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

Chapter: Stars & the Universe

What this practice covers

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

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

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

Problem 1

A cool but very luminous star may be a…
  1. red giant
  2. small white dwarf only
  3. dark planet only
  4. star with no surface area
Answer and explanation

red giant

A large radiating area can produce high luminosity at lower temperature.

Problem 2

Why can apparent brightness not determine luminosity by itself?
Answer and explanation

Distance and intervening material affect the observed brightness.

Problem 3

On many H–R diagrams, temperature increases toward the…
  1. left
  2. right always
  3. top only
  4. bottom only
Answer and explanation

left

The conventional horizontal temperature axis is often reversed; verify labels.

Problem 4

A star is cool but has high luminosity. What does this suggest about its size compared with a hot dim star? Explain the reasoning in your own words.
Answer and explanation

It likely has a much larger radiating surface, consistent with a giant-type star.

Cool surfaces emit less energy per unit area than hot ones.

High total luminosity at low temperature requires a large emitting area.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 5

On an H–R diagram, a star is cool but very luminous. What broad size inference follows? Show the reasoning, not just the conclusion.
Hints

Luminosity is intrinsic output, distinct from apparent brightness.

Answer and explanation

It likely has a large radiating surface, consistent with a giant or supergiant region rather than a small main-sequence star.

Problem 6

Why must the temperature-axis direction be checked on an H–R diagram?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Many diagrams place hot temperatures on the left, opposite the usual increasing numerical axis.

Problem 7

At equal surface temperature, greater emitting area raises luminosityAt equal surface temperature, greater emitting area raises luminosityEqual surface temperature, different radiusRadius RRadius 3RArea factor = (3R)² / R² = 9; luminosity factor is 9 in this model.
Original matched-temperature radius comparison. Disk colors are a shared temperature label, not exact observed stellar colors. The drawn circle area represents the radius-squared relation; the physical emitting surface is spherical and the inference depends on matched conditions.
Interpret the supplied visual model. If two equal-temperature stars differ in luminosity by a factor of 16, what radius factor follows in the basic model?
Answer and explanation

A factor of four, because area and luminosity scale with radius squared at equal temperature/emissive conditions.

Compare radius rather than diameter of the illustrated disks, and use the written three-to-one ratio for the spherical model.

A factor of four, because area and luminosity scale with radius squared at equal temperature/emissive conditions.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 8

Two ideal stars have equal surface temperature; star B has radius twice A’s. Find B/A luminosity. At the same observer distance and unobscured path, compare received brightness. Name one condition that would break the simple comparison.
Answer and explanation

Luminosity factor is 2² = 4 and received brightness is also four at equal distance/path conditions. Different temperatures, extinction or emission/measurement conditions would require additional factors.

Luminosity factor is 2² = 4 and received brightness is also four at equal distance/path conditions. Different temperatures, extinction or emission/measurement conditions would require additional factors.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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