White Dwarfs
• Low mass stars are unable to reach high enough temperatures to ignite elements heavier than LECTURE 15: hydrogen in their core and become white dwarfs. Planetary nebulae reveal them. Can be He or CO. WHITE DWARFS Rarely NeOMg. AND THE ADVANCED EVOLUTION • Hot exposed core of an evolved low mass star. OF MASSIVE STARS • Supported by electron degeneracy pressure. M ≥ 8M • Unable to produce energy by contracting or igniting http://apod.nasa.gov/apod/astropix.html nuclear reactions
• As white dwarfs radiate energy, they become cooler and less luminous gradually fading into oblivion, but it can take a long time….
http://en.wikipedia.org/wiki/Ring_Nebula
A white dwarf is the remnant of stellar evolution for stars between 0.08 and 8 solar masses (below 0.08 one can have brown dwarfs). They can be made The Ring Negula in Lyra (M57) out of helium, or more commonly carbon and oxygen 700 pc; magnitute 8.8 (rarely NeOMg). MASS RADIUS RELATION FOR WHITE DWARFS if supported by GM ρ 13 5/3 Pc = = 1.00× 10 (ρYe ) non-relativistic electron 2R degeneracy pressure ⎛ 3M ⎞ GM ⎜ 3 ⎟ 5/3 5/3 GM ρ ⎝ 4π R ⎠ 13 ⎛ 3M ⎞ ⎛ 1⎞ ≈ = 1.00× 10 ⎜ 3 ⎟ ⎜ ⎟ 2R 2R ⎝ 4π R ⎠ ⎝ 2⎠ 2 5/3 5/3 ⎛ 3G ⎞ M 13 ⎛ 3 ⎞ M ⎜ ⎟ = 1.00×10 ⎜ ⎟ 5 ⎝ 8π ⎠ R4 ⎝ 8π ⎠ R 2/3 1/3 13 ⎛ 3 ⎞ 1 M = 1.00×10 ⎜ ⎟ ⎝ 8π ⎠ GR 19 1/3 R = 3.63×10 / M More massive 1/3 white dwarfs 8 ⎛ M ⎞ Earth R = 6370 km R = 2.9×10 cm are smaller ⎝⎜ ⎠⎟ M
Mass versus radius relation More massive white dwarf stars are denser
for the radius
How far can this go? Mass distribution!
Most WDs cluster around 0.6 M!. Narrow mass distribution" (most are fainter) White dwarfs are known with temperatures ranging from 4000 K to 200,000 K
Madej et al. 2004
Luminosity function!
The typical luminosity is 10-3 that of the sun but a broad range is observed,
LF of disk " white dwarfs" IK Pegasi A Class A star Maximum white dwarf mass P = 21.7 days • As mass increases, electron speed The sun approaches c. Pressure becomes due relativistic electrons. Proportional to 4/3
• Electron degeneracy cannot support a white dwarf heavier than 1.4 solar masses, the Chandrasekhar limit