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Flip to reveal answersWhat is a star's 'main-sequence lifetime'?
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All 12 Flashcards — Stellar lifetime and mass loss
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Question
What is a star's 'main-sequence lifetime'?
Answer
How long the star spends steadily **fusing hydrogen into helium** — the long, stable middle of its life.
Question
What does 'luminosity (L)' mean?
Answer
The total energy a star radiates **every second** — its power output, in watts (W = J s⁻¹).
Question
How do you estimate a star's main-sequence lifetime?
Answer
Lifetime = energy the fusible hydrogen releases ÷ luminosity: **t = E ÷ L**. Then convert seconds to years.
Question
Is t = E ÷ L given in the data booklet?
Answer
**No** — you build it yourself from 'luminosity = energy used per second', so lifetime = energy available ÷ luminosity.
Question
Why is the fusible fuel far less than the star's mass?
Answer
Only the **core's** hydrogen fuses (~10–12% of the mass), and only **~0.7%** of that mass becomes energy. Multiply by both.
Question
Which equation turns the fuel mass into energy?
Answer
$E = mc^{2}$ — mass-energy equivalence (given in the data booklet).
Question
Why does a brighter star have a shorter lifetime?
Answer
A high luminosity means it **burns through its fuel faster**, so even with lots of fuel it runs out sooner.
Question
How do you find the mass a star loses by radiating energy?
Answer
**Δm = E ÷ c²**, where E is the total energy it radiates. (Rearranged from E = mc².)
Question
Name one assumption behind a lifetime estimate.
Answer
The **luminosity stays constant**; or only the core hydrogen fuses; or a fixed ~0.7% of the mass is converted; or the fusion rate is steady.
Question
How do you convert a lifetime from seconds into years?
Answer
**Divide by about 3.16 × 10⁷** — the number of seconds in one year.
Question
A star's fuel is worth E = 1.8 × 10⁴⁴ J and its luminosity is L = 5.0 × 10²⁶ W. Lifetime?
Answer
t = E ÷ L = 3.6 × 10¹⁷ s ≈ **1.1 × 10¹⁰ years** (÷ 3.16 × 10⁷).
Question
A star radiates E = 1.8 × 10⁴⁴ J over its life. Mass lost?
Answer
Δm = E ÷ c² = 1.8×10⁴⁴ ÷ (3.00×10⁸)² ≈ **2.0 × 10²⁷ kg**.
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Topic 5.5 hub
Fusion and stars
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