What are White Dwarfs?

White Dwarf

White dwarfs are the bare cores of low-mass stars such as the Sun. A low-mass Main Sequence star becomes a white dwarf when the star uses up all its hydrogen, swells, and ejects its outer layers.

If a white dwarf has a binary companion…

  • Mass-Transferring Binary Star System (e.g. white dwarf and red giant)
  • Gas “spills over” from red giant’s atmosphere and is gravitationally pulled into the white dwarf
  • Nova– explosion powered by fusion of hydrogen to helium on the surface of a white dwarf star; caused by matter spilling onto the star from its binary companion
    • Star brightens rapidly, then fades over weeks or months
    • Nova explosions can recur in the same binary system

Maximum Mass

  • S. Chandrasekhar(1930): calculated the maximum mass of white dwarf
    • Electron degeneracy pressure can only support a white dwarf less than 1.4 M☉
    • If a white dwarf accreted enough mass that overcomes this limit, gravity would win and something dramatic would happen
    • Chandrasekhar’s Limit: a white dwarf’s mass can only be less than 1.4M☉

After the Type Ia Supernova, the white dwarf is completely destroyed; no solid remnant is left, although the companion star might remain.

Type Ia Supernovae

  • Brightens over 2 weeks, reaches a peak, and then fades
  • At its peak, the supernova is 10 times more luminous than the Sun (e.g. 1994 D in galaxy NGC 4526)
  • Composed of mostly iron and other heavy metals
  • Core collapse supernova produces carbon, oxygen, neon, magnesium, silicon, and other lighter elements, and iron and other heavy elements
  • The ejected material is “recycled” into new generations of stars and planets

*Note: Without supernova explosions, earth-like planets, organic chemistry, and life wouldn’t exist.

  • All heavy elements were created inside stars or during supernova explosions, and then expelled into interstellar space
  • Heavier elements in supernova form by neutron capture: in an dense environment of  free neutrons, atoms absorb neutrons, beta (β) decays, and a proton forms — when an atom gains a proton, its identity changes and it moves one atomic number on the periodic table
  • Carbon, nitrogen, and oxygen are winners in the burning (release tons of energy)
  • Lithium, beryllium, boron are destroyed and not created in stars
  • Iron, the most stable element, is the end of nuclear burning
  • More massive elements formed by neutron capture followed by β-decay
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Evolution/ Death of Low-Mass Stars

Evolution of Low-Mass Stars

  • For stars on the main sequence, luminosity is proportional to the star’s mass to the 3.5th power (L α M3.5)
  • For the Sun: original composition about 30% helium in mass –> today – 65% helium –> 5 billion years later: 100% helium
  • The total amount of hydrogen “fuel” in a star is proportional to the star’s mass; the rate of fuel use is proportional to luminosity; lifetime α mass α 1/L
  • The Sun’s main sequence lifetime is 10¹º years; its entire lifetime is 10¹º + 1/M2.5 years, where M = the star’s mass in solar masses

How Lifetime, Luminosity, and Mass Compare Among Various Spectral Types

Spectral Type  Mass (M☉)   Luminosity (L☉)  Lifetime (years)

  • O5                        60                          800,000                        3 million
  • A5                         3                                55                              4 million
  • G2                        1                                  1                             10 billion
  • M0                      0.5                           0.08                             70 billion
  • M5                      0.2                           0.01                           190 billion

Stars with masses below 0.8 M☉ have never left the main sequence and have lifetimes longer than the current age of the Universe.

Post- Main Sequence

  • Core can’t maintain its balance between gravity and pressure; gravity compresses the star to a much smaller size
  • Electron Degeneracy Pressure: halts collapse of the star
  • Core’s radius swells to several thousand km, or about the size of Earth
  • Hydrogen converts to helium at  a very rapid rate; luminosity more than 1000 times greater than before; star swells to enormous size

Red Giant

Red Giants: 50x Sun’s radius, 1000x Sun’s luminosity

  • H –> He in a shell
  • Helium core swells to the size of Earth; no fusion anymore
  • Non-burning hydrogen atmosphere
  • Helium fusion needs higher velocities and energy to overcome repulsion
  • At 100 million K, helium atoms yield carbon atoms, also known as “helium flash”
  • Triple-Alpha Process“: 4He + 4He –> 8Be; 4He + 8Be –> ¹²C
  • In half an hour, half the helium yields carbon in the core
  • Horizontal Branch Star: after the core expands and the star enters a steady phase (50-100 million years) of helium burning and becomes less luminous

    Evolution of Low-Mass Stars: H-R Diagram

  • Core contracts again, He –> C in a shell around the core; hydrogen burning shell around that layer
  • Asymptotic Giant Brand“: star moves upward toward the H-R Diagram “Red Giant” area, exceeds 10,000 L☉

*Blue-Stragglers: stars in a dense environment; when two stars collide, the core could be “rejuvenated,” giving the star extra lifetime

Planetary Nebula

Planetary Nebulae

  • The outer layers, about 20% of the star’s mass, are ejected in a strong wind
  • Gas is ionized by UV protons from hot, exposed stellar core
  • Star shines for 50,000 years before gases disperse and fade
  • About 1,000 in the Milky Way Galaxy
  • Have many shapes and sizes because of binary star systems’ different orbits, temperature, rotation, luminosity, mass

Binary System: Sirius A (brighter, Main Sequence) & Sirius B (dimmer, white dwarf)

White Dwarfs:  (0.6 – 0.7 M☉) bare core of a star often all fusion reactions ended, supported against gravity by electron degeneracy pressure; density at 1 million grams/cm³

  • e.g. Sirius A: Main Sequence, A1, -1.5; Sirius B: white dwarf, 8.5

End States: Initial Mass and White Dwarf Composition

  • >0.45 M☉: helium
  • 0.45-4 M☉: carbon, oxygen –> (Sun)
  • 4-8 M☉: oxygen, neon, magnesium