lecture 4 Flashcards
(38 cards)
What is the main-sequence stage of a low-mass star?
The main-sequence stage is when a star fuses hydrogen into helium in its core via the proton–proton chain, shining steadily for about 10 billion years.
This stage occupies about 90% of a star’s total lifetime.
What is the significance of the solar thermostat in a low-mass star?
The solar thermostat maintains gravitational equilibrium and energy balance, keeping the fusion rate and overall luminosity steady.
This process ensures a consistent output of energy from the star.
How long does the main-sequence phase last for low-mass stars?
About 10 billion years for stars like the Sun.
Stars with mass lower than the Sun have longer lifetimes.
What happens when a low-mass star like the Sun depletes its core hydrogen?
Nuclear fusion ceases, causing the core to shrink and the outer layers to expand, leading to the red giant stage.
This phase occurs after about 10 billion years of stable fusion.
What characterizes the red giant stage of a low-mass star?
The outer layers expand while the core shrinks, and the star becomes more luminous, growing over 100 times larger in radius.
Hydrogen shell fusion begins around the inert helium core.
What is hydrogen shell fusion?
Hydrogen fusion that occurs in a shell surrounding the inert helium core, producing significant energy and causing the star to expand.
This process contributes to the increased luminosity of red giants.
What is the triple-alpha reaction in helium core fusion?
A process where three helium nuclei fuse to form one carbon nucleus, releasing energy due to mass loss.
This process requires much higher temperatures than hydrogen fusion.
What initiates a helium flash in a low-mass star?
The rapid increase in temperature in the inert helium core leads to a spike in the helium fusion rate.
This occurs when degeneracy pressure is overcome by thermal pressure.
What happens to a low-mass star after it exhausts core helium?
The star expands again due to helium fusion in a shell around an inert carbon core, becoming a double shell-fusion star.
This stage occurs after about 100 million years of helium fusion.
What is a planetary nebula?
A glowing shell of gas ejected from a dying star, ionized by the exposed hot core.
Despite the name, planetary nebulae have no relation to planets.
What is a white dwarf?
The remnant core of a low-mass star that has exhausted its nuclear fuel, supported by degeneracy pressure.
White dwarfs cool over time and eventually fade from view.
What is the CNO cycle in high-mass stars?
A series of reactions where hydrogen is fused into helium using carbon, nitrogen, and oxygen as catalysts, allowing faster fusion rates.
This process contributes to the higher luminosity of high-mass stars compared to low-mass stars.
How does the life of a high-mass star differ from that of a low-mass star?
High-mass stars fuse heavier elements and have shorter, more intense lifetimes, culminating in a supernova explosion.
They proceed through stages more rapidly than low-mass stars.
Fill in the blank: The _______ stage occurs when a low-mass star’s core hydrogen is depleted.
red giant
True or False: Low-mass stars can produce elements heavier than helium during their lifetime.
False
What ultimately happens to a high-mass star when fusion stops?
The core implodes, leading to a supernova explosion.
This marks the dramatic end of a high-mass star’s life.
How does the luminosity of high-mass stars compare to low-mass stars?
High-mass stars have much higher luminosities due to faster fusion rates in their cores.
This is a result of the CNO cycle.
What is the final fate of a white dwarf?
It will cool over time and eventually become too cold to emit visible light.
White dwarfs are remnants of low-mass stars.
What is the proton chain?
Four hydrogen nuclei fuse into one helium-4 nucleus.
How does the CNO cycle differ from the proton-proton chain?
The CNO cycle allows hydrogen fusion to proceed at a much faster rate.
Why do high-mass stars have higher luminosities than low-mass stars?
High-mass stars fuse hydrogen faster, leading to higher luminosities.
What happens to a 25-solar-mass star as it runs low on hydrogen fuel?
It develops a hydrogen-fusing shell and expands into a supergiant.
What occurs when the core hydrogen of a high-mass star runs out?
The core contracts, raising the temperature to fuse helium into carbon.
Do high-mass stars experience a helium flash?
No, high-mass stars do not experience a helium flash.
