Lecture Notes: HR Diagram and Plotting Stars

Overview of the HR Diagram

• The HR (Hertzsprung-Russell) Diagram is a graphical representation of stars.
• Vertical Axis: Represents luminosity or power (in comparison to the Sun’s luminosity).
• Horizontal Axis: Represents surface temperature (temperature increases to the left).
  • Units of thousands of Kelvins (e.g., 5000 K, 10000 K, etc.).

Understanding the Diagram

• Luminosity is measured in solar luminosities.
• The scale is logarithmic:
  • 1 (Sun's luminosity), 10, 100, 1000, 10000, up to a million.
  • Divisions between major marks represent smaller incremental increases.

Plotting Stars on the HR Diagram

Example 1: Sirius

• Luminosity: 38 solar luminosities.
• Temperature: 9700 K (about 10,000 K).
• Position: Plot between 30 and 40 on luminosity scale, slightly right of the 10,000 K mark.
• Classification: Main sequence star, as most stars are (>90% are main sequence).

Example 2: Arcturus

• Luminosity: 200 solar luminosities.
• Temperature: 4600 K.
• Position: Between 100 and 200 on luminosity scale, at around 4600 K on temperature scale.
• Classification: Not a main sequence star; it's in the giant range, indicating it's in the last 10% of its life cycle.

Example 3: Betelgeuse

• Luminosity: 110,000 solar luminosities.
• Temperature: 3100 K.
• Position: Near 100,000 on luminosity scale, around 3100 K on temperature scale.
• Classification: Supergiant (luminosity class 1-a), nearing the end of its life.

Additional Notes

• Surface Area and Luminosity: Larger surface area increases luminosity.
• Mnemonic for Star Classification: O B A F G K M.
• Plot and classify the remaining 13 stars as practice.

Conclusion

• Understanding the HR diagram is crucial for classifying stars.
• The lecture covers how to plot stars based on their luminosity and temperature, as well as draw conclusions about their life stages.