Flux, Magnitudes and the Distance Ladder · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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From faint glimmers to far distances

Science · Astronomy & Astrophysics · ages 20-21
Name ______________________   Date ____________

  1. In the same observing band and magnitude system, star A has an apparent magnitude 5 lower than star B. How much flux do we receive from A compared with B?

    • 5 times the flux received from star B
    • 100 times the flux received from star B
    • 2.512 times the flux received from star B

  2. What is the difference between flux and luminosity?

    • Flux is received power per area; luminosity is emitted power.
    • Flux is emitted power per area; luminosity is received power.
    • Flux and luminosity both measure received power per unit collecting area.

  3. Within the same observing band and magnitude system, a larger apparent magnitude means a brighter object.

    Circle one:   True   False

  4. What is a star's absolute magnitude in a specified observing band and magnitude system?

    • Its magnitude at Earth, with no adjustment for its actual distance
    • Its total emitted power, measured across all wavelengths of light
    • Its magnitude at 10 parsecs, with no intervening extinction

  5. Two nearby stars emit isotropically with equal luminosity in the measured band. There is no extinction. B is 3 times as far away as A. What is the ratio F_A/F_B?

    Answer: ______________

  6. With the observing band matched and extinction accounted for, how can a Cepheid's pulsation help estimate its distance?

    • Use period with a calibrated luminosity relation, then compare mean luminosity and mean flux.
    • Use period alone to find an exact luminosity without calibration, then compare it with the measured mean flux.
    • Use mean flux alone to find distance without a luminosity estimate, then use period only to check that distance.

  7. A ladder route uses only one parallax distance scale to calibrate Cepheids and then Type Ia supernovae. The common anchor distance scale is revised. With all other data and fitting choices fixed, what follows?

    • Only parallax distances shift; Cepheid and supernova scales stay fixed.
    • Dependent Cepheid and supernova distance scales shift.
    • Only supernova distances shift; the dependent Cepheid scale stays fixed.

  8. A nearby star has apparent magnitude m = 10 and absolute magnitude M = 5 in the same observing band and magnitude system. Neglect extinction. What is its distance in parsecs?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

From faint glimmers to far distances W1-mt_q1vEUas3AY-s1

  1. 100 times the flux received from star B · A decrease of 5 magnitudes corresponds to 100 times the received flux in that band.
  2. Flux is received power per area; luminosity is emitted power. · Flux measures arriving power per unit area, in W/m². Luminosity measures emitted power, in W.
  3. False · Larger apparent magnitude numbers describe smaller received fluxes, so the statement is false.
  4. Its magnitude at 10 parsecs, with no intervening extinction · Absolute magnitude is the apparent magnitude the star would have at 10 parsecs without intervening extinction.
  5. 9 · Equal luminosities cancel. The inverse-square law gives F_A/F_B = (d_B/d_A)^2 = 3² = 9.
  6. Use period with a calibrated luminosity relation, then compare mean luminosity and mean flux. · A calibrated period-luminosity relation estimates mean luminosity. Comparing it with mean received flux gives a distance estimate.
  7. Dependent Cepheid and supernova distance scales shift. · Both later scales depend on the revised anchor. Uncertainty in that shared calibration can come from random errors in anchor measurements as well as systematic biases.
  8. 100 · m - M = 10 - 5 = 5. The distance is 10^[(5 + 5)/5] = 10² = 100 parsecs.
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