Lecture 11
Stellar Evolution Stars are chemical factories The Earth and all life on the Earth are made of elements forged in stars
A Spiral Galaxy (Milky Way Type) 120,000 ly A few hundred billion stars plus gas and dust
Stellar Birth Stars are born in molecular clouds of mostly hydrogen molecules Stars form in clouds where gravity overcomes thermal pressure Star Forming region along the border of Canis Major and Monoceros
Molecular Clouds Small cloud with much less gas-may form only one star About 15 M sun of gas each-enough for a few stars About 1.4 light-years 10-30 K 300 molecules/cm 3
Star Birth Conservation of energy Cloud heats up Contraction continues if thermal energy is radiated away Infrared Emission Conservation of angular momentum Cloud flattens and spins up Jets from protostar Fragmentation into binary
The Orion Nebula
The Orion Nebula Infrared
Model: A Circumstellar Accretion Disk and Jets A Circumstellar Accretion Disk and Jets distance - 460 ly HST - visible Jets in HH 30 distance - 1,500 ly HST visible Time Lapse: 6 years
Stellar Birth Protostar to Main-Sequence M star: 0.5 M sun 150 million years G star: 1.0 M sun 50 million years A star: 9.0 M sun 3 million years B star: 10.0 M sun 150,000 years O star: 15.0 M sun 60,000 years
Relative number of stars Demographics of Newborn Stars Red Dwarfs numerous! 100 10 Hot O-Type rare! 1 0.1 200 stars 50 stars 10 stars 1 star 0.01 0.08 0.5 2 10 150 stellar mass (M sun )
Life Track of a 1 Msun
Lives in the Balance Gravity and Pressure: Two opposing forces. Thermal Pressure Gravitational Contraction Nuclear Fusion Pressure increases with temperature Degeneracy Pressure Two particles cannot occupy the same state (electrons, neutrons, protons) Depends only on density, not on temperature.
Brown Dwarfs Less than 0.08 M Sun Radiates infrared Thermal energy --from gravitational contraction Electron degeneracy pressure --stops gravitational contraction before fusion can begin Cools after contraction stops
Brown Dwarfs
What are the life stages of a low mass star?
Main-sequence Hydrogen core burning 90% of a star s life Life of a Low Mass Star
Life of a Low Mass Star Main-sequence Red Giant Stage (thermostat breaks)
Life of a Low Mass Star Main-sequence Red Giant Stage Helium Burning Horizontal branch (helium burning carbon) helium flash (100 million K) + energy 3 4 He 1 12 C
Life of a Low Mass Star
Life of a Low Mass Star Main-sequence Red Giant Stage Helium Burning 2 nd Red Giant Stage Planetary Nebula White Dwarf
Dying Stars - HST
Life of a Low Mass Star HR Diagram of a Cluster
Low-Mass Star Summary 1. Main Sequence: H fuses to He in core 2. Red Giant: H fuses to He in shell around He core 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 4. Double Shell Burning: H and He both fuse in shells Not to scale! 5. Planetary Nebula: Leaves white dwarf behind
What are the life stages of a high mass star?
Massive Naked-eye Stars
15 Solar Mass Star H fusion core 10 million years
15 Solar Mass Star H fusion shell He fusion core 1 million years
15 Solar Mass Star H fusion shell He fusion shell C fusion core 1000 years
15 Solar Mass Star H fusion shell He fusion shell C fusion shell Ne fusion core
15 Solar Mass Star H fusion shell He fusion shell C fusion shell Ne fusion shell O fusion core 1 year
15 Solar Mass Star H fusion shell He fusion shell C fusion shell Ne fusion shell O fusion shell Si fusion core 1 week
15 Solar Mass Star H fusion shell He fusion shell C fusion shell Ne fusion shell O fusion shell Si fusion shell Iron core Iron core Diameter: 500 km Mass: 1.3 suns
The Origin of Elements 3 4 He 1 12 C + energy
Neutron star Shock wave rips through outer layers Supernova Energy released can briefly outshine an entire galaxy! Iron core forms Reaches Chandrasekhar s Limit (1.4 solar masses) Thermal pressure and electron degeneracy can no longer support star Core collapse Triggers a star-destroying explosion Neutron star forms
Fe Photo-disintegration e n p e e e e e n n n n p p p p p n n n n n n n n n n n n n Gamma-Rays p e n Electron Capture neutrino 1 H + e - + Energy neutron + neutrino Reaction absorbs energy
Main-sequence Brief but brilliant Hydrogen fusion CNO cycle radiation pressure strong fast moving interstellar winds Red Giant/Supergiant advance nuclear burning requires high temperatures (helium capture) Supernova Supernova remnant Life of a High-Mass Star
The Origin of Elements Model of Elemental Creation Oldest stars: metal poor (0.1 %) Youngest stars: metal rich (2-3%) Elements with even number protons outnumber those with odd number of protons. Elements heavier than iron extremely rare. 3 4 He 1 12 C + energy
The Crab Nebula - 1054 AD Supernova distance - 6,500 ly
Progenitor Star : B3 supergiant Supernova 1987A (Before and After)
SN 1987A - HST visible image
ALMA (red, mm-radio) Hubble (green, visible) Chandra (blue, x-ray)
Light curve of Supernova 1987A
Supernova 1987A
Supernova 1987A Neutrinos
Life Stages of High-Mass Star 1. Main Sequence: H fuses to He in core 2. Red Supergiant: H fuses to He in shell around He core 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 4. Multiple Shell Burning: Many elements fuse in shells Not to scale! 5. Supernova leaves neutron star behind
Exam 2 Thursday, March 8 About 60 Multi-guess Questions Text Chapters 5, 11-13 Lectures (online slides): 6-11 Study Hints (see web calendar) Review Quizzes (basic/visual/conceptual) Class notes/web slides Tutorials/web & text study aids Homework!!
The End.