Energy Transport and Nuclear Burning in Stars · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How starlight gets out

Science · Astronomy & Astrophysics · ages 20-22
Name ______________________   Date ____________

  1. What does the CNO cycle need?

    • Only cold hydrogen gas
    • A hotter heavier core plus carbon, nitrogen, and oxygen as catalysts
    • No fuel at all

  2. Which fusion route powers a Sun-like star?

    • The proton-proton chain
    • The CNO cycle
    • Gravitational contraction

  3. Core photons reach the solar surface within days.

    Circle one:   True   False

  4. What do solar neutrinos tell us?

    • Fusion is powering the Sun right now
    • The Sun will cool tomorrow
    • Photons travel faster than neutrinos

  5. What decides between radiative and convective transport?

    • The star's name
    • The observer's telescope
    • Temperature and opacity of the gas

  6. Four protons fuse into one helium nucleus. Type how many protons go in.

    Answer: ______________

  7. A star has twice the Sun's mass. Which fusion route dominates?

    • Helium burning only
    • Gravitational contraction
    • CNO cycle dominates

  8. A student says contraction still powers today's Sun. What is wrong?

    • Contraction would cool the Sun instantly
    • Contraction funds only about 100 million years
    • Gravity cannot make any heat
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How starlight gets out W1-mt_2rc4UAMyl--s1

  1. A hotter heavier core plus carbon, nitrogen, and oxygen as catalysts · CNO catalysts hurry hydrogen fusion, but only in hotter massive cores.
  2. The proton-proton chain · Cooler Sun-like cores burn through the proton-proton chain.
  3. False · Swallowed and re-emitted constantly, they wander for tens of thousands of years.
  4. Fusion is powering the Sun right now · Neutrinos escape in minutes, so they report today's core burning.
  5. Temperature and opacity of the gas · Clear hot gas lets photons through; cool opaque gas must overturn instead.
  6. 4 · The chain starts with four protons and ends with one helium nucleus.
  7. CNO cycle dominates · Heavier cores run hotter, where the CNO cycle outpaces the chain.
  8. Contraction funds only about 100 million years · Contraction falls billions of years short, so fusion must supply the Sun.
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