8.2.5The Periodic Table
Hertzsprung-Russell Diagrams — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 23 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Axes and Scales
The H-R diagram plots luminosity against surface temperature, with an unusual temperature axis.
Vertical axis
- Vertical axis Luminosity, usually in solar units, with the Sun having a value of 1.
Horizontal axis
- Horizontal axis Surface temperature in kelvin, which decreases from left to right, so the hottest stars are on the left.
Colour link
- Colour link Hot blue stars lie towards the left of the diagram and cool red stars lie towards the right.
Upper area
- Upper area Stars near the top are very luminous, because they are either very large, very hot, or both.
Lower area
- Lower area Stars near the bottom are faint, meaning they emit very little light energy every second.
Section 2 of 5Groups of Stars
Groups of Stars
Main sequence
- Main sequence A diagonal band from the top left to the bottom right that contains most stars, including the Sun.
Red giants
- Red giants Above the main sequence on the right; they are cool but very luminous because of their large size.
Red supergiants
- Red supergiants Found above the red giants; they are the coolest and most luminous stars on the diagram.
White dwarfs
- White dwarfs Below the main sequence, towards the left; they are hot but small, so they are faint.
Red dwarfs
- Red dwarfs The coolest and dimmest stars on the main sequence, found at the bottom right of the band.
| Region | Typical stars | Key property |
|---|---|---|
| Top left | Hot, luminous main sequence stars | Hot and very luminous |
| Top right | Red giants and red supergiants | Cool and very luminous |
| Bottom left | White dwarfs | Hot but very faint |
| Bottom right | Red dwarfs | Cool and faint |
Section 3 of 5Life Cycles on the Diagram
A star's life cycle can be traced as a path across the diagram.
1
A star spends most of its life at a fixed position on the main sequence, and that position depends mainly on its mass.
2
When the hydrogen in a solar-mass star runs out, it moves up and to the right to become a red giant.
3
After its helium fusion ends, the star moves down to the lower left of the diagram as a white dwarf.
4
A high-mass star moves up and to the right to become a red supergiant, which is the most luminous type of cool star.
Section 4 of 5Worked Example
Question
- Star Y has a surface temperature of 3000 K and a luminosity of 0.01 times that of the Sun.
- State the type of star Y is and where it is located on the H-R diagram.
- Step 1: A temperature of 3000 K is cool, so the star lies towards the right-hand side of the diagram.
- Step 2: A luminosity of 0.01 is much less than 1, so the star lies near the bottom of the diagram.
- Step 3: A cool, faint star that lies on the main sequence is a red dwarf.
Answer Star Y is a red dwarf, located at the bottom right of the main sequence.
Section 5 of 5Exam Checklist & Tips
8.2.5 – Key Points for the Exam
- Hertzsprung-Russell diagram A graph of luminosity against surface temperature used to classify stars.
- Main sequence The band on the H-R diagram where most stars lie, running from top left to bottom right.
- Luminosity The total energy emitted by a star as light every second.
- Red dwarf A cool, faint star found at the bottom right of the main sequence.
- White dwarf A hot but faint remnant found at the bottom left of the H-R diagram.
Common Mistakes
- Plotting temperature increasing from left to right;
- on the H-R diagram temperature decreases from left to right.
- Confusing white dwarfs with red dwarfs; white dwarfs are hot and faint, while red dwarfs are cool and faint.
- Thinking the main sequence is a horizontal line; it slopes from the top left to the bottom right.
- Reading the vertical axis as temperature; the vertical axis shows luminosity.
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