The Hertzsprung-Russell diagram and stellar classification
Practice Flashcards
Flip to reveal answersWhat does a Hertzsprung-Russell (H-R) diagram plot?
Track your progress — Sign up free to save your progress and get smart review reminders based on spaced repetition.
All 12 Flashcards — The Hertzsprung-Russell diagram and stellar classification
Sign up free to track progress and get spaced-repetition review schedules.
Question
What does a Hertzsprung-Russell (H-R) diagram plot?
Answer
A star's **luminosity** (vertical, up = brighter) against its **surface temperature** (horizontal).
Question
Which way does the temperature axis run on an H-R diagram?
Answer
**Backwards** — **hot stars on the LEFT**, cool stars on the right. (A classic exam trap.)
Question
What is luminosity?
Answer
The **total power** a star radiates, in watts. (Different from apparent brightness, which also depends on distance.)
Question
Where do main-sequence stars sit, and what are they doing?
Answer
On the **diagonal band** through the middle; they are fusing **hydrogen into helium**. The Sun is one.
Question
Where is a red giant on the H-R diagram, and why is it bright?
Answer
**Top-right** — cool but very luminous. It is bright because it is **huge** (large radius), not because it is hot.
Question
Where is a white dwarf on the H-R diagram?
Answer
**Bottom-left** — **hot** surface but **very dim**, because it is **tiny** (small radius).
Question
Which equation links a star's luminosity to its size and temperature?
Answer
$L = \sigma A T^{4}$ (given). With A = 4πr² it becomes **L ∝ r²T⁴**.
Question
How do you find the ratio of two stars' radii from L and T?
Answer
$R_{\text{star}}/R_{\text{sun}} = (T_{\text{sun}}/T_{\text{star}})^{2}\sqrt{L_{\text{star}}/L_{\text{sun}}}$ — from L ∝ r²T⁴.
Question
Two stars have equal luminosity; the cooler one is...
Answer
**Larger**. For fixed L, r ∝ 1/T², so a lower temperature means a bigger radius.
Question
How do you state a star's type on the H-R diagram?
Answer
From its **position**: diagonal band = main sequence; top-right = red giant/supergiant; bottom-left = white dwarf.
Question
A star has L = 16 L_{sun} and the Sun's temperature. Its radius?
Answer
Equal T makes the bracket 1, so R/R_{sun} = √16 = **4 R_{sun}**.
Question
Why can a cool star still be very luminous?
Answer
Because L ∝ r²T⁴ — a large enough **radius** makes up for the low temperature, so a big cool star (red giant) is still bright.
Read the notes
Full study notes for The Hertzsprung-Russell diagram and stellar classification
Topic 5.5 hub
Fusion and stars
More from Topic 5.5
All flashcards in this topic
Physics exam skills
Paper structures & tips
Track your progress with spaced repetition
Sign up free — Aimnova tells you exactly which cards to review and when, so you remember everything before your IB exam.
Start Free