Astronomy Chapter 17

Earth's Fate

1 billion years: M-S brightening will cause oceans to begin evaporating 3-4 billion years: brightening will boil away oceans, Venus-like runaway greenhouse effect; 5 billion years: Sun exhausts hydrogen in its core, temperatures rise dramatically; Helium Flash: Earth's surface reaches 1000 K, begins to lower during helium-core fusion; 100 million years later: core helium exhausted, radius expands until it engulfs the Earth, planetary nebula destroys it

Death of Low Mass Stars

1. Hydrogen core fusion (M-S lifetime) 2. Core interior runs out of hydrogen, core contracts slightly 3. Hydrogen shell fusion (around inert helium core), star swells into subgiant / red giant 4. Helium flash! (core expands, outer layers shrink) 5. Helium core fusion + hydrogen shell fusion, stellar winds 6. Core interior runs out of helium, core contracts slightly 7. Helium shell fusion + hydrogen shell fusion, e.g. "double shell-fusion" (around an inert carbon core), star swells into a red giant 8. Outer layers puff off into planetary nebula - carbon white dwarf core is left behind

Death of High Mass Stars

1. Hydrogen core fusion (M-S lifetime; p-p chain & CNO cycle) 2. Core interior runs out of hydrogen, core contracts slightly 3. Hydrogen shell fusion (around inert helium core), star swells into red supergiant 4. Helium core fusion ignites (no helium flash) 5. Core interior runs out of helium, core contracts slightly 6. Carbon core fusion ignites (+ hydrogen shell and helium shell fusion) 7. Core (& shells) fuses heavier elements (He capture + other reactions) until... 8. IRON cannot fuse! (without input of energy) 9. Core collapses into a neutron star (e- + p = n + neutrinos) 10. Gravitational collapse of core (no longer supported by thermal pressure) releases enormous amount of energy = SUPERNOVA EXPLOSION! 11. Stellar core either remains a neutron star or further collapses into a black hole 12. In the energetic "furnace" of this SN explosion, elements heavier than iron are produced à the SN spreads these elements (and other produced in the star earlier in its lifetime) into the interstellar medium 13. Expanding material from SN becomes a "supernova remnant"

White Dwarf

After the core cools, planetary nebula fades, and ejected gas disappears, the carbon core will be left behind. The balance between gravity and degeneracy pressure is indefinite as the core slowly cools until it no longer emits light.

Supergiant

As core hydrogen runs out, the star develops a hydrogen-fusing shell, expanding its outer layers; the core contracts and releases energy raising its temperature until it can fuse helium, the process repeats when the core helium runs out, increasing core pressure, temperature and density until it can fuse carbon with a helium fusion cell

Hydrogen Shell Fusion

As the helium core contracts, the outer becomes hot enough for fusion, at a much faster rate than in the core, causing a buildup of thermal energy pushing the surface outward

CNO Cycle

Carbon, nitrogen, and oxygen act as catalysts for hydrogen fusion, making it proceed at a far higher rate, leading to enormous luminosities and short lifetimes. Enormous fusion rates lead to massive power generation; many photos bounce around exerting a significant amount of radiation pressure that drives solar winds expelling large amounts of mass

Carbon Stars

Degeneracy pressure halts core contraction before it can reach the 600 million K required for carbon fusion. As luminosity and radius continue to rise expel increasing amount of matter through stellar winds while convection dredges up carbon from the core and enriches the photosphere of the red giant

Fusion of Heavier Nuclei

Helium Capture Reactions - reactions in which a helium nucleus fuses with some other nucleus (C, O, Ne, Mg) Other Reactions - at high enough temperatures, heaver nuclei can b e fused together (Si, S, Fe) Core depletion creates multiple layers of shell-fusion; as each stage of core fusion ceases, shell-fusion intensifies and inflates outer layers; each time the core flares up, outer layers contract; life track zig-zags at same luminosity (Greater Mass = Faster Core Changes, Fewer Zig-Zags)

Thermal Pulses

Helium fusion never reaches equilibrium but instead proceeds in a series of pulses during which the fusion rate spikes upward every few thousand years

Low Mass Star M-S Lifetime

Hydrogen core fusion through proton-proton chain, energy is released through nuclear fusion and is released by radiative diffusion and convection. Convection and rotation twist and stretch magnetic field lines causing flares. As the number of the particles in the core drops, it contracts to maintain equilibrium

Formation of Elements Heavier than Iron

Iron and heavier elements can only generate energy through fission, as fusion creates an element with a greater mass and therefore requires energy to occur.

Supernova

Once gravity pushes electrons past the quantum mechanical limit, they disappear by combining with protons to form neutrons, eliminating degeneracy pressure. Gravitational collapse releases enormous amounts of energy, driving the outer layers of the star into space in a titanic explosion. The ball of neutrons left behind is a neutron star. if the remaining gas is large enough, gravity overcomes neutron degeneracy pressure and the core collapses into a black hole.

Helium Fusion

Once the core temperature reaches 100 million K, the star begins fusing helium. The triple alpha reaction fuses 3 helium nuclei (alpha particles) into 1 carbon nucleus, releasing energy. Energy production falls after the helium flash, reducing the luminosity and contracting the core

Double Shell Fusion

Once the star fuses all the helium in its core, it begins helium-shell fusion while continuing hydrogen-shell fusion. The core and two shells contract, increasing the temperature and fusion rate and causing massive expansion in size and luminosity

Low Mass Stars

Stars born with less than about 2 solar masses of material. Smaller, less luminous, cooler core temperature, slower fusion rates, deeper convection zones, most dramatically active cycles

High Mass Stars

Stars born with masses greater than about 8 solar masses. Have greater fusion rates, come into energy balance with higher core temperature, more luminous, hotter interiors and shallow convection zones or convection cores

Stellar Mass

The mass of a main-sequence star determines both its luminosity and its lifetime because it determines the core temperature and fusion rate at which the star can remain in gravitational equilibrium. It also determines whether the star ever becomes hot enough to fuse elements heavier than helium

Iron

The one element from which it is not possible to generate any kind of nuclear energy

Planetary Nebula

The star ejects its outer layers, creating a huge shell of gas expanding away from the carbon core; the core will emit intense ultraviolet radiation which ionizes the gas in the shell making it glow brightly

Helium Flash

The temperature and helium fusion rate spike drastically, releasing enormous amounts of energy into the core, raising the temperature enough for thermal pressure to surpass degeneracy pressure, push back against gravity, and expand the core. The star drops down and to the left on the H-R diagram

Red Giant Stage

When the star runs out of hydrogen, fusion will stop and the core will contract. The outer layers will expand, increasing luminosity and the star will move horizontally to the right to become a subgiant then upward to become a red giant, 100 times larger and 1,000 times brighter. The increased radius weakens the pull of gravity and allows large amounts of mass to escape via solar winds