H-R
Diagrams
An H-R Diagram is…
• A graph of stars’ BRIGHTNESS and
TEMPERATURE
– It also shows color since color is related to
temperature
– It was made by two astronomers who plotted the
data for thousands of stars and noticed some
trends.
– It stands for Hertzsprung-Russell
Diagram
Notice…..
• Temperature DECREASES as you move
to the Right on the X-axis
• Absolute Magnitude INCREASES as you
move up on the Y-axis.
• NEGATIVE absolute magnitude values are
BRIGHTER than POSITIVE absolute
magnitude values
A star…
• Starts off in the lower right hand corner in
MAIN SEQUENCE when it first becomes a
star.
• As it gets hotter and brighter, it moves up
the main sequence.
• Then it moves to upper right section when it
changes to Giants/Supergiants.
• It ends in the lower left section when it
changes to a white dwarf
Arrival on the Main Sequence
• The mass of the
protostar determines:
– how long the protostar
phase will last
– where the new-born star
will show up on the Main
Sequence
– i.e., what spectral type
the star will have while
on the main sequence
• Limits
– Upper ~ 100 M
– Lower = 0.08 M
Supergiants on the HR Diagram
• As the shells of fusion
around the core increase in
number:
– thermal pressure overbalances
the lower gravity in the outer
layers
– the surface of the star expands
– the surface of the star cools
• The star moves toward the
upper right of HR Diagram
– it becomes a red supergiant
– example: Betelgeuse
• For the most massive stars:
– the core evolves too quickly for
the outer layers to respond
– they explode before even
becoming a red supergiant
Summary of Differences
Questions
1. What is the relationship between
temperature and brightness for a
main sequence star?
2. What kinds of stars are the
hottest?
3. What kinds of stars are the
brightest?
4. Why do White dwarves have a low
brightness when they are so hot?
Questions
5. Why do Giants have such high
brightness when they are not
especially hot?
6. What is the temperature of the
sun?
7. What is the absolute magnitude of
the sun?