Mid-Sized Stars (con't.)
This H-R diagram the shows the evolutionary track of the Sun, which is halfway through its lifetime of ~9 billion years on the main sequence. It is a spectral type G star, has an effective surface temperature of ~5800K, and one solar luminosity. When the Sun runs out of hydrogen fuel in its core and fusion stops, it will begin its journey to the red giant branch. The Sun will contract, heat up until a shell of hydrogen is fusing around the helium core, and become cooler, ~3000K, reddish in color, and more luminous - in excess of 500 solar luminosities. After ~one billion years, the hydrogen shell fusion stops and the Sun contracts again, becoming less luminous, hotter, and less red in color. During this phase it is sometimes referred to as a yellow giant. The contraction will cause the core to heat up until helium fusion begins in the core. The fusion of helium nuclei to carbon nuclei causes the Sun to expand again, becoming more luminous. The core will contract again when it runs out of helium and fusion stops again; this time there is not enough mass for the shrinking core to achieve the temperature necessary for the fusion of carbon to begin. The Sun will throw off its outer atmospheric layers into a planetary nebula and the remaining carbon core - called a white dwarf - will then reside on the white dwarf branch of the H-R diagram. The white dwarf is very dim and very hot - with a temperature of ~20,000K. The white dwarf will radiate away its heat over the next ~12 billion years and become a burnt out carbon cinder called a black dwarf.
Accretion Disk Binary System
Credit: ST ScI, NASA
Animation: Triggered Star Formation
Credit: JPL-Caltech
A white dwarf is not the end product of stellar evolution of a mid-sized star if it is in a binary system. Suppose two stars, one with one solar mass and the other with five solar masses are in a binary system. The five solar mass star runs out of hydrogen faster than its less massive companion, becomes a red giant, shrugs off a planetary nebula, and collapses into a white dwarf. Eventually the companion star runs out of hydrogen and enters the red giant stage. The outer layers of the red giant are loosely held by the star, and the extreme gravitational field of the white dwarf starts pulling the material from the red giant into an accretion disk around the white dwarf. The mass transfer continues, with the material orbiting the white dwarf in the accretion disk. Friction slows the matter's orbital motion, which causes the matter to spiral through the disk down to the surface of the white dwarf. The falling and spiraling of the matter toward the white dwarf releases large amounts of gravitational energy and heats the accretion disk.
Tycho's SNR
Credit: NASA/CXC/SAO
The white dwarf is predominately carbon and oxygen, and accretes matter from its companion relatively rapidly. Consequently, the white dwarf grows in mass. When the accretion has raised the white dwarf's mass to the critical mass of 1.4 solar masses, the density and temperature in the center of the white dwarf become so severe that carbon starts fusing explosively. Within one second the fusion front moves all the way to the surface, making the entire white dwarf one huge nuclear catastrophic event. The white dwarf explodes and is completely destroyed. There is no stellar remnant. All of the core's matter - namely, the products of the nuclear fusion (iron, nickel, silicon, magnesium, and other heavy elements) plus remaining carbon and oxygen - are ejected into space at speeds upwards of 48,000,000 km/hr. Tycho's supernova remnant is the result of a Type Ia supernova event; the core was completely destroyed by the explosion.
Revised: March 19, 2008