Low Mass Stars (< 8 MSun) - Outline Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf Mommy The more massive the star, the faster it does everything From Main Sequence to Planetary Nebula, each stage goes faster than the previous Fetus Adult Old Woman Q. 71: Length of Stages Cancer Corpse

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

Molecular Clouds – Keyhole and Orion

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

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 Double Shell-Burning Core Helium-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

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

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

Mass Distribution of Stars Stars Range from about 0.08 – 250 Msun Lighter than 0.08 – they don’t get hot enough for fusion Heavier than 250 – 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 called brown dwarfs Brown Dwarf Small Star

High Mass Stars Eta Carinae About 150 MSun HDE 269810 Peony Nebula Star R136a1 265 MSun

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

Structure of Main Sequence Stars All burn hydrogen to helium at their cores Solar mass: Convection on the outside High mass: Convection on the inside Low mass: Convection everywhere

Evolution on the Main Sequence 4H + 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 Double Shell-Burning Core Helium-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 72. Duration of Main Sequence Lifetime Cl M life O5 60 360 ky B0 18 10 My A0 3 400 My A5 2 1.1 Gy G2 1 10 Gy G5 0.9 15 Gy M7 0.2 500 Gy Age of Universe Stars lighter than Sun still main sequence

Giant Stages – Low Mass Stars Main Sequence  Red Giant At the center, Hydrogen is gone – there is only Helium “ash” As more Helium accumulates, gravity pulls the core together – it shrinks and heats up Hydrogen continues to burn in a layer around the center High temperature – it burns fast Luminosity rises This dumps lots of heat into the outer layer It expands and cools Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf

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

Red Giant Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf Double Shell-Burning Core Helium-Burning Star moves up and right on H-R diagram

73. Duration of Red Giant Stage Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf The star is incredibly bright and incredibly large Goodbye Mercury It is using up fuel faster than ever It evolves fast 200 Myr for Sun The core keeps getting more massive, more compressed, and hotter It accelerates faster and faster 73. Duration of Red Giant Stage

More Nuclear Physics There are other processes besides hydrogen burning At 100 million K, three helium atoms can join to make carbon plus a little energy 3He  C + Energy With a little higher temperature, they can add one more to make oxygen C + He  O + Energy These processes produce far less energy than hydrogen burning

Red Giant  Core Helium Burning Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf At 100 million K, the helium core in a red giant star ignites Suddenly for light stars (< 3 MSun) Gradually for heavy stars (> 3 MSun) New heat source in core It expands and cools Hydrogen, still burning in a shell, burns more slowly now Less heat going into hydrogen envelope Hydrogen envelope shrinks and heats up Star gets hotter but less luminous 74. Core Helium Burning On H-R Diagram

Red Giant  Core Helium Burning Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary Nebula White Dwarf Hydrogen Helium Carbon/Oxygen

Core Helium Burning Molecular Cloud Protostar Main Sequence Red Giant Double Shell-Burning Planetary Nebula White Dwarf Double Shell-Burning Core Helium-Burning Star gets hotter and dimmer: down and left on H-R diagram