1. Agenda Review magnitude scale and HR diagrams
Another way to measure distance: Spectroscopic Parallax
Please pick up essays after class
Stars are born from H gas (nebula) and spend 90% of their lives on the main sequence. After becoming a red giant, the fate of a star depends on its mass.

2. Apparent Magnitude scale
Sun
Naked
eye
Polaris
Faintest object
With binoculars
Venus
Sirius
Full moon
4 m telescope

3. Absolute magnitude Luminosity = how much light a star is giving off
(like wattage for light bulbs)
Compared to the Sun (Lsun)
Absolute magnitude = how bright (what magnitude) a star would appear at 10 parsecs (32.6 light years)

4. Standard Candles
The Sun has an apparent magnitude of –26. What might its absolute magnitude be?
–26
–23
4.85
–33
Cannot determine

5. Recall from last time… We know temperature
O B A F G K M
We can determine luminosity (if we know distance)
Temperature and luminosity are related!
main sequence 15 5 10
-10 -5

6. The HR Diagram!! Bright Dim Main Sequence Stars Red Giant Stars15 5 10
-10 -5
Spectral Class
HOT
COOL
Bright
Dim
Sun
Main Sequence Stars
White
Dwarf Stars
Red
Giant Stars
Absolute magnitude
O B A F G K M

7. Given two stars, which one looks brighter in the night sky?
Star A
Star B
Look the same
Cannot
conclude
-10 -5 B
Absolute magnitude 5 10 A
Let them talk to their neighbor --- convince your neighbor and try again! 15
O B A F G K M
Spectral Class

8. Given two stars, which is larger (more surface area)?
Star A
Star B
Look the same
Cannot
conclude
-10 -5 B
Absolute magnitude 5 10 A 15
O B A F G K M
Spectral Class

9. Spectroscopic “parallax”
Another way to measure distance
It’s not parallax!
How does it work?
Measure star’s spectral type
Get luminosity (absolute magnitude) from spectral type (how?)*
Use the “standard candle” method to get distance.

10. *Temperature => Luminosity15 5 10
-10 -5
Spectral Class
HOT
COOL
Bright
Dim
Sun
Main Sequence Stars
White
Dwarf Stars
Red
Giant Stars
Absolute magnitude
Luminosity
O B A F G K M
Spectral type
Works only for main sequence!

11. Summary We can measure apparent magnitude and spectral type.
The main sequence lets us infer absolute magnitude from spectral type.
Comparing apparent magnitude to absolute magnitude gives us distance.

12. Something to ponder...
Astronomy, as an observational science, has limitations:
Can’t build stars in our laboratory
Can only observe what happens to be available
Can’t go back in time to watch stars form, or go forward in time to see them die.
How, then, can we learn about the life cycles of stars?
Sample stars that exist now in various stages of development.

13. Star Clusters — and how they hint at Stellar Life Cycles
Next time
Star Clusters — and how they hint at Stellar Life Cycles

14. Spectroscopic Parallax
Activity #7
Spectroscopic Parallax