The slide features the title "The Life and Death of Stars: From Protostars to Black Holes," outlining the journey of stars from their formation as protostars to their ultimate fates, such as black holes. Its subtitle, "Exploring Cosmic Cycles for Seasoned Astronomers," indicates a focus on advanced astronomical concepts for experienced viewers.

The Life and Death of Stars: From Protostars to Black Holes

Exploring Cosmic Cycles for Seasoned Astronomers

Source: Astronomy Presentation for Retired Professors

The slide outlines a presentation on stellar evolution, starting with the HR Diagram and star formation processes like molecular clouds and fusion ignition. It covers main sequence and post-main sequence stages, low-mass outcomes such as white dwarfs and novae, high-mass paths leading to supergiants, supernovae, neutron stars, and black holes, before concluding with stars' role in enriching the universe with heavy elements.

Presentation Outline

1. HR Diagram and Star Formation

Axes, sequences, molecular clouds, and fusion ignition.

2. Main Sequence and Post-MS Evolution

Hydrogen burning, transitions to giants, helium fusion.

3. Low-Mass End: White Dwarfs and Novae

Formation, properties, binary system explosions.

4. High-Mass End: Supergiants to Black Holes

Heavy fusion, supernovae, neutron stars, singularities.

5. Conclusion: Stellar Legacy

Enriching universe with heavy elements. Source: The Life and Death of Stars: From Protostars to Black Holes

This section header slide introduces "The Hertzsprung-Russell Diagram" as the first topic in exploring the life and death of stars, from protostars to black holes. It presents the HR diagram as a fundamental tool for classifying stars based on their luminosity, temperature, and evolutionary stages.

The Life and Death of Stars: From Protostars to Black Holes

01

The Hertzsprung-Russell Diagram

Introduction to the HR diagram as a stellar classification tool

The HR Diagram features temperature on the x-axis, ranging from hot stars on the left to cool ones on the right, and luminosity on the y-axis, increasing from low at the bottom to high at the top. Key stellar populations include the main sequence as a diagonal band of stable hydrogen-burning stars, giants as expanded cool luminous bodies, and white dwarfs as dense faint remnants in the lower left.

HR Diagram Axes and Features

• Temperature (x-axis): Hot to cool, left to right.
• Luminosity (y-axis): Low to high, bottom to top.
• Main sequence: Stable hydrogen-burning stars, diagonal band.
• Giants: Expanded, cool, highly luminous stars.
• White dwarfs: Dense, faint remnants in lower left.

The HR diagram visualizes stars by plotting their temperature against luminosity, with the main sequence forming a diagonal band of hydrogen-burning stars. It also highlights red giants in the cool, highly luminous upper right, white dwarfs in the hot, low-luminosity lower left, and curved evolutionary paths from the main sequence to these stages.

Visualizing the HR Diagram

!Image

• HR diagram plots stellar temperature vs. luminosity.
• Main sequence forms diagonal band of hydrogen-burning stars.
• Red giants occupy upper right: cool, highly luminous.
• White dwarfs in lower left: hot, low luminosity.
• Evolutionary paths curve from main sequence to giants and dwarfs.

Source: Hertzsprung–Russell diagram

This slide serves as a section header titled "Star Formation," marking it as the second section in the presentation. It features a subtitle describing the birth of stars from cosmic clouds.

Star Formation

02

Star Formation

Birth of stars from cosmic clouds.

Molecular clouds collapse under gravity, leading to the formation of protostars that accrete surrounding mass. As the core heats to 10 million K, hydrogen fusion ignites through the proton-proton chain, marking the birth of a star.

From Clouds to Fusion

• Molecular clouds collapse under gravitational forces.
• Protostars form and accrete surrounding mass.
• Core heats to 10 million K, igniting fusion.
• Hydrogen fusion begins via proton-proton chain.

The Star Formation Timeline slide outlines the key stages of a star's birth, beginning with the collapse of molecular clouds under gravity at around 10^6 years to form dense cores. It progresses through the emergence of a heated protostar with an accretion disk at 10^5 years, pre-main sequence contraction along the Hayashi track at 10^4 years, and culminates in hydrogen fusion igniting in the core at 0 years, stabilizing the star on the main sequence.

Star Formation Timeline

10^6 years: Molecular Cloud Collapse Interstellar gas clouds fragment and collapse under gravity, forming dense cores. 10^5 years: Protostar Emerges Accretion heats the core, creating a protostar with surrounding envelope and disk. 10^4 years: Pre-Main Sequence Evolution Protostar contracts along Hayashi track, cooling surface while core heats up. 0 years: Main Sequence Entry Hydrogen fusion ignites in core, stabilizing star on Hertzsprung-Russell main sequence.

This slide introduces the "Main Sequence Life" section, numbered 03, focusing on a key phase in stellar evolution. It describes the stable period of core hydrogen burning that represents a star's prime and most enduring stage.

Main Sequence Life

03

Main Sequence Life

The stable phase of core hydrogen burning that defines a star's prime.

Low-mass stars fuse hydrogen primarily through the proton-proton chain, while high-mass stars rely on the faster CNO cycle, which demands hotter stellar cores. These mechanisms significantly influence the lifetimes and stability of stars.

Hydrogen Burning Mechanisms

• Low-mass stars fuse hydrogen via proton-proton chain.
• High-mass stars use CNO cycle for faster fusion.
• CNO cycle requires hotter cores than pp chain.
• These mechanisms dictate stellar lifetimes and stability.

Stellar rotation rapidly flattens stars into oblate spheroids, leading to equatorial bulging, polar flattening, and impacts on internal mixing, surface temperatures, mass loss, and overall evolution. In stellar interiors, convective motions drive the dynamo effect to generate magnetic fields, which appear as sunspots, prominences, and flares, while influencing stellar winds, angular momentum loss, and long-term development.

Rotation and Magnetic Fields

This section header slide introduces "Post-Main Sequence Evolution" as the fifth topic in the presentation. It explains that stars transition from the main sequence phase once their core hydrogen begins to deplete.

Post-Main Sequence Evolution

05

Post-Main Sequence Evolution

Stars transition from main sequence after core hydrogen depletion begins.

Source: Astronomy Presentation

When a star exhausts hydrogen in its core, the core contracts while a shell of hydrogen burning ignites around it, causing the outer layers to expand and form the subgiant phase. This expansion continues, leading to further growth into a red giant with a convective envelope.

Subgiant to Red Giant

• Core hydrogen exhaustion causes contraction
• Shell hydrogen burning ignites around core
• Outer layers expand, forming subgiant phase
• Further growth creates red giant with convective envelope

Source: Core contracts, shell H burns. Star expands into subgiant, then red giant phase with convective envelope.

In low-mass stars, helium fusion ignites suddenly at around 100 million Kelvin following the exhaustion of core hydrogen, triggered by the degenerate core causing a rapid "helium flash." This flash burns helium intensely, releasing enormous energy that stabilizes the star and shifts it to the horizontal branch.

Helium Fusion and Flash

• Helium fusion ignites at ~100 million Kelvin.
• Follows core hydrogen exhaustion in low-mass stars.
• Degenerate core causes sudden helium flash ignition.
• Flash rapidly burns helium, releasing vast energy.
• Stabilizes star, transitioning to horizontal branch.

After helium core exhaustion, shell helium burning begins, propelling the star up the Asymptotic Giant Branch (AGB). This phase features thermal pulses that trigger periodic helium flashes, along with intensified mass loss and deeper surface convection.

Second Red Giant and Shell Burning

• Helium core exhaustion initiates shell helium burning
• Star ascends to Asymptotic Giant Branch (AGB)
• Thermal pulses cause periodic helium flashes
• Enhanced mass loss and surface convection

This slide introduces the section on the end stages of low-mass stars, focusing on their evolution into white dwarfs. It specifies that this fate applies to stars with masses below 8 solar masses.

Low-Mass Star End: White Dwarfs

Fate of stars with masses less than 8 solar masses

White dwarfs form when a star ejects its outer envelope as a planetary nebula, leaving behind a dense, Earth-sized core composed of carbon and oxygen. With no nuclear fusion occurring, the core cools slowly over billions of years and eventually fades into an inert black dwarf.

White Dwarf Formation and Properties

• Ejects outer envelope as planetary nebula
• Leaves dense carbon-oxygen core, Earth-sized
• No nuclear fusion; cools slowly over time
• Eventually fades to inert black dwarf

In binary systems, a white dwarf accretes hydrogen from its companion star, leading to a thermonuclear runaway that ignites explosive fusion on its surface. This ejects the outer layers in a bright nova outburst, with the white dwarf surviving intact and potentially enabling recurrent events.

Nova in Binary Systems

!Image

• Accretion of hydrogen onto white dwarf from companion star.
• Thermonuclear runaway ignites explosive hydrogen fusion on surface.
• Ejection of outer layers causes bright nova outburst.
• White dwarf survives intact, allowing potential recurrent events.

Source: Nova

This section header slide introduces "High-Mass Star Evolution" as the 10th part of "The Life and Death of Stars: From Protostars to Black Holes." It highlights that stars exceeding eight solar masses experience short, intense, and explosive lives.

The Life and Death of Stars: From Protostars to Black Holes

10

High-Mass Star Evolution

Stars exceeding eight solar masses live short, intense, and explosive lives.

The timeline outlines the late evolutionary stages of a high-mass star leading to supernova, starting around 10 million years when it exhausts hydrogen and expands into a blue supergiant, fusing helium in its core. Over the next 10 million years, successive shell burnings occur—neon to oxygen, oxygen to silicon, and silicon to iron-group elements—culminating in an iron core that halts fusion and triggers rapid collapse at about 20 million years.

High-Mass Path to Supernova

~10 Myr: Supergiant Phase Begins High-mass star exhausts hydrogen, expands into blue supergiant with helium core fusion. ~15 Myr: Neon and Oxygen Burning Core contracts; neon fuses to oxygen, oxygen to silicon in successive shells. ~18 Myr: Silicon Fusion Stage Silicon burning produces iron-group nuclei; energy release peaks before decline. ~20 Myr: Iron Core Formation Iron core accumulates; no further fusion energy, leading to rapid collapse.

Source: Stellar Evolution Presentation

Core-collapse supernovae occur when an iron core implodes and rebounds violently, generating a shock wave that ejects stellar material into the interstellar medium. This process drives the r-process nucleosynthesis of heavy elements beyond iron, seeding the universe with essential metals for life.

Supernovae and Nucleosynthesis

• Core-collapse supernovae: iron core implodes and rebounds violently.
• Shock wave ejects stellar material into interstellar medium.
• R-process rapidly synthesizes heavy elements beyond iron.
• Supernovae seed universe with metals essential for life.

Neutron stars are the dense remnants of massive stars after a supernova, with masses between 1.4 and 3 solar masses but a radius of just about 10 km, where atomic nuclei are packed extraordinarily tightly. Pulsars are rotating neutron stars that emit magnetic beams, pulsing like a lighthouse as these beams sweep across Earth.

Neutron Stars and Pulsars

• Post-supernova remnants of massive stars
• Mass: 1.4–3 solar masses, radius ~10 km
• Incredibly dense: nuclei packed tightly
• Pulsars: rotating neutron stars with magnetic beams
• Lighthouse analogy: beams pulse as they sweep Earth

Black holes form event horizons in stars exceeding three solar masses, leading to a central singularity where density becomes infinite. Theoretical Hawking radiation allows particles to potentially escape, challenging the notion of total inescapability.

Black Holes and Singularities

!Image

• Event horizon forms in stars >3 solar masses.
• Singularity: infinite density point at black hole center.
• Hawking radiation enables theoretical particle escape.

Source: Black hole Wikipedia image search

Stars forge heavy elements through fusion and supernovae, enriching the cosmos to enable the formation of new worlds and life. The deaths of stars give birth to the universe's future, as highlighted in this conclusion slide.

Stellar Evolution's Cosmic Role

Stars forge heavy elements through fusion and supernovae, enriching the cosmos for new worlds and life. Stellar deaths birth the universe's future.

Thank you!

Source: The Life and Death of Stars: From Protostars to Black Holes