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The Story so far
Big Bang Model – Three major pieces of evidence
- Problems – Horizon and Flatness problems
Inflationary Big Bang – introduced by Guth (1981)
- solves these problems
History of Universe in terms of the Big Bang.
Finally – Formation of stars and galaxies.
Superforce reigns
Electroweak era
QGP/Hadron phase
Universe becomes
Voids on the largest Scale
The Evolution of Stars
Ideally we would follow a star’s “life” from its genesis in a cloud
of gas and dust to its “death”.
However the timescale is way beyond our life span.
The alternative approach is to realise that the numbers of stars we
can see is very large. We assume that a) the lifetimes of stars are
shorter than the age of the Universe and b) we can observe stars at
all stages of development.
We have seen now how to measure stellar distances, masses, surface
temperatures, luminosities etc. We can now ask whether these
quantities are correlated in any way.
One way to look at this is the Hertzsprung-Russel diagram.
Hertzsprung-Russell Diagram
1.The H-R diagram plots Luminosity against Surface Temperature.
Note:-Log Luminosity is used because of the large range and it is
plotted against decreasing temperature.
2.Each star is represented by a point on the diagram.
3.The results depend to some extent on the sample of stars.They could
be from stars within a limited volume round the Solar system,
members of a cluster,stars of apparent brightness above a certain
4.Any H-R diagram shows that only a limited combination of values of
T and L are allowed.
5.Most stars lie on a thin strip running diagonally across the diagram
This is the Main Sequence.
6.Top right is also populated with brighter stars with lower T.Red Giants
7.Lower left is also rich in stars.They are bluish-white and small.The size
is comparable to that of the Earth but with approx. the same mass as
the Sun. They are White Dwarfs.
Hertzsprung-Russell Diagram
By restricting the range of stars plotted
one can test ideas of stellar evolution.
Here we see stars from a particular
globular cluster.
These groups of stars are very old and
different from open clusters. The ages
are such that only stars of 1 solar mass
or less are left.They are close to the age
of the Universe. Most other stars not on
the main Sequence here are White
dwarfs or brown/black dwarfs.In
general they are Population 2 stars with
less than 1% of heavy elements
compared with Population 1 stars where
it is 2% or so.
[Contrast with stars in the disc.]
M31-Andromeda Galaxy-2.2Mly from Earth, Part of our Local
cluster of galaxies.
Stellar Birth
1.Seeing the early stages is difficult. It starts with a collapsing cloud of
gas and dust and it is not hot enough to shine so we don’t see it.
As it collapses half of the potential energy is turned into kinetic energy
[Heat]. [Virial Theorem] Triggering of such collapses is not fully
2.If the temperature of the gas cloud reaches high enough temperature
the particles[protons]will have enough energy to interact and nuclear
reactions will begin at about 8 million Kelvin .As we will see this
releases energy which heats the gas and raises its pressure.
3.If heated enough, the gas pressure will countermand the gravitational
contraction and the star will stabilise under these two opposing forces.
4.At this stage the star will be moving to the left on the H-R diagram
and will end up on the Main Sequence.
The proton-proton chain
+ 1H = 2H + e+ +
+ 1H = 2H + e+ +
1H + 2H = 3He + 
+ 2H = 3He + 
3He + 3He = 4He + 1H + 1H + 
Thus the sequence of reactions turns 4 protons into an alpha particle.
+ 1H + 1H + 1H
+ 2e+ + 2e + 3
Since the alpha particle is particularly tightly bound this process of
turning 4 protons into an alpha releases about 26MeV of energy.
It is this energy which heats the stellar interior,allows it to withstand
the gravitational pressure and causes it to shine!
The p-p chain;the reactions which power the Sun
Overall - 4p  4He + 2e- +2 + 26.7 MeV
The CNO-Cycle:
In stars where we already have
C,N and O we can get hydrogen
 + 2e- + 2 +26.4 MeV
The C,N and O nuclei act as
catalysts for the burning process
Hans Bethe-1938
Life Cycle of Stars and Nucleosynthesis
1. Formation from large clouds of gas and dust.
2. Centre of cloud is heated as it collapses under gravity
3. When it reaches high enough temperature then nuclear reactions
can start.
4p 4He + 2e + 2ν + 26.7 MeV
4. This raises temperature further and star eventually reaches
equilibrium under heating internally and gravitational collapse.
5. The process of making heavier nuclei occurs in the next stage.
Zero Age Main Sequence – Temperatures and magnitudes at which different
mass stars first reach equilibrium.
After the Main Sequence
1.Once a star’s hydrogen is used up its future life is dictated by its mass.
2.During the H-Burning phase the star has been creating He in the core
by turning 4 protons into a He nucleus plus electrons and neutrinos.
Once the H burning stops in the centre the star contracts and some
of the potential energy is turned into heat. If the core temperature
rises far enough then He-burning can begin. Coulomb(electrostatic)
barrier is 4 times higher for two He nuclei compared with protons.
3.Now we face again the problem of there being no stable A = 5 or 8
4.It turns out that we can bypass these bottlenecks but it depends
critically on the properties of the properties of individual levels in
Be and C nuclei.
The Creation of 12C and 16O
• H and 4He were made in the Big Bang.Heavier nuclei were
not produced because there are no stable A = 5 or 8 nuclei.
There are no chains of light nuclei to hurdle the gaps.
• How then can we make 12C and 16O?
• Firstly 8Be from the fusion of
two alphas lives for 2.6 x 10-16 s
cf. scattering time 3 x 10-21 s.
They stick together for a
significant time.
• At equilibrium we get a concentration
of 1 in 109 for 8Be atoms in 4He.
• Salpeter pointed out that this meant that C must be produced
in a two step process.
• Hoyle showed that the second step
must be resonant.He predicted that
since Be and C both have 0+
s-wave fusion must lead to a
0+ state in 12C close to the Gamow
peak at  3 x 108K.
• Experiment shows such a state at
7654 keV with  = 5 x 10-17s
The 7654 keV state
has /  1000
A rare set of
circumstances indeed!
Red Giant
H burning
The Earth
will be
++ 12C + 16O
Path of Solar Mass Star on Hertzsprung Russell Diagram
White Dwarf
H, N, O
Helix Planetary Nebula in the constellation of Aquarius
The End of Fusion Reactions in Stars
A = 56
Binding Energy per nucleon as a
function of Nuclear Mass(A)
[Remember E = mc2]
•When two nuclei fuse together energy is released up to mass A = 56
Beyond A = 56 energy is required to make two nuclei fuse.
•As a result we get the burning of successively more massive nuclei
in stars.First H, then He, then C,N,O etc.
•In massive stars we eventually end up with different materials burning
in layers with the heaviest nuclei burning in the centre where the
temperature is highest.
•When the heaviest(A = 56) fuel runs out the star explodes-Supernova
If Etoile
the star is massive
eight times more
massive than the Sun
H He
Ne Na Mg
Al Si P S
Death of a Red Giant:
October 1987
1056 Joules of energy
This happened 170000 years ago in the nearest galaxy
The Destiny of the Stars…
Massive Stars
100 kg
Spectrum of Cassiopeia
We see here the remnants of a
supernova in Cassiopeia.This
radio telescope picture is taken
with theVery Large Array in
New Mexico.
From the measured rate of
expansion it is thought to have
occurred about 320 years ago.
It is 10,000 ly away.
With optical telescopes almost
nothing is seen.
The inset at the bottom shows a small part
of the gamma ray spectrum with a clear
peak at 1157 keV,the energy of a gamma
ray in the decay of 44Ti.
e.g., Diehl et al., Astron. Astrophys 97, 181 (1993);
Publications of the Astr. Society of the Pacific 110:637 (1999)
Full-sky Comptel
map of 1.8 MeV
gammas in 26Mg
following 26Al GS
(a) Spin traps, eg. 26Al, (N=Z=13) 0+ state b-decaying spin-trap.
0+, T=1
5+, T=0
(decays direct to 26Mg GS
228.3 keV, T1/2=6.3 secs via superallowed Fermi
b+…forking in rp-process
0 keV, T1/2=7.4x105 yrs (decays to 2 states in Mg
via forbidden, Dl=3 decays).
Principe de
of laNucleosynthesis
28 Ni
58 59 60 61 62
27 Co
26 Fe
29 Cu
54 55 56 57 58
Il y a compétition
between twoentre
a neutron
••b Radioactivité
n  p + e-+ 
Part of the Slow Neutron Capture Pathway
In Red Giant Stars neutrons are produced in the 13C( 4He,n) 16O or
22Ne(4He,n)25Mg reactions.
The flux is relatively low.As a result there is time for beta decay before
a second neutron is captured.
The boxes here indicate a stable nuclear species with a particular Z & N.
Successive neutron captures increase N. This stops when the nucleus
created is unstable and beta decays before capture.
The pathways for the s- and r-processes
S-process:Neutron flux is low so beta decay occurs before a second
neutron is captured.We slowly zigzag up in mass.
R-process:Neutron flux is enormous and many neutrons are captured
before we get beta decays back to stability.
The Abundances of the
Elements for A = 70 - 210
Note the double peaks at
N = 46/50, 76/82, 116/126
They are due to production
by the two separate
S – process
M74 Gemini
M31-Andromeda Galaxy-2.2Mly from Earth, Part of our Local
cluster of galaxies.
The Solar System
The Solar System
The formation of the Solar System has been a topic of great interest for a long time.
As yet there is no definitive theory but there is an emerging consensus.
There have been (are) theories that start with a) A comet colliding with the Sun and
knocking the material that composes the planets out of it, b) A close encounter with
another large body, with the resulting tidal effects causing part of the Sun’s material
to be ripped out.
These theories face a variety of problems such as the differences in composition
between the Sun and the planets.
Other theories rely on the accretion of material from interstellar space. This solves
the difference in composition from the Sun but not between planets.
The basis of the models that are popular now is the idea that Sun and planets all
formed from the same material. Differences in composition arise during the formation
of the system. [Does not preclude a mixture of these ideas]
Many problems remain but now there seems to be convergence on a theory of this
The Solar System
Before looking at the theories we should remind ourselves of some of the facts.
The Solar System consists of a very large number of objects, held together by
gravity and obeying Kepler’s Laws.
The picture is not to scale. It
shows the Sun with the four, inner
Terrestrial planets, followed further
out by the Asteroid belt then the
Gas giants.
Then we have comets , a large
number of moons etc.
The Solar System
Sun - a Main Sequence Star of mass 2 x 1030 kg
- radius = 696,000 km
- Luminosity = 3.86 x 1026 W
- Distance to centre of galaxy = 8000pc = 26,000ly
- density = 1410 kg/m3
Nine planets
137 known moons
Gas and dust
We again see the solar system below but this time without
the Sun. On the right we see the scales of the orbits
of the various planets.
Distance-109 m
The Solar System
Kepler’s Laws
1.The planets orbit the Sun in ellipses
with the Sun at one focus.
2.The line joining the Sun and a planet
sweeps out equal areas in equal times.
3.The square of the period of a planet
is proportional to the cube of the
semi-major axis of the ellipse.
P2  a3
Convenient Measure of distance
-Astronomical Unit(1 au)
= Average Earth-Sun distance
= 1.496 x 1013 cm
= 1.496 x 1011 m
Note:- Elliptical orbits were an essential innovation but for simple
calculations one can assume that the orbits are circles. In
general it is a good approximation.
Kepler’s Third Law
Plot of a3 versus P2 for the planets
in the Solar system
- Here a is in AU and P is in
Earth Years.
Clearly P2  a3
Asteroids lie in belt from 2-3.5AU from Sun.
Reminder:All three of Kepler’s Laws are rigorously obeyed
wherever two objects move under their mutual gravitational
Planetary Orbits
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
Semi-major axis
(106 km)
Sidereal Period
Orbital Eccent.
57.9 108.2 149.6 227.9 778.4 1424
Surface Temp.
0.241 0.615 1.0 1.88
11.9 29.5
0.05 0.06
Direction of revn
Angle to plane
to ecliptic(degs.)
Angle of Plane
to spin axis(degs)
Rotation Period
2871 4499
0.01 0.02 009
They are all the same apart from Venus
1.3 2.5
178 23.5 25.2
3.1 26.7
0.41 0.43
Mass(1024 kgm) 0.33
4.87 5.98 0.64 1900 569
Formation of Solar System
Key piece of evidence:- Sun and planets orbit in same direction and lie
almost in the same plane.
This suggests Sun and planets all formed at the same time from a mass
of gas and dust, which was rotating.
Why did this mass come together?
What triggered this process?
Here there is no clear answer – Perhaps a shock wave from a supernova
or some other event.
Basic Idea of Nebular Hypothesis
Here is the idea with which we are already familiar.
The system forms from a collapsing cloud
of gas and dust
If the whole cloud is spinning slowly when
the collapse starts then it will speed up
as it gets smaller in order to conserve
angular momentum.
Formation of Solar System
Protoplanetary disc
The initial cloud or nebula must have had a small rate of rotation. As
the cloud collapses it would speed up to conserve angular momentum.
Two results:- the rotation we see today and the formation of a disc with
the planets forming in the outer part of the disc as the material clumps
Solar System – Role of Condensation Temperature
Temperature would play a large role in determining the composition of the planets.
For the inner planets T is high so molecules have higher average velocities and
light gases escape from the gravitational field.
Metals condense out at higher T so the inner planets have more metals or heavy
For outer planets T is lower and masses higher so they retain the light gases.
The angular momentum leads to a flattened disc which explains why all the planets
are in the same plane. T rose in centre and stayed at say 50K in the outer reaches.
Rocky material stayed solid near the protosun and gases and other icy substances
vaporised. The planetesimals of rock coagulated to form inner planets. The icy
grains on outside grew together and then accumulated gases.
Note:- Chemical differentiation vs. heterogeneous accretion
Former - material accumulated and radioactive decay caused melting and Fe-rich
Minerals sank to centre.
Latter- Fe and Fe oxides were first to condense so cores formed early. Later Si-rich
Material condensed on top.
Formation of Solar System
Here is another view of the same process. Initially T = 50K so solar nebula would have
been filled with dust grains, small ice particles etc plus H and He as gases. As protosun
formed it heated central part leaving outer parts at 50K. In inner section everything
except materials with high condensation temperatures were vaporised i.e.Fe,Si,Mg
S,Al,Ca and Ni and their oxides remained.
Protoplanets formed from planetesimals by accretion, which collided to form planets.
Solar System
Question of angular momentum.
Most of the angular momentum is in the disc-the of the planets. The Sun
has only 0.5% of the total. Why?
Rotating solar nebula was gaseous and hot. The molecules move quickly and are
ionised in collisions – thus a plasma of ions and free electrons forms. The motion of
charged particles creates a magnetic field. The nucleus of the solar nebula thus had
a magnetic field associated with it. Matter close to the nucleus was also partially
ionised and moved with it.
T for disc fell as we moved away from the nucleus so that more and more electrically
neutral molecules would have been found as we moved out from the centre.
As the charged particles were dragged round they collided with uncharged particles
and so they dragged the uncharged particles with them and transferred
to the disc.
A particle whose thus increased would move out from the that
total is conserved.
Nucleus continued to contract and increase its rotational velocity(although at a slower
rate due to the transfer) while the matter moving away slowed as its
orbital radius increased. In principle all of the ang. Mom. could be transferred to disc.
Eagle Nebula – Star Formation
Hubble space telescope pictures of star-forming region in the Eagle Nebula.
About 1 light year
Enlargement of regions where young stars
are forming. Tips are about size of the
Solar system.
Origin of the Solar System
A small part of the ORION nebula
In which young stars are being
formed from small pieces of the
giant interstellar cloud.
Our solar system presumably
formed as a small part of a gas
Cloud collapsed under gravity.
This piece of collapsed cloud we
call the solar nebula.
Before the collapse began it
would have been spread out
over a few Light years in
diameter. It was cold and had
a low density.
Why did it start to collapse?
Perhaps it was the result of a
shock wave from an exploding
Size of Solar
Protoplanetary discs forming in the ORION Nebula. Insets show examples. They are
In false colour and the picture is a mosaic of HST pictures. A young star is at centre
of each proplyd.
Jovian Planets
Outer planets probably began in same way with accretion of planetesimals. Since T
was low this included ice particles. Gas was moving slowly so it got attracted by
gravitational force. Process stopped when gas ran out.
Result - a small solid core with a large gas envelope.
This is thought to be origin of four Jovian planets.
Initially they would have been hotter and would have behaved like a miniature solar
System and we can imagine their satellites forming like the planets in the solar system.
Solar wind plus accretion would have scavenged all of the gas and dust and planets
would then have stabilised at present sizes.
Extra-Solar Planets
Are there other “solar systems”? The unequivocal answer is YES. We now have
evidence of at least 100 planets around other stars.
There is a systematic search for such objects. For example at the 3.9m
Anglo-Australian telescope.
As the planet orbits the star it will cause it to “wobble” back and forth in space. This
will also cause the light from the star to be Doppler shifted. The AAT team can detect
a Doppler shift to an accuracy of 3m/sec. This is the basis of their planet-hunting
The latest such planet was observed around a star called Tau Gruis and is of the
size of Jupiter. It is about 100ly away. It is three times further from its star than
Earth is from the Sun.
Extra-Solar planets
Variety of methods used to look for them.
- radial velocity measurement
- astrometry – looking for slight variation
in position
- Imaging – looking for reflection of light from
- Photometry (occultations)
So far we have only been able to detect the effects of Jovian-like planets.
Earth-like planets are too small to detect by these methods.
Extrasolar planets known
within 200 pc of Earth.
This picture shows their distances
from the stars they orbit in AU
Formation of Binary Star Systems
A large fraction of all stars are binary systems.
They are important for astronomers because they allow us to measure masses.
The collapsing nebula idea gives a natural explanation.
The Sun
Sun - a Main Sequence Star of mass 2 x 1030 kg
- radius = 696,000 km
- Luminosity = 3.86 x 1026 W
- Distance to centre of galaxy = 8000pc = 26,000ly
- density = 1410 kg/m3
Aside:- Source of energy. Typical chemical reaction-1eV = 1.6 x 10-19 Joules
No.of atoms needed to provide Sun’s luminosity = 3.9 x 1026 / 10-19
= 3.9 x 1045 atoms
Length of time to consume all of Sun = 1057 / 3.9 x 1045 = 3 x 1011 s
= 10,000 Years!!!
Surface of the Sun
Photosphere = layer at which photons finally escape from the surface. Average T is
5850K but close up we see it is granulated. This is the result of convection. The
bright areas are where hot gas bubbles upwards and the dark edges are where
cool gas descends. It is like the surface of water boiling in a pan.
Sunspots and Solar Surface
One of the most striking features of the solar surface-sunspots. T is ~ 4000K, rather
cooler than normal surface temp of 5850K. Why is the region not heated? It turns out
that these are regions with strong magnetic fields and the fields cause charged
particles to spiral along magnetic field lines. No easy motion at rt. angles to field lines.
Solar Prominences
Fields from two sunspots often go high above the photosphere. These loops of
Magnetic field sometimes appear as solar prominences in which the field traps gas
that can glow for days or even weeks. They can rise to 100,000 kms above the surface
Solar flares are even more
dramatic – they usually occur
in vicinity of sunspots
Suggesting they may be due to
a collapse in the magnetic field
with a large release of energy.
This heats the plasma and
accelerates the charged particles
to high velocity.
Solar Prominence
UV photo of this very
Large solar prominence.
It is 20 times Earth size.
Courtesy of A..King
Courtesy of A.King
Mass = 3.3 x 1023 kg
Distance from Sun = 0.307 – 0.467 AU
Orbital period = 87.97 days
Rotational Period = 58.6 days
Density = 5430 kg/m3
Average surface Temp. = 350 – (-170) degrees centigrade
Decent photographs only from Mariner 10 spacecraft in 1974
Surface looks like the moon. With the results of many impacts clearly visible.
It has a large iron core and a magnetic field.
Mercury is not in synchronous
rotation round the Sun. It makes
three rotations on its axis for
every twice it orbits the Sun.
This is related to the large
eccentricity in its orbit.
Distance from Sun = 0.723 AU
Mass = 4.869 x 1024 kg
Orbital Period = 224.7 days
Rotational period = 243 days
Density = 5243 kg/m3
Surface temp = 733K
Surface Pressure = 90 atmospheres
Covered by a thick, unbroken layer of clouds.
It rotates in retrogade rotation
Clouds are transparent to radiowaves and
Large number of space probes aimed at
Strong greenhouse effect so surface is hot.
Satellite landed
And we see
Venus-Second Planet out
Terrestrial Type planet. Covered in thick cloud of ammonia etc. Surface is rocky
As we can see on the radar maps. Density similar to Earth.
Volcanic activity is probably responsible for
Injecting substantial amounts of sulphuric
Acid and sulphur dioxide in atmosphere of
Lava flows clearly visible via radar
Lots of volcanic activity
No evidence of plate tectonics.
Mass = 5.97 x 1024 kg
Distance to Sun = 1.496 x 108 km
Density = 5515 kg/m3
Surface Temperature = 333K
Orbital period = 365.256 days
Rotational period = 23.9345 hours.
Troposphere-heated only indirectly by Sun.
Stratosphere – Lot of ozone so it absorbs
UV heavily. T increases with height.
Mesosphere – Little ozone so UV is not
absorbed. T decreases with height.
No defining edge to atmosphere.
Earth Observed from Space
Earth seen from Apollo 11
Galileo shot of Earth
and its Moon
Galileo shot of South America
Structure formed in Differentiation process.
After approx 109 years Earth melted due to a) gravitational energy.
from formation, b) Meteor bombardment and c) radioactive decay.
Whilst molten, gravity concentrated denser material near the centre. When it solidified
again apart from outer liquid core it had a layered structure.
As the outer layers cooled large cracks developed in the lithosphere because of thermal
Stress-this leads to favourable conditions for plate tectonics.
We can study Earth’s interior with seismic waves.
P-waves:-Longitudinal waves which propagate
in liquids and solids.
S-Waves:-transverse waves propagate in
solids but not liquids
Seismic studies plus “theory” suggest the structure
on the left. Solid inner core (Fe + Ni), Liquid outer
core(Fe+Ni).Diameter 7000km.
Crust = tens of km.
Mantle = region between core and crust.
Lithosphere = crust + upper part of mantle.
Aesthenospshere = region of plasticity
Plate tectonics.
Crust is thin (tens of km). Lithosphere is broken into large plates. Aesthenosphere
Is plastic and kept so by heating from radioactive decay.. Very slow convection then
provides horizontal force on plates to make them move.
Seismic activity
 laser ranging can detect few cms. per century.
 fossil record supports theory.
Australia will join Asia. Parts of
California will “leave” USA.
Africa will separate from Middle
East. Italian boot will disappear.
Earth’s Atmosphere
Sunlight warms surface which heats lower part of troposphere. Resulting vertical
T variation causes convection currents which lead to the large variation in the weather.
The atmosphere is also strongly affected by the Earth’s rotation, namely by the Coriolis
Oxygen all came from plants. H and He gone at an early stage
Coriolis Effects
Solar Heating and Coriolis Forces
Winds are driven by solar heating. This would suggest N-S pattern of air flow.
Coriolis forces deflect air to the right in N.hemisphere and to left in S.hemisphere.
In other words you might expect a natural air
flow of hot air from the equator towards the
However the Coriolis effect deflects the air
molecules. We end up with a very general
pattern as shown.
Earth’s Magnetic Field
It is like a simple bar magnet.
Axis is tilted relative to rotation axis
Remember Magnetic field is the result of electrical currents.
Van Allen belts
Field is thought to be due to electrical currents in the spinning liquid outer core.
This Is called the dynamo effect. Rocks formed from molten state retain their
magnetism from that time. Accordingly fossil records show field has reversed every
million years or so.
Charged particles spiral along
the field lines and are reflected at
Mirror points.
Primary source of these particles
is the solar wind.
They are responsible for Aurora.
Earth’s Magnetosphere
Solar wind = stream of ionised gas from Sun. velocity = approx 400 km/second
It varies in intensity depending on solar activity.
When it encounters Earth’s field it is deflected.
Region behind the Bow Shock is called the Magnetosphere. It largely prevents
the solar wind entering. Leakage causes Van Allen belts, Aurora etc.
Aurora over Circle, Alaska
Delicate colours are due to collisions between energetic electrons and O and N
molecules in the atmosphere.
Aurora in UV in Northern hemisphere from Nasa’s
Polar satellite.
Earth Observation
U.S.A. at night from space.
Mount Etna from space
Earth Observation can be at any wavelength. Here it is in Infra-red and we see
the distribution of water vapour.
The Moon
Mass = 1/80 x Earth’s mass
Mean distance = 384,000 km
Diameter = ¼ x Earth’s diameter.
Orbit eccentricity is approx 0.05
Daytime T = 373 K
Nightime T = -160K No atmosphere to store heat.
Density = 3.4 g/cc
Apollo rock samples show material is as old as Solar
system. They are older than Earth rocks because of
Volcanic activity here.
Structure of Interior
Largely dead geologically
No magnetic field in essence
Maybe in past it was bigger.
Any seismic activity due to tidal effects.
Craters from meteor impact
Early molten stage
Vulcanism ended some 3 billion years ago
How did Moon form?
1.Fission Theory:-Once part of Earth and separated in some way-Pacific basin
is favourite site for this.
2.The Moon formed somewhere else and was captured by Earth’s field.
3.Condensation theory:- Moon and Earth condensed together.
4.Colliding planetesimal theory:- Moon condensed from debris.
5.Ejected ring theory:- Large Planetesimal struck Earth and ejected material
that formed Moon.
First three are essentially ruled out because of differences in the
Material on Moon and Earth.
Fifth is the currently favoured theory.
Mars-Fourth Planet Out
Mass = 6.418 x 1023 kg
Distance from Sun = 1.381 – 1.666 AU
Orbital Period = 686.98 days
Rotational Period = 24 hours 37 min.
Diameter = 6794 km.
Mars from Viking 2
Average density = 3934 kg/m3
Surface Temperature = 133-293K
Mars-Fourth Planet Out
Prominent features on surface
- Meteor craters
- Huge volcanic cones
Gorges larger than Grand canyon
Vast sedimentary deposits
Valleys that look as if they were formed
in water flow
No plate tectonics.
Note:- craters are thought to have been formed at a very early stage as in the case
of the Moon. This process stopped when all the debris in the solar system had been
Mopped up.
Surface Atmospheric pressure = 1/200 atmosp.
pressure on Earth
Atmosphere = 95% CO2 plus 5% N
Large Dust Clouds due to seasonal heating
Frozen carbon dioxide at Pole
Variation in temperature at site of Viking 1
Valles Marineris 500 m wide and up to 6 km deep.
Mass = 1.899 x 1027 kg
Distance from Sun = 4.95 – 5.455 AU
Orbital period = 11.86 years
Rotational period = 9 hours 50 mins
Diameter = 133.7 – 142.98 x 106 m
Average density = 1326 kg/m3
Average Temperature = 165K
at cloud tops.
Largest object in solar system
Large number of Moons
Great Red spot is strong feature of
Weather patterns are due to solar
and internal heating and differential
Shape is oblate (6.5%). This due to
rotation of core.
Jupiter-The largest Gas Giant
Jupiter has a volume approx 1000 times the Earth’s volume..The mass = 1.9 x 1027 kgm
Diameter is 142,800 kms.
It has a very dynamic weather system-atmospheric clouds,storms and latitudinal bands.
The Great Red Spot is a complex storm moving in a counter-clockwise direction. At the
outer edge material appears to rotate in 4-6 days. Near the centre motions are small
and random.
Atmosphere is very deep, maybe including the
whole planet. It is very like the Sun.
It is composed mainly of H and He with small
amounts of Methane, ammonia, water vapour
and other compounds.
At great depths the pressure is very high and
atoms are broken up. In this state H becomes
a metal.
The Great Red Spot
The four Galilean Satellites
They all orbit more or less in plane
of planet’s orbit. Their motions are
closely linked. The tidal effects are
very strong.
They all rotate in same direction as orbit.
Large number of other satellites.
Although all of “publicity” is for Saturn’s
rings we see here that there are rings
for Jupiter.
Jupiter has 28 known satellites, four of which were observed as long ago as 1610.
They are Callisto, Europa, Ganymede and Io. There is also a faint ring system.
The image below is a collage of images acquired by Voyager and Galileo spacecraft.
We see the Valhalla region of Callisto in the lower right. Inside the four Galilean
Moons are Amalthea(top), Metis and Adrastea(to right) and Thebe(left).
Jupiter’s rings and moons
exist within an intense radiation belt
of ions and electrons trapped in the
planet’s magnetic field.
This field stretches out 3-7 Mkms
towards the Sun and 750 Mkms
towards Saturn.
Saturn-Another gas Giant
Saturn’s rings are complex
Pluto and Charon
Nucleus = mixture of ice and dust.
Ion tail = Ions from comet are swept directly away from comet by solar wind.
Dust tail = photons hitting dust particles are absorbed and hence exert a pressure
on dust.
Tails always point away from Sun.
Comet Hale-Bopp photographed over Boulder Colorado (1997)
Approx 105 asteroids spread
Over 1017 square km.
Largest is CERES with diameter
Of 934 km. It accounts for 1/3 of total
Asteroid mass.
Probably planet did not form
because of huge pull of Jupiter.
They occasionally collide with each other
and with Earth
Photograph of EROS asteroid from NEAR spacecraft. It is about 40 kms in length.
Appearance is probably typical of most asteroids. Note its non-spherical shape, also
typical of such small objects.
Solar System
Schematic view of the solar
The insets show a COMET
Note the asteroid belt between
Mars and Jupiter
Further out we have the
Kuiper belt and much further
away the Oort Cloud.
Solar System Montage
Saturn-Another Gas Giant
Solar System
The planets of the Solar system are classified as Terrestrial or Jovian.
The four inner, terrestrial planets are close to the Sun. They are quite warmNoon on Mercury = 600K and on Mars = 300K
At top of clouds on Jupiter = 150K cf 63K on Neptune.
Inner planets – densities 5.4-3.9 gcm-3 . Masses typically 1024kg
Outer planets0.7-2.0
Conclusion-terrestrial planets contain a large amount of material denser than rock.
Whilst outer planets probably have solid cores of Earth dimensions but with
extensive gaseous atmospheres.
All the planets except Mercury and Venus have satellites. At least 50 are known.
Seven are comparable in size to Mercury – Moon, Io, Europa, Ganymede, Callisto,
Titan, and Triton
Artist’s Impression
This how the possible scene
from a moon around the recently
Discovered planet. The star is 6th
magnitude and takes 4 years to
Orbit at a distance three times the
Earth-Sun distance.
There is,of course, no evidence of
a moon.
So far we only see planets of
Jupiter-like mass but they fall into
two groups-those very close in
and those a long way out.
So far we have no explanation of
1022 m
1019 m
10-15 m
10-14 m
10-10 m
10-9 m
1012 m
107 m
10-6 m
10-5 m