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Brief Overview
of Stellar Evolution
• Pre-Main Sequence (really short time):
The phase in which a protostar forms out of a cloud of
gas that is slowly contracting under gravity
• Main Sequence (long time):
The phase in which a star-wannabe becomes hot
enough to initiate and maintain nuclear fusion of
hydrogen in its core to become a true star.
• Post-Main Sequence (sorta short time):
H-burning ceases, and other kinds of burning may
occur, but the star is destined to become a White
Dwarf, Neutron Star, or Black Hole
Formation of Stars and
Planets
Observational Clues
from the Solar
System:
1. Orbits of planets lie
nearly in ecliptic plane
2. The Sun’s equator lies
nearly in the ecliptic
3. Inner planets are rocky
and outer ones gaseous
4. All planets orbit
prograde
5. Sun rotates prograde
6. Planet orbits are nearly
circular
7. Big moons orbit planets
in a prograde sense,
with orbits in equatorial
plane of the planet
8. Rings of Jovians in
equatorial planes
9. S.S. mass in Sun, but
angular momentum in
planet orbits
Accretion and Sub-Accretion
Collection of
Planetesimals
into Planets
Solar Nebula Theory
Immanuel Kant (German): 1775, suggested that a rotating
cloud that contracts under gravity could explain
planetary orbit characteristics
Basic Modern View –
1. Oldest lunar rocks ~4.6 Gyr
2. Planets formed over brief period of 10-100 Myr
3. Gas collects into “disk”, and cools leading to formation
of condensates
4. Growth of planetesimals by collisions
a) Build up minor bodies and small rocky worlds
b) Build up Jovian cores that sweep up outer gases
Share Question
If an interstellar cloud contracts to become a star, it is due
to which force?
a) electromagnetic
b) nuclear
c) gravitational
d) centrifugal
The Chaotic Early Solar System
• Recent computer models are
challenging earlier views that
planets formed in an orderly
way at their current locations
• These models suggest that the
jovian planets changed their
orbits substantially, and that
Uranus and Neptune could
have changed places
• These chaotic motions could
also explain a ‘spike’ in the
number of impacts in the inner
solar system ~3.8 billion years
ago
The Moon and terrestrial planets were
bombarded by planetesimals early in solar
system history.
Cosmic Billiards
• The model predicts:
100 Myr
1.After formation, giant planet
orbits were affected by
gravitational ‘nudges’ from
surrounding planetesimals
2. Jupiter and Saturn crossed a
1:2 orbital resonance (the ratio
of orbital periods), which made
their orbits more elliptical. This
suddenly enlarged and tilted the
orbits of Uranus and Neptune
3.Uranus / Neptune cleared away
the planetesimals, sending some
to the inner solar system
causing a spike in impact rates
20 AU
880 Myr
planetesimals
883 Myr
~1200 Myr
N
J
S
U
The early layout of the solar system may have
changed dramatically due to gravitational
interactions between the giant planets. Note how
the orbits of Uranus and Neptune moved
outwards, switched places, and scattered the
planetesimal population.
The Big Picture
• The current layout of our solar
system may bear little resemblance
to its original form
• This view is more in line with the
“planetary migration” thought to
occur even more dramatically in
many extrasolar planet systems
• It may be difficult to prove or
disprove these models of our early
solar system. The many
unexplained properties of the
nature and orbits of planets,
comets and asteroids may provide
clues.
Artist’s depiction of Neptune
orbiting close to Jupiter (courtesy
Michael Carroll)
Bode’s Law
Planet
4 + {0,3,6,12,24,...}
d(AU) =
10
Bode’s
Actual
Error
Mercury
0.4
0.4
<1%
Venus
0.7
0.7
<1%
Earth
1.0
1.0
Perfect
Mars
1.6
1.5
7%
Asteroids
2.8
2.8
<1%
Jupiter
5.2
5.2
<1%
Saturn
10.0
9.5
5%
Uranus
19.6
19.2
2%
Neptune
---
30.0
Miserable
Pluto
38.8
39.4
2%
??
77.2
---
---
Radiative Equilibrium
Global Temperatures of
Planets
Planet
Predicted
Actual
Error
Mercury
Venus
Earth
(K)
440
230
250
(K)
400
730
280
(%)
10
68
11
Mars
Jupiter
Saturn
220
105
80
210
125
95
5
16
16
Uranus
Neptune
Pluto
60
45
40
60
60
40
<1
25
<1
Share Question
As the Sun, changes how will we?
Density and Composition
<r>
(kg/m3)
<r>
(kg/m3)
Water
1000
Ices
1000
Rock
3000
2800 - 3900
Air
1.3
Volcanic rock
and stony
meteorites
Brass
8600
5000 - 6000
Steel
7830
Iron rich
minerals
Gold
19300
iron
~7900
Ex:
Moon – r(surf) ~ 2800 and <r> ~ 3300
Earth – r(surf) ~ 2800 but <r> ~ 5500