The Hertzsprung-Russell (HR) diagram (A-level only) Flashcards – AQA A-level Physics (7408)

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

Luminosity or absolute magnitude, and surface temperature or spectral class.

What does absolute magnitude measure, and what does a more negative value indicate?

It measures intrinsic brightness; a more negative value indicates a more luminous star.

What is the typical range of an HR diagram’s absolute-magnitude axis, from top to bottom?

Approximately $-10$ at the top to $+15$ at the bottom.

How does surface temperature vary across an HR diagram, and what is its typical range?

It decreases from left to right, typically from about $50\,000\ \mathrm{K}$ to $2\,500\ \mathrm{K}$.

What is the order of the seven spectral classes from hotter to cooler stars?

O, B, A, F, G, K, M.

Where is the main sequence on an HR diagram?

It forms a band from hot, luminous stars at the upper left to cool, faint stars at the lower right.

Where are giants on an HR diagram, and what are their temperature and luminosity characteristics?

At the upper right: relatively cool at their surfaces but luminous.

Where are white dwarfs on an HR diagram, and what are their temperature and luminosity characteristics?

At the lower left: hot but faint.

Where does the Sun lie on an HR diagram?

On the main sequence.

What is the Sun’s approximate surface temperature?

$5\,800\ \mathrm{K}$

What is the Sun’s approximate absolute magnitude?

$+4.8$

What is the Sun’s spectral class?

G

How does a Sun-like protostar form?

A cloud of gas and dust collapses under gravity, producing a contracting protostar.

How does contraction affect a Sun-like protostar as it approaches the main sequence?

Its interior heats up.

When does a Sun-like star reach the main sequence?

When hydrogen fusion in its core supplies the energy that sustains it.

What happens to a Sun-like star when its core hydrogen is depleted?

It expands into a red giant.

How does a Sun-like star move on the HR diagram as it becomes a red giant?

Its surface cools and its luminosity increases, moving it towards the upper right.

What fuel undergoes fusion during the giant stages of a Sun-like star?

Helium.

How does a Sun-like star form a white dwarf?

It sheds its outer layers, leaving a hot, dense core.

Where does a newly formed white dwarf lie on the HR diagram, and what does that position indicate?

In the lower-left region: it is hot but faint.

Why does a white dwarf cool and fade over time?

It has no sustained fusion to replenish its energy.