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Flip to reveal answersWhat is a star's 'main-sequence lifetime'?
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All 15 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**.
Question
How do you find a star's energy supply by counting reactions?
Answer
Fuel mass ÷ (4 × proton mass) = number of p–p chains; × energy per chain = total energy E.
Question
Do more massive main-sequence stars live longer or shorter lives?
Answer
**Shorter**: they have more fuel, but their luminosity is far greater, so they use it up much faster.
Question
Roughly what fraction of the Sun's hydrogen fuses before it leaves the main sequence?
Answer
About **10 %**: the hydrogen in its core.
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Topic 5.5 hub
Fusion and stars
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