What two quantities are plotted on the axes of a Hertzsprung-Russell diagram?

Absolute magnitude (or luminosity) on the vertical axis and temperature (or spectral class) on the horizontal axis.

On an HR diagram, what is the typical range of absolute magnitudes shown on the vertical axis?

From –10 (brightest) at the top to +15 (faintest) at the bottom.

Why is –10 positioned at the top of the absolute magnitude scale on an HR diagram rather than at the bottom?

Because lower (more negative) absolute magnitude values correspond to greater luminosity, so the brightest stars appear at the top.

What is the typical temperature range shown on the horizontal axis of an HR diagram?

From approximately 50,000 K (hottest) on the left to 2,500 K (coolest) on the right.

Why does temperature decrease from left to right on an HR diagram?

By convention, the hottest stars (highest temperatures) are placed on the left and the coolest on the right, matching the historical use of spectral class ordering.

AQA A-Level Physics 7408

Measurements and Their Errors

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Astrophysics

The Hertzsprung-Russell (HR) diagram

5 things to know
Paper 3, section B

Overview

The axes and their unusual scales, the main sequence, and where giants and dwarfs sit.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What ranges do the axes of an HR diagram cover, and why is −10 at the top?

Process

Where does the main sequence lie on an HR diagram, and what process powers those stars?

Key term

Why are white dwarfs dim, where do they sit on the diagram, and what class is the Sun?

Numbers

What is the approximate absolute magnitude of the Sun?

Key term

What is the name given to the ejected outer layers of a Sun-like star after the red giant phase?

Questions from this subtopic

Each one with the model answer underneath.

1
What two quantities are plotted on the axes of a Hertzsprung-Russell diagram?
Model answer
Absolute magnitude (or luminosity) on the vertical axis and temperature (or spectral class) on the horizontal axis.
2
On an HR diagram, what is the typical range of absolute magnitudes shown on the vertical axis?
Model answer
From –10 (brightest) at the top to +15 (faintest) at the bottom.
3
Why is –10 positioned at the top of the absolute magnitude scale on an HR diagram rather than at the bottom?
Model answer
Because lower (more negative) absolute magnitude values correspond to greater luminosity, so the brightest stars appear at the top.
4
What is the typical temperature range shown on the horizontal axis of an HR diagram?
Model answer
From approximately 50,000 K (hottest) on the left to 2,500 K (coolest) on the right.
5
Why does temperature decrease from left to right on an HR diagram?
Model answer
By convention, the hottest stars (highest temperatures) are placed on the left and the coolest on the right, matching the historical use of spectral class ordering.
6
List the spectral classes in order from hottest to coolest as they appear left to right on an HR diagram.
Model answer
O, B, A, F, G, K, M.

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