Stars, radiation and scale
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| flux/flʌks/ | Thông lượng |
| luminosity/ˌluːmɪˈnɒsɪti/ | Độ sáng |
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
- Stars transfer energy through nuclear fusion.
- A star life cycle depends on its initial mass.
- Light spectra and redshift provide evidence used in models of the universe.
- luminosity 光度: Total emitted power.
- flux: Power received per unit area.
At fixed luminosity, what happens to flux when distance doubles?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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: Compare source observations with the life-cycle model and identify the relevant initial mass. State limitations of evidence rather than treating a model diagram as a direct observation of one star whole lifetime.
Which two habits make the investigation or model in this case more defensible?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
- Stars transfer energy through nuclear fusion.
- 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 9 units
- A source gives a flux of 36 units. The distance then doubles and the luminosity stays fixed.
- Find the new flux. Apparent brightness is not the star's luminosity.
Apparent brightness alone always determines a star luminosity.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
luminosity means Total emitted power.
A source gives a flux of 36 units. The distance then doubles and the luminosity stays fixed. What is the new flux, and why is apparent brightness not the luminosity?
At twice the distance the flux is one quarter. Luminosity is the source output, not the flux at the observer.
What is the flux after the distance doubles?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
36 divided by 4 is 9 units. Doubling the distance does not halve the flux.