| The actual process of star formation remains | | | | 5. Post Main Sequence |
| shrouded in mystery because stars form in dense, | | | | · Age: About 1 billion years from Point |
| cold molecular clouds whose dust obscures newly | | | | 4 |
| formed stars from our view. For reasons which are | | | | · R ~ 2.6Rsun |
| not fully understood, but which may have to do with | | | | · Tsurface = 4500K |
| collisions of molecular clouds, or shockwaves passing | | | | · Energy Source: P-P Chain in shell, |
| through molecular clouds as the clouds pass through | | | | Gravitational contraction of core. |
| spiral structure in galaxies, or magnetic-gravitational | | | | 6. Red Giant - Helium Flash |
| instabilities (or, perhaps all of the above) the dense | | | | As the Helium core of the star contracts, nuclear |
| core of a molecular cloud begins to condense under | | | | reactions continue in a shell surrounding the core. |
| its self-gravity, fragmenting into stellar mass clouds | | | | Initially the temperature in the core is too low for |
| which continue to condense forming protostars. As | | | | fusion of helium, but the core-contraction liberates |
| the cloud condenses, gravitational potential energy is | | | | gravitational energy causing the helium core and |
| released - half of this released gravitational energy | | | | surrounding hydrogen-burning shell to increase in |
| goes into heating the cloud, half is radiated away as | | | | temperature, which, in turn, causes an increase in the |
| thermal radiation. Because gravity is stronger near the | | | | rate of nuclear reactions in the shell. In this instance, |
| center of the cloud (remember Fg ~ 1/distance2) the | | | | the nuclear reactions are producing more than enough |
| center condenses more quickly, more energy is | | | | energy to satisfy the luminous energy output. This |
| released in the center of the cloud, and the center | | | | extra energy output pushes the stellar envelope |
| becomes hotter than the outer regions. As a means | | | | outward, against the pull of gravity, causing the outer |
| of tracking the stellar life-cycle we follow its path on | | | | atmosphere to grow by as much as a factor of 200. |
| the Hertzsprung-Russell Diagram. | | | | The star is now cool, but very luminous - a Red |
| 1. Protostar | | | | Giant. |
| The initial collapse occurs quickly, over a period of a | | | | (You do the arithmetic: 200 x 700,000km = ?; where |
| few years. As the star heats up, pressure builds up | | | | will the outer radius of the sun be?) |
| following the Perfect Gas Law: | | | | · Age: 100 million yrs from Point 5 |
| PV = NRTwhere, most importantly P=pressure and | | | | · R ~ 200Rsun |
| T=Temperature. The outward pressure nearly | | | | · Tcore = 200,000,000K |
| balances the inward gravitational pull, a condition called | | | | · Tsurface = 3500K |
| hydrostatic equilibrium. | | | | · Energy Source: P-P Chain in shell |
| · Age: 1--3 yrs | | | | around core; |
| · R ~ 50 Rsun | | | | Ignition of Triple-Alpha Process. |
| · Tcore = 150,000K | | | | The contraction of the core causes the temperature |
| · Tsurface = 3500K | | | | and density to increase such that, by the time the |
| · Energy Source: Gravity | | | | temperature is high enough for Helium nuclei to |
| The star is cool, so its color is red, but it is very large | | | | overcome the repulsive electrical barrier and fuse to |
| so it has a high luminosity and appears at the upper | | | | form Carbon, the core of the star has reached a |
| right in the H-R Diagram. | | | | state of electron degeneracy. Degeneracy comes |
| 2. Pre-Main Sequence | | | | about due to the Pauli Exclusion Principle, which |
| Once near-equilibrium has been established, the | | | | prohibits electrons from occupying identical energy |
| contraction slows down, but the star continues to | | | | states. The core of the Red Giant is so dense that all |
| radiate energy (light) and thus must continue to | | | | available lower energy states are filled up. Because |
| contract to provide gravitational energy to supply the | | | | only high-energy states are available, the core resists |
| necessary luminosity. The star must continue to | | | | further compression -- there is a pressure due to the |
| contract until the temperatures in the core reach high | | | | electron degeneracy. This pressure has a significant |
| enough values that nuclear fusion reactions begin. | | | | difference from pressure produced by the Ideal Gas |
| Once nuclear reactions begin in the core, the star | | | | Law -- it is independent of temperature. This |
| readjusts to account for this new energy source | | | | removes a key element in the feedback-stability |
| Gravity releases its potential energy throughout the | | | | mechanism that regulates hydrogen burning on the |
| star, but due to the very high temperature | | | | main sequence. |
| dependence of the nuclear fusion reactions, the | | | | H-R Diagram from Helium Burning to White Dwarf. |
| proton-proton chain is highly centrally concentrated. | | | | 7. Helium Burning Main Sequence |
| During this phase the star lies above the main | | | | Once again the core of the star readjusts to allow |
| sequence; such pre-main sequence stars are | | | | for a new source of energy, in this case fusion of |
| observed as T-Tauri Stars, which are going through a | | | | Helium to form Carbon via the Triple-Alpha Process. |
| phase of high activity. Material is still falling inward | | | | The Triple alpha process releases only about 20% as |
| onto the star, but the star is also spewing material | | | | much energy as hydrogen burning, so the lifetime on |
| outward in strong winds or jets as shown in the HST | | | | the Helium Burning Main Sequence is only about 2 |
| Photo below. | | | | billion years. |
| · Age: 10 million yrs | | | | · Age: About 10,000 yrs from point 6. |
| · R ~ 1.33 Rsun | | | | · Tsurface = 9000K |
| · Tcore = 10,000,000K | | | | · Tcore = 200,000,000K |
| · Tsurface = 4500K | | | | · Energy Source: Triple-alpha process in |
| · Energy Source: P-P Chain turns on. | | | | core; |
| 3. Zero Age Main Sequence | | | | P-P Chain in shell |
| It takes another several million years for the star to | | | | During this phase some Carbon and Helium will fuse |
| settle down on the main sequence. The main | | | | 12C + 4He --> 16Oresulting in the formation of a |
| sequence is not a line, but a band in the H-R Diagram. | | | | Carbon-Oxygen core. When the Helium is exhausted |
| Stars start out at the lower boundary, called the | | | | in the core of a star like the sun, no further reactions |
| Zero-Age Main Sequence referring to the fact that | | | | are possible. Helium burning may occur in a shell |
| stars in this location have just begun their main | | | | surrounding thecore for a brief period, but the |
| sequence phases. Because the transmutation of | | | | lifetime of the star is essentially over. |
| Hydrogen into Helium is the most efficient of the | | | | 8. Planetary Nebula |
| nuclear burning stages, the main sequence phase is | | | | When the helium is exhausted in the core of a star |
| the longest phase of a star's life, about 10 billion yrs | | | | like the sun, the C-O core will begin to contract again. |
| for a star with 1 solar mass. | | | | Central temperatures will never reach high enough |
| · Age: 27 million yrs | | | | values for Carbon or Oxygen burning, but the Helium |
| · R ~ Rsun | | | | and Hydrogen burning shells will conyinue burning for a |
| · Tcore = 15,000,000K | | | | while. Throughout the star's lifetime it is losing mass |
| · Tsurface = 6000K | | | | via a stellar wind, like the solar wind. This mass loss |
| · Energy Source: P-P Chain in core. | | | | increases when the star swells up to the size and low |
| During the main sequence phase there is a | | | | gravity of a Red Giant. During Helium Burning, thermal |
| "feedback" process that regulates the energy | | | | pulses, caused by the extreme temperature |
| production in the core and maintains the star's | | | | sensitivity of the 3-alpha Process, can cause large |
| stability. The basic physical principles are: | | | | increases in luminosity with accompanying mass |
| - The thermal radiation law, L ~ R2T4, determines | | | | ejection. During Helium Shell Burning, a final thermal |
| the energy output, which fixes requirement for | | | | pulse produces a giant "hiccough" causing the star to |
| nuclear energy production. | | | | eject as much of 10% of its mass, the entire outer |
| - The nuclear reaction rates are very strong | | | | envelope, revealing the hot inner regions with |
| functions of the central temperature; Reaction Rate | | | | temperatures in excess 100,000K, shown in this |
| ~ T4 for the P-P Chain. | | | | animation of the Helix, below. The resulting Planetary |
| - The inward pull of gravity is balanced by the gas | | | | Nebuala is the interaction of the newly ejected shell |
| pressure which is determined by the Ideal Gas Law: | | | | of gas with the more slowly moving ejecta from |
| PV=NRT. | | | | previous events and the ultraviolet light from the hot |
| A good way to see the stability of this equilibrium is | | | | stellar remnant, which heats the gas and causes it to |
| to consider what happens if we depart in small ways | | | | fluoresce. |
| from equilibrium: Suppose that the amount of energy | | | | |
| produced by nuclear reactions in the core is not | | | | 9. White Dwarf |
| sufficient to match the energy radiated away at the | | | | As the nebula disperses, the shell nuclear reactions |
| surface. The star will then lose energy; this can only | | | | die out leaving the stellar remnant, supported by |
| be replenished from the star's supply of gravitational | | | | electron degeneracy, to fade away as it cools down. |
| energy, thus the star will contract a bit. As the core | | | | The white dwarf is small, about the size of the earth, |
| contracts it heats up a bit, the pressure increases, | | | | with a density of order 1 million g/cm3, about |
| and the nuclear energy generation rate increases until | | | | equivalent to crushing a volkswagen down to a cubic |
| it matches the energy required by the luminosity. | | | | centimeter or a "ton per teaspoonful." |
| Similarly, if the star overproduces energy in the core | | | | · R ~ Rearth (a few thousand km) |
| the excess energy will heat the core, increasing the | | | | · Tsurface = 30000K - 5000K |
| pressure and allowing the star to do work against | | | | · Energy Source: "Cooling Off". |
| gravity. The core will expand and cool a bit and the | | | | A white dwarf star will take billions of years to |
| nuclear energy generation rate will decrease until it | | | | radiate away its store of thermal energy because of |
| once again balances the luminosity requirement of the | | | | its small surface area. The white dwarf will slowly |
| star. | | | | move down and to the right in the H-R Diagram as it |
| 4. End of Main Sequence | | | | cools until it fades from view as a "black dwarf". To |
| · Age: 10 billion yrs | | | | the right is the white dwarf companion to the nearby |
| · Energy Source: P-P Chain in shell | | | | star Sirius. |
| around core. | | | | |