Formation of Stars - 3 (Chapter 5 – Universe)

Stage 10: Carbon Core There now exist a stable helium burning star. The inner core is now burning helium to form a non-burning carbon core (i.e. carbon ash). Outside of that helium burning shells is the hydrogen burning shell, forming helium. The outermost shell is still the non-burning envelope composed of hydrogen. The star rests in this stage for a while before it evolves any further.

Figure 20-1 The Post-Main-Sequence Evolution of a 1-M Star These H-R diagrams show the evolutionary track of a star like the Sun as it goes through the stages of being (a) a red-giant star, (b) a horizontal-branch star, and (c) an asymptotic giant branch (AGB) star. The star eventually evolves into a planetary nebula (described in Section 20-3).

Stage 11: Attempting the Giant Branch again Similar to stages 8, because there are no more nuclear reactions occurring in the pure carbon core, its pressure drops and the core begins to collapse. Again, this collapsing of the core causes gravitational energy to radiate outwards. This energy speeds up the hydrogen and helium burning in the outer layers.

Stage 11: Attempting the Giant Branch again The outer layers expands as the inner core continues to shrink. The gravitational energy due to shrinkage causes the star’s luminosity to rise once again. It becomes very bright and starts to climb up the H-R diagram again.

Figure 20-1 The Post-Main-Sequence Evolution of a 1-M Star These H-R diagrams show the evolutionary track of a star like the Sun as it goes through the stages of being (a) a red-giant star, (b) a horizontal-branch star, and (c) an asymptotic giant branch (AGB) star. The star eventually evolves into a planetary nebula (described in Section 20-3).

Figure 20-2 The Structure of an Old, Moderately Low-Mass AGB Star Near the end of its life, a star like the Sun becomes an immense, red, asymptotic giant branch (AGB) star. The star’s inert core, active helium-fusing shell, and dormant hydrogen-fusing shell are all contained within a volume roughly the size of the Earth. Thermonuclear reactions in the helium-fusing shell are so rapid that the star’s luminosity is thousands of times that of the present-day Sun. (The relative sizes of the shells in the star’s interior are not shown to scale.)

Figure 20-3 Stellar Evolution in a Globular Cluster In the old globular cluster M55, stars with masses less than about 0.8 M are still on the main sequence, converting hydrogen into helium in their cores. Slightly more massive stars have consumed their core hydrogen and are ascending the red-giant branch; even more massive stars have begun helium core fusion and are found on the horizontal branch. The most massive stars (which still have less than 4 M ) have consumed all the helium in their cores and are ascending the asymptotic giant branch. (Compare with Figure 21-11.) (Adapted from D. Schade, D. VandenBerg, and F. Hartwick)

Figure 20-6 Planetary Nebulae (a) The pinkish blob is a planetary nebula surrounding a star in the globular cluster M15, about 10,000 pc (33,000 ly) from Earth in the constellation Pegasus. (a: NASA/Hubble Heritage Team, STScI/AURA)

Figure 20-6 Planetary Nebulae (b) The planetary nebula Abell 39 lies about 2200 pc (7000 ly) from Earth in the constellation Hercules. The almost perfectly spherical shell that comprises the nebula is about 1.5 pc (5 ly) in diameter; the thickness of the shell is only about 0.1 pc (0.3 ly). (b: WIYN/NOAO/NSF)

Figure 20-6 Planetary Nebulae (c) This infrared image of the planetary nebula NGC 7027 suggests a more complex evolutionary history than that of Abell 39. NGC 7027 is about 900 pc (3000 ly) from Earth in the constellation Cygnus and is roughly 14,000 AU across. (c: William B. Latter, SIRTF Science Center/Caltech, and NASA)

Stage 12: A Planetary Nebula One could say the carbon core is now “dead”. No more nuclear reactions are now happening within it. But at this stage, there is still a large amount of energy being radiated outwards from the core in the form of gravitational energy. The outer hydrogen and helium envelopes begin to separate from the non-burning carbon core, and drift outwards. What was once a Red Giant, now is made up as two distinct parts: Very Hot and Bright (i.e. Luminous) exposed core. A surrounding cloud of dust and gas

... continued The left over carbon core is still very hot, and since there are no nuclear reactions, it continues to contract and heat up. The remaining core is hot enough to heat up the surrounding cloud of gas and dust, and make it glow (through ionizing the gas). This produces a colorful display called a planetary nebula Planetary Nebula: The ejected envelope of a red-giant star spread over the volume roughly the size of our solar system.

Figure 20-8 Sirius A and Its White Dwarf Companion Sirius, the brightest-appearing star in the sky, is actually a binary star: The secondary star, called Sirius B, is a white dwarf. In this Hubble Space Telescope image, Sirius B is almost obscured by the glare of the overexposed primary star, Sirius A, which is about 104 times more luminous than Sirius B. The halo and rays around Sirius A are the result of optical effects within the telescope. (NASA; H. E. Bond and E. Nelan, STScI; M. Barstow and M. Burleigh, U. of Leicester; and J. B. Holberg, U. of Arizona)

Stage 13: White Dwarf Eventually, the left over carbon core becomes hot enough to build up enough internal pressure so that it cannot contract anymore. All that is left is this rotating, high density, high pressured, ball of carbon. It is approximately the size of earth, but is around 150,000 times the mass of earth. THAT’S DENSE! It does shine, but only from stored heat. There are no more nuclear reactions. This is called a White Dwarf. White Dwarf: It is a star with a radius equal to our sun’s (or smaller), that is hot enough to glow white.

... continued In most cases, a White Dwarf is the last stage for sun-type stars. Eventually, they will fade from white to yellow to red, and then become a Black Dwarf. Black Dwarf is basically a cold, dense, burned out ash is space.