• During helium shell flashes ejection of outer
atmosphere regions
Planetary Nebula
• In the exposed stellar core, no nuclear reactions
• It becomes a dense sphere about the size of the
Earth and is called a white dwarf
• It is so dense that electrons are degenerate
• The degenerate-electron pressure supports the
star against further collapse
• There is still thermal radiation
• As the sphere cools, it becomes dimmer
• One teaspoon white dwarf matter weighs about
5.5 tons (density about 109 kg/m3)
Burned-out core becomes a White
Dwarf
• The Chandrasekhar Limit is the upper limit of the
mass a white dwarf can have
• The limit is 1.4 M
• Beyond this limit, the degenerate electron
pressure can no longer hold the gravitation
contraction
Chandrasekhar Limit
• The more massive a white dwarf, the stronger
the gravitation, and the smaller the size
White Dwarfs
White Dwarfs
Optical
X-ray
• Up to 20% of the “visible mass” in the solar
neighbourhood are white dwarfs
Sirius
White Dwarfs
3.0 M =>
1.2 M
1.5 M =>
0.8 M
0.8 M =>
0.6 M
• Types of white dwarfs
White Dwarfs