The Life History of Stars – Young StarsThe Importance of Mass

•The entire history of a star depends on its mass and almost nothing else•The more mass a star has, the faster it does everything•The stages of a star differ based on what is happening in the core of the star

•The properties of a star vary wildly as it passes through different stages

•Qualitatively, stars have similar histories, with one big split:•Low mass stars (< 8 MSun) have quiet deaths

•High mass stars (> 8 MSun) go out with a bang

Low Mass Stars (< 8 MSun) - Outline•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary nebula•White Dwarf

MommyFetusAdult

Old Woman

CancerCorpse

•The more massive the star, the faster it does everything•From Main Sequence to Planetary Nebula, each stage goes faster than the previous

Which stage takes the largest amount of time?A) Main SequenceB) Red GiantC) Core Helium-BurningD) Double Shell-BurningE) Planetary Nebula

Molecular Clouds•Huge, cool, relatively dense clouds of gas and dust•Gravity causes them to begin to contract•Clumps begin forming – destined to become stellar systems•Composition:

•75% hydrogen (H2), 23% helium (He), < 2% other

•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

Molecular Clouds – Eagle Nebula

Molecular Clouds – Keyhole and Orion

Formation of Protostars•Cloud fragments to form multiple stars

•Stars usually form in clusters•Often, two or more stars remain in orbit

•The stars are a balance of pressure vs. gravity•Heat leaks out – they cool off•Reduced pressure – gravity wins – it contracts

•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

Negative Heat Capacity

What happen as heat leaks out•They cool off

•By P = knT, they have less pressure•Gravity defeats pressure

•They contract•Energy is converted

•Gravitational Energy Kinetic energy•Kinetic energy Heat

•Net effect: When you remove heat, a star gets:•Smaller•Hotter (!)

H-R diagram: Protostar•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

Core Helium-Burning

Double Shell-Burning

Stellar Winds

•Stars are still embedded in molecular clouds of gas and dust•Stars begin blowing out gas - winds

•Wind blows away the dust – we see star

•The interior of the star is getting hotter and hotter•At 10 million K, fusion starts

•This creates energy•It replaces the lost heat – the star stops getting dimmer

•The surface continues shrinking for a while

•Left and a little up on the H-R diagram

•It becomes a Main Sequence star

A Star is Born•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

H-R diagram: To the Main Sequence•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

Core Helium-Burning

Double Shell-Burning

Mass Distribution of Stars•Stars Range from about 0.08 – 150 Msun

•Lighter than 0.08 – they don’t get hot enough for fusion•Heavier than 150 – they burn so furiously they blow off their outer layers

•Light stars much more common than heavy ones•Objects lighter than 0.08 MSun are calledbrowndwarfs

Small Star

Brown Dwarf

Eta CarinaeAbout 150 MSun

High Mass Stars

HDE 269810Peony Nebula Star

Life on the Main Sequence•The star is now in a steady state – it is “burning” hydrogen

4H + 2e- He + 2 + energy•It burns at exactly the right rate to replace the energy lost•For the Sun, there is enough fuel in the central part to keep it burning steadily for 10 billion years•All stars are in a balance of pressure vs. gravity•To compensate for larger masses, they have to be bigger•They have lower density, which lets heat escape faster•They have to burn fuel faster to compensate•To burn faster, they have to be a little hotter

R M

3.5L M0.4T M

Structure of Main Sequence Stars

•All burnhydrogen tohelium at theircores•Solar mass: Convection on the outside•High mass: Convection on the inside•Low mass: Convection everywhere

Announcements

6/15

Date ReadToday Sec. 12.1, 12.2Thursday Sec. 12.3Friday Sec. 13.2, 11.3, 13.1, 13.3Monday Study for Test

Lab Tonight•Out-4, In-8

Evolution on the Main Sequence4H + 2e- He + 2 + energy

•Number of particles decreased:•The neutrinos leave•6 particles 1 particle•Reduced pressure: P = knT

•Core shrinks slightly•Temperature rises slightly•Fuel burns a little faster•Star gets a little more luminous

•Up slightly on H-R diagram

Evolution on the Main Sequence•Molecular Cloud•Protostar•Main Sequence•Red Giant•Core Helium-Burning•Double Shell-Burning•Planetary Nebula•White Dwarf

Core Helium-Burning

Double Shell-Burning

Lifetime on the Main Sequence•The amount of fuel in a star is proportional to the mass•How fast they burn fuel is proportional to the Luminosity

•Massive stars burn fuel much faster

Which stars run out of fuel first?A) Massive stars B) Light starsC) Same timeD) Insufficient information

3.5L M

Mt

L 2.5

3.5

MM

M

Cl M lifeO5 60 360 kyB0 18 10 MyA0 3 400 MyA5 2 1.1 GyG2 1 10 GyG5 0.9 15 GyM7 0.2 500 Gy

Age of Universe

•Stars lighter than Sun still main sequence