Stars, radiation and scale
Введено| English | Значение |
|---|---|
| flux/flʌks/ | поток |
| luminosity/ˌluːmɪˈnɒsɪti/ | светимость |
What would explain this observation?
- Two stars can have the same apparent brightness while having different luminosities. Distance changes the flux 通量 reaching an observer.
Build the model
- Luminosity 光度 is total power emitted; flux is power received per area.
- A stellar spectrum carries information about surface temperature and composition.
- Fusion transfers energy as light nuclei combine.
- luminosity: Total emitted power.
- flux: Power received per unit area.
At fixed luminosity, what happens to flux when distance doubles?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
Flux follows an inverse-square law, F proportional to 1/d^2, because the same luminosity spreads over a sphere of area 4 pi d^2. Doubling d gives 1/4 of the flux - not a half.
Match each technical term to its precise meaning.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For isotropic emission without absorption, flux follows an inverse-square relationship with distance.
- Observed brightness alone therefore cannot establish luminosity.
In the lab: Keep distance units consistent, identify which quantities are intrinsic to the star, and distinguish observational evidence from a model of stellar evolution. Do not confuse a red giant stage with every possible final remnant.
Which two habits make the investigation or model in this case more defensible?
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: distance doubles while luminosity remains fixed.
- Use F proportional to 1/d².
- Flux ratio = 1/2² = 1/4.
- A flux of 12 units becomes 3 units.
- This assumes no change in absorption or source output.
Your turn: A source gives flux 18 units. Find flux at three times the distance.
A source gives flux 18 units. Find flux at three times the distance.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
The result is 2 . Known: flux 18 units at the original distance; distance triples. Use F proportional to 1/d^2. New flux = 18/3^2 = 18/9 = 2 units. This assumes constant luminosity and no extra absorption.
Check the conclusion and its limits
- The Sun is not expected to become a supernova.
- Redshift can support cosmological expansion; it does not mean every nearby object must move away from every observer.
Not true: Apparent brightness alone always determines a star luminosity. Apparent brightness depends on distance as well as luminosity (F = L/4 pi d^2). A dim-looking star may be very luminous but far away, so brightness alone cannot fix luminosity without a distance.
- Luminosity is total power emitted; flux is power received per area.
- For isotropic emission without absorption, flux follows an inverse-square relationship with distance.
- Watch for: the Sun is not expected to become a supernova.
Write from a flux of 5 units
- A source gives flux 45 units.
- Find the flux at three times the distance. Apparent brightness alone does not give luminosity.
Apparent brightness alone always determines a star luminosity.
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
Apparent brightness depends on distance as well as luminosity (F = L/4 pi d^2). A dim-looking star may be very luminous but far away, so brightness alone cannot fix luminosity without a distance.
Total emitted power: write the technical term.
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
luminosity means Total emitted power.
A source gives flux 45 units. What is the flux at three times the distance, and why does apparent brightness alone not give the luminosity?
The same power spreads over an area that grows with the square of the distance.
What is the flux at three times the distance?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
3 squared is 9, and 45 divided by 9 is 5. Other options — 2 units (the lesson flux of 18 units at three times the distance); 45 units (the original flux).