Hertzsprung-Russell Diagram

Astrophysics Lesson 11

Learning Objectives

To recap:- Analysis of absorption spectra gives

clues as to the make up and surface temperature of stars. To know:-

How to draw a Hertzsprung-Russell Diagram

The range on each axis. The position of white dwarfs, the

main sequence and red giants

Homework

Open book exam to do over the next week.

Real mock exam the first week after half term (will be in Mr. Moore’s lesson).

Intensity of Lines

Main ClassesSpectral Class

Intrinsic Colour

Temperature (K)

Prominent Absorption Lines

O Blue 25,000 - 50,000 He+, He, H

B Blue 11,000 - 25,000 He, H

A Blue-White 7,500 - 11,000 H (strongest) ionised metals

F White 6,000 - 7,500 Ionised metals

G Yellow-White 5,000 - 6,000 Ionised & neutral metals

K Orange 3,500 - 5,000 Neutral metals

M Red <3,500 Neutral atoms, TiO

Oh Be A Fine Girl, Kiss Me!

For Example

Therefore in the Sun, the spectral lines would be seen for iron and calcium, indicating a surface temperature of about 6000 K.

Very hot stars show spectral lines for light elements while cool stars will show up heavy elements, and spectra for molecules as well.

Sub-divisions

The classifications of stars according to spectra are also subdivided further with numbers (e.g A5) to allow the surface temperature to be determined within about 5 %.

Question

What would you not see when looking at the spectrum of the red giant Betelgeuse? What elements would you expect to see? 

Answer

You would see little evidence of Balmer Lines.

You would see spectral lines for iron and calcium.

You would also see evidence for molecules of titanium dioxide.

Classification

• In the last topic we saw how we can classify stars according to their apparent and absolute magnitude, and their temperature and spectral analysis. 

• However these classifications do not tell us a great deal about the age of the star or how it has evolved.

HR Diagram

• Hertzsprung and Russell noticed that a plot of absolute magnitude against temperature (or spectral class) showed certain patterns.

• This is referred to as a Hertzsprung-Russell diagram.

HR Diagram

HR Diagram Notes

• Notice that:-• the temperature scale is decreasing

(50,000 K to 2,500 K).  • the classes of star are placed

alongside the temperature scale; • the luminosity scale is logarithmic to

compress it;• But AQA seem to prefer the absolute

magnitude scale going from -15 to 10.

Main Sequence

• The long, diagonal band is called the main sequence.

• This is the long stable phase of a star’s life where they are fusing hydrogen into helium.

• The Sun is a main sequence star.

Red Giants

• Objects that have a high luminosity and a low surface temperature must have a large surface area. (Stefan’s Law)

• These stars are called red giants and are found in the top right corner of the HR diagram.

• These are stars that have moved off the main sequence and fusion reactions other than H He are taking place.

White Dwarfs

• Objects that have a low luminosity and a high surface temperature must have a low surface area. (Stefan’s Law).

• These stars are called white dwarfs and lie in the bottom right hand corner of the HR diagram.

• White dwarfs are stars at the end of their lives, where all of their fusion reactions have stopped and are they are just cooling down.

• Luminosity (Sun = 1) Surface Temp (K)Group

• Sun1.05800Main sequence

• Betelgeuse20 000 Red Supergiant

• Aldebaran2004700 

• Regulus 14000 Main Sequence

• Rigel 20000 Main Sequence

• Sirius B 0.00 220000 

Star