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Star Formationand Main

SequenceEvolution

Condensation Theory

Dark Nebulae Raw material for star and planet formation!

Interstellar clouds are normally stable!

Because the inward force of gravity is balanced by the outward force of pressure!

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To form a star and planetary system we need the cloud to become unstable:

gravity > pressure

causing the cloud to collapse under gravity

size ↓ temp ↑ spin ↑

This can happen when clouds are compressed externally!

This can be caused by a nearby supernova explosion! Cloud Fragmentation

The cloud does not fragment into equal-sized pieces but fragments into clumps with a range of masses

Formation of Binary Stars The Conservation of Angular Momentum

As a rotating object gets smaller it spins faster!

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Similarly, as a cloud collapses, it

spins faster forming a rotating

protoplanetary disk (proplyd) around a

central clump which will

eventually become a star

Particles of gas and dust stick together within the disk

A process called accretion….. Leading to the formation….

of a family of planets….

ProtostarsCollapsing clumps of gas on there way

to becoming stars are known as protostars as they have not yet started

nuclear fusion!

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Protostars are large cool and

luminous!

Found in the same region of the H-R diagram as red giants but

they are not the same!

How can a collapsing clump of gas produce more energy than the Sun when it has not yet started nuclear

fusion?

Why does a collapsing clump of gas heat up anyway?

Kelvin-Helmholtz ContractionF = GMAMB/d2

d2 < d1

so

d ↓ F ↑ a ↑ T ↑

where:

d = distanceF = gravity

a =accelerationT = temperature

Conversion of gravity into heat!

Instability and Mass LossProtostars transfer energy from their hot interiors to their cool surfaces via

convection

This makes their surfaces very unstable

As they heat up they eventually start to eject their outer layers into space

leading to mass loss

Up 50% of the original mass of the clump can be lost in this way

Mass Loss from Protostars

Mass loss can only occur perpendicular to the protoplanetary disc leading to the formation of a

bipolar outflow

Evolutionary Tracks

As protostars collapse and heat up they move on the H-R diagram towards the main

sequence

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A Star is BornEventually nuclear fusion begins in the core when it reaches a temperature of

around 10 million K

A Stable Main Sequence StarOnce nuclear fusion begins the star

attains hydrostatic and thermal equilibrium producing a newborn

stable zero-age main sequence star

The Sun took around 20 million years to form Formation of Stars of Different Masses

Final position on main sequence determined by mass luminosity relation

Formation of High Mass Stars1. Form much faster due to stronger gravitational

attraction

2. Move horizontally rather than diagonally onto the main sequence

3. Produce high luminosity stars at the top of the main sequence (mass-luminosity relation)

End result: a cluster of newborn stars with a range of masses

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Distribution of newborn star masses

Low mass stars are much more common!

If a young cluster contains one or more hot, massive O- or B-type stars an emission nebula will

be produced

Observational Evidence?

The Orion Nebula

A star formation region 1500 ly away

Protostars and Protoplanety disks are seen inside interstellar clouds!

Infrared (IR) Image

Disks of gas and dust are seen around other, mature solar-type stars!

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Extrasolar Planets

Main Sequence Evolution

Stellar AdulthoodA star spends more than 90% of its total lifetime on the main sequence

This this the most stable phase of a star’s life similar to adulthood in

humans

Definition of a Main Sequence Star:

Core hydrogen to helium fusion

In hydrostatic and thermal equilibrium

During the Main Sequence:hydrogen → helium

so

time ↑ hydrogen ↓ helium ↑

Eventually the hydrogen fuel runs out in the core!

When the Sun formed its core contained roughly 75% H and 25% He by mass

After 4.6 billion years of fusion the amount of hydrogen in the core has decreased

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Main Sequence Evolution of the Sun

The Sun is gradually heating up and expanding

Main Sequence Lifetime

Expect:

High mass stars will live longer since they have more fuel!

Depends on:

1. The amount of nuclear fuel = mass

2. Rate which fuel is consumed = luminosity

Find:

t ~ M / L

where:

M = mass and L = luminosity

Mass-luminosity relation:

L ~ M3.5

so

t ~ M / L ~ M / M3.5 ~ 1 / M2.5

Inverse relation!

mass ↑ lifetime ↓

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High mass stars have shorter lifetimes even though they have more fuel to

fuse!

Mass and Main Sequence Lifetime

Why?They have much higher central temperatures

(and hence luminosities) so they burn through their greater amount of fuel in a shorter amount of time!

AnalogyA hybrid car with a small fuel tank and good fuel economy can typically drive more miles than an

SUV with a large fuel tank but poor fuel economy

Low mass stars are like hybrids while high mass stars are like SUV’s!

Energy Transport in Main Sequence Stars