Birth and Death of Stars. Birth of Stars. Gas and Dust Clouds. Astronomy 110 Class 11

Transcription

1 Birth and Death of Stars Astronomy 110 Class 11 Birth of Stars Start in cloud of gas and dust Contraction and Fragmentation Gravitational collapse and heating Protostar and disk Main Sequence Star Gas and Dust Clouds 1

2 Gravity Versus Thermal Motion Gravity pulls inward Thermal Motion (heat) resists Trigger to Collapse Shock wave from exploding star? Fragmentation The cloud splits up into smaller blobs which ultimately form individual stars 2

3 Collapse in progress Size decreasing Density increasing Radiation leaks out A protostar Protosun Collapse increases spinning Spinning causes flattening Planetary Disks Stars and disks in creation Protostars formed Glowing balls of gas Powered by gravitational collapse No nuclear reactions yet 3

4 Compression causes Heating As more gas is forced into a smaller volume The gas temperature rises What stops collapse? Gravity pulls inward Temperature builds up Thermal pressure pushes outward Protostars Newly formed Stars Glowing balls of gas Powered by gravitational collapse No nuclear reactions yet 4

5 A star is born When central temperature rises to 10 7 K Nuclear reactions start The star arrives on the Main Sequence Forces in Nature Gravity (weakest) Between all mass Electrical (medium) Between charged particle Nuclear (strongest) Between nucleons Energy Sources Gravitational Collapse Source of initial heating Lasts only 30 Million years for Sun Chemical sources e.g. burning much too weak relies of electron forces which are much too small Nuclear Fusion The joining together (fusing) of atoms Proposed in the 1930s 5

6 Atoms and Nuclei Proton + Neutrons nuclei Electrons orbit #protons == # electrons Elements Number of protons determines element Also known as Atomic Number Hydrogen 1- proton Helium 2- protons Carbon 6- protons Neutrons Protons repel Nuclear force attracts Needs neutrons as glue Neutron as massive as proton Total particles = atomic weight 6

7 Ions Atoms with electrons removed No longer electrically neutral Ions occur at high temperature Isotopes Same element may have different number of neutrons Neutrons act as nuclear glue These are isotopes Total number of particles determines atomic weight Nuclear Fusion Fusion means joining together Joins together elements to form new elements Difficult because protons don t like each other Occurs only at very high pressure and temperature Very hard to do on Earth 7

8 Nuclear Fusion Combine Hydrogen to form Helium Missing Mass Combine Hydrogen to form Helium 0.7% less mass Atomic Weight H He * H = E=mc2 Proposed by Einstein Mass can be converted energy Energy can be converted mass Missing mass in fusion is energy 8

9 Nucleosynthesis Generation of new elements This is not possible in chemistry Energetically possible up to Iron Takes very high temperature Proton-proton cycle Proposed in the 1930s Occurs in our Sun Generates Gamma rays light Neutrinos escape Proton-proton Details 9

10 Hotter Stars i.e. more massive stars Still nuclear fusion But different (CNO) cycle generates elements Iron Main Sequence Nuclear burning is prime of life Star on Main Sequence Elements beyond Iron Elements with atomic weight greater than Iron are not made in stars Created in supernova explosions 10

11 Brown Dwarfs Jupiter Mass Too small to be star Internal temperatures are too low A star is a battle with Gravity The mass on the outside Is supported by pressure on the inside What supplies the pressure? Heat is needed Prime of Life Main Sequence Nuclear burning of hydrogen Really nuclear reactions convert all the hydrogen 11

12 Heavy Element Fusion Energy is released by fusion of elements lighter than Iron However higher temperatures are needed because nuclear repulsion is stronger All Stars will Run Out of Fuel Two fates depending on Mass Lightweight stars less than 10 M sun Heavyweight stars greater than 10 M sun Sun is a Lightweight Fate of Lightweight Stars Stars with mass less than 10 M sun The Sun is a lightweight 12

13 Fuel Exhaustion Runs out in core Burning in shell Core can fuse He Carbon He Oxygen Fate of the Earth Red Giants Sun will grow 20 times bigger Convert He Carbon and Oxygen Live 1 Billion years Like Arcturus (α-boo) Aldebaran (α-tau) 13

14 Planetary Nebula Core collapses Envelope expands Shoots off gas Hot (100,000K) star Glowing gas shell Lasts ~50,000 years Planetary Nebula Glowing gas Excited by UV radiation Called planetary as some look like Uranus White Dwarf Predicted by Chandrasekhar Fuel is gone Collapse to tiny star Cools Mass can t be over 1.4 M sun 14

15 Electron Degeneracy Two electrons can t be pushed into the same space They create a sort of pressure Enough to hold a star without needing heat White Dwarf Material Very dense (teaspoon is a ton!) Maximum star mass is 1.4 M sun White Dwarf Cool They don t generate energy They just cool to oblivion 15

16 Nova and Supernova White Dwarf Matter added from companion star If little added it brightens If over 1.4 M sun BOOM! 16