Evolution of High-Mass Stars Notes
High-Mass Stars: Evolution
Characteristics
Greater gravitational force leads to higher temperature and pressure.
Increased nuclear fusion rate.
More luminous.
Shorter lifespans.
Nuclear Fusion
Convert hydrogen to helium via the carbon-nitrogen-oxygen (CNO) cycle.
CNO cycle: hydrogen fuses to helium using carbon as a catalyst; more efficient than the proton-proton chain.
Convection mixes hydrogen in the core, increasing mass available for fusion.
Post-Main-Sequence Evolution
Evolve into supergiants.
Ignite helium in a nondegenerate core.
Radius increases, temperature decreases, luminosity remains constant.
Heavier elements (C, Ne, etc.) fuse with rising central temperatures.
Fusion shells build up like onion layers until iron is created.
Instabilities
Can pass through the instability strip on the HR diagram.
Become variable stars with regular luminosity changes.
Period-luminosity relationship used for determining distances.
Types of pulsating stars:
Cepheid variables: high-mass stars becoming supergiants, periods from 1 to 100 days.
RR Lyrae variables: low-mass stars on the horizontal branch, less luminous than Cepheid variables.
Final Days
Many stages of nuclear fusion, each stage progressively shorter.
Example: Silicon burning lasts only a few days.
Neutrino production carries away energy (neutrino cooling).
End of Fusion
Binding energy: energy required to break a nucleus.
Iron cannot generate energy by fusion because it has the highest binding energy.
Fusion stops once an iron core is formed, leading to core collapse.
Core Collapse
Photodisintegration: gamma rays break apart iron.
Charge destruction: electrons forced into protons, producing neutrons and neutrinos.
Type II Supernova
The core collapses until nuclear forces become repulsive.
The overcompressed core bounces, driving outer layers outward.
A lot of energy is emitted (1 billion Suns’ worth of light).
Kinetic energy is transferred to the interstellar medium (ISM) as a shock wave.
Nucleosynthesis: new elements are created in the explosion.
All atoms heavier than iron are made in supernova explosions.
Neutron Stars
Type II supernova leaves behind a neutron-degenerate core: a neutron star.
Mass: , radius: ~6 miles.
Neutron Stars: Pulsars
Strong magnetic fields.
Electrons and positrons produce radiation beamed from the poles.
The star appears to pulse on and off as the beams sweep past an observer.
Neutron Stars: X-ray Binary Systems
An evolving star overflows its Roche lobe, pouring matter onto its neutron star companion.
Infalling matter heats the accretion disk to X-ray-emitting temperatures.
Relativistic jets are emitted from the rotating neutron star.
Crab Nebula
Remnant of a Type II supernova witnessed in 1054 CE.
Powered by a pulsar.
Star Clusters
Bound groups of stars formed at the same time from the same material.
Globular clusters: very dense, up to millions of stars.
Open clusters: looser, a few dozen to a few thousand stars.
Star Clusters: H-R Diagram
The presence of massive, hot stars indicates a young open cluster.
The absence of massive, hot stars indicates an old globular cluster.
Cluster age is determined from the main-sequence turnoff.
Composition of Stars
Young stars have more massive elements.
Supernovae seed the universe with massive elements.
Process of Science
Occam’s razor: The simplest answer is often the correct one.
Binding Energy of Atomic Nuclei
Net energy released is the difference between the binding energy of products and reactants.
Gravity on a Neutron Star
High surface gravity and escape velocities due to incredible density.
Composition of Planets
Supernovae seed the universe with massive elements; elements heavier than boron form from dying, massive stars.