Main Sequence Stars and their Lifetimes

1
Main Sequence Stars and their Lifetimes

1. Luminosity (from Brightness and Distance)
2. Mass (from Doppler shifts in
   Binary Stars)
3. Temperature (from absorption lines Doppler
   Thermometer)
4. Radius (from Luminosity and
   Temperature deduced)

Soon, we will learn how to measure the Lifetimes
of Main Sequence Stars!
2
HR Diagram for Stars
big cool
big hot
Units Of Suns
radius
small hot
small cool
3

• Luminosity, L, depends upon
• Surface flux, F
• Total Surface Area

L Area x Flux
Surface flux, F, is the light energy radiated
from every unit area of the stars surface, it is
temperature dependent (i.e. blackbody spectrum)
Flux T 4
Surface area is radius dependent
Area R2
We have
L R2T 4
4
Main Sequence Mass Relationship
As temperature increases, mass increases! As
mass increases, luminosity increases!
L M3.5
5
Main Sequence is
O, B, A, F, G, K, M
decreasing temperature
increasing temperature
decreasing size
increasing size
decreasing mass
increasing mass
decreasing luminosity
increasing luminosity
This applies to the Main Sequence Only Other
Luminosity Classes do NOT obey all these trends.
6
The Life Expectancies of Main Sequence Stars,
Part 1
BASIC PRINCIPLES OF LIFETIMES The more fuel the
longer the life time The slower the consumption
of fuel, the longer the life time
Amount of Fuel
Lifetime
Rate of Fuel Consumption
The Amount of Fuel is proportional to the stars
mass, M
The Rate of Fuel Consumption is proportional to
the stars luminosity, L
The luminosity depends upon mass! L M3.5
7
The Life Expectancies of Main Sequence Stars,
Part 2
Therefore, the lifetime can be written using only
the stars mass! If we write the mass in solar
units, then the lifetime is given in solar
lifetimes. The suns lifetime is roughly 10
billion years, or Lifetime 10 x 109 yr
Large Mass Stars Have Shorter Lives
M
1
M
Lifetime


M3.5
M2.5
L
0.5 solar mass star lives 5-6 solar life times,
or 56 x 109 years
M
A
4.0 solar mass star lives 0.031 solar life
times, or 310 x 106 years
O
40.0 solar mass star lives 0.0001 solar life
times, or 1 x 106 years
8
Main Sequence is
O, B, A, F, G, K, M
decreasing temperature
increasing temperature
decreasing size
increasing size
decreasing mass
increasing mass
decreasing luminosity
increasing luminosity
increasing lifetime
decreasing lifetime
This applies to the Main Sequence Only Other
Luminosity Classes do NOT obey all these trends.
The most massive stars live short lifetimes.
9
Evolutionary Tracks
When Main Sequence Stars are newly born they are
called Zero Age Main Sequence stars. (the red
line on the graph) As they convert hydrogen to
helium in their cores, their temperatures
decrease and their luminosities increase
slightly. This is due to an increase in the
stars radii. This moves the star to the upper
right on the HR diagram as they age. The path
of migration on the HR diagram is called an
evolutionary track.
Zero Age Main Sequence