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Homework #10:
Chp.14: Prob 1, 3
Chp. 15: Thought Question 1
Prob 1
Final Exam scheduled for May 22nd @
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Exam #3: average=70%
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Stellar Remnants:
White Dwarfs, Neutron Stars, & Black Holes
(Chp. 14)
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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.
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White dwarfs: remnants of solar-like stars
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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

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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.

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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
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In a binary system, a white dwarf may gravitationally
capture gas expelled from its companion
Result: Nova or Supernova (type I)
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Two types of supernovae: type I and II
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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

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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

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P=0.1 s
P=0.7 s
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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

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Neutron stars likely to be spinning rapidly.
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What generates the regular radio pulses?
Free electrons spiraling around magnetic field lines
emit radiation.
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