Announcements Angel Grade update Friday April 2 Reading for next class: 17.4, chapter 18 Star Assignment 7, due Monday April 5 ÜDo Angel quiz, ÜAstronomy 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 LOWHIGHLuminosity

Distribution of Stellar Masses

Luminosity - Mass Relation L~M 3.5

Hertzsprung- Russell Diagram Luminosity ->

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

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 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

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

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 (10 7 K) for Nuclear Fusion: 4H -> He + 2  + 2  e +

Main Sequence Star = Fusing H -> He in Core

Main Sequence Stars Convective CoreConvective EnvelopeAll Convective Radiative Envelope

Temperature Luminosity Very massive stars are rare Low-mass stars are common Why no stars with less than 0.08 M sun ?

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 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 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 M Sun Low-Mass Stars < 2 M Sun Intermediate- Mass Stars Brown Dwarfs

Star becomes a RED 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 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.