Day 13 notes Chapter 12

Louis Lackey

Day 13 Notes

Chapter 12: Star Stuff

Section 12.1: Star Birth

            Stars form in dark clouds (nebula=cloud) of dusty gas in interstellar space. The gas is called the interstellar medium. Gravity can only create stars if it can overcome the thermal pressure in the cloud. Gravity becomes stronger as the gas becomes denser. The typical cloud is 30K, 300ppcm^2. It must contain a few hundred solar masses to overcome the pressure. The cloud can convert pressure into infrared and radio photons that escape. As stars begin to form, dust grains heat up and emit infrared light. The more light, the more stars are forming.

            The cloud heats up as it contracts and rotates, and spins faster, because of conservation of energy/angular momentum. Spin hampers collapse perpendicular to the axis, forming a disk. The disk forms in the direction of the spin.

            Protostar to Main Sequence- protostar contracts and heats until the core temperature creates fusion. Contraction ends when the fusion energy balances energy lost from surface radiation. It takes 30 million years for a star like our sun to form.

            There is an upper limit on star size because of photon pressure. If it gets too bright more photons cannot fit in. It’s about 300 solar masses. The lower limit-quantum mechanics prohibit two electrons from occupying the same state in the same place. The pressure cannot get high enough in a too small area to create fusion. The lower limit is 0.08 solar masses. Objects too small to have fusion are brown dwarfs. A brown dwarf emits heat from contraction but no light.

Section 12.2: Life as a low mass star

            A star remains on the main sequence as long as it can perform hydrogen fusion. Stars become redder, larger, and brighter after it leaves the main sequence. At the end the fusion occurs in a shell around the core, breaking the thermostat effect. It no longer stops the core from contracting. A higher temperature causes He fusion into carbon, 3He-1C. This causes a helium flash. The core temperature rises rapidly and the He fusion skyrockets, thermal pressure takes over again, and the core expands again. The star stops shrinking or growing because the core is temporarily fixed. The red giant should shrink and get dimmer after the helium flash. This cycle happens again as a helium core star, and then explodes into a planetary nebula, which leaves behind a cooling white dwarf.

H fusion main sequence

H shell red giant

He core horizontal branch

Double shell fusion red giant

            Combining models of stars of similar age but different mass helps us to age-date star clusters.

Section 12.3: Life as a high mass star

            Large stars follow a similar path as low mass stars except they do it much faster. Higher mass stars use carbon nitrogen and oxygen as catalysts.

            Helium fusion can make carbon. The CNO cycle can change carbon into nitrogen and oxygen. High core temperature can fuse helium into heavier elements, neon, and magnesium. Even higher temperatures can create up to silicon, sulfur, and iron. The heavier elements can be made in the explosion of a supernova.

            High mass stars continue into multiple shell burning, with the core having layers of heavier materials from fusion, with heavier in the middle. The high mass star dies in a supernova. The core builds until it cannot resist gravity. When it collapses it explodes, producing complex elements and a supernova (nebula) cloud. The core left behind is a Neutron star, from electrons combining with protons to form neutrons and neutrinos.