Stellar Evolution Notes

How Stars Change Over Time

• Stars undergo significant changes throughout their existence.

Types of Stars

• Stars vary greatly in size within our galaxy.

Initial Conditions

• The Big Bang: Occurred approximately 13.7 billion years ago.

  • Characterized by extreme heat and density.

  • The universe expanded and cooled over time.

• Elemental Composition:

  • Primarily Hydrogen (around 92% of atoms) and Helium (around 8% of atoms) were formed.

• First-Generation Stars:

  • Formed solely from Hydrogen and Helium.

  • Star formation began roughly 200 million years after the Big Bang, as the universe expanded and cooled, allowing for atom clumping.

Nebulae: Star Nurseries

• Nebulae: Large clouds of gas and dust, spanning light-years, where stars are born.

• Formation Process:

  • Gravity draws gas and dust together, increasing density.

  • Example: The Crab Nebula.

Accretion Disks and Star Birth

• Accretion Disk Formation: As mass concentrates, a flat, spinning disk forms.

• Star Ignition: Sufficient mass accumulation in the center leads to high heat and pressure, initiating star birth.

Stellar Life Cycle and Mass

• Mass Ranges:

  • Brown dwarfs: less than 0.08 M_{\odot}

  • Low mass stars: less than 8 M_{\odot}

  • High mass stars: greater than 8 M_{\odot}

• Stellar Endpoints:

  • Brown dwarf

  • White dwarf (from low mass stars)

  • Neutron star (from high mass stars)

  • Black hole (from very high mass stars)

Stellar Lifecycles Overview

• A star's life cycle depends on its initial mass, affecting its lifespan and ultimate fate.

Brown Dwarfs

• Definition: Lowest mass objects that emit light, but are not true stars.

• Mass: Less than 8% of the mass of our sun (0.08 M_{\odot}).

• Energy Source: Glow due to heat from friction caused by high gravity compression.

• Fusion: Lack sufficient mass to initiate nuclear fusion.

• Prevalence: Estimated to be numerous, but difficult to detect.

• Analogy: If Jupiter were ~13 times heavier, it would be a brown dwarf.

Low Mass Stars - Main Sequence

• Definition: Stars with mass less than 8 M_{\odot}.

• Main Sequence: The period where Hydrogen fusion is the primary energy source, lasting billions of years.

• Lifespan Variation: Smaller stars burn fuel more slowly and last longer.

• Red Dwarfs: Very low mass stars can exist for up to 5500 billion years.

Low Mass Stars - Red Giant Phase

• Fuel Depletion: Occurs when Hydrogen fuel is exhausted, leading to fusion of heavier elements.

• Timescale: A relatively rapid process, lasting 0.1 - 2 billion years.

• Expansion: High energy production causes the star to expand significantly, becoming a Red Giant.

• Solar Example: In approximately 5 billion years, our sun will become a Red Giant, consuming Mercury and Venus.

• Earth's Fate: Earth will be too close to the sun's surface, making life as we know it impossible.

Low Mass Stars - White Dwarf Phase

• Fusion Ceases: The star can no longer fuse heavier elements.

• Planetary Nebula Formation: Outer layers are expelled, potentially initiating a new star formation cycle.

• White Dwarf Characteristics:

  • A very dense, hot core, approximately the size of Earth.

  • No longer produces energy through fusion.

  • Radiates remaining heat over an extremely long period (billions of trillions of years - 10^{100} years).

  • These will be among the last radiating objects in the universe.

High Mass Stars - Main Sequence

• Definition: Stars with mass greater than or equal to 8 M_{\odot}.

• Lifespan: Shorter and more dramatic lives compared to low mass stars.

• Appearance: Burn hotter, producing a blue glow.

• Fuel Consumption: Rapid Hydrogen burning due to extreme temperatures and pressures.

• Timescale: Main sequence lasts from 3 million to 2 billion years.

High Mass Stars - Red Supergiant/Hypergiant

• Post-Hydrogen Burning: Continue fusing heavier elements up to iron.

• Size: Can become extremely large; if the sun were Betelgeuse's size, it would extend beyond Jupiter's orbit.

• Lifespan: Short-lived phase, lasting only 3 - 100 million years.

• Fate: End their lives in a SUPERNOVA explosion.

Supernova

• Process:

  • Fuel exhaustion stops energy production, leading to gravitational collapse.

  • The star's material collapses inward at approximately 10% of the speed of light.

• Supernova Explosion: The largest explosion in the universe.

• Brightness: For a brief period, the supernova can outshine an entire galaxy.

• Historical Examples:

  • Supernova in 1604: Visible to the naked eye for 3 weeks, even during the day.

  • Supernova in 1054: Created the Crab Nebula, which is still visible today.

High Mass Stars - Neutron Stars

• Formation: Result from supernova events in stars with mass 10-25 times our sun's mass.

• Composition: Composed almost entirely of neutrons (formed from protons + electrons under extreme pressure).

• Density: Extremely densely packed; a teaspoon of neutron star material has the mass of Mount Everest.

• Size: Small radius of only 10-15km.

High Mass Stars - Black Holes

• Formation: Occur in stars with mass > 25-30 times our sun's mass.

• Density: Continue collapsing to an even denser state than neutron stars, forming a singularity.

• Gravity: Possess incredibly strong gravitational force, preventing even light from escaping.

Black Holes

• Theoretical Prediction: Predicted by Albert Einstein.

• Direct Observation: First image captured in 2019.

Stellar Life Cycle Summary

• Low mass stars (< 8 M_{\odot}): Become white dwarfs.

• High mass stars (> 8 M_{\odot}): Become neutron stars or black holes.

• Brown dwarfs: Lowest mass objects (< 0.08 M_{\odot}).