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All 15 Flashcards — Stellar evolution and nucleosynthesis
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Question
What decides how a star evolves and what it becomes?
Answer
Its **mass**. Low-mass stars end as white dwarfs; high-mass stars end in supernovae, leaving neutron stars or black holes.
Question
Give the life cycle of a low-mass star like the Sun.
Answer
main sequence → **red giant** → **planetary nebula** → **white dwarf**.
Question
Give the life cycle of a high-mass star.
Answer
main sequence → **red supergiant** → **supernova** → **neutron star** (or **black hole** if heavy enough).
Question
What is a planetary nebula?
Answer
The glowing shell of gas a dying **low-mass** star gently puffs off (it has nothing to do with planets).
Question
What is a white dwarf?
Answer
The small, hot, dense leftover core of a **low-mass** star after it sheds its outer layers; it just cools over time.
Question
What is a supernova?
Answer
The violent explosion that ends a **massive** star's life, leaving a neutron star or a black hole.
Question
What is nucleosynthesis?
Answer
The making of **heavier elements** by fusion inside stars (e.g. helium → carbon → ... up to iron in massive stars).
Question
How does fusion in a massive evolved star differ from the Sun's?
Answer
The Sun fuses only **hydrogen into helium**. A hotter, massive star fuses **heavier elements** (carbon, oxygen...) up to **iron**.
Question
Why can only massive stars fuse heavier elements?
Answer
Heavier nuclei repel more strongly, so fusing them needs a **hotter** core — only a massive star's core gets that hot.
Question
Why does fusion in stars stop at iron?
Answer
Fusing up TO iron releases energy, but fusing iron into heavier elements would **cost** energy — so even massive stars can go no further by fusion.
Question
How do we know which elements a star contains?
Answer
From its **absorption spectral lines** — each element absorbs its own wavelengths, leaving a unique pattern of dark lines (a fingerprint).
Question
Why does each element make its own absorption lines?
Answer
Its electrons only absorb photons whose energy exactly matches the gaps between its **energy levels**, which are unique to that element.
Question
How does the Sun move on the HR diagram as it becomes a red giant, then a white dwarf?
Answer
**Up and right** to the red giant, then **down and left** to the white dwarf.
Question
What will a three-solar-mass main-sequence star end as?
Answer
A **white dwarf**: it is below the mass needed for a supernova (about 8 solar masses).
Question
Order these by stage: red giant, main-sequence star, white dwarf.
Answer
**Main sequence → red giant → white dwarf.**
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Full study notes for Stellar evolution and nucleosynthesis
Topic 5.5 hub
Fusion and stars
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