IB Physics
Option E SL Component
Astrophysics
2012
1
Structure of the solar system
The solar system is made up of a star that we call the sun and all the celestial bodies
that orbit it.
The sun
 Mass: 1.99 x 1030 kg
 Radius: 6.96 x 108 m
 Surface temperature: 5800 K
Planets orbit the Sun in ellipses. Satellites (which we call moons) orbit planets
Planet
Distance to Diameter Orbital Orbital
the Sun (km) (km)
period period
around
its axis
Mercury 58 million
4 878 km 59 days _______
Venus
Surface Density
day temp (water=1)
(ºC)
Satellites
167
5,430
0
5,240
0
5,520
1
3,040
2
1,320
+63
690
+56
1,270
27
1,770
13
2000
1
108 million
12 104 km -243
225 days 464
days
Earth 149.6 million 12 756 km 23, 93 h 365,2
15
days
Mars
228 million 6 794 km 24h
687 days -65
37min
Jupiter 778 million 142 800
9h
12 years -110
km
50min
30s
Saturn 1 427 million _______ 10h
29,5
-140
14min years
Uranus 2 870 million 51 800 km 16h
84 years -195
18min
Neptune 4 497 million 49 500 km 15h
164 years -200
48min
Pluto
5 900 million 2 400 km 6 days 248 years -225
Inner planets (terrestrial)
Mercury, Venus, Earth and Mars make up the inner planets.
They are composed primarily of rock.
Between Mars and Jupiter there exists an asteroid belt. The largest asteroid is called
Ceres (900km diameter). The belt is at 2-3.5AU from the Sun.
An AU is the astronomical unit, the mean distance from the Earth to the Sun (1.496 x
1011 m)
If 1 AU is the ______________ distance between the Earth and the sun how far away
is the asteroid belt?
2
Outer planets (Jovian)
Jupiter, Saturn, Uranus and Neptune are the outer planets.
They are composed mainly of gases.
Beyond the orbit of Pluto are chunks of rock and ice called comets. They are frozen
balls of ice and dust that can resemble a “dirty snowball”. They orbit the Sun is highly
elliptical orbits. And their orbital periods can range from a few years to several
thousand years. Halley's Comet is famous due to the fact that everyone has a chance
to see it in their lifetime (Orbital Period of 77 years).
Lightyear
Astronomers like to measure distances in lightyears
1 lightyear is the distance that light would _________ in one year.
c = distance/time
300000000 = distance / 365 * 24*60*60
Distance = 1 lightyear =
The distance across our galaxy, The Milky Way is 80 000 light years. Our nearest
neighbouring galaxy, The Andromeda galaxy, is 2.2 million light years away. The
furthest galaxies that can be detected with the Hubble Telescope are over 10 billion
light years away. This marks the edge of the detectable Universe. It is a big place!
Stellar cluster
A number of stars that are held together in a group by a gravitational attraction. They
were created at about the same time There may be many thousands of stars in a group.
3
Galaxies
A galaxy is a collection of a very large number of stars mutually attracting each other
through the gravitational force and staying together. The number of stars varies
between a few million and hundreds of billions. There approximately 100 billion
galaxies in the observable universe.
There are three types of galaxies:
- Spiral (Milky Way)
- Elliptical (M49)
- Irregular (Magellanic Clouds)
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a
central bulge of generally older stars. Extending outward from the bulge are relatively
bright arms.
Elliptical cross-section and no spiral arms. They range in shape from nearly spherical
to highly flattened ellipsoids and in size from hundreds of millions to over one trillion
stars. In the outer regions, many stars are grouped into globular clusters.
Irregular galaxies have no specific structure. The Large and Small _______________
Clouds, the nearest galaxies, are an example of irregular galaxies.
Constellations
A group of stars in a recognizable pattern that appear to be near each other in space.
Nebulae
Nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of
a star's cycle but it can also refer to the remains of a dying star (planetary nebula).
Originally nebula was a general name for any extended astronomical object, including
galaxies beyond the Milky Way (some examples of the older usage survive; for
example, the Andromeda Galaxy was referred to as the Andromeda Nebula before
galaxies were discovered by Edwin Hubble). Nebulae often form star-forming
regions, such as in the Eagle Nebula.
Planetary nebulae are nebulae that form from the gaseous shells that are ejected from
low-mass giant stars when they transform into white dwarfs.
Apparent motion of night sky
At night the star field appears to “rise in the east and set in the west”. The Sun and the
Moon also follow a similar path across the sky. Early astronomers felt that the Earth
was the centre of the Universe and all stars, planets, the Moon and the Sun orbited the
Earth. This was called the Geocentric Solar System. Several problems arose in the
1600’s in regards to this model of the universe. One was the apparent “wandering” of
the planets, what is now called retrograde motion. Planets sometimes “drift” back
towards the eastern horizon over the course of a few nights.
Eventually ____________ (1600’s) put forward a new theory that had the Sun as the
centre of our Solar System. This was called the Heliocentric Solar System.
 The 9 planets orbit the Sun.
 The Asteroid Belt orbits the Sun between Mars and Jupiter.
 Comets also orbit the Sun.
 The moons orbit the planets.
4
It is the Earth ____________ (spinning) once every 24 hours that gives the
impression that the Sun, the Moon, the planets and the entire star field are moving
from east to west. The Earth rotates from WEST to EAST. This is why we call it
“apparent motion”.
There is also a shift in the star pattern of the night sky over the course of a year. The
Earth REVOLVES around the sun once every 365 days. We will see a different
background of stars on the Celestial Sphere during each season. Orion is a
constellation that is prominent in the winter sky. It is not visible in the summer.
Stars
Stars are formed by interstellar dust coming together through mutual gravitational
attraction. The loss of potential energy is responsible for the initial high temperature
necessary for fusion. The fusion process releases so much energy that the pressure
created prevents the star from collapsing due to gravitational pressure. Very high
temperatures are needed in order to begin the fusion process: usually 107 K. A typical
reaction is:
2
2
4
1 H + 1H 2 He + 25 MeV
It must overcome the coulomb (electrostatic) ____________ between the nuclei so
that they can fuse together. In Stable Stars there is an equilibrium between the
gravitational attraction of all of the gas and dust particles and the outward pressure
exerted by the nuclear fusion process. This keeps a stable star from collapsing or
exploding.
The mass of a normal star almost completely determines its LUMINOSITY and
TEMPERATURE!
5
Luminosity (L) is the total ____________ emitted per unit time from the surface of a
star.
Luminosity depends on the temperature and the surface area of the star. The energy
the Sun emits is generated by the fusion in its core. The mass of a star determines the
pressure in its core. Gravity pulls outer layers in, gas pressure pushes them out. The
more mass the star has, the higher the central pressure. The core pressure determines
the rate of fusion. Mass + Pressure & Temperature = Rate of fusion which in turn
determines luminosity. Luminosity is an intrinsic property it doesn’t depend on
distance.
The ____________ of a star is the energy that it releases per second. Our sun has a
luminosity of 3.90 X 1026 W (often written as L): it emits 3.90x1026 joules per
second in all directions. The energy that arrives at the Earth is only a very small
amount when compared will the total energy released by the Sun.
TOK
The ancient Greeks classified stars by their brightness using the naked eye. They were
quite good at it. Have we lost skills because of our reliance on technology? Is this a
concern?
Apparent brightness
When the light from the Sun reaches the Earth it will be spread out over a sphere of
radius d. The energy received per unit time per unit area is b, where:
b
L
4d 2
b = apparent brightness of the star
The apparent brightness is directly proportional to the star’s luminosity and varies as
the inverse square of the stars distance.
Question
The Sun is a distance d=1.5 x 1011 m from the Earth. Estimate how much energy falls
on a surface of 1m2 in a year.
Black body radiation
A black body is a perfect emitter. A good model for a black body is a filament light
bulb: the light bulb emits in a very large region of the electromagnetic spectrum.
There is a clear relationship between the temperature of an object and the wavelength
for which the emission is maximum. That relationship is known as Wien’s law:
max T  constant
max T  2.9x10 -3 m K
By analysing a star’s spectrum, we can know in what ____________ the star emits
more energy.
6
The Sun emits more energy at λ=__________ nm.
According to Wien’s law, the temperature at the
Sun’s surface is inversely proportional to the
maximum wavelength.
So:
T
2.9x10 -3
max
2.9x10 -3

 5800 K
500x10 -9
Apart from temperature, a radiation spectrum can also give information about
luminosity. The area under a black body radiation curve is equal to the total energy
emitted per second per unit of area of the black body. Stefan showed that this area was
proportional to the fourth power of the absolute temperature of the body.
The total power emitted by a black body is its luminosity.
According to the Stefan-Boltzmann law, a body of ____________ area A and
absolute temperature T has a luminosity given by:
L  σAT 4
where, σ = 5.67x108 W m-2 K-4 , A = 4πr2
The spectrum of stars is similar to the spectrum emitted by a black body. We can
therefore use Wien Law to find the temperature of a star from its spectrum. If we
know its temperature and its luminosity then its radius can be found from StephanBoltzmann law.
7
Real spectra are more complicated than this (remember emission and absorption
lines?)
Example 1
The apparent brightness of our Sun is 1,393 Wm-2. This can be determined using light
sensors on Earth.
We know that the Earth is 1AU from the Sun.
The Sun has an approximate black body spectrum with most of the energy radiated at
a wavelength of 5.0 X 10-7 m. This is done using a spectrometer on Earth.
Use the above information to find out the
1. Luminosity of the Sun
2. Surface temperature of the Sun
3. Radius of the Sun
Atomic Spectra
The spectrum of atomic hydrogen was discussed and accounted for using the Bohr
model of the atom. Remember that the electron shells of a given atom can absorb a
____________ frequency of energy. E = hf
Lets look at Hydrogen as an example
8
An electron transition downwards leads to an emission of a specific frequency of
light. This produces an emission spectrum if observed through a spectrometer.
Another good example of line emission spectra is the burning of sodium. The gaseous
sodium’s electrons produce two distinct spectral lines in the yellow region of the E-M
spectrum.
A particular gas, like Hydrogen can also ____________ specific frequencies of light.
This removes particular frequencies from a continuous spectrum. This is called an
ABSORPTION SPECTRUM.In all cases the absorption and the emission spectra will
match perfectly. The absorption spectrum also tells us the outer temperature of the
sun’s surface. For every element there is a temperature range which will produce
strong absorption lines.
9
The spectrum seen from a star is due to the presence of a particular chemical element
in the outer atmosphere of the star. The sun produces ____________ lines of
Hydrogen, iron, calcium and sodium. The absorption spectrum also tells us the outer
temperature of the sun’s surface. For every element there is a temperature range
which will produce strong absorption lines. Examples would include Hydrogen
absorption lines occur at temperatures of 4000 to 12 000 K. Helium lines require
temperatures of between 15 000 and 30 000 K in order to get their electrons to absorb
energy.
Different atoms are sensitive to different temperatures. It is possible to determine a
star’s temperature by the absorption spectra that the star is producing. The chemical
composition of stars due to their line absorption spectra are found to be remarkably
similar. The average composition of stars is 74% Hydrogen, 25% Helium and only
1% other elements.
In summary, line absorption spectra tell us more about a star’s temperature rather
than its chemical composition (as most stars have the same composition).
Stars can be arranged into categories based on the features in their spectra. This is
called “Spectral Classification” We categorise stars by three different methods
1. by the “strength” (depth) of the absorption lines in their spectra
2. by their color as determined by their blackbody curve
3. by their temperature and luminosity
The first attempts to classify stars used the strength of their ____________ lines. Stars
were labeled “A, B, C…” in order of increasing strength of Hydrogen lines Later,
these categories were reordered according to temperature/color.
Oh Be A Fine Girl Kiss Me Lots Tonight
10
Eventually, the connection was made between the observables and the ___________.
Observable:
• Strength of Hydrogen Absorption Lines
• Blackbody Curve (Color)
Theoretical:
• Using observables to determine things we can’t measure:
Temperature and Luminosity
Class
O
B
A
F
G
K
M
L
T
Spectrum
Color
bluish
ionized and
neutral helium,
weakened
hydrogen
neutral helium,
blue-white
stronger hydrogen
strong hydrogen,
ionized metals
white
weaker hydrogen,
ionized metals
yellowish
white
yellowish
still weaker
hydrogen, ionized
and neutral
metals
weak hydrogen,
neutral metals
little or no
hydrogen, neutral
metals, molecules
no hydrogen,
metallic hydrides,
alkalai metals
methane bands
Temperature
31,00049,000 K
10,00031,000 K
7400-10,000
K
6000-7400 K
5300-6000 K
orange
3900-5300 K
reddish
2200-3900 K
red-infrared
1200-2200 K
infrared
under 1200 K
11
Hertzsprung Russell Diagram
This diagram shows a correlation between the luminosity of a star and its temperature.
The scale on the axes is not linear as the temperature varies from 3000 to 25000 K
whereas the luminosity varies from 10-4 to 106, 10 orders of magnitude.
The stars are not randomly distributed on the diagram.
There are 3 features that emerge from the H-R diagram:
 Most stars fall on a strip extending diagonally across the diagram from top left
to bottom right. This is called the MAIN SEQUENCE.
 Some large stars, reddish in colour occupy the top right – these are red giants
(large, cool stars).
 The bottom left is a region of small stars known as white ____________
(small and hot)
12
Star Types
Red Giants
Very large, cool stars with a reddish appearance. All main sequence stars __________
into a red giant. In red giants there are nuclear reactions involving the fusion of
helium into heavier elements. The fuel is expended much faster than in stars like our
sun. Within a red giant is a core still increasing in temperature. When the temperature
rises to 100 million degrees Kelvin helium fusion takes place. There are now two
layers of energy production;
 the hydrogen burning shell,
 the helium-burning core.
This process eventually yields a carbon and oxygen core,that may eventually produce
an iron core,in the most massive stars. The fusion process stops with iron; Iron
represents the most stable form, in which protons and neutrons can exist. Once the
iron core is formed energy production comes to an end. The pressure forcing the star
to expand no longer is present, gravity takes over
Within seconds, iron core collapses with such a force, not even the space within the
orbital structure of the atom is preserved. The layers within the iron core fall into the
centre, at different rates, an outward shock wave is produced. This shock wave is
capable of driving off most of the mass of the star. For a star of size 10 solar masses,
85% of the mass is lost, the star goes supernova.
White dwarfs
A red giant at the end stage of its evolution will throw off ____________ and leave
behind a very small size (the size of the Earth), very dense star in which no nuclear
reactions take place. It is very hot but its small size gives it a very small luminosity.
As white dwarfs have mass comparable to the Sun's and their volume is comparable to
the Earth's, they are very dense.
Neutron stars
A neutron star is formed from the collapsed remnant of a massive star (usually
supergiant stars – very big red stars). Models predict that neutron stars consist mostly
of neutrons, hence the name. Such stars are very hot. A neutron star is one of the few
possible conclusions of stellar evolution.
Supernovae
A supernova is a stellar explosion that creates an extremely luminous object.
The explosion expels much or all of a star's material at a velocity of up to a tenth the
speed of light, driving a shock wave into the surrounding interstellar medium. This
shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
A supernova causes a burst of radiation that may briefly outshine its entire host galaxy
before fading from view over several weeks or months. During this short interval, a
supernova can radiate as much energy as the Sun would emit over 10 billion years.
Pulsars
Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable
electromagnetic radiation in the form of radio waves. Periods of rotation vary from a
few milliseconds to seconds.
13
Black Holes
A black hole is a region of space in which the ____________ field is so powerful that
nothing can escape after having fallen past the event horizon. The name comes from
the fact that even electromagnetic radiation is unable to escape, rendering the interior
invisible. However, black holes can be detected if they interact with matter outside the
event horizon, for example by drawing in gas from an orbiting star. The gas spirals
inward, heating up to very high temperatures and emitting large amounts of radiation
in the process.
Cepheid variables
Cepheid variables are stars of variable luminosity. The luminosity increases sharply
and falls of gently with a well-defined period. The period is related to the absolute
luminosity of the star and so can be used to estimate the distance to the star.
A Cepheid is usually a giant yellow star, pulsing regularly by expanding and
contracting, resulting in a regular oscillation of its luminosity. The luminosity of
Cepheid stars range from 103 to 104 times that of the Sun.
Binary stars
A binary star is a stellar system consisting of two stars orbiting around their centre of
mass. For each star, the other is its companion star. A large percentage of stars are
part of systems with at least two stars.
Binary star systems are very important in astrophysics, because observing their
mutual orbits allows their mass to be determined. The masses of many single stars can
then be determined by extrapolations made from the observation of binaries.
There are three types of binary stars
Visual binaries – these appear as two separate stars when viewed through a telescope
and consist of two stars orbiting about common centre. The common rotation period is
given by the formula:
4 2 d 3
T2 
G(M1  M 2 )
where d is the distance between the stars.
Because the rotation period can be measured directly, the sum of the ____________
can be determined as well as the individual masses. This is useful as it allows us to
determine the mass of singles stars just by knowing their luminosities.
Eclipsing binaries – some binaries are two far to be resolved visually as two separate
stars (at big distances two stars may seem to be one).
But if the plane of the orbit of the two stars is suitably oriented relative to that of the
Earth, the light of one of the stars in the binary may be blocked by the other, resulting
in an eclipse of the star, which may be total or partial
14
Spectroscopic binaries – this system is detected by analysing the light from one or
both of its members and observing that there is a periodic ____________ shifting
of the lines in the spectrum.
A blue shift is expected as the star approaches the Earth and a red shift as it moves
away from the Earth in its orbit around its companion.
If λ0 is the wavelength of a spectral line and λ the wavelength received on earth,
the shift, z, is defined as:
  0
z
0
If the speed of the ____________ is small compared with the speed of light, it can
be shown that:
v
z
c
The speed is proportional to the shift.
15
H-R Diagram
The stars are not randomly distributed on the diagram. There are 3 features that
emerge from the H-R diagram:
 Most stars fall on a strip extending diagonally across the diagram from top left
to bottom right. This is called the MAIN SEQUENCE.
 Some large stars, reddish in colour occupy the top right – these are red giants
(large, cool stars).
 The bottom left is a region of small stars known as white dwarfs (small and hot)
22 000 stars are plotted from the Hipparcos catalog and 1000 from the Gliese catalog
of nearby stars. An examination of the diagram shows that stars tend to fall only into
certain regions on the diagram. The most predominant is the diagonal, going from the
upper-left (hot and bright) to the lower-right (cooler and less bright), called the main
sequence. In the lower-left is where white dwarfs are found, and above the main
sequence are the subgiants, giants and supergiants. The Sun is found on the main
sequence at luminosity 1 and temperature 5780K (spectral type G2).
Astronomical distances
The SI unit for length, the metre, is a very small unit to measure astronomical
distances. The units usually used in astronomy:
The Astronomical Unit (AU) – this is the average distance between the Earth and the
Sun. This unit is more used within the Solar System.
1 AU = 150 000 000 km
or
1 AU = 1.5x1011m
The light year (ly) – this is the distance travelled by the light in one year.
1 ly = 9.46x1015 m
The parsec (pc) – this is the distance at which 1 AU subtends an angle of 1 arcsecond.
1 pc = 3.086x1016 m
Or 1 pc = 3.26 ly
Our nearest star is 1.3 pc away. This is 206 000 times further than the Earth is from
the Sun.
Parallax
Parallax, more accurately motion parallax, is the change of ____________ position of
two observations of a single object relative to each other as seen by an observer,
caused by the motion of the observer.
Simply put, it is the apparent shift of an object against the background that is caused
by a change in the observer's position.
16
Stellar parallax
tan p (Parallax) 
R (Baseline)
d (Distance)
For very small angles
R
p 
d
In conventional units it means that
1.5 x 1011
1 pc 
m  3.086 x 1016 m
 2   1 



 360   3600 
1 pc 
p 
1.5 x 1011
m  3.986 x 1016 m
 2   1 



 360   3600 
R
d
d (parsec) 



d
R
p
1
p ( arcsecond)
360 degrees (360o) in a circle
60 arcminutes (60’) in a degree
60 arcseconds (60”) in an arcminute
17
Parallax has limits
The farther away an object gets, the ____________ its shift. Eventually, the shift
is too small to see.
Question
The parallax angle for Barnards star from the Earth is 0.545 arc secs. What is its
distances in ly, parsecs and AU
The parallax angle for 61 Cygni star from the Earth is 0.333 arc secs. What are its
distances in parsecs and AU
Magnitude scale
We’ve figured out brightness, but stars don’t put out an equal amount of all light,
some put out more blue light, while others put out more red light! Usually, what
we know is how bright the star looks to us here on Earth. We call this its
____________ Magnitude
Magnitudes are a way of assigning a number to a star so we know how bright it is
Similar to how the Richter scale assigns a number to the strength of an earthquake
The Greeks ordered the stars from brightest to dimmest in clusters of twenty.
18
Magnitude
Description
1st
The 20 brightest stars
2nd
stars less bright than the 20
brightest
3rd
and so on...
4th
getting dimmer each time
5th
and more in each group, until
6th
the dimmest stars (depending on
your eyesight)
Because stars have such a wide range in brightness, ____________ are on a “log
scale” Every one magnitude corresponds to a factor of 2.5 change in brightness. Every
5 magnitudes is a factor of 100 change in brightness (because (2.5)5 = 2.5 x 2.5 x 2.5
x 2.5 x 2.5 = 100)
Object
Apparent Magnitude
The Sun
-26.8
Full Moon
-12.6
Venus (at brightest)
-4.4
Sirius (brightest star)
-1.5
Faintest naked eye stars
6 to 7
Faintest star visible from
Earth telescopes
~25
However knowing how ____________ a star looks doesn’t really tell us anything
about the star itself. We’d really like to know things that are intrinsic properties of the
star such as Luminosity (energy output) and Temperature
19
In order to get from how bright something looks to how much energy it is putting out,
we need to know its distance.
The whole point of knowing the distance using the parallax method is to figure out
luminosity. It is often helpful to put luminosity on the magnitude scale.
Absolute Magnitude:
The magnitude an object would have if we put it 10 ____________ away from Earth.
This scale removes the effect of distance and puts stars on a common scale.
 The Sun is -26.5 in apparent magnitude, but would be 4.4 if we moved it far
away
 Aldebaran is farther than 10pc, so it’s absolute magnitude is brighter than its
apparent magnitude
Knowing the apparent magnitude (m) and the distance in pc (d) of a star its absolute
magnitude (M) can be found using the following equation:
m  M  5 log d  5
Example: Find the absolute magnitude of the Sun.
The apparent magnitude is -26.7
Question: What is the absolute magnitude of Sirius which is 2.7 parsecs away and has
an apparent magnitude of -1.46
So we have three ways of talking about brightness:



Apparent Magnitude - How bright a star looks from Earth
Luminosity - How much energy a star puts out per second
Absolute Magnitude - How bright a star would look if it was 10 parsecs away
Spectroscopic parallax is an astronomical method for measuring the distances to
stars. Despite its name, it does not rely on the apparent change in the position of the
star.
This technique can be applied to any main sequence star for which a spectrum can be
recorded.
The ____________ of a star can be found using an absorption spectrum. Using its
spectrum a star can be placed in a spectral class. Also the star’s surface temperature
can determined from its spectrum (Wien’s law).Using the H-R diagram and knowing
both temperature and spectral class of the star, its luminosity can be found.
Cepheid variables
Cepheid variables are stars of variable luminosity. The luminosity increases sharply
and falls of gently with a well-defined period.
The period is related to the absolute luminosity of the star and so can be used to
estimate the distance to the star.
A Cepheid is usually a giant yellow star, pulsing regularly by expanding and
contracting, resulting in a regular oscillation of its luminosity. The luminosity of
Cepheid stars range from 103 to 104 times that of the Sun.
20
The relationship between a Cepheid variable's luminosity and variability period is
quite precise, and has been used as a standard candle (astronomical object that has a
known luminosity) for almost a century.
This connection was discovered in 1912 by Henrietta Swan Leavitt. She measured the
brightness of hundreds of Cepheid variables and discovered a distinct periodluminosity relationship.
A three-day period Cepheid has a luminosity of about 800 times that of the Sun.
A thirty-day period Cepheid is 10,000 times as bright as the Sun.
The scale has been calibrated using nearby Cepheid stars, for which the distance was
already known.
This high luminosity, and the precision with which their distance can be estimated,
makes Cepheid stars the ideal standard candle to measure the distance of clusters
and external galaxies.
Distance to a Cepheid variable
L = 4πd2b for the Cepheid and the star…if they are same luminosity then you can
equate them to find distance (see slide 9)
Newton’s model of the universe
The universe is
a) infinite in extent,
b) contains an infinite number of stars,
c) is static and
d) exists forever
Olbers Paradox
Why is the night sky dark?
If the Universe is eternal and infinite and if it has an ____________ number of stars,
then the night sky should be bright.
21
Very distant stars contribute with very little light to an observer on Earth but there are
many of them. So if there is an infinite number of stars, each one emitting a certain
amount of light, the total energy received must be infinite, making the night sky
infinitely bright, which is not.
If we consider the Universe finite and expanding, the radiation received will be small
and finite mainly for 2 reasons:
There is a finite number of stars and each has a finite lifetime (they don’t
radiate forever) and
Because of the finite age of the Universe, stars that are far away have not yet
had time for their light to reach us. Also,
The Universe is expanding, so distant stars are red-shifted into obscurity
(contain less energy).
The Big Bang model
In astronomy, the Doppler effect was originally studied in the ____________ part of
the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to
electromagnetic waves in all portions of the spectrum.
Also, because of the inverse relationship between frequency and wavelength, we can
describe the Doppler shift in terms of wavelength. Radiation is redshifted when its
wavelength increases, and is blueshifted when its wavelength decreases.
Astronomers use Doppler shifts to calculate precisely how fast stars and other
astronomical objects move toward or away from Earth. In 1920’s Edwin Hubble and
Milton Humanson realised that the spectra of distant galaxies showed a redshift,
which means that they are moving away from Earth. So, if galaxies are moving away
from each other then it they may have been much closer together in the past
Background radiation
In 1960 two physicists, Dicke and Peebles, realising that there was more He than it
could be produced by stars, proposed that in the beginning of the Universe it was at a
sufficiently high temperature to produce He by fusion.
In this process a great amount of highly energetic ____________ was produced.
However, as the Universe expanded and cooled, the energy of that radiation decreased
as well (wavelength increased). It was predicted that the actual photons would have an
maximum λ corresponding to a black body spectrum of 3K. So, we would be looking
for microwave radiation.
Shortly after this prediction, Penzias and Wilson were working with a microwave
aerial and found that no matter in what direction they pointed the aerial it picked up a
steady, continuous background radiation.
In every direction, there is a very low energy and very uniform radiation that we see
filling the Universe. This is called the 3 Degree Kelvin Background Radiation, or the
Cosmic Background Radiation, or the Microwave Background. These names come
about because this radiation is essentially a black body with temperature slightly less
than 3 degrees Kelvin (about 2.76 K), which peaks in the microwave portion of the
spectrum.
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Why is the background radiation an evidence for the Big Bang?
The cosmic background radiation (sometimes called the CBR), is the afterglow of the
big bang, cooled to a faint whisper in the microwave spectrum by the expansion of the
Universe for 15 billion years (which causes the radiation originally produced in the
big bang to redshift to longer wavelengths).
The Big Bang Model is a broadly accepted theory for the origin and evolution of our
universe. It postulates that 12 to 14 billion years ago, the portion of the universe we
can see today was only a few millimetres across.
It has since expanded from this hot dense state into the vast and much cooler cosmos
we currently inhabit. We can see remnants of this hot dense matter as the now very
cold cosmic microwave background radiation which still pervades the universe and is
visible to microwave detectors as a uniform glow across the entire sky.
The big bang- The singular point at which space, time, matter and energy were
created. The Universe has been expanding ever since.
Main evidence:
Expansion of the Universe – the Universe is expanding (____________) --- it
was once smaller --- it must have started expanding sometime --- “explosion”
Background radiation --- evidence of an hot Universe that cooled as it
expanded
He abundance --- He produced by stars is little --- there is no other explanation
for the abundance of He in the Universe than the Big Bang model.
Fate of the Universe
Closed
Not enough matter 
density is not enough to
allow an infinite expansion
 gravity will stop the
Universe expansion and
cause it to contract (Big
Crunch)
Open
Enough matter 
density is such that
gravity is too weak to
stop the Universe
expanding forever
Flat
Critical density 
Universe will only
start to contract after
an infinite amount of
time
Critical density
The density of the Universe that separates a universe that will ____________ forever
(open universe) and one that will re-collapse (closed universe).
A universe with a density equal to the critical density is called flat and it will expand
forever at a slowing rate.
Measure?
If we take in account all the matter (stars) that we can see then the total mass would
not be enough to keep the galaxies orbiting about a cluster centre. So, there must be
some matter that can not be seen – dark matter. This dark matter cannot be seen
because it is too cold to irradiate. According to the present theories dark matter
consists in MACHO’s and WIMPS
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MACHO’s - Massive compact halo objects – brown and ____________ dwarfs or
similar cold objects and even black holes
WIMP’s - Non-barionic weakly interacting massive particles (neutrinos among other
particles predicted by physics of elementary particles)
It seems that there is also what is called “ ____________ energy”
TOK
Scientists claim our knowledge of the universe is based upon 5% of what is in the
universe. Can we claim to know anything about the universe?
Are there other ways besides Science to explain the universe? What happens when
these alternatives meet? Is one right and the other wrong?
Current research
Suggests universe is open
An example of the international nature of astrophysics
Evaluate arguments related to investing significant resources into researching the
nature of the universe
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Astro extension/consolidation material:
1.Neutron stars/Pulsars
http://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsars.html
"Gravitational Waves from an Orbiting Pulsar", Weisberg, J.M., Taylor, J.H.
and Fowler, L.A., 1981, Scientific American Oct, 74
2.Supernovae
http://imagine.gsfc.nasa.gov/docs/science/know_l1/supernovae.html
3.Black holes
http://imagine.gsfc.nasa.gov/docs/science/know_l1/black_holes.html
4.Quickly check your current understanding of the universe
http://www.sciencewithmrnoon.com/projectarise/physics1st/universequiz.htm
5.Quasar reading:
J. Narlikar, "What if the Big Bang Didn't Happen? New Scientist, pp. 48-51,
Mar. 2 1991.
6. Galaxy reading:
http://www.newscientist.com/article/mg20327171.300-how-does-your-galaxygrow.html
Our Growing, Breathing Galaxy; January 2004; Scientific American
General websites:
www.universetoday.com
hubblesite.org/newscenter
www.spaceref.com
science.discovery.com/games/space-games.html
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