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 Planetesima

ls into Planets

Solar Nebula TheoryImmanuel 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 Gyr2. Planets formed over brief period of 10-100 Myr3. Gas collects into “disk”, and cools leading to

formation of condensates4. Growth of planetesimals by collisions

a) Build up minor bodies and small rocky worldsb) 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.

• The model predicts:

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

Cosmic Billiards

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.

20 AU

planetesimals

100 Myr 880 Myr

883 Myr ~1200 Myr

JS

UN

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 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

(K) (K) (%)

Mercury 440 400 10

Venus 230 730 68

Earth 250 280 11

Mars 220 210 5

Jupiter 105 125 16

Saturn 80 95 16

Uranus 60 60 <1

Neptune 45 60 25

Pluto 40 40 <1

Share Question

As the Sun, changes how will we?

Density and Composition<>

(kg/m3)

Water 1000

Rock 3000

Air 1.3

Brass 8600

Steel 7830

Gold 19300

<>

(kg/m3)

Ices 1000

Volcanic rock and

stony meteorites

2800 - 3900

Iron rich minerals

5000 - 6000

iron ~7900

Ex: Moon – (surf) ~ 2800 and <> ~ 3300Earth – (surf) ~ 2800 but <> ~ 5500