Stellar Remnants: White Dwarfs, Neutron Stars & Black Holes - Compact Star States

1. Announcements Homework #10: Chp.14: Prob 1, 3 Chp. 15: Thought Question 1 Prob 1 Final Exam scheduled for May 22nd @ 12:15. Exam #3: average=70%

2. Stellar Remnants: White Dwarfs, Neutron Stars, & Black Holes (Chp. 14)

3. Introduction There are three end states of stars, all of which are known as compact objects: 1. White Dwarf an ice-cube of WD material would weigh about 16 tons. 2. Neutron Star a dime-sized piece of neutron star would weigh as much as 400 million SUV’s. 3. Black Hole a lot of mass in an infinitely small volume.

4. White dwarfs: remnants of solar-like stars

5. White Dwarfs • Mass: similar to the Sun’s • Diameter: about that of the Earth • Hot (at least initially): 25,000 K; Dim (very small) • Light they emit comes from heat (blackbody) • Carbon and Oxygen; thin H/He surface layer • White dwarf will cool over time (many billion of years) until it becomes a black dwarf emitting no visible light • Very-low mass stars (0.4-0.5 M¤) become white dwarfs on a time scale longer than the Universe’s age

6. Structure of White Dwarfs • White dwarfs are in hydrostatic equilibrium • Gravity is balanced by the pressure of electron degeneracy (no fusion!) • A white dwarf’s mass cannot exceed a certain limit (Chandrasekhar limit) – if it does, it will collapse M < 1.4 Msun!! • A white dwarf’s high density (106 g/cm3) implies that atoms are separated by distances less than the normal radius of an electron orbit.

7. Would you weigh more or less on a white dwarf compared to what you weigh here on Earth? a) more b) less c) this is a trick question; actually, I would weight exactly the same on a white dwarf because of its size. How much more? M=300,000 times the mass of the Earth R= radius of the Earth

8. In a binary system, a white dwarf may gravitationally capture gas expelled from its companion Result: Nova or Supernova (type I)

9. Two types of supernovae: type I and II

10. Neutron Stars • A neutron star is one possible end state of a supernova explosion • Theoretically derived in the 1930s by Walter Baade and Fritz Zwicky • Radius: 10 km (size of a city) • Mass: 1.4-3 times that of the Sunf • Because of their small size, they were thought to be unobservable small size, neutron stars were thought to be unobservable

11. Pulsars and the Discovery of Neutron Stars • In 1967, Jocelyn Bell, a graduate student of Anthony Hewish, detected an odd radio signal with a rapid pulse rate of one burst per 1.33 seconds • Over the next few months, more pulsating radio sources were discovered and eventually were named pulsars

12. P=0.1 s P=0.7 s

13. Pulsars and the Discovery of Neutron Stars (continued) • The key to explaining pulsars turned out to be a rotating neutron star, not a pulsating one • By conservation of angular momentum, an object as big as the Sun with a one-month rotation period will rotate more than 1000 times a second if squeezed down to the size of a neutron star • Such a size reduction is exactly what is expected of a collapsing massive star’s iron core

14. Neutron stars likely to be spinning rapidly.

15. What generates the regular radio pulses? Free electrons spiraling around magnetic field lines emit radiation.