The Stellar Graveyard

1. The Stellar Graveyard AST 112

2. Review: Stellar Evolution(Low Mass)

3. Review: Stellar Evolution(High Mass)

4. Stellar Remnants • White Dwarfs • Supported by electron degeneracy pressure • Left over from low mass stars • What are they made of? • Neutron Stars • Supported by neutron degeneracy pressure • Left over from high mass stars • Black Holes • State of matter is not well understood • Left over from really high mass stars

5. White Dwarfs • Exposed core of expired low mass star • Outer layers of the star were expelled as a planetary nebula White dwarf

6. White Dwarfs • Small (~Size of Earth) • Massive (~1 Sun) • Dense • 1 teaspoon weighs as much as a truck • Made of carbon • Hot • Some shine in UV and X-ray • Sirius is a binary system • Sirius A: Brightest star in the sky • Sirius B: White Dwarf • Which is which? Photo from Chandra X-ray Observatory

7. White Dwarfs • No fusion • Can’t fuse the carbon • Electron degeneracy pressure: • Electrons can’t cram together any closer • Supports the star against gravity

8. Chandrasekhar Limit • Compress a white dwarf, electrons go faster • Eventually the electrons approach the speed of light • Nothing can exceed the speed of light • At 1.4 MSun, electrons would reach the speed of light • No white dwarfs have ever been observed with masses > Chandrasekhar Limit

9. Accretion Disks • White dwarf by itself: • Slowly fades away • White dwarf with a friend: • More interesting! • Steals mass from its friend

10. Accretion Disks • Mass starts off with small rotation • As it falls in: • Conservation of angular momentum makes it rotate faster • Strong gravity of white dwarf makes disk spin fast • Faster rotation on inside, slower on outside • LOTS of friction; the disk gets very hot

11. Nova • White dwarf has strong gravity • If it is accretes enough H, can fuse the H • Nova: burst of fusion blows the layer off! • 100,000x weaker than a supernova

12. White Dwarf Supernova • Even with the recurring novas, white dwarfs gain mass • Can suddenly fuse the carbon! • Just before Chandrasekhar Limit (1.4 MSun) • “This day is the white dwarf’s last.” ** Sometimes called a “carbon bomb”

13. Type 1a Supernova and Distance • White Dwarf supernovae are also called Type 1a Supernovae • Because they are thought to occur when a white dwarf begins carbon fusion, we are always dealing with the same 1.4 MSun ball of carbon and therefore the same explosion! • By looking at the brightness of a Type 1a Supernova, we can figure out how far away it is. • They are bright; we use these to measure distances across the universe

14. Neutron Stars • An iron core that collapsed into neutrons • Product of a supernova • Around 10 miles in diameter • Spinning FAST! • Supported by neutron degeneracy pressure • DENSE!

15. Neutron Star Gravity BIG

16. Neutron Star Gravity • A teaspoon of neutron star weighs 10 million tons • If it ran into Earth, Earth would be “squashed into a shell no thicker than your thumb on the surface of the neutron star”. • A paper clip with the density of a neutron star would weigh as much as Mount Everest

17. Neutron Stars: Pulsars • Jocelyn Bell built a radio telescope to study fluctuating radio sources • She noticed a periodic signal in Cygnus • Precisely 1.337301 seconds apart! • Actually thought to be aliens

18. Neutron Stars: Pulsars • Two of these objects were found in the Vela Nebula and the Crab Nebula (supernova remnants) • So they’re neutron stars • But why don’t they all have this periodic signal?

19. Neutron Stars: Pulsars • Neutron stars have strong, narrow cones of radiation • Not always pointed along axis of rotation • If the cones sweeps across us, it’s a pulsar

20. Neutron Stars: Pulsars • Just like a lighthouse. • All pulsars are neutron stars, but not all neutron stars are pulsars.

21. Neutron Stars: Pulsars • Why does it spin so fast? • Sun spins once every 26 days • The neutron star (pulsar) spins up to 625 times per second! (fastest) • Conservation of angular momentum: • It went from 109 Earths across to 5 miles across

22. Black Holes • Gravity wins. Hands down. • No thermal pressure. No electron degeneracy. No neutron degeneracy. • This is a frontier of physics. It is NOT simple.

23. Black Holes • Gravity is so strong that light cannot escape • Light does not respond to gravity like, say, a ball. • It responds to space-time being curved. It follows what it thinks is a straight path.

24. Black Holes • Event Horizon: • Anything inside the Event Horizon must move faster than light to escape. • Nothing moves faster than the speed of light. • (Think about the name! We can’t see events beyond the event horizon.)

25. Black Holes • A solar-mass black hole is about 2 miles across • A single, infinitely dense point at the center of the event horizon

26. Trip to a Black Hole • Say we visit one. • 1000 miles away: • Throw a blue clock out the window: • Slows down, turns red! • At 5 miles from event horizon, ticks half as fast • Near event horizon: • Clock pretty much stops • Blue light has shifted to radio frequencies!

27. Trip to a Black Hole • Your friend wants to go inside! • Falls toward event horizon. • Feels time passing normally • Sees YOUR time going fast. Also sees you turning blue.

28. Trip to a Black Hole • Friend’s point of view: Your friend passes through the event horizon • Your point of view: Your friend approaches the event horizon more and more slowly • Their time slows from your perspective • Right at the event horizon, takes an infinite amount of time for anything to happen; they just hang there • They disappear because the light is infinitely red-shifted

29. Trip to a Black Hole But really, did your friend survive the tripinto the event horizon?

30. Observing Black Holes • The night sky is black. A black hole is black. • …? • Look for X-ray binaries with unseen companions • Measure mass of unseen companion; exceeds allowed neutron star mass