Overview
This lecture explains the life cycles of stars, how their mass determines their fate, and how elements heavier than hydrogen and helium are formed and distributed in the universe.
Lifecycle of Low-Mass Stars
- Stars form from clouds of gas and dust, primarily hydrogen and helium, due to gravitational collapse.
- Nuclear fusion in the core fuses hydrogen into helium, releasing energy and stabilizing the star as a main sequence star.
- After billions of years, hydrogen runs out, core contracts, and outer layers expand into a red giant.
- A helium flash allows the fusion of helium into carbon and oxygen via the triple-alpha process.
- Further evolution leads to the ejection of outer layers as a planetary nebula, leaving behind a white dwarf core.
- White dwarfs are Earth-sized, very dense, and gradually cool as they cannot fuse heavier elements.
Lifecycle of High-Mass Stars
- High-mass stars start similarly but form from much larger gas clouds, resulting in brighter, hotter, blue main sequence stars.
- These stars burn fuel rapidly, exhausting hydrogen in tens to hundreds of millions of years.
- When hydrogen depletes, the core contracts and heats up, enabling fusion of heavier elements like carbon, oxygen, neon, silicon, up to iron.
- Once an iron core forms, fusion ceases to provide energy, and the star collapses, triggering a supernova explosion.
- Supernovae produce elements heavier than iron and outshine their entire galaxy temporarily.
Stellar Remnants and Outcomes
- Stars below about 8 solar masses leave behind white dwarfs, supported by electron degeneracy pressure.
- If the core remnant after supernova is between 1.4 and 3 solar masses, it becomes a neutron star, extremely dense and small.
- Cores above roughly 3 solar masses collapse into black holes, objects with gravity so strong that not even light can escape.
Creation and Distribution of Heavy Elements
- Elements heavier than hydrogen and helium are formed inside stars and during supernovae.
- Supernovae and neutron star collisions are responsible for elements heavier than iron (such as gold and silver).
- Ejected material from dying stars becomes part of the interstellar medium, contributing to new star and planet formation.
Key Terms & Definitions
- Nuclear Fusion — Process where atomic nuclei combine to form heavier nuclei, releasing energy.
- Main Sequence Star — Stable star fusing hydrogen in its core.
- Red Giant — Expanded, cooled star nearing the end of its fuel supply.
- Helium Flash — Sudden onset of helium fusion in a star’s core.
- Triple-alpha Process — Fusion process that converts helium into carbon and oxygen.
- Planetary Nebula — Ejected shell of gas from a dying low-mass star.
- White Dwarf — Dense, Earth-sized stellar remnant not hot enough for further fusion.
- Supernova — Explosive death of a high-mass star, spreading heavy elements.
- Neutron Star — Extremely dense remnant composed mostly of neutrons.
- Black Hole — Remnant with gravity so strong that not even light can escape.
- Chandrasekhar Limit — Maximum mass (~1.4 solar masses) for a white dwarf.
Action Items / Next Steps
- Prepare to study the properties and mysteries of black holes in the next chapter.