HR Diagram• Read Your Textbook: Foundations of Astronomy

– Chapter 9, 10

• Homework Problems Chapter 9– Review Questions: 1, 4, 5, 7– Review Problems: 1-5– Web Inquiries: 1

• Homework Problems Chapter 10– Review Questions: 1, 2, 4, 6-8– Review Problems: 1-4, 8– Web Inquiries: 2

Intrinsic Brightness vs Spectral TypeA.K.A. Luminosity vs Temperature

• HR diagram

Stars within 5 parsecs

• HR diagram

100 Brightest Stars

• HR diagram

Luminosity Function

Spectral Frequency

Luminosity Classes CLASS DESCRIPTION

I Supergiants

II Bright giants

III Giants

IV Subgiants

V Main-sequence stars/dwarfs

Luminosity Classes

Spectral Luminosity Class SURFACE TEMPERATURE (K) LUMINOSITY (Lsun) RADIUS (Rsun) OBJECT

4000 0.1 0.7 K7 V MS

4000 20 10 K7 III giant

4000 3000 100 K7 I supergiant

Main Sequence Masses

Using The HR Diagram• Example 1: Spectroscopic Parallax

• Same Spectral Type– means same temperature

• Same Luminosity Class– means same size

• m-M = 5 log d - 5

• Determine Distance by observing (m ), and utilizing the HR diagram to assume (M ).

L/Lsun

O B A F G K M

Using The HR Diagram• Example 2: Size Determination

• Same Spectral Type– means same temperature

• Different Luminosity Class– means different size

L = 4R2T4 M = 0, Msun = 5

L/Lsun

O B A F G K M

Using The HR Diagram• Example 2: Size Determination

• Same Spectral Type– means same temperature

• Different Luminosity Class– means different size

L = 4R2T4 M = 0, Msun = 5

A difference in 5 magnitudes is 100x in brightness

4R2T4 = 100 4Rsun2T4

R2 = 100 Rsun2

L/Lsun

O B A F G K M

Using The HR Diagram• Example 2: Size Determination

• Same Spectral Type– means same temperature

• Different Luminosity Class– means different size

L = 4R2T4 M = 0, Msun = 5

A difference in 5 magnitudes is 100x in brightness

4R2T4 = 100 4Rsun2T4

R2 = 100 Rsun2 Therefore, R/Rsun = (100)1/2

L/Lsun

O B A F G K M

Fundamental Stellar Parameters