1. Evolutionary Stages A Summary

2. STAGE 1 A large volume of a dust/gas nebula begins contracting, falling together under the influence of the mutual gravitational pull of all the material; as the cloud gets smaller, fragmentation probably occurs, but the "stellar pieces" each get smaller and denser, as part of a large cluster of collapsing fragments. [Note : extended clusters of very young stars are known as associations and are always found in nebulous regions.]nebula

3. STAGE 2 As the "cloud fragment" shrinks, the outer "edge" stays cool and much less dense than the central region, which will get denser and hotter much faster, although it is still very cold.

4. STAGE 3 When the central region becomes so dense that its radiation cannot easily escape, the temperature of the gas/dust mixture increases dramatically (although the outer "edge" is still very cold); the object's radiation (from its photosphere) is now detectable and we call such an "embryonic star" a protostar - a hot gaseous ball, much larger and brighter than our Sun, that has not yet achieved a balance in its life.

5. STAGE 4 Even with a central temperature now over 1 million K and a surface a few thousand Kelvins hot, the inward gravity pull of the slowly collapsing object is still not quite balanced by the escaping heat generated by the squeezed gases.

6. STAGE 5 At an ever-slowing rate, the contraction continues, but at almost constant temperature, so the size and thus the luminosity decrease, as the star slowly approaches the stability of the Main Sequence; this period is called the T Tauri phase, as a vigorous outflowing "protostellar wind" keeps the surface active.

7. STAGE 6 Finally the contraction has raised the core temperature to that required to cause the highly energetic core protons to fuse together in nuclear reactions - we can say that "a star is born", although is is not quite exactly stable. The core temperature is at least 10 million degrees.

8. STAGE 7 Slight contraction over millions of years finally results in a perfect, continued balance between outward luminosity and inward gravity. Our star has reached the Zero Age Main Sequence, where it will stay for over 90% of its life, virtually unchanged externally. [Note : Stars of different masses experience similar evolutionary tracks on the H-R Diagram, but end up at different points on the ZAMS; recall that mass => gravity => squeezing => core T => fusion E => luminosity.] Although low- mass stars seem to vastly outnumber their high-mass relatives, a star with too small a mass (<.08 suns) will not have enough "squeeze" in its core to initiate fusion; such objects (termed brown dwarfs) will be dim and cool and, as they grow older, will only grow dimmer and cooler, ultimately becoming black dwarfs (see STAGE 14). Astronomers have identified several brown dwarf candidates, and even have evidence for the presence of Jupiter-like planets in orbit around several nearby stars. Recently, objects "in between" the two groups have been detected by the extra-solar planet hunting teams. A star on the Main Sequence is undergoing (for over 90% of its life) steady core hydrogen burning, due to its interior structure. While lower-mass M.S. stars (<8 suns, according to your text, although the exact value is not known) are producing sufficient energy by just using the proton-proton chain of nuclear reactions, higher-mass stars must primarily rely on the carbon cycle to produce their greater requirements of energy to balance their greater gravity. The carbon atoms help the hydrogen atoms to fuse and produce a much greater output of energy, but this causes the star to deplete its hydrogen supply much faster than the lower-mass stars. Every star must eventually use up most of its core hydrogen "fuel" and increase its core helium "ash", until the core energy production diminishes and the star is no longer in equilibrium. As the interior structure changes, the exterior appearance must also change, and the star is said to evolve "off the Main Sequence".

9. STAGE 8 At the point where the amount of core helium is just sufficient to reduce the energy output, the hot core begins to contract, which raises the temperature all through the inside of the star, and it begins hydrogen shell-burning, actually producing more energy than ever before. The outer envelope of the star begins to slowly expand and the star "leaves" the Main Sequence region, becoming larger, brighter, and cooler as it expands along the subgiant branch.

10. STAGE 9 While the star's temperature doesn't change too much, its luminosity greatly increases as more shell hydrogen is fused as the star's core gets hotter; the star reaches the red giant branch of the H-R Diagram; the small, very dense (termed degenerate), very hot core is surrounded by an enormous "bloated" envelope. As the temperature of the core increases, it finally reaches the ignition temperature of helium (about 100 million K), and the core undergoes a "helium flash".

11. STAGE 10 The "flash" results in two things : #1 - a hot carbon core begins to form inside the helium core, and #2 - the star physically changes, moving to the hotter, but somewhat dimmer, horizontal branch of the H-R Diagram, where it stays for many years, at a precise place determined by its mass (sort of a "main sequence of giant stars"). But the star continues to change, both internally and externally, since the hot carbon core cannot support the star's mass and soon begins to contract, increasing the central temperature and accelerating the hydrogen- and helium-burning in the shells -- the star expands even more and cools as it does so.

12. STAGE 11 The expansion carries the star up the asymptotic giant branch into the red super- giant region; the star could continue the nuclear reaction sequence and fuse the carbon atoms, but its gravity is not high enough to generate the temperatures needed (about 600 million K) for this to happen, so it has essentially reached the end of its nuclear-burning lifetime, and death is inevitable.

13. STAGE 12 As the degenerate carbon core heats up, the fusion reactions increase in intensity so much that the outer envelope continues to swell until it actually leaves the core - a two-part object is formed : the central hot (some have been identified by the H.S.T. at over 200,000 K) degenerate core remains in the center of an expanding cloud of hydrogen and helium known as a planetary nebula, which gradually disperses, recycling its atoms (mostly hydrogen, but with some helium and carbon) back into the nebulae while the core cools and dims and is finally left all alone...to die.

14. STAGE 13 The small, hot, core that remains is known as a white dwarf. For stars with masses comparable to that of the Sun, the composition will be a mixture of carbon and oxygen nuclei. It is extremely dense (degenerate) and incapable of producing any new energy output by itself. Thus it can only slowly cool and fade, eventually to become...

15. STAGE 14... a cold, dark, dense black dwarf (none of these have probably ever had time to form - the universe isn't old enough to have allowed this to happen). This is the expected final end of our Sun and any similar solitary stars (which includes a high percentage of all stars).

16. Very massive stars can go directly to the red giant stage. They end their life cycle with explosive consequences. More Massive Stars M > M 

17. The Final Stages - Summary End Points of Evolution for Stars of Different Masses Initial Mass (Solar Masses) Final State Less than 0.08 Hydrogen brown dwarf 0.08 – 0.25 Helium white dwarf 0.25 - 8 Carbon-Oxygen white dwarf 8 – 12 (approx) Neon-Oxygen white dwarf Greater than 12 supernova