Supernovae, the tremendous explosions that are seen across the universe, are believed to have two origins: the gravitational collapse of a massive star, and the thermonuclear explosion of a white dwarf star. With this issue of the web site, pages describing the latter origin are added to the web site. The first page replaces an older page describing the basic theories behind these supernovae. The second page presents the energetics of thermonuclear supernovae.
The thermonuclear explosion of a white dwarf is believed to be the source of the type Ia supernova, which is a supernova with a spectrum that show no hydrogen lines but strong silicon lines. Type Ia supernovae are extremely valuable to observers, because they are standard candles; the luminosity of a type Ia supernova can be derived from its spectrum, and its distance can be derived by comparing its luminosity to its peak brightness. Type Ia supernovae are used by cosmologists to test the big bang theory for cosmological expansion by measuring how the velocity of the most distant galaxies changes with distance. Observations of type Ia supernovae suggest that the gravitational deceleration from the mass of the universe is offset to some extent by a repulsion provided by the vacuum itself.
This unexpected result in cosmology raises the question of whether type Ia supernovae remain standard candles at very large distances. The chemical composition of the early universe is much different from what it is today, with the early universe having a much lower concentration of elements heavier than helium. If the relationship between luminosity and spectral shape is somehow related to the chemical composition of a galaxy, the more distant type Ia supernovae could be less luminous than the nearby type Ia supernovae, causing the observed effect. This possibility drives much of the current theoretical research on these supernovae. In truth, though, theorists still have not sorted out which theory out of the three current theories is the correct theory for type Ia supernovae, so while cosmology drives much of the current interest in these events, uncertainty in the theory justifies the research.
Next Issue: The web site will next be updated some time after May 13.
Thermonuclear Supernovae. Most type Ia supernovae are attributed to the thermonuclear explosion of white dwarfs. A star becomes a white dwarf before it has completely consumed its thermonuclear fuel. The amount of thermonuclear energy locked within a white dwarf is of order 0.1% of the white dwarf's rest mass energy. Theorists have three theories that explain what triggers the release of this energy, with each theory relying on the white dwarf being a member of a binary system. The preferred theory is that the white dwarf grows in mass by pulling gas from its companion onto itself until it becomes gravitationally unstable; when the white dwarf collapses, its internal pressure and temperature rise until thermonuclear reactions cause it to explode. (continue)
Energetics of Thermonuclear Supernovae. The thermonuclear energy locked inside a white dwarf is sufficient to blow the star apart. In particular, white dwarfs composed of carbon and oxygen, which are more common and contain more thermonuclear energy than those composed of oxygen, neon, and magnesium, can release up to 0.1% of the star's rest mass energy as the carbon and oxygen are converted into an unstable isotope of nickel. The energy released in the explosion goes into expanding the debris from the white dwarf to velocities approaching 10% of the speed of light. The power we see radiated from a thermonuclear supernovae comes from the decay of radioactive nickel to iron. The light we see from a thermonuclear supernovae is about 10% of the energy released in the explosion, or 0.01% of the white dwarf's rest mass energy. (continue)