star formation and main sequence...
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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