Stellar Evolution Summary

Study Notes

Stars are categorized by mass: low (<2 solar masses), intermediate (2-8 solar masses), and high (>8 solar masses). These categories influence their lifespans.

Red Giant Phase: Hydrogen in the core is depleted, causing the core to contract and heat. The outer layers expand, lowering surface temperature, but increasing luminosity. Stars can lose up to 80% of their mass during this phase.
Planetary Nebula Formation: Outer layers are expelled, forming an expanding shell of gas. The core is exposed as a hot, dense white dwarf.
White Dwarf Stage: The final stage for low-mass stars. The core is composed of very dense carbon. Supported by electron degeneracy pressure, which prevents further collapse.

Intermediate-Mass Stars: (2-8 solar masses). These stars evolve off the main sequence quickly. They briefly become supergiants and fuse helium faster than low-mass stars. They end as white dwarfs after the ejection of the outer layers.

Massive Stars: (>8 solar masses). Develop an onion-like shell structure through successive fusion stages (e.g., hydrogen, helium, carbon, oxygen...) Each stage is progressively shorter.
Supernova: Core fusion eventually stops at iron. Iron fusion absorbs energy thus leading to collapse and explosion in a supernova. Supernovae produce more common heavy elements in space.
Neutron Star or Black Hole: Depending on the mass, a neutron star or black hole forms after the supernova.

Both low-mass and high-mass stars return enriched material to interstellar space. This recycled material forms new stars and planets.
Supernovae play a key role in enriching the interstellar medium with heavy elements made in their cores.

Low-Mass stars: have lifespans in billions of years
High-Mass stars: have lifespans in millions of years
The evolutionary stages and timescales are different for low-mass, intermediate-mass, and high-mass stars, reflecting their different initial masses.

Core remnants of low or intermediate-mass stars. Comparable in size to Earth, but far more dense, with masses roughly 0.6 solar masses (for Sun-like stars).
Composed of helium, carbon or oxygen. Supported by electron degeneracy pressure.
Have a maximum mass limit of approximately 1.4 solar masses (Chandrasekhar limit).

Nova: Binary systems where hydrogen from a companion star accumulates on a white dwarf. Surface fusion causes outer layers to be expelled. May occur multiple times.
Type Ia Supernova: A white dwarf that exceeds 1.4 solar masses, undergoes carbon fusion, leading to an extremely bright explosion destroying the entire star. Very visible in distant galaxies.

Formed from the core collapse of massive stars during a supernova. Highly dense with masses between 1.4 and 3 solar masses and sizes around 10 kilometers.
Rapid rotation due to conservation of angular momentum. Strong magnetic fields. Visible as pulsating sources of radiation (Pulsars).

Galaxy Rotation Curves: Reveal more mass than visible matter, indicating the existence of "dark" matter.
Galaxy Cluster Motion: High velocities of galaxies in clusters suggest substantial invisible mass (dark matter).
Gravitational Lensing: Distortion of light from background galaxies by mass, allows measurement of precise masses.
Observable effects on environment: X-ray emission from accreting matter, orbital measurements of companion stars, gravitational waves from merging black holes are used to detect them.

Galaxies grow through gas acquisition. Collisions trigger starbursts or merging.
Mergers can result in bulge-disk or elliptical galaxies.

Light emitted from hot plasma about 380,000 years after the Big Bang. Shows subtle temperature variations indicative of early density fluctuations.

Dark Matter: Accounts for roughly 84% of the universe's matter. Cannot be observed directly, only through its gravitational effects.
Dark Energy: Represents the accelerating expansion of the universe. Its nature is unknown.

Current estimations place the age of the universe at approximately 14 billion years.
Space itself is expanding, not just objects moving through space. Observed redshifts and the Hubble constant indicate this.