The Life History of Stars – Young Stars

1. 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 MSun) have quiet deaths • High mass stars (> 8 MSun) go out with a bang

2. Low Mass Stars (< 8 MSun) - Outline Mommy Fetus Adult Old Woman Cancer Corpse • Molecular Cloud • Protostar • Main Sequence • Red Giant • Core Helium-Burning • Double Shell-Burning • Planetary nebula • White Dwarf • Which stage takes the largest amount of time? • Main Sequence • B) Red Giant • C) Core Helium-Burning • D) Double Shell-Burning • E) Planetary Nebula • The more massive the star, the faster it does everything • From Main Sequence to Planetary Nebula, each stage goes faster than the previous

3. 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

4. Molecular Clouds – Eagle Nebula

5. Molecular Clouds – Keyhole and Orion

6. 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

7. 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 (!)

8. 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

9. 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

10. 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

11. 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

12. 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 Brown Dwarf Small Star

13. High Mass Stars Eta Carinae About 150 MSun HDE 269810 Peony Nebula Star

14. 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

15. 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

16. Announcements DateRead Today Sec. 12.1, 12.2 Thursday Sec. 12.3 Friday Sec. 13.2, 11.3, 13.1, 13.3 MondayStudy for Test • Lab Tonight • Out-4, In-8 6/15

17. 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

18. 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

19. Lifetime on the Main Sequence Age of Universe • 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 ClMlife 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 Which stars run out of fuel first? A) Massive stars B) Light stars C) Same time D) Insufficient information • Stars lighter than Sun still main sequence