How does light from a star change when that light travels through dust? Select all that apply.

Question

BlackTom AI · Accepted Answer

Interstellar dust both removes and selectively scatters light: it causes extinction (dimming) and wavelength-dependent attenuation that makes the transmitted light redder.

1) Relevant concepts
- Extinction: dust absorbs and scatters photons, reducing observed intensity. Quantitatively I = I0 e^(-τ), where τ(λ) is the optical depth at wavelength λ.
- Wavelength dependence (reddening): τ is larger at shorter (bluer) wavelengths, so blue light is extinguished more than red light. In magnitudes, Aλ = −2.5 log10(I/I0) = 1.086 τ(λ). Color excess E(B−V) = A_B − A_V > 0 indicates reddening.

2) Step-by-step reasoning
- Start with intrinsic flux I0(λ). After passing through dust the flux is I(λ) = I0(λ) e^(−τ(λ)).
- Because τ(blue) > τ(red), the blue flux is reduced more: I_blue/I0_blue < I_red/I0_red.
- In magnitudes, the extinction is larger in blue: A_B > A_V so E(B−V) = A_B − A_V > 0, measured as the object appearing redder.

3) Example calculation (intermediate steps)
- Suppose τ_B = 1.0 and τ_V = 0.6. Then I_B = I0_B e^(−1.0), I_V = I0_V e^(−0.6).
- Magnitude extinctions: A_B = 1.086×1.0 = 1.086 mag, A_V = 1.086×0.6 = 0.652 mag.
- Color excess E(B−V) = 1.086 − 0.652 = 0.434 mag (positive → redder).

4) Why the correct answers are correct
- “The star appears dimmer.” is true because dust reduces total received flux (extinction).
- “The light appears redder.” is true because dust attenuates blue light more strongly than red, producing reddening (positive color excess).

5) Why other options are incorrect
- “The star appears smaller” / “larger”: dust does not change the star’s intrinsic angular size; scattering can produce faint halos but does not make the star itself appear larger in typical observations.
- “The star appears brighter”: dust cannot increase the received flux (except rare special cases like lensing or forward-scattering into line of sight, which are not the general effect).
- “The light appears bluer”: opposite of reality — selective extinction removes blue light first, so the observed spectrum becomes redder.

Conclusion: The star appears dimmer and the light appears redder, matching the provided answers.

How does light from a star change when that light travels through dust? Select all that apply.多项选择题

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