Download Astrophysics 11 - HR Diagram

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 Classes
Spectral 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