Transcript The HR Diagram

The HR Diagram
L ~ R2T4
What one sees in the HR diagram
What one sees in the HR diagram
What one sees in the HR diagram
Brightness, size, and the HR diagram
We know two things about objects that are approximately blackbodies
•Hotter things are brighter.
•Energy radiated per unit time per unit area is proportional to T4,
•so bigger T means more energy radiated
•Bigger things are brighter.
•Energy radiated per unit time per unit area is proportional to 4πR2,
•so bigger surface area means more energy radiated.
Putting this together, we have
Temperature, size and luminosity
Temperature, size and luminosity
Mass, luminosity, and the HR diagram
Our conclusion:
Different stars have
different masses.
Among all stars with
the same mass,
almost all have the
same properties.
The most massive
stars are the most
luminous.
Conclusion
Based on this evidence, we
conclude
•Stars spend most of their lives
as main sequence stars.
•During its lifetime, the surface
temperature and luminosity
stays pretty much constant.
•Something else
could happen in
the star birth
process.
•Something else
could happen in
the star death
process.
•The star's mass determines
what the temperature and
luminosity is during the star's
main sequence lifetime.
•More mass -> hotter.
•More mass -> more
luminous.
•Also, more mass ->
bigger.