What two properties are plotted on the axes of the Hertzsprung-Russell (HR) diagram?

Distance from Earth (x-axis) and apparent brightness (y-axis) Surface temperature (x-axis, hot on the left) and luminosity or absolute magnitude (y-axis, bright at top) Age (x-axis) and mass (y-axis) Colour (x-axis) and size (y-axis)

What is the Main Sequence on the HR diagram?

A horizontal line of stars all with the same luminosity A small cluster of stars in the upper right corner of the diagram A diagonal band where about 90% of stars lie, running from hot-bright stars in the upper left to cool-dim stars in the lower right The vertical axis of the HR diagram

Where on the HR diagram would you find red giants?

In the upper left — hot and very luminous In the lower right — cool and dim In the lower left — hot but dim In the upper right — they are cool (so red) but very luminous because they are enormously large

Where do white dwarfs appear on the HR diagram, and why?

In the upper right — they are hot and very large On the Main Sequence — they still fuse hydrogen slowly In the lower left — they are hot (remnant stellar cores, 10,000-100,000 K) but very dim because they are only about the size of Earth They are not found on the HR diagram — only living stars are plotted

When a star like the Sun exhausts its core hydrogen, what happens to its position on the HR diagram?

It disappears from the HR diagram entirely It moves down the Main Sequence toward lower luminosity It stays on the Main Sequence but slowly moves to the left It leaves the Main Sequence and moves to the upper right, becoming a red giant

Astronomers plot the HR diagram for a cluster of stars to determine the cluster's age. What feature of the cluster's HR diagram reveals the age?

The colour of the most common star in the cluster The total number of stars visible on the diagram The turnoff point — where the most massive remaining Main Sequence stars are just starting to leave the Main Sequence and become giants Whether white dwarfs are present in the lower left of the diagram

Blue supergiants like Rigel in Orion appear in the upper left of the HR diagram. What does this tell us about them?

They are cool but extremely large — the same as red giants but hotter They are extremely hot (blue, hence left) and extremely luminous (hence top) — among the most powerful stars in the galaxy They have used up all their fuel and are about to become white dwarfs They are dim stars at short distances that appear brighter than they are

What is the luminosity class system, and what does a Roman numeral in a star's spectral classification indicate?

It ranks stars by mass: I = least massive, V = most massive It ranks stars by size: I = supergiant, II = bright giant, III = giant, IV = subgiant, V = main sequence dwarf It indicates a star's age in billions of years It is a historical classification used before the modern HR diagram was developed

Why do more massive stars spend less time on the Main Sequence than less massive ones?

More massive stars rotate faster, consuming their fuel through turbulence More massive stars are cooler, so fusion is less efficient More massive stars are far more luminous — they burn through their hydrogen fuel much faster despite having more of it Less massive stars have no hydrogen left after formation, so they leave the Main Sequence immediately

Proxima Centauri is a red dwarf star (M-type, V luminosity class) located 4.2 light-years away. Where does it sit on the HR diagram compared to the Sun?

Above and to the left of the Sun — hotter and more luminous In the upper right red giant region — it is a giant star In the white dwarf region — it is very old and cooling Below and to the right of the Sun on the Main Sequence — cooler (about 3,000 K) and much less luminous (about 0.0015 solar luminosities)