Requiem for a Star

1. Requiem for a Star Stellar Collapse

2. Gravity Nebula Star Nebula Gravity is an inexorable force always trying to cause further collapse Nebulae → Protostars Protostars → Main Sequence Stars Main Sequence Stars → ? At each stage stability is only achieved if there is a balancing force. Protostar

3. Main Sequence Stars • Force of gravity is balanced by internal pressure of star • Pressure increases with depth • Internal pressure is maintained by heat produced by the fusion of light elements into heavier elements • If fuel is exhausted then internal pressure is not maintained, and gravity causes collapse of star

4. Main Sequence Stars Question Will anything other than hydrostatic equilbrium stop inevitable collapse? It all depends on the mass

5. Options • Fusion of elements other than hydrogen • Electrons • Neutrons • Nothing

6. Fusion Reactions • 4 H → He • 3 He → C • 2 C → Mg • C + He → O • O + He → Ne • O + O → Si • O + He → Ne • Si + 7 He → Ni • Ni → Co (radioactive decay) • Co → Fe (radioactive decay) • Cannot have fusion reactions for elements above Fe

7. Fusion Reactions • Fusion of heavier elements becomes increasingly difficult • Impossible for elements above iron • Requires increasingly higher temperatures and pressures • Requires more gravitational collapse = more mass • All stars must eventually run out of fuel

8. Stars Comparable to Sun • M up to about 3 or 4 solar masses • As a Main Sequence star can only use hydrogen as a fuel • When hydrogen is exhausted collapse of interior is inevitable • Increase in temperature caused by collapse suddenly ignites unprocessed hydrogen, causing star to expand to become red giant

9. Stars Comparable to Sun • Red Giant star pushes its outer atmosphere into space • Planetary nebula • Unused hydrogen • Helium residue in core ‘ignites’ and rapidly turns to carbon • Carbon core cannot be used. Remains as a hot carbon remnant, slowly cooling in space • White dwarf Ring Nebula

10. White Dwarf Stars What causes further collapse? • Electrons • Electron degeneracy • Can only be packed so far • Prevents further collapse when white dwarf is approximately the size of the Earth • Only possible if the mass is less than about 1.4 solar masses • Chandrasekhar limit • Above 1.4 solar masses electrons are not strong enough to balance out gravity

11. Heavier stars • If mass > 3-4 solar masses the core left behind would be larger than Chandrasekhar limit • Cannot use electrons to stabilize collapse of star • Cannot form white dwarfs • Can use elements heavier than hydrogen as fuel during main sequence life • Onion like structure

12. Cores of Heavier Stars He C O Si Fe

13. Final products fromDeath of Heavier Stars • M < 3-4 solar masses • Core less than 1.4 solar masses • Collapse stabilized by electrons • White dwarf • 3 < M < 6 solar masses • Core between 1.4 and 3 solar masses • Elements disappear, only neutrons remain • Collapse stabilized by neutrons • Neutron star = Pulsar • M > 6 solar masses • Core greater than 3 solar masses • Collapse cannot be stabilized • Continues indefinitely • Black hole

14. ← Black hole

15. Main sequence Star Red Giant Red SuperGiant Red SuperGiant Planetary Nebula Black Hole Planetary Nebula White Dwarf Planetary Nebula Neutron Star M = 3 M = 6