12: Stellar Evolution
- Last updated
- Jul 23, 2025
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I like the night. Without the dark, we’d never see the stars.
Stephenie Meyer
Twilight
“I think that we sare like stars. Something happens to burst us open; but when we burst open and think we are dying; we’re actually turning into a supernova. And then when we look at ourselves again, we see that we’re suddenly more beautiful than we ever were before!”
C. JoyBell C.
Upon completion of this module, the student will be able to:Upon completion of this module, the student will be able to:
- Define the biology-like terms astronomers use when describing stars.
- Identify the characteristics of low-mass stars.
- Describe how white dwarfs, novae, and Type 1a supernovae are formed in low-mass stars.
- Identify the characteristics of high-mass stars.
- Describe how neutron stars, supernovae, and black holes are formed in high-mass stars.
- Define GRBs.
- Describe GRB characteristics
This module looks at how stars form and develop over time, including the less-massive stars, like the Sun, and more-massive stars.
- 12.1: Star Life
- This page examines the similarities between biological and astronomical terms, focusing on the life cycle of stars: their formation, evolution, and demise. It details star formation in molecular clouds, emphasizing the historical debate over stellar composition, notably Cecelia Payne-Gaposhkin's 1925 work suggesting stars are mostly hydrogen.
- 12.2: Stellar Birth
- This page discusses the interplay among pressure, temperature, and volume in gases, particularly the interstellar medium. It outlines how increased pressure raises temperature and reduces volume, hindering thermal energy loss, which aids in protostar formation. A star's birth occurs when the protostar's core temperature hits 10,000,000 K, triggering fusion via the proton-proton cycle.
- 12.3: Stellar Mass
- This page discusses how a star's characteristics and lifespan are determined by its initial mass, which is divided into low-mass, intermediate-mass, and high-mass categories. Each category has unique life cycles, with high-mass stars living only a few million years, whereas low-mass stars can last thousands of billion years. The content emphasizes comparing low- and high-mass stars regarding their mass, lifespan, and spectral classification.
- 12.4: Stellar Evolution
- This page discusses stellar evolution, highlighting that a star's life cycle is influenced by its initial mass. Massive stars have shorter lifespans of a few million years, whereas smaller stars can last for trillions of years, significantly outliving the universe's age of 13.7 billion years.
- 12.5: Low-Mass Stars
- This page explains the lifecycle of low-mass stars, such as the Sun, starting with hydrogen fusion via the proton-proton cycle. After hydrogen depletion, the star expands into a Subgiant and then a Red Giant, where outer layers expand and the core compresses, leading to Hydrogen Shell Burning. Eventually, helium fusion produces carbon, causing the star to dim and contract. This fusion cycle continues for millions of years until the core mainly comprises carbon.
- 12.6: White Dwarf
- This page discusses the evolution of a star into a White Dwarf after it stops core nuclear fusion, resulting in a planetary nebula. The remaining White Dwarf is a highly dense core, weighing about 5 tons per teaspoon, formed from the collapse of the star's core under gravity, following the process of hydrogen fusion into helium and then carbon.
- 12.7: Planetary Nebula
- This page explains the formation of planetary nebulae from low-mass stars ejecting gas shells illuminated by a central White Dwarf. Coined by William Herschel in the 1780s due to their planet-like shapes, these nebulae emit ultraviolet energy and are visible for tens of thousands of years. Eventually, they cool into a carbon sphere, sometimes referred to as a black dwarf, although the existence of black dwarfs in the Universe is debated.
- 12.8: White Dwarfs and Neighbors
- This page describes a White Dwarf star with an accretion disk, alongside a companion star being cannibalized, noting their color and size differences. It highlights a nova illuminating a reddish nebula, which will become less visible as the nova's light fades. The images are credited to NASA and the Hubble Space Telescope.
- 12.9: Nova, Novae, and Supernova
- This page explains that a nova occurs from a White Dwarf's fusion event, shining as bright as 100,000 Suns and forming a planetary nebula. This can repeat until reaching a limit of about 1.33 solar masses, leading to a Type 1a Supernova. This supernova results in an explosive event, creating elements up to iron and heavier elements like gold and uranium. Additionally, they serve as 'standard candles' for astronomers and may produce cosmic rays, including ultra-high energy particles.
- 12.10: High-Mass Stars
- This page discusses high-mass stars, which have shorter lifespans and undergo fusion through the CNO cycle, with Betelgeuse as an example. Upon exploding in a supernova, these stars release immense energy, briefly outshining galaxies. Their cores collapse into neutron stars, creating heavier elements and potentially harmful particles. The Crab Nebula is noted as a remnant of a supernova, highlighting the impact such events can have, particularly if they occur within 30 light-years of Earth.
- 12.11: Neutron stars
- This page discusses neutron stars, dense objects formed from the collapse of a massive star's core during a supernova. Proposed in 1934, these stars are about 10 miles in diameter and are kept intact by gravity. They include binary neutron stars, which are two orbiting each other, and pulsars, rapidly rotating neutron stars that emit regular electromagnetic radiation and sometimes gamma rays.
- 12.12: Neutron Star and Companion Star Scenario
- This page discusses X-ray binaries, which are binary systems of a neutron star and a companion star. The neutron star, being hotter and more luminous than a white dwarf, forms an accretion disk from the material taken from the companion star. This activity generates intense X-ray sources, known as X-ray binaries, capable of emitting bursts of energy called X-ray bursters.
- 12.13: High-Mass Star Stellar Endings
- This page discusses the fates of stars based on their mass. Stars under three solar masses will cool or accrete in binaries until their fuel runs out. In contrast, stars over three solar masses may collapse into black holes after a Type 2 Supernova. Black holes, which form an event horizon preventing light escape, can also expel energy when overloaded, eventually becoming singularities.
- 12.14: Gamma-Ray Bursts (GRBs)
- This page discusses the development of satellites in the 1960s to monitor gamma radiation due to nuclear testing concerns. These satellites detected gamma-ray bursts (GRBs), which are powerful events linked to massive star explosions, particularly hypernovae. GRBs can last from milliseconds to minutes and are followed by an afterglow. Research, including results from the Compton Gamma Ray Observatory, established that GRBs originate from outside the Milky Way galaxy.
