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Lecture 6

PCS 181 Lecture Notes - Lecture 6: Neutrino, Planetary Nebula, Carbon-Burning Process


Department
Physics
Course Code
PCS 181
Professor
Raffi Karshafian
Lecture
6

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Unit 6: Death of Stars
-stars die in several different ways
-manner of death is determined by star’s mass
Death of Lightweight Stars
-these are ≤ ≈ 4 Mo
-these are normal stars…95% of stars are in this category
1. core runs low on hydrogen fuel
-star leaves main sequence (=hydrogen fusion) phase
2. core contracts and at 100 x 106 K, star fuses Helium (3α12C) in core
3. star runs low on helium fuel in core which now consists of…
4. mainly Carbon and some Oxygen
5. gravity causes outer layers to collapse onto carbon core, squashing it and raising core
temperature
6. core temperature never becomes high enough to initiate carbon-based nuclear fusion reactions
7. computer simulations energy is released by gently blowing off star’s outer layers at ≈25
km/s
Planetary Nebula
-no more nuclear reactions in core of star
-star dies with a compressed carbon (some oxygen) core
-planetary nebula= dying/dead star remnant surrounded by a shell of ejected gases
-central remnant core= white dwarf…small, very hot, plots on lower left of HR diagram
Typical White Dwarf
-mass ≈0.8 Mo 1 Mo
-mass limit is 1.4 Mo, Chandrasekhar Limit
-size ≈ same size as Earth
-density ≈103 kg/cm3
-magnitude= M ≈ +11 (very dim)
-temperature ≈15 000 K on surface
-no longer has a central energy source
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