Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 1 Announcements Homework #10: Chp.14: Prob 1, 3 Chp. 15: Thought Question 1 Prob 1 Final Exam scheduled for May 22 12:15. Exam #3: average=70%

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 2 Stellar Remnants: White Dwarfs, Neutron Stars, & Black Holes (Chp. 14)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 4 White dwarfs: remnants of solar-like stars

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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 6 Structure of White Dwarfs White dwarfs are in hydrostatic equilibrium White dwarfs are in hydrostatic equilibrium Gravity is balanced by the pressure of electron degeneracy (no fusion!) 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 A white dwarf’s mass cannot exceed a certain limit (Chandrasekhar limit) – if it does, it will collapse M < 1.4 Msun!! M < 1.4 Msun!! A white dwarf’s high density (10 6 g/cm 3 ) implies that atoms are separated by distances less than the normal radius of an electron orbit. A white dwarf’s high density (10 6 g/cm 3 ) implies that atoms are separated by distances less than the normal radius of an electron orbit.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8 In a binary system, a white dwarf may gravitationally capture gas expelled from its companion Result: Nova or Supernova (type I)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 9 Two types of supernovae: type I and II

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 10 Neutron Stars A neutron star is one possible end state of a supernova explosion A neutron star is one possible end state of a supernova explosion Theoretically derived in the 1930s by Walter Baade and Fritz Zwicky Theoretically derived in the 1930s by Walter Baade and Fritz Zwicky Radius: 10 km (size of a city) Radius: 10 km (size of a city) Mass: times that of the Sun Mass: 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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 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 Over the next few months, more pulsating radio sources were discovered and eventually were named pulsars

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12 P=0.1 s P=0.7 s

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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 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 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 Such a size reduction is exactly what is expected of a collapsing massive star’s iron core

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 14 Neutron stars likely to be spinning rapidly.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 15 What generates the regular radio pulses? Free electrons spiraling around magnetic field lines emit radiation.

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