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The Webfooted Astronomer - June 1999

 

The Making of a White Dwarf

by David C. Irizarry

What are white dwarf stars and where do they come from? We can define a white dwarf star as a very hot stellar remnant consisting of a sphere of degenerate gas resulting from the end stage evolution of a low- mass star. The Sun is destined to become a white dwarf at the end of its life.

Becoming a White Dwarf
When the Sun first became a star on the main sequence, its core was approximately 90% hydrogen and 10% helium. Hydrogen-to-Helium fusion is the predominant mechanism used for internal energy production. As it ages, the abundance of hydrogen within the Sunís core will decrease and the helium abundance will increase.

After about 10 billion years, the Sun will form a core of helium "ash," which at this stage is not hot enough to fuse into heavier elements. Eventually, the non-burning helium core begins to contract, releasing enough gravitational energy to heat the shell of hydrogen around the core. Hydrogen shell burning begins and becomes even more fervent as the helium core continues to contract. The excess energy put out by the star heats up the outer layers and the star swells into what is called a red giant.

At this point, the star is no longer on the main sequence, but has taken up position in the upper right corner of the H-R Diagram. Red giants can swell up to 100 times the current radius of the Sun, and although they have a lower surface temperature than our sun currently does, their larger size makes these giant stars hundreds of time more luminous than our Sun.

Continued contraction of the core eventually leads to a core density of 108 kg/m3, which causes the central helium core to ignite and begin fusing into carbon. At the onset of helium burning, a phenomenon known as the helium flash takes place within the starís core. Large quantities of free electrons are now being squeezed together within the core. The strong negative repulsion of these electrons exerts an outward pressure that acts to support the stellar core. Hydrostatic equilibrium eventually gives way to electron degeneracy pressure, which inhibits the expansion or contraction of the stellar core; thereby, inhibiting the cooling of the core.

As the temperature continues to rise, a runaway helium fusion reaction (the helium flash) takes place, which heats the core to the point where the thermal/pressure relationship is restored. The star is now steadily burning helium at the central core and fusing hydrogen within a shell around the core.

After a few tens of millions of years in this state, the helium in the central core is all burned to carbon in a nuclear reaction called the triple alpha cycle. The core further contracts, igniting a shell of helium around the carbon ash. A new hydrogen shell is formed around the helium shell, heating the starís outer layers even more and resulting in a red super giant star.

As the core continues to contract, it heats up again but doesn't get hot enough to fuse carbon into heavier elements. Electron degeneracy now supports the inner core while hydrogen and helium continue to burn in the outer shells of the stellar core. Helium shell flashes, coupled with energy freed by the recombination of electrons and nuclei near the outer layers of the star, liberate sufficient energy over time to blow off the starís outer layers into space forming a planetary nebula. Examples of such objects are M57 (Ring Nebula), M27 (Dumbbell Nebula), and M97 (Owl Nebula).

Such nebulae are illuminated by the remaining stellar core, which is no longer undergoing nuclear fusion reactions, but instead is shining due to its residual temperature. The planetary nebula shines because it absorbs high-energy ultraviolet radiation from the central star, which raises the energy level of electrons within the gas envelope. When the electrons return to their normal state, they emit photons at characteristic wavelengths, which cause the nebula to shine.

As the outer layers of the planetary nebula dissipate, the white dwarf, a sphere of ultra dense degenerate matter about the size of the earth, is left to cool over billions of years, eventually becoming a burned-out cinder known as a black dwarf. White dwarf stars have enormous densities with one cubic meter of material weighing in at 1010 Kg!

Well-Known White Dwarf Stars
Due to their small size, white dwarfs have relatively low luminosity; therefore, they can be difficult to observe. Amateur astronomers often attempt to glimpse the elusive central star of the Ring nebula (M57), which shines at an approximate visual magnitude of 15. Another nearby white dwarf is the companion to the bright star Sirius; however, consider yourself fortunate if you can glimpse this companion. Sirius, being extremely bright (visual magnitude -1.43) is certain to overwhelm the light of the nearby companion, Sirius B, which shines at apparent visual magnitude 8.65. The estimated separation between the two stars in 1999 should be about 4 to 5 arc seconds. Good luck in seeing this one!

References
Astronomy Today, Eric Chaisson & Steve McMillan, Prentice Hall, 1999

Burnhamís Celestial Handbook (Volume I-II), Robert Burnham Jr., Dover Publications, 1978

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