Stellar Evolution: Birth, Life, and Death Notes

How do stars form?

They are born within a diffuse nebulae

What is a diffuse nebulae?

vast clouds of gas, mostly hydrogen and helium, spread across many light-years

What forces act within a diffuse nebula?

gravity, thermal pressure, magnetic fields, and cosmic rays

What leads to the gravitational collapse of gas within a nebula to form stars?

Regions within a nebula can become dense enough that gravity overcomes other forces, leading to collapse and star formation

What are two key forces that are a balance for the formation of stars?

gravity pulling inward and thermal pressure pushing outward

What is the Jeans limit?

a critical mass where self-gravity overcomes thermal pressure in a cloud of gas, leading to collapse. It varies with the temperature and density of the gas.

What happens if the cloud's mass is greater than the Jeans limit?

Gravity wins over thermal pressure, and the cloud collapses

What happens if the cloud's mass is less than the Jeans limit?

Thermal pressure pushes outward and the cloud expands

What happens if the cloud's mass is equal to the Jeans limit?

Gravity and pressure balance, and the cloud remains in equilibrium

How do magnetic fields affect star formation?

It can prevent gas contraction in some regions, influencing where and how stars can form.

What marks the transition of a collapsing cloud of gas to a protostar?

A collapsing cloud becomes a protostar when it becomes dense and hot enough to convert gravitational energy into heat, marking a key stage in star development.

What causes a protostar to heat up as it collapses?

Kelvin-Helmholtz contraction

What kind of variable star forms from pulsations during protostar collapse?

Tauri variable star

What must happen for a protostar to become a true star?

Its core must get hot enough to ignite hydrogen fusion

When is a star considered "born"?

When hydrogen fusion balances gravity

True or False: All stars are born as main-sequence stars.

True

What determines a star's position on the main sequence?

Its birth mass

What spectral types are high-mass stars born as?

hot and luminous, often O or B-type stars

What spectral types are low-mass stars born as?

cooler and dimmer, often K and M-type stars

What is the most important property of a star?

Its mass

What spectral types do intermediate-mass stars typically have?

moderate temperature and brightness, often A, F, or G-type stars

Why are high-mass stars hot and bright?

Their strong gravity requires high pressure and high temperature to balance, making them hot and luminous.

Why are low-mass stars cool and dim?

Their weak gravity needs only weak pressure to balance, resulting in low temperatures and brightness.

What is the Eddington limit?

The maximum mass (roughly 100 M☉) a star can have before radiation pressure prevents collapse into a main sequence star.

What happens if a protostar exceeds the Eddington limit?

Radiation pressure overwhelms gravity, causing the star to blow itself apart before reaching the main sequence.

What is the most massive type of main sequence star?

O0V-type stars, with masses near the Eddington limit (~100 M☉).

What is another name for main sequence stars?

Hydrogen-burning stars

What is the lower mass limit for a star to sustain hydrogen fusion?

About 0.08 M☉ (8% of the Sun's mass).

Why can't objects below 0.08 M☉ become main sequence stars?

Their cores never get hot enough to ignite hydrogen fusion due to low mass and insufficient gravity.

What stabilizes low-mass protostars that don't undergo fusion?

Electron degeneracy pressure

What are objects called that fall below the hydrogen fusion mass limit?

Brown dwarfs

Why aren't brown dwarfs considered true stars?

Because they do not sustain hydrogen fusion in their cores.

What is the lifespan of brown dwarf stars?

Being too cool to fuse hydrogen, may live indefinitely, essentially remaining "alive" for as long as the universe exists.

What is the spectral type of the coolest, lowest-mass true main sequence stars?

M9V

What distinguishes main sequence stars from other stellar objects?

Main sequence stars fuse hydrogen into helium in their cores, defining their status on the Hertzsprung-Russell diagram.

Brown dwarfs are cooler and dimmer than

M-type main sequence stars

Brown dwarfs are classified as

L-type stars and are further to the right and down on the Hertzsprung-Russell diagram.

What kind of light do brown dwarfs primarily emit?

Infrared light

What happens if a protostar's mass is greater than 0.08 M☉?

It becomes a main sequence star that fuses hydrogen into helium.

What happens if a protostar's mass is less than 0.08 M☉?

It becomes a brown dwarf.

What is the key difference between a brown dwarf and a gas-giant planet?

Their formation—brown dwarfs form from collapsing gas clouds, while gas giants form in protoplanetary disks.

What do brown dwarfs and gas giants have in common?

They are cool, low-mass spheres made mostly of hydrogen and helium, and they do not undergo nuclear fusion.

Is Jupiter a failed star? Why or why not?

No, Jupiter is not a failed star; its mass (0.001 M☉) is far below the 0.08 M☉ threshold needed for fusion.

Why does a protostar spin faster as it collapses?

Due to the Law of Conservation of Angular Momentum.

Protostars lose angular momentum through the formation of

protoplanetary disks and the ejection of ionized gases via strengthening magnetic fields, preventing centrifugal forces from overpowering gravitational collapse

How is angular momentum distributed in our Solar System?

The Sun holds ~99.9% of the mass but less than 4% of the angular momentum; over 96% is in the planets and disk.

What are Herbig-Haro objects (HH objects)?

Glowing clouds of gas formed when jets from protostars collide with surrounding material.

Fragmentation occurs when a protostar, rotating faster due to collapse,

undergoes enough centrifugal force to split into two or more protostars, leading to the formation of binary or multiple star systems.

What is a trinary star system?

A system where three stars form—usually two orbit closely while the third orbits farther out.

What kind of star is α Centauri A?

A G2V star, like our Sun.

What is the closest star to Earth besides the Sun?

Proxima Centauri (α Centauri C).

Name three other nearby star systems.

Barnard's Star (nearly 2 parsecs away), Luhman 16 binary (roughly 2 parsecs away), Wolf 359 (about 2.4 parsecs away), and Sirius binary (nearly 3 parsecs away).

What is a double binary system?

A quadruplet star system where two binaries orbit each other.

Can fragmentation form systems with more than four stars?

Yes, it can form quintuplets (five), sextuplets (six), or even larger systems, though these are rare

What is unusual about the Sun's formation?

It formed alone, without fragmentation, which is rare.

Why are high-mass main sequence stars rare?

High-mass protostars often fragment into smaller, lower-mass stars during formation.

What does it mean that the main sequence is a "population-abundance sequence"?

Earlier-type stars (O, B, A, etc.) are less common, while later-type stars (K, M) are more abundant.

List the spectral types from most to least abundant.

M, K, G, F, A, B, O.

What is true of a star earlier in the OBAFGKM sequence compared to one later?

It is hotter, brighter, larger, more massive, and rarer.

Is the main sequence an evolutionary sequence?

No, it is not based on a star's age or evolution.

What six types of sequences does the main sequence represent?

Temperature, luminosity, size (radius), mass, population-abundance, and spectral type.

high-mass stars have shorter main sequence lifetimes because their

intense luminosity speeds up nuclear fusion reactions, consuming hydrogen more quickly

low-mass stars have longer main sequence lifetimes because they

burn hydrogen more slowly

Why is it that low mass stars can outlive high mass stars, even with less hydrogen?

Low mass stars burn their hydrogen very slowly, allowing them to sustain themselves over much longer periods.

How do the temperatures and luminosities of early-type main sequence stars affect their fusion rates?

Early-type stars are hotter and more luminous, which causes them to burn hydrogen rapidly in their cores.

How do late-type main sequence stars differ in terms of fusion compared to early-type stars?

Late-type stars are cooler and less luminous, so they burn hydrogen much more slowly.

What is the key distinction between high mass and low mass stars in terms of fuel consumption?

High mass stars burn their fuel rapidly due to their higher temperatures and luminosities, while low mass stars burn fuel slowly.

How does nuclear fusion rate correlate with temperature?

Fusion occurs faster at higher temperatures and slower at cooler temperatures.

What is a helpful analogy to understand the difference in fuel usage between high mass and low mass stars?

High mass stars are like millionaires who spend quickly, while low mass stars are like people with modest incomes who spend slowly and last longer.

The death of a low-mass star takes

millions of years

The death of a high-mass star takes

thousands of years

Earlier main sequence stars are

hotter, more luminous, larger, more massive, less abundant, and have shorter lifetimes than later main sequence stars

How do high-mass stars (like O-type stars) burn their hydrogen fuel?

High mass stars burn their hydrogen fuel rapidly due to their intense heat and luminosity, leading to much shorter lifespans of around a million years.

How long do B-type main sequence stars live?

Around 10 million years

What is the main-sequence lifetime of A-type stars?

Live around 100 million years

How long do F-type main-sequence stars live?

About 1 billion years

What is the main-sequence lifetime of G-type stars like the Sun?

About 10 billion years

How long do K-type main sequence stars live?

About 100 billion years

How long do M-type main sequence stars live?

Around 1 trillion years

What is the relationship between stellar mass and main-sequence lifetime?

Higher mass stars burn their fuel quickly and have shorter lifespans, while lower mass stars burn fuel slowly and have much longer lifespans.

What determines a star's main-sequence lifetime and death duration?

A star's mass

How does mass affect the protostar phase?

Higher mass stars have shorter protostar stages, while low-mass stars have longer protostar phases.

Which phase of a star's life is longer: the main-sequence phase or the birth and death phases?

The main-sequence

Can a high-mass star live longer than it takes for a low-mass protostar to collapse?

No, a high-mass star could be born, live, and die in less time than it takes for a low-mass protostar to collapse.

How do a star's luminosity, temperature, and radius change during the main-sequence phase?

Gradual changes in luminosity, temperature, and radius occur, but these changes are relatively small compared to the significant changes during birth and death.

Where do stars remain on the Hertzsprung-Russell diagram during their main-sequence lifetime?

Stars remain at nearly the same position during most of their main-sequence lifetime.

What makes it possible to compare stars along the main sequence?

Since a star's physical quantities (like temperature, luminosity, radius, and mass) remain relatively constant, we can compare them based on spectral type.

What does the main sequence represent?

Temperature, luminosity, radius, mass, population abundance, and lifetime.

What marks the beginning of stellar death for a main-sequence star?

Stellar death begins when a main-sequence star has exhausted the hydrogen in its core, marking the end of its hydrogen-burning lifetime.

What is the mass threshold that distinguishes low mass stars from high mass stars?

Low mass stars have a mass less than 7M☉, 8M☉, or 9M☉, while high mass stars have a mass greater than 7M☉, 8M☉, or 9M☉.

Which spectral types are associated with low mass stars?

A, F, G, K, or M types

Which spectral types are associated with high mass stars?

O or B types

How does the death process of high mass stars compare to low mass stars?

High mass death is much more violent, while low mass death is more gradual and gentle.

Why is it important to discuss high mass stellar death, despite it being rare?

It plays a crucial role in the creation of elements necessary for life, including our own existence.

What happens to a low mass star after it exhausts hydrogen in its core?

The helium core begins to collapse, generating heat, and a surrounding hydrogen layer becomes hot enough to begin fusing into helium, causing the outer layers to expand.

What is the result of a low mass star's outer layers expanding and cooling?

The star grows larger and cooler, giving it a redder appearance, and it becomes a red giant.

What is the sequence of changes as a low mass star evolves from main sequence to red giant?

The star first becomes an orange subgiant before transitioning into a red giant.

What causes the pulsations in a red giant star?

The imbalance between gravitational forces and thermal pressure causes the star to pulsate, leading to changes in luminosity.

What is the helium flash?

The event that marks the onset of helium fusion when the core reaches a high enough temperature.

What is the triple-alpha process?

The fusion of three helium nuclei into a carbon nucleus, which occurs during the helium-burning phase of a star.