Star Properties & H–R Diagrams — Drills · Grade 9 Science

Chapter: Stars & the Universe

What these drills covers

By the end of Star Properties & H–R Diagrams you should be able to:

8 questions, written answer, numeric and multiple choice, each with a worked explanation.

Vocabulary used: luminosity, apparent brightness, H–R diagram, kelvin (K), logarithmic scale.

Problem 1

How can a white dwarf be hot but faint in total output?
Answer and explanation

Its small surface area limits luminosity despite high surface temperature.

Problem 2

A luminosity scale marks 1, 10, 100, 1000 at equal spacing. By what factor does each step change?
Answer and explanation

10

It is a logarithmic scale with tenfold steps.

Problem 3

An H–R diagram plots a star's luminosity against its
  1. distance
  2. temperature or colour
  3. age
  4. mass only
Answer and explanation

temperature or colour

Temperature runs along the horizontal axis, conventionally hottest on the left.

Problem 4

Most stars lie along the
  1. main sequence
  2. white dwarf region
  3. supergiant branch
  4. edge
Answer and explanation

main sequence

The main sequence is where stars spend most of their lives fusing hydrogen.

Problem 5

A star that is cool yet very luminous must be
  1. small
  2. very large in radius
  3. very close
  4. a white dwarf
Answer and explanation

very large in radius

Low temperature per unit area with high total output requires a huge surface, so it is a giant.

Problem 6

A blue star is
  1. cooler than a red star
  2. hotter than a red star
  3. the same temperature
  4. always smaller
Answer and explanation

hotter than a red star

Colour indicates surface temperature, with blue hottest and red coolest.

Problem 7

Explain how an H–R diagram lets you infer a star's size without measuring it directly.
Answer and explanation

Luminosity depends on both surface temperature and surface area. Two stars at the same temperature emit the same energy per unit area, so the one far more luminous must have a much larger surface. Reading a star's position on the diagram therefore gives its radius relative to others, which is an inference from two measurable quantities rather than a direct measurement.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Why is the H–R diagram useful for studying how stars change over time?
Answer and explanation

Although no star can be watched through its whole life, a cluster contains stars of many masses formed at the same time. Plotting them shows which have left the main sequence and which remain, so the diagram captures different stages at once. Comparing clusters of different ages then reveals the sequence a star passes through.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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