The Life History of Stars – Young Stars The 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 M Sun ) have quiet deaths High mass stars (> 8 M Sun ) go out with a bang

Low Mass Stars (< 8 M Sun ) - Outline Molecular Cloud Protostar Main Sequence Red Giant Core Helium-Burning Double Shell-Burning Planetary nebula White Dwarf Mommy Fetus Adult Old Woman Cancer Corpse 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 Sequence B) Red Giant C) Core Helium-Burning D) Double Shell-Burning E) 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 (H 2 ), 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 M sun 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 M Sun are called brown dwarfs Small Star Brown Dwarf

Eta Carinae About 150 M Sun High Mass Stars HDE Peony Nebula Star

Life on the Main Sequence The star is now in a steady state – it is “burning” hydrogen 4H + 2e -  He 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

Announcements 6/15 Date Read Today Sec. 12.1, 12.2 Thursday Sec Friday Sec. 13.2, 11.3, 13.1, 13.3 Monday Study for Test Lab Tonight Out-4, In-8

Evolution on the Main Sequence 4H + 2e -  He 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 stars C) Same time D) Insufficient information ClMlife O ky B01810 My A My A Gy G2 110 Gy G Gy M Gy Age of Universe Stars lighter than Sun still main sequence