Download 1 Star Formation and Main Sequence Evolution Condensation

Star Formation
and Main
Sequence
Evolution
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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This can happen when clouds are compressed
externally!
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 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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Particles of gas and dust stick together within the
disk
Similarly, as a
cloud collapses, it
spins faster
forming a rotating
protoplanetary disk
(proplyd) around a
central clump
which will
eventually become
a star
A process called accretion…..
Leading to the formation….
of a family of planets….
Protostars
Collapsing 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!
Kelvin-Helmholtz
Contraction
F = GMAMB/d2
d2 < d1
so
d ↓F ↑a ↑T↑
where:
d = distance
F = gravity
a =acceleration
T = temperature
Conversion of gravity
into heat!
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?
Instability and Mass Loss
Protostars 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
Evolutionary
Tracks
As protostars
collapse and heat
up they move on
the H-R diagram
towards the main
sequence
Mass loss can only occur perpendicular to the
protoplanetary disc leading to the formation of a
bipolar outflow
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A Star is Born
A Stable Main Sequence Star
Eventually nuclear fusion begins in the
core when it reaches a temperature of
around 10 million K
Once 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
End result: a cluster of newborn stars with a range
of masses
Formation of High Mass Stars
1. 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)
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Distribution of newborn star masses
If a young cluster contains one or more hot,
massive O- or B-type stars an emission nebula will
be produced
Low mass stars are much more common!
The Orion Nebula
Observational Evidence?
A star formation region 1500 ly away
Protostars and Protoplanety disks are seen inside
interstellar clouds!
Disks of gas and dust are seen around other,
mature solar-type stars!
Infrared (IR) Image
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Extrasolar Planets
Main Sequence Evolution
Stellar Adulthood
A 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
When the Sun formed its core contained roughly
75% H and 25% He by mass
During the Main Sequence:
hydrogen → helium
so
time ↑ hydrogen ↓ helium ↑
Eventually the hydrogen fuel runs out
in the core!
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
Main Sequence Lifetime
The Sun is gradually heating up and expanding
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
t~M/L
where:
M = mass and L = luminosity
Mass-luminosity relation:
Find:
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!
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!
Mass and Main
Sequence Lifetime
Analogy
A 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
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