21.2 The H–R Diagram and the Study of Stellar Evolution
3 min read•june 12, 2024
The is a stellar roadmap, showing how stars' brightness and temperature relate. It reveals where stars are in their life cycles, from birth as to their prime and eventual fate as , neutron stars, or black holes.
A star's mass is its destiny, determining its position on the and its evolutionary path. Heavier stars burn brighter but die faster, while lighter stars live long, quiet lives. The H-R diagram helps us understand these cosmic lifecycles and the creation of elements.
The H-R Diagram and Stellar Evolution
Interpretation of H-R diagram
Hertzsprung-Russell (H-R) diagram plots stars based on (vertical axis, more luminous stars at top) and (horizontal axis, hotter stars on left, cooler stars on right)
Protostars form from gravitational collapse of not yet hot enough to fuse hydrogen in cores located to right of main sequence on H-R diagram indicating lower temperatures
As protostar contracts, core temperature increases when reaching about 10 million K, hydrogen fusion begins marking star's arrival on main sequence
Main sequence is diagonal band on H-R diagram where stars spend most of their lives in with inward force of gravity balanced by outward pressure from (, )
Stars on the main sequence are classified based on their spectral characteristics ()
Mass influence on stellar evolution
Star's initial mass determines position on main sequence with more massive stars being hotter and more luminous at top left (, ) and less massive stars being cooler and less luminous at bottom right (, )
Mass also determines main sequence lifetime with more massive stars having shorter lifetimes due to higher rate of nuclear fusion and less massive stars having longer lifetimes due to lower rate of nuclear fusion
After main sequence, evolutionary path on H-R diagram depends on mass:
Low-mass stars (less than 8 ) become , then ()
High-mass stars (more than 8 solar masses) become , then undergo explosion leaving behind () or ()
The relationship between a star's mass and its is described by the
Timescales of stellar formation
Time for star formation depends on mass with lower-mass stars taking longer to form due to slower accumulation of material from molecular cloud and higher-mass stars forming more quickly due to greater gravitational attraction rapidly drawing in material
Pre-main sequence phase from protostar to main sequence also varies with stellar mass:
Lower-mass stars spend more time as protostars before reaching main sequence (0.5 solar masses may take about 100 million years)
Higher-mass stars spend less time as protostars and reach main sequence more quickly (5 solar masses may take only about 1 million years)
Stellar Evolution and Nucleosynthesis
is the process by which stars create heavier elements through nuclear fusion
As stars evolve, they follow specific paths on the H-R diagram known as
The is the maximum mass a white dwarf can have before electron degeneracy pressure can no longer support it against gravitational collapse
Key Terms to Review (39)
Betelgeuse: Betelgeuse is a red supergiant star located in the constellation Orion, known for its distinctive reddish-orange hue. As one of the largest and most luminous stars visible to the naked eye, Betelgeuse has become an important subject of study in various fields of astronomy, from understanding stellar evolution to exploring the nature of interstellar matter.
Black hole: A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. They are formed from the remnants of massive stars after they undergo supernova explosions.
Black Hole: A black hole is an extremely dense and massive object in space from which nothing, not even light, can escape due to its immensely strong gravitational pull. Black holes are formed when a massive star collapses in on itself at the end of its life cycle, creating a singularity with an event horizon that marks the point of no return.
Chandrasekhar limit: The Chandrasekhar limit is the maximum mass (approximately 1.4 times the mass of the Sun) that a white dwarf star can have before it collapses under its own gravity. Beyond this limit, the white dwarf will undergo further gravitational collapse to form a neutron star or black hole.
Chandrasekhar Limit: The Chandrasekhar limit is the maximum mass above which a star can no longer support itself against gravitational collapse after exhausting its nuclear fuel. It is a critical threshold that determines the fate of a star's evolution and the type of stellar remnant it will leave behind.
Crab Pulsar: The Crab Pulsar is a rapidly rotating neutron star located at the center of the Crab Nebula, a supernova remnant. It is one of the most studied and well-known pulsars, providing crucial insights into the nature of neutron stars and the processes that govern stellar evolution.
Cygnus X-1: Cygnus X-1 is a well-known binary star system located in the constellation Cygnus, approximately 6,070 light-years from Earth. It is considered one of the strongest candidates for a black hole, as it exhibits characteristics that strongly suggest the presence of a compact, high-mass object with a gravitational field so strong that not even light can escape it.
Giant molecular clouds: Giant molecular clouds are vast regions of gas and dust in space, primarily composed of molecular hydrogen. They are the primary sites for star formation within galaxies.
H-R Diagram: The H-R diagram, also known as the Hertzsprung-Russell diagram, is a graphical representation that plots the relationship between the intrinsic brightness (absolute magnitude) and the surface temperature (spectral class) of stars. It is a fundamental tool used in the study of stellar evolution and the classification of stars.
H–R diagram: The H–R diagram, or Hertzsprung-Russell diagram, is a scatter plot of stars showing the relationship between their absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. It is a fundamental tool in understanding stellar evolution and properties.
Hertzsprung-Russell diagram: The Hertzsprung-Russell (H-R) diagram is a scatter plot that illustrates the relationship between the luminosity, or absolute brightness, and the surface temperature or spectral type of stars. It is a fundamental tool in the study of stellar evolution and the classification of stars.
Hydrostatic equilibrium: Hydrostatic equilibrium is the balance between the inward gravitational force and the outward pressure within a star. This balance maintains the star's spherical shape and prevents it from collapsing or expanding uncontrollably.
Hydrostatic Equilibrium: Hydrostatic equilibrium is a state of balance where the gravitational force acting on a body is exactly balanced by the buoyant force, resulting in a stable, stationary state. This concept is fundamental to understanding the composition and structure of planets, the sources of energy in stars, and the evolution of stellar objects.
Kepler’s Supernova: Kepler’s Supernova is a Type Ia supernova that was observed in 1604 within the Milky Way galaxy. Named after astronomer Johannes Kepler, it is one of the few supernovae visible to the naked eye in recorded history.
Luminosity: Luminosity is the total amount of energy a star emits per unit of time, measured in watts. It depends on both the star's temperature and radius.
Luminosity: Luminosity is a measure of the total amount of energy emitted by a celestial object, such as a star, over a given period of time. It is a fundamental property that describes the intrinsic brightness of an object and is closely related to its size and temperature.
Main sequence: The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. Stars spend the majority of their lifetimes in this phase, where they are fusing hydrogen into helium in their cores.
Main Sequence: The main sequence is a band on the Hertzsprung-Russell (H-R) diagram where the majority of stars spend most of their lives. It represents a stage in a star's life cycle where nuclear fusion of hydrogen into helium is the dominant energy-producing process occurring in the star's core.
Molecular Clouds: Molecular clouds are vast, dense regions of the interstellar medium composed primarily of molecular hydrogen and other molecules. These clouds serve as the birthplace for new stars and play a crucial role in the life cycle of cosmic material throughout the universe.
Neutron Star: A neutron star is an extremely dense, collapsed stellar remnant that forms when a massive star runs out of fuel and undergoes a supernova explosion, leaving behind a core so dense that the electrons are forced to combine with protons, creating a star composed almost entirely of neutrons. These incredibly dense objects have immense gravitational fields and are some of the most extreme objects in the universe.
Nuclear Fusion: Nuclear fusion is the process in which two or more atomic nuclei collide at very high temperatures and fuse together to form a new, heavier nucleus. This release of energy is the fundamental source of power for the Sun and other stars, as well as a potential future source of energy for human use.
Protostars: Protostars are the earliest stage of stellar evolution, where a dense cloud of gas and dust begins to collapse under its own gravity to form a new star. They are the precursors to main-sequence stars, the most common type of stars in the universe.
Proxima Centauri: Proxima Centauri is the closest star to the Sun, located just over 4 light-years away in the constellation of Centaurus. It is a small, low-mass red dwarf star that is part of the Alpha Centauri triple star system, which is the closest stellar system to our solar system.
Red Giants: Red giants are large, cool, and luminous stars that have evolved from the main sequence stage of their life cycle. They are characterized by their reddish appearance, expanded size, and decreased surface temperature compared to their earlier main sequence phase.
Rigel: Rigel is a prominent blue supergiant star located in the Orion constellation. It is one of the brightest stars in the night sky and holds significance in various aspects of stellar astronomy, including the brightness of stars, stellar census, measuring stellar masses, diameters of stars, the Hertzsprung-Russell (H-R) diagram, and the study of stellar evolution.
Sirius: Sirius, also known as the Dog Star, is the brightest star in the night sky. It is a binary star system located in the constellation Canis Major, approximately 8.6 light-years from Earth. Sirius has been an important astronomical object throughout human history, with its prominence in the night sky and its significance in various cultural and religious traditions.
Sirius B: Sirius B is a white dwarf star that is the smaller and denser companion to the bright star Sirius, the Dog Star. It is a collapsed, extremely dense stellar remnant that represents the final stage of a medium-sized star's life cycle.
Solar Masses: Solar masses are a unit of measurement used to express the mass of celestial objects, particularly stars, in relation to the mass of the Sun. It provides a convenient way to quantify the size and scale of stellar objects within the context of the H-R Diagram and the study of stellar evolution.
Spectral Classification: Spectral classification is a scheme used to categorize stars based on their observed spectral characteristics, which are directly related to their surface temperature and chemical composition. This classification system is a fundamental tool in the study of stellar properties and evolution.
Stellar Evolution Tracks: Stellar evolution tracks refer to the paths that stars follow on the Hertzsprung-Russell (H-R) diagram as they progress through different stages of their life cycle. These tracks illustrate how a star's properties, such as luminosity and surface temperature, change over time as it undergoes various nuclear fusion processes and structural changes.
Stellar Mass-Luminosity Relation: The stellar mass-luminosity relation is a fundamental relationship that describes the connection between the mass and luminosity of stars. It is a crucial concept in the study of stellar evolution and the understanding of the H–R diagram.
Stellar Nucleosynthesis: Stellar nucleosynthesis is the process by which new atomic nuclei are created inside stars through nuclear fusion reactions. This process is responsible for the creation and distribution of the elements that make up the universe, from the lightest elements like hydrogen and helium to the heavier elements like carbon, oxygen, and iron.
Sun: The Sun is the star at the center of the solar system, providing light, heat, and energy that sustains life on Earth. As the closest star to our planet, the Sun's gravitational influence shapes the orbits of the planets and other objects in the solar system, and its nuclear fusion powers the processes that drive the evolution of the universe.
Supergiants: Supergiants are a class of the most luminous and largest stars in the universe. They are extremely bright and massive, with diameters hundreds of times larger than the Sun, making them some of the most prominent celestial objects in the night sky.
Supernova: A supernova is a powerful and luminous stellar explosion that occurs at the end of a massive star's life cycle. It is one of the most energetic and dramatic events in the universe, releasing an immense amount of energy and ejecting vast amounts of material into space.
Surface Temperature: Surface temperature refers to the temperature of the outermost layer or surface of a celestial body, such as a planet or star. It is a crucial parameter that helps understand the physical characteristics and atmospheric conditions of these objects.
TRAPPIST-1: TRAPPIST-1 is a planetary system located approximately 40 light-years from Earth, consisting of an ultra-cool dwarf star and at least seven Earth-sized exoplanets orbiting it. This system has become a focus of study in various areas of astronomy, including the comparison of planetary systems, the understanding of stellar evolution, the formation of planets, and the potential for habitable worlds in the cosmic context of life.
White dwarfs: White dwarfs are dense, compact remnants of low to medium-mass stars that have exhausted their nuclear fuel and expelled their outer layers. They are roughly the size of Earth but contain a mass comparable to that of the Sun.
White Dwarfs: White dwarfs are the dense, compact remnants of low- to medium-mass stars that have exhausted their nuclear fuel and shed their outer layers, leaving behind a core composed primarily of degenerate matter. They are one of the final stages in the life cycle of many stars and play a crucial role in our understanding of stellar evolution, the H-R diagram, and gravitational wave astronomy.