Sunday, April 21, 2013

Jump Starting Stellar Evolution


Those familiar with science fiction know that stars play a key role in the setting, lore, and sometimes even the plot of almost all science fiction. This makes them an object of fascination for those who dream of one day sailing through the heavens (like me). However current stellar evolution theory seems to possess a fatal flaw that prevents it from being viable, thus requiring a tweak to the theory.

Conventional Theory
As it stands at the moment the theory of stellar evolution follows a simple formula for the life cycle of a standard (main sequence) star.
1. Proto-Star Formation – The first stage in the stellar evolution process is the formation of the proto-star within a nebula. This process involves the gravitational collapse of gas and dust within the nebula, typically spread out over the area of about 100 light-years. This process can take a long time, but once it’s begun and the proto-star grows in size the pressures generated by the gas and dust begin to heat the star
The Sun (Sol)
2. Main Phase – Once the stars core reaches 10,000,000° Kelvin the stars gravitational pressure is high enough to start forcing small atoms to combine into larger atoms (a process called fusion). This occurs first with hydrogen atoms which are fused into deuterium in very early stages and then later into helium. Depending on the amount of hydrogen the star begins with the life of the star in this phase can extend from only a few million years to hundreds of billions of years. Counter to what seems the logical conclusion, the larger stars with more fuel (and thus more mass and pressure) actually burn more quickly and thus have shorter lives than do those that are smaller which burn more slowly. Our sun is expected to remain in its main phase for roughly 10 billion years.
3. Red Giant – A typical mass star at the end of its main phase will move into the red giant phase of its life where hydrogen fusion only occurs in a shell around the core of the star and the sudden collapse of material in the star’s core due to the stalling of hydrogen fusion creates enough pressure and heat to begin helium fusion combining helium atoms to form oxygen and carbon. Because of the extra energy being released from the fusion of these heavier elements the star expands and brightens as the outward force of the fusion attempts to reach equilibrium with the inward force of the star’s gravity. It is predicted that this will be the fate of our star at the end of its main phase.
4. White Dwarf – Upon using up all the fuel that the star is large enough to fuse it will collapse upon itself (though not enough to create a neutron star or black hole) creating an extremely dense solid mass called a white dwarf. These objects are no longer producing energy through fusion, but simply radiating their remaining heat out into space, eventually becoming black dwarfs upon having radiated all their remaining energy, assuming no intervention of additional mass occurs.

High Mass Stars
A high mass star tends to work a bit differently. As they have much higher temperatures and pressures these stars finish the main phase portion of their lives much faster and move to the giant phase very quickly (for a star). They typically will not become red giants as they’re burning too hot for the outer shell to cool enough for the color shift to the red spectrum. These stars have enough pressure to move on to oxygen and carbon fusion continuing to produce heavier and heavier elements until they reach iron. Once these stars start creating iron in their cores the fusion process will stall out and one of three results will occur depending upon the star’s mass.
1. White Dwarf – The smallest of the high mass stars will cast off its outer layers in a nova and become a white dwarf, ending its life much the same way as a lower mass star.
Simulated image of a supernova
2. Neutron Star – The next possibility for a larger star is that it will cast off its outer layers in a massive supernova explosion and its core will contract so much that the electrons and protons of the atoms composing the core will actually combine forming a large super dense object composed entirely of neutrons.
3. Black Hole – For the most massive stars their fate is that of the black hole. Like the less massive stars these will cast off their outer layers in a massive supernova explosion with the remnants collapsing down upon themselves with such force that they become compressed into a singular point called a singularity. The mass of these objects is so great that anything that gets too near, even light falls victim to the intense gravitational collapse.

The Glitch in the Theory
Over all, despite never having observed the ignition phenomenon of a newly forming star (due to long time spans involved in the proto-star phase and obscuring nebula clouds) the theory of stellar evolution is sound and verifiable in computer models. However, though I’m by no means an astrophysicist, it seems to me that the laws of thermodynamics contradict the beginnings of the stellar evolution process. As gravity pulls the gas and dust together the increase in pressure causes its temperature to rise, which thus forces it back apart. Because of this, it seems likely that some type of catalyst is necessary to kick off the proto-star formation process; a chunk of matter dense enough to pull in the gas and dust with enough force to overcome the expansive force that their coalescence would cause. Of course correct me if I’m wrong, but it seems to me that these “star seeds” need to be added to the theory in order to make it function in reality as advertised.

Conclusion
While I feel it’s true the theory of stellar evolution could use a minor tweak, it seems like a solid working theory that deserves the acknowledgement and acceptance that it receives from the scientific community. I am pleased at the fact that it’s found its way into popular science fiction and as such, also has a place in my N3rd C0rn3r.