Announcements
• Angel Grade update Friday April 2
• Reading for next class: 17.4, chapter 18
• Star Assignment 7,
due Monday April 5
Do Angel quiz,
Astronomy Place tutorial “Stellar Evolution”
lessons 1 & 2 only (not exercises yet)
Objectives
• Explain the evolution of a star in terms of loss of
energy to space, consumption of nuclear fuel,
upsets in the equilibrium balance conditions and
gravitational contraction.
• Describe the sequence of evolutionary stages of a
star and how they depend on the mass of the star.
• Describe the properties of stars in different
evolutionary stages: pre-main sequence, main
sequence, red giant, white dwarf, supernova,
neutron star and black hole.
• Describe the evolutionary state of the 16 brightest
northern-hemisphere stars.
What do we know?
What do we want to explain?
Distribution of Luminosities
100
HIGH
1
0.01
Luminosity
0.0001
LOW
Distribution
of
Stellar
Masses
Luminosity - Mass Relation
HertzsprungRussell
Diagram
Regions of
the H-R
Diagram
First
Theory:
Energy source
gravitational
PE.
Star evolves by
contracting
from giant to
MS & then
down MS
from larger to
smaller
Current Theory
• Energy Source Nuclear Fusion
Question:
Why must
stars evolve?
Star Birth
The Orion
Nebula is one
of the closest
star forming
clouds
Gravitational
Potential
Energy ->
Kinetic (heat)
Energy as cloud
shrinks & then
into Infrared
light
Infrared light from Orion
Disks
around
newborn
stars in
the
Orion
Nebula
Evidence for
spinning disks
around newly
forming stars
More
disks &
jets
Infrared light reveals protostar and jets
embedded within a dark star-forming cloud
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
• Cloud of H (70%), He (28%) gas + dust &
other elements (2%)
• Pressure - Gravity balance is upset
Gravity > Pressure
CONTRACTS
Gets denser
Converts gravitational PE -> KE
Gets hotter
• Conserves Angular Momentum
Spins faster
Flattens into a disk (rotation prevents
contraction perpendicular to spin axis)
Disks & Jets
The
Orion
Nebula:
How many
protostellar
disks can
you find?
Stars position in H-R diagram changes as it evolves
Life tracks for protostars
Star birth similar for all
stars, but massive stars
pass through the stages
faster
Star Birth
• Protostar contracts -> gets denser
Converts gravitational PE -> Thermal KE
• Insulation increases -> Energy loss decreases
Star heats up
• If Mass large enough, convert enough
gravitational PE -> Thermal KE to
Heat Core to temperature (107 K) for
Nuclear Fusion: 4H -> He + 2 g + 2 n + 2e+
Main Sequence Star =
Fusing H -> He in Core
Main Sequence Stars
Radiative Envelope
Convective Core
Convective Envelope
All Convective
Luminosity
Very massive
stars are rare
Low-mass
stars are
common
Why no stars
with less
than 0.08
Msun ?
Temperature
What do we want to Explain?
•
•
•
•
Mass - Luminosity Relation
Main Sequence
Distribution of Stellar Masses
Distribution of Stellar-Luminosities
What do we want to Explain?
• Mass - Luminosity Relation
Larger Mass stars have larger Gravity pulling in
Need larger Pressure pushing out
Larger Pressure requires higher Temperature
Higher Temperature produces much greater Energy
Generation Rate
Energy Loss balances Energy Generation
L ~ M 3. 5
What do we want to Explain?
• Main Sequence
Balance between Pressure pushing out &
Gravity pulling in
and between
Energy Generation & Energy Loss
Determines relation between stellar
Mass & Radius & Luminosity
& Surface Temperature
What do we want to Explain?
• Distribution of Stellar Masses
Not yet clear, seems to be due to sizes of
random motions in the gas between stars
• Distribution of Stellar - Luminosities
Due to Mass - Luminosity relation
What happens as Hydrogen is fused
into Helium in core of a star?
1. What happens to the number of particles?
a) The number remains the same
b) The number increases
c) The number decreases
What happens as Hydrogen is fused
into Helium in core of a star?
1. What happens to the number of particles?
a) The number remains the same
b) The number increases
c) The number decreases (4 H -> 1 He)
(the e+ annihilate with the e-)
(the n stream unimpeded out of the star)
(the g random walk out of the star exerting some pressure)
What happens as Hydrogen is fused
into Helium in core of a star?
2. As a result of the decrease in the number
of particles in the core, due to the fusion
of 4 H -> 1 He, What happens to the
Pressure?
a) Pressure increases
b) Pressure remains the same
c) Pressure decreases
What happens as Hydrogen is fused
into Helium in core of a star?
2. As a result of the decrease in the number
of particles in the core, due to the fusion
of 4 H -> 1 He, What happens to the
Pressure?
(the e+ annihilate with the e-)
(the n stream unimpeded out of the star)
(the g random walk out of the star exerting some
pressure)
a) Pressure increases
b) Pressure remains the same
c) Pressure decreases
What happens as Hydrogen is fused
into Helium in core of a star?
3. As a result of the decrease in Pressure,
what happens to the size of the core?
a) It shrinks
b) It stays the same size
c) It expands
What happens as Hydrogen is fused
into Helium in core of a star?
3. As a result of the decrease in Pressure,
what happens to the size of the core?
a) It shrinks (pressure < gravity)
b) It stays the same size
c) It expands
What happens as Hydrogen is fused
into Helium in core of a star?
4. As a result of the contraction of the core,
what happens to the core’s temperature?
a) It decreases
b) It remains the same
c) It increases
What happens as Hydrogen is fused
into Helium in core of a star?
4. As a result of the contraction of the core,
what happens to the core’s temperature?
a) It decreases
b) It remains the same
c) It increases
(contraction converts gravitational PE -> thermal KE)
What happens as Hydrogen is fused
into Helium in core of a star?
5. As a result of the increase in the core’s
temperature, what happens to the rate of
nuclear energy generation?
a) It decrease
b) It remains the same
c) It increases
What happens as Hydrogen is fused
into Helium in core of a star?
5. As a result of the increase in the core’s
temperature, what happens to the rate of
nuclear energy generation?
a) It decrease
b) It remains the same
c) It increases
(Rate of nuclear fusion reactions increases
rapidly with increasing temperature)
What happens as Hydrogen is fused
into Helium in core of a star?
6. As a result of the increase in the rate of
nuclear energy generation, what happens
to the Luminosity
a) It increases
b) It remains the same
c) It decreases
What happens as Hydrogen is fused
into Helium in core of a star?
6. As a result of the increase in the rate of
nuclear energy generation, what happens
to the Luminosity
a) It increases
b) It remains the same
(until the pressure outside the core builds up
enough to make the surrounding envelope
expand which reduces the insulation and
allows more energy to escape, which
increases the Luminosity)
c) It decreases
Star
Expands
&
becomes
more
Luminous
Star
expands
&
becomes
more
Luminous
What happens when all the H in the
core is finally converted to He?
• Can He fuse into heavier elements?
• He nuclei has 2 protons & 2 neutrons,
has twice the charge of H
stronger repulsion
needs to move faster to overcome repulsion
and touch
needs higher Temperature
What happens when all the H in the
core is finally converted to He?
• No nuclear fusion energy released in inert He core
Core still losing energy
Core tends to cool
Pressure tends to decrease
Pressure < Gravity
Star contracts
Converts gravitational PE -> thermal KE
Star Heats up (both core and surrounding envelope)
What happens when all the H in the
core is finally converted to He?
• Outside inert He core, H still exists
• As star contracts, H gets hotter
H in shell surrounding He core gets hot enough to
fuse H -> He
Adds more He to inert He core
Increases Mass and Gravity of He core
Core continues to shrink
H fusing shell also shrinks along with core
Converts gravitational PE -> thermal KE in core
and shell
What happens when all the H in the
core is finally converted to He?
He core & H fusing shell get Hotter
Rate of H fusion into He increases
Energy generation > Energy Loss
Shell gets hotter
Pressure increases
Makes surrounding envelope Expand
Reduces insulation
Increases Luminosity
Star becomes a RED
GIANT
High-Mass Stars
> 8 MSun
IntermediateMass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs
Star
becomes
a
RED
GIANT
Star
becomes
a
RED
GIANT
Main Sequence to Red Giant
Helium Fusion
Helium fusion requires higher temperatures than hydrogen fusion
because the larger charge leads to greater repulsion. Eventually, core
gets hot enough to fuse helium.
Fusion of two helium nuclei doesn’t work, so helium fusion must
combine three He nuclei to make carbon
Small mass stars can not get hot enough
to fuse Carbon
Evolution
of low
mass star
Large
mass
stars
get hot
enough
to fuse
heavy
nuclei
Evolution
of high
mass star
Betelgeuse:
Red Supergiant
Life History of a Star
Loss of Energy to Space
Gravitational Contraction of Core
Contraction is halted temporarily
by nuclear fusion
Energy generation in core
Test:
Cluster
HR
Diagrams
Same Distance
Same Age
Question:
Why must
stars evolve?
Think about this and submit answer as part of
Angel assignment due Monday 4/5.