The Hertzsprung-Russel Diagram

• Who independently plotted the luminosity of stars against their temperature?

  Ejnar Hertzsprung and Henry Norris Russell

  
• What does the y-axis of the Hertzsprung-Russell Diagram represent?

  Luminosity relative to the Sun

  
• How does luminosity change on the y-axis of the Hertzsprung-Russell Diagram?

  It goes from dim at the bottom to bright at the top
• What does the x-axis of the Hertzsprung-Russell Diagram represent?

  Temperature in degrees Kelvin

  
• How does temperature change on the x-axis of the Hertzsprung-Russell Diagram?

  It goes from hot on the left to cool on the right
• What is the main sequence in the Hertzsprung-Russell Diagram?

  A band where most stars are clustered

  
• How does luminosity change for main sequence stars?

  It increases with surface temperature
• What are the two areas above the main sequence called?

  Red giants and red supergiants
• Why do red giants and red supergiants have increased luminosity at cooler temperatures?

  They are much larger than main sequence stars
• Where are white dwarf stars located in the Hertzsprung-Russell Diagram?

  Below and to the left of the main sequence

  
• What is the luminosity and temperature characteristic of white dwarf stars?

  Hot but not very luminous
• What types of stars does the Hertzsprung-Russell Diagram show?

  Stars in stable phases

  
• Why are transitory phases like supernovae not shown on the Hertzsprung-Russell Diagram?

  They happen quickly relative to a star's lifetime
• Why can't black holes be seen on the Hertzsprung-Russell Diagram?

  They emit no light
• What is the evolutionary path of Sun-like stars on the Hertzsprung-Russell Diagram?

  1. Protostar to Main Sequence (A to B)

  • Collapses from a cold gas cloud
  • Initially dim and cool

  1. Main Sequence to Red Giant (B to C)

  • Fuses hydrogen into helium
  • Expands into a red giant after hydrogen fusion stops

  1. Red Giant to White Dwarf (C to D)

  • Collapses into a white dwarf after helium runs out

  
• What happens to a star when hydrogen fusion stops?

  It begins to collapse under gravity
• How much larger can a red giant be compared to the Sun?

  Up to 100 times the current diameter
• What is the surface temperature of a red giant?

  About 3000 K
• What is the surface temperature of a white dwarf?

  Generally very hot, around 10,000 K
• How do the brightest stars compare in terms of lifetimes?

  They have very short lifetimes
• How do the dimmest stars compare in terms of lifetimes?

  They have extremely long lifetimes
• How does a star's brightness relate to its nuclear fuel consumption?

  Brighter stars use fuel at a higher rate
• How does the mass of a star affect its lifespan?

  A star 100 times the Sun's mass lives about $10^6$ years
• What are the key steps to label and draw an H-R diagram?

  1. Label axes:

  • Absolute magnitude from +15 to -10
  • Temperature from 50,000 K to 2,500 K

  1. Draw the main sequence as a band with curvature
  2. Distinctively label regions for white dwarfs, red giants, and supergiants

  
• What is the absolute magnitude of the Sun?

  +5 on the relative scale
• What is the surface temperature of the Sun?

  5800 K
• How should the evolutionary path of the Sun be represented on the H-R diagram?

  From protostar to red giant to white dwarf

  
• What is the luminosity of Star X compared to the Sun?

  10,000 times greater than the Sun
• What is the surface temperature of Star X?

  20,000 K
• What are the key points to remember when drawing an H-R diagram?

  • Absolute magnitude on y-axis (+15 to -10)
  • Temperature on x-axis (50,000 K to 2,500 K)
  • Main sequence as a band with curvature
  • Distinct regions for giants and dwarfs