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8.2.4The Periodic Table

The Brightness of Stars — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Luminosity

8.2.4 The Brightness of Stars

Luminosity describes how much energy a star emits, regardless of where it is.

Luminosity
  • The total amount of light energy emitted by a star every second, which is a measure of its power output.
Brightness from Earth
  • The brightness seen from Earth depends on both the luminosity of the star and its distance from the observer.
Solar units
  • Luminosity is often given relative to the Sun, so the Sun has a luminosity of 1.
Brighter than the Sun
  • A luminosity greater than 1 means the star emits more energy every second than the Sun does.
Dimmer than the Sun
  • A luminosity less than 1 means the star emits less energy every second than the Sun does.

Part 2 of 5Apparent Magnitude

Apparent Magnitude
Definition
  • Apparent magnitude is the perceived brightness of a star as seen from Earth.
Scale direction
  • The scale runs backwardsa bright star has a small or even negative magnitude, and a faint star has a large one.
Distance effect
  • A distant star with high luminosity can look fainter than a nearby star with lower luminosity.
Measurement
  • Apparent magnitude is what an observer on Earth measures directly.

Part 3 of 5Absolute Magnitude

Definition
  • The brightness a star would appear to have if it were placed at a standard distance of 10 parsecs from Earth.
Standard distance
  • 10 parsecs is equivalent to about 32.6 light-years, and it is a fixed reference distance for comparing stars.
Purpose
  • It allows the true brightness of stars to be compared fairly, without the effect of their distances.
Link to luminosity
  • Absolute magnitude depends on the luminosity of the star, not on its distance from Earth.

Part 4 of 5Worked Example

Question

  • Star P has an apparent magnitude of 1.0 and Star Q has an apparent magnitude of 4.0, both seen from Earth.
  • Which star appears brighter, and can we conclude which star has the greater luminosity?

  1. Step 1: Recall that on the apparent magnitude scale a lower number means a brighter star.
  2. Step 2: Compare the values: 1.0 is lower than 4.0.
  3. Step 3: Conclude that Star P appears brighter from Earth.
  4. Step 4: Note that the distances to the stars are needed before comparing their luminosities.

Answer

  • Star P appears brighter from Earth.
  • Its luminosity cannot be concluded from apparent magnitudes alone, because the distance of each star is unknown.

Part 5 of 5Exam Checklist & Tips

8.2.4 – Key Points for the Exam

  • Luminosity The total energy emitted by a star as light every second.
  • Apparent magnitude
    • The brightness of a star as seen from Earth;
      • lower values mean brighter stars.
  • Absolute magnitude The brightness a star would have at a standard distance of 10 parsecs.
  • Parsec A distance of about 3.26 light-years, used in astronomy.
  • Light-year The distance light travels in one year.

Common Mistakes

  • Saying that a star is bright because it is close;
    • apparent brightness also depends on the luminosity of the star.
  • Thinking that a higher apparent magnitude means a brighter star; the scale runs backwards.
  • Confusing luminosity, the total energy emitted per second, with the brightness seen from Earth.
  • Stating that absolute magnitude depends on the distance from Earth;
    • it is a comparison at a standard distance.

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