it expanded.
3. Helium Abundance – predicted by the Big Bang theory when H atoms fused to form Helium.
Scientists have measurements that show there is 25% by mass of He in the universe.
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Option D: Astrophysics
Review Questions
1.
Fi. E31 p.19 of packet. Tsokos pp. 506-07
2.
3.
1 parsec (pc) = 3.26 light years (ly) = 3.09 ´1016 m
4.
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Option D: Astrophysics
5. The parallax of a star is 0.025 “. Calculate the distance to the star.
6.
b=
L
4p d 2
7.
L ∝ M and m
b=
L
4p d 2
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Option D: Astrophysics
8.
Tsokos p. 517
9.
Tsokos p. 516: Note : eventually gravity will cause collapsing of the universe, increasing pressure and thus
temp.
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Option D: Astrophysics
10. Read on Own
The big bang signifies the beginning of time and space. At the big bang the
universe was a point and so the big bang happened everywhere in the universe.
The question is meaningless within the big bang model since by definition time
started with the big bang. It is as meaningless as to ask for a place 1 km north of the
north pole. However, recent developments within string theory suggest that the
question may not be as meaningless as it appears. See the very interesting article
“The time before time”, by Gabriele Veneziano (one of the true greats of theoretical
physics) in the May 2004 Scientific American.
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Option D: Astrophysics
Cosmology: Galaxies and the Expanding Universe – Hubble’s Law:
pp. 533-541 Tsokos.
ALSO: Read Oxford New Text pp. 660 - 665
E.6.1:
Describe the distribution of galaxies in the Universe.
Galactic Cluster:
A cluster of galaxies. Our galaxy belongs to what is called the Local Cluster.
Galactic Supercluster:
E.6.2:
A cluster of galactic clusters.
Explain the red-shift (z) of light from distant galaxies.
The light received by distant galaxies is all red-shifted, implying that galaxies are moving away from us.
Since galaxies are moving AWAY form each other this is evidence that the universe is EXPANDING.
The velocity of a galaxy relative to earth can be determined using the following equation:
where
Dl
v
@
l c
Dl = change in wavelength (m)
l = original wavelength (m)
v = relative velocity
c = speed of light
OR
E.6.3:
Solve problems involving red-shift and the recession of galaxies.
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Option D: Astrophysics
E.6.4: State Hubble’s Law. P. 537
In 1929 Hubble discovered that the speed of a receding galaxy is proportional to its distant from us.
v = H od
where
v = velocity of galaxy (ms -1 )
H o = Hubble's constant of proportionality
d = distance (m)
E.6.4:
Explain how the Hubble constant may be determined.
The slope of the velocity vs distance graph gives us Hubble’s constant. It is currently estimated to be:
H o = 72 km s -1 Mpc -1
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Option D: Astrophysics
E.6.7:
Explain how the Hubble constant may be used to determine the age of the
Universe. Pp. 538 - 539
At the time of the big bang all parts of the universe were in the same place, so if we know how fast any two
parts are moving apart and how far apart they are now, we can calculate the age of the universe (for a constant
rate of expansion).
v=
d
t
d seperation distance
=
v recessional velocity
1
or t =
Ho
\t =
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Option D: Astrophysics
E.6.5:
Discuss the limitations of Hubble’s law.
Hubble Law derived form the calculated observations of Edwin Hubble contain significant random errors.
The line of
best fit is therefore a rough approximation leading to a rough approximation of the Hubble constant
E.6.8:
Solve problems involving Hubble’s law.
E.6.9:
Explain how the expansion of the Universe made possible the formation of light
nuclei and atoms. Pp. 539-541
Students should appreciate that, at the very high temperatures of the early Universe, only elementary
(fundamental) particles could exist and that expansion gave rise to cooling to temperatures at which light
nuclei could be stable.
E.5.4:
Apply the mass-luminosity relation. P. 522
For stars in the main sequence only, there exists a relationship between the star’s mass and its luminosity:
L µ mn where 3 < n < 4
(Given)
Basically , the higher the mass the more luminosity and faster it will burn up. So stars with lower mass have a
longer life span.
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Option D: Astrophysics
MORE DETAILS
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Option D: Astrophysics
Red Shift and the Cosmic Scale Factor
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Option D: Astrophysics
Questions
1.
v = Ho d
2.
Hint : nearby galaxies may move toward us.
3.
State age of universe in years: Assume 1 year = 365 days. Round off to 1 sig. digit.
1 parsec (pc) = 3.26 light years (ly) = 3.09 ´1016 m
1Mpc = 1x 106 pc
Time = 1
H
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4.
5.
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Stellar Evolution pp. 521-522 Tsokos
E.5.1:
Describe the conditions that initiate fusion in a star.
S
tars are formed when huge clouds of gas and dust are compressed. They can’t form on their own because
the gravitational force is not big enough to pull the particles together. However, if something causes the cloud
to be compressed, such as an exploding supernova or a collision between two dust clouds, the particles get
closer and the gravitational force becomes sufficient to start pulling the particles together. As the gas atoms
are pulled together by the gravitational force, they gain KE, and the temperature of the gas increases. This
increase in temperature causes an outward pressure that pushes against the gravitational attraction. However,
as the atoms get closer, the gravitational attraction increases so the gas continues to collapse and get hot at an
ever-increasing rate.
As the cloud collapses, a dense core is formed surrounded by a cloud of gas and dust. The center of the dense
core rapidly contracts, resulting in high temperature and pressure. This star, called a protostar, gives out light
due to its high temperature, but isn’t visible because it is surrounded by a cloud of gas. After about 105 years
of mass increase the protostar contains enough pressure and heat to allow for fusion to start. The star is now a
main sequence star.
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E.5.5:
Explain how the Chandrasekhar and Oppenheimer-Volkoff limits are used to
predict the fate of stars of different masses.
If a star has the mass m <1.4 Solar Masses it will become a white dwarf.
This is called the Chandrasekhar limit
Stars with large masses, m > 4 Solar Masses will go through fusion processes until their core is iron.
As
the iron can no longer fuse to create outward energy the outer layers rush into the iron core and explode
outwards in what is known as a super novae.
What is left is the neutron core of the star. This neutron star will be stable as long as m < 3 Solar Masses .
This is called the Oppenheimer-Volkoff limit.
E.5.6:
Compare the fate of a red giant and a red supergiant- vocabulary
A red giant forms a planetary nebula and then becomes a white dwarf.
A white dwarf is stable due to electron degeneracy pressure (electrons cannot collapse past a certain
point).
A red supergiant experiences a supernova and becomes a neutron star, or collapses into a black hole
A neutron star is stable due to neutron degeneracy pressure. (neutrons cannot collapse past a certain
point).
Birth and Death of a Star
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Birth and Death of a Low Mass Star
Birth and Death of a High Mass Star
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E.5.7:
Draw evolutionary paths of stars on HR diagrams. P. 525
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Option D: Astrophysics
Questions
1. Example Problem HR Diagram :
a) State the necessary condition of the Sun in order to for it to move from position S.
b) State and explain the change in the luminosity of the Sun that occurs between positions S
and I.
c) Explain, by reference to the Chandrasekhar limit, why the final stage of the evolutionary
path of the Sun is at F.
d) On the diagram, draw the evolutionary path of a main sequence star that has a mass of 30
solar masses.
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2.
L ∝ mn (mass)
3.
4.
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Option D: Astrophysics
ASTROPHYSICS OPTION D – PART 3 : HL ONLY
STELLAR PROCESSES
Stellar Evolution Review pp. 521-522
E.5.1:
Describe the conditions that initiate fusion in a star.
S
tars are formed when huge clouds of gas and dust are compressed. They can’t form on their own because
the gravitational force is not big enough to pull the particles together. However, if something causes the cloud
to be compressed, such as an exploding supernova or a collision between two dust clouds, the particles get
closer and the gravitational force becomes sufficient to start pulling the particles together. As the gas atoms
are pulled together by the gravitational force, they gain KE, and the temperature of the gas increases. This
increase in temperature causes an outward pressure that pushes against the gravitational attraction. However,
as the atoms get closer, the gravitational attraction increases so the gas continues to collapse and get hot at an
ever-increasing rate.
As the cloud collapses, a dense core is formed surrounded by a cloud of gas and dust. The center of the dense
core rapidly contracts, resulting in high temperature and pressure. This star, called a protostar, gives out light
due to its high temperature, but isn’t visible because it is surrounded by a cloud of gas. After about 105 years
of mass increase the protostar contains enough pressure and heat to allow for fusion to start. The star is now a
main sequence star.
E.5.4:
Apply the mass-luminosity relation. P. 522
For stars in the main sequence only, there exists a relationship between the star’s mass and its luminosity:
L µ mn where 3 < n < 4
(Given)
Basically , the higher the mass the more luminosity and faster it will burn up. So stars with lower mass have a
longer life span.
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IB Physics II
Option D: Astrophysics
Nucleosynthesis
E.5.3:
Outline the changes that take place in nucleosynthesis when a star leaves the
main sequence and becomes a red giant.
As the main sequence continues, the star’s hydrogen center will eventually all fuse into helium.
When this happens, the
star no longer produces the core energy required to combat the gravitational forces of the star. This will cause
the core to compress. This compression increases the core temperature, which increases the temperature of
the surround hydrogen, causing the hydrogen to begin to fuse. When fusion starts the star swells into what is
called a Red Giant. See below
If the mass of a star is large enough, the process described above will result in various other elements fusing,
and will produce different kinds of element at its core. The tables below provide examples of the possibilities
for different masses.
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IB Physics II
Option D: Astrophysics
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Option D: Astrophysics
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Option D: Astrophysics
Questions
1. Describe the evolution of a main sequence star of 1 solar mass. See table - nucleosysnthesis
2. a) Explain why a star having a mass of 50 times the solar mass would be extected to have a lifetime of many times less
than that of the Sun.
b) By referring to the mass-luminosity relationship , suggest why more massive stars will have shorter lifetimes.
3. Summarize the Jeans Criterion
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IB Physics II
Option D: Astrophysics
Supernovae
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IB Physics II
Option D: Astrophysics
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IB Physics II
Option D: Astrophysics
Questions
1. Outline the difference between a Type Ia and a Type II supernova.
2. What is meant by a standard candle?
3. Explain how a Type Ia supernova can be used as a standard candle?
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IB Physics II
Option D: Astrophysics
Further Cosmology – The Development of the Universe
Tsokos pp. 517-519. Also read New Text Oxford pp. 676-679
The Cosmological Principle
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Option D: Astrophysics
Closed , Open and Flat Universes
1. Closed Universe
2. Open Universe
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Option D: Astrophysics
3. Flat Universe
Critical Density ( pc)
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Option D: Astrophysics
Equation for Critical density
Critical Density:
The density at which a closed Universe becomes open. In other words, the density that
would result in a flat Universe that continues to expand with a rate that tends towards zero.
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Option D: Astrophysics
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IB Physics II
Option D: Astrophysics
Dark Matter and Why the Expansion of Universe is considered to be
Accelerating
Discuss the problems with determining the density of the Universe.
To determine the density of the Universe physicist must determine all stellar mas (starts, dust, planets, etc.).
However, when estimates are made, the total amount of mass is only 4% of the mass that is necessary to
perform the gravitational effects that are needed to keep the Universe “together”. Therefore, there is a lot of
unaccounted mass!
Scientists call this dark matter. Dark matter is matter we cannot see because it has low enough energy not to
radiate. Some examples of places that contain dark matter are WHIMPS (and acronym for small particles that
contain dark matter - like neutrinos for instance) and MACHOS (an acronym for larger objects that contain
dark matter - like dark stars)
State that current evidence suggests that the Universe is open.
It has been determined that not only is the Universe expanding (from red shifted spectrum) but that the rate
of the expansion is increasing – it’s accelerating!
This is surprising, however, because the density of the Universe is currently thought to be that of the critical
density, which would imply not an accelerating Universe but a flat Universe.
The current theory on why the Universe expansion is accelerating is that there exists a “dark energy” that fills
the vacuum of space. This energy is able to over-power the effects of gravity and “push” galaxies away from
one another at ever increasing rates. Wicked!
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Option D: Astrophysics
Discuss an example of the international nature of recent astrophysics research.
NASA, the space agency of the United States of America, is involved with many international projects, such
as the Gamma-ray Large Area Space Telescope (GLAST), which involves the U.S. Department of Energy and
institutions in France, Germany, Japan, Italy and Sweden.
Evaluate arguments related to investigating significant resources into researching the nature
of the Universe.
Exploring the known Universe is expensive!
You should be able to demonstrate an understanding of how this could be
an issue, and be able to formulate coherent arguments presenting your own opinion
Questions
1. Explain what is meant by a closed universe.
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Option D: Astrophysics
2. On the diagram above draw the variation based on the open model of the universe.
3. Explain , by reference to your answer in 2, why the predicted age of the universe
depends upon the model of the universe chosen.
4. a) What evidence suggests that the expansion of the universe is accelerating?
b) What is believed to be the cause of the acceleration?
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