Announcements
•Homework: Supplemental Problems
•Make your selection from the Second Project
List. The project is due at the final exam:
Monday May 2 @ 4:00pm
The HR Diagram shows that
stars group together by type
Most stars are on the Main
Sequence
Stars on the Main
Sequence are fusing
hydrogen into helium
4 H  He   
1
4
Stars on the Main Sequence
are in equilibrium: outward
pressure of the energy
produced balances the
inward pressure of gravity
The luminosity of a star on
the main sequence depends
on its mass
A log(luminosity) versus
log(mass) plot shows a
straight line for main
sequence stars above
about 0.5 MSun
A straight line on a log-log
plot means there is a power
relationship between the two
quantities. The slope of the
line gives the power.
The mass-luminosity relationship
is a simple power law
LM
3.5
If the mass is given in Msun then the luminosity is in LSun
Examples
The star Sirius has a luminosity 23 times that of the Sun.
What is its mass?
What is the luminosity of a star with a mass of 0.75 Msun?
Example Solution
Solve the Mass-Luminosity relationship for mass
and plug in the numbers
L  M 3.5  M  3.5 L  3.5 23LSun  2.45M Sun
Given the mass, just plug in the value to get
the luminosity
LM
3.5
  0.75M Sun 
3.5
 0.365LSun
How long a
star lives
on the Main
Sequence
also
depends on
the mass
fusion rate  T or T
4
pp
17
CNO
As with mass-luminosity,
mass-lifetime is also a power
relationship
 MS
 M 
 10 years  

 M Sun 
10
2.5
Examples
How long will a 3.5 solar mass star live on the main sequence?
How long will a 0.5 solar mass star live on the main sequence?
Example Solution
Just plug in the numbers and grind away on the
calculator
 MS
 MS
 M 
 1010 years  

M
 Sun 
 440 million years
2.5
 M 
 1010 years  

M
 Sun 
 57 billion years
2.5
 3.5M Sun 
 1010 years  

M
Sun


2.5
 0.5M Sun 
 1010 years  

M
Sun


 0.044 1010 years
2.5
 5.7 1010 years