Announcements• Angel Grade update Friday April 2• Reading for next class: 17.4, chapter 18• Star Assignment 7,

due Monday April 5Do 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

LOWHIGH Luminosity1100 0.01 0.0001

Distribution of

Stellar Masses

Luminosity - Mass Relation

L~M3.5

Hertzsprung-Russell

Diagram

Lum

inos

ity

->

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

Infrared light from Orion

Gravitational Potential Energy ->

Kinetic (heat) Energy as cloud shrinks & then into Infrared

light

Disks around

newborn stars in

the Orion

Nebula

Evidence for spinning disks around newly forming stars

More disks &

jets

QuickTime™ and aMPEG-4 Video decompressor

are needed to see this picture.

Infrared light reveals protostar and jets embedded within a dark star-forming cloud

• Cloud of H (70%), He (28%) gas + dust & other elements (2%)

• Pressure - Gravity balance is upsetGravity > Pressure CONTRACTSGets denser

Converts gravitational PE -> KEGets hotter

• Conserves Angular MomentumSpins 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

Star birth similar for all stars, but massive stars pass through the stages faster

Life tracks for protostars

Star Birth

• Protostar contracts -> gets denserConverts 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 + 2 e+

Main Sequence Star = Fusing H -> He in Core

Main Sequence Stars

Convective Core Convective Envelope All Convective

Radiative Envelope

Temperature

Lu

min

osi

tyVery massive stars are rare

Low-mass stars are common

Why no stars with less than 0.08 Msun ?

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 inNeed larger Pressure pushing outLarger Pressure requires higher TemperatureHigher Temperature produces much greater Energy

Generation RateEnergy 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 increasesc) The number decreases (4 H -> 1 He)

(the e+ annihilate with the e-)(the stream unimpeded out of the star)(the 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 stream unimpeded out of the star)(the random walk out of the star exerting some pressure)

a) Pressure increasesb) Pressure remains the samec) 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 shrinksb) It stays the same sizec) 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 sizec) 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 decreasesb) It remains the samec) 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 decreasesb) It remains the samec) 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 decreaseb) It remains the samec) 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 decreaseb) It remains the samec) 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 increasesb) It remains the samec) 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 increasesb) 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 repulsionneeds to move faster to overcome repulsion

and touchneeds higher Temperature

What happens when all the H in the core is finally converted to He?

• No nuclear fusion energy released in inert He coreCore still losing energyCore tends to coolPressure tends to decreasePressure < GravityStar contractsConverts 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 hotterH in shell surrounding He core gets hot enough to

fuse H -> HeAdds more He to inert He coreIncreases Mass and Gravity of He coreCore continues to shrink H fusing shell also shrinks along with coreConverts 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 HotterRate of H fusion into He increasesEnergy generation > Energy LossShell gets hotterPressure increasesMakes surrounding envelope ExpandReduces insulationIncreases Luminosity

Star becomes a RED GIANT

High-Mass Stars> 8 MSun

Low-Mass Stars< 2 MSun

Intermediate-Mass Stars

Brown Dwarfs

Star becomes

a RED

GIANT

Star becomes

aRED

GIANT

Main Sequence to Red Giant

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

Helium Fusion

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 SpaceGravitational Contraction of CoreContraction is halted temporarily

by nuclear fusionEnergy generation in core

Test:Cluster

HR Diagrams

Same DistanceSame Age

Question:

Why must stars evolve?

Think about this and submit answer as part of Angel assignment due Monday 4/5.