2. What are the properties of Stars?
Chapter 12 Measuring the Stars
What are the properties of Stars?
What are the patterns among Stars?

3. From the Sun we’ve learned:
stars are far away
stars are bright
stars are hot
stars are massive
How FAR AWAY? (DISTANCE)
How BRIGHT? (LUMINOSITY)
How HOT? (SPECTRAL TYPE)
How MASSIVE? (MASS)

4. Stellar Brightness -- what is it?
The brightness of an object depends on
both distance and energy output

5. Amount of energy output a star radiates is called the
Luminosity (L): the energy per second
Amount of starlight that reaches Earth is called the
Apparent brightness (m)

6. Two Kinds of Brightness
Apparent Magnitude (m): How bright the object appears to us on Earth.
Absolute Magnitude (M): How bright a star actually is, its intrinsic brightness

7. Two Kinds of Brightness
Apparent Magnitude: How bright the object appears to us on Earth.

8. Two Kinds of Brightness
Apparent Magnitude: How bright the object appears to us on Earth.
Which would look brighter?
Vega, m = 0.03
Antares, m = 1.06
Sirius, m = -1.4
Venus, m = -4.4

9. Two Kinds of Brightness
Apparent Magnitude: How bright the object appears to us on Earth.
Which would look brighter?
Vega, m = 0.03
Antares, m = 1.06
Sirius, m = -1.4
Venus, m = -4.4

10. Two Kinds of Brightness
Absolute Magnitude: How bright a star actually is, its intrinsic brightness
PROBLEM: stars are at different distances from Earth and so it’s hard to know which stars are ACTUALLY brighter versus which APPEAR bright

11. Which star looks brightest from Earth? Which star is brightest?
Compare some stars:
Absolute Apparent
MSun = mSun = -26
MSirius = mSirius = -1.46
MBetelgeuse = mBetelgeuse = 0.50
Which star looks brightest from Earth? Which star is brightest?

12. Which star looks brightest from Earth? Sun Which star is brightest?
Compare some stars:
Absolute Apparent
MSun = mSun = -26
MSirius = mSirius = -1.46
MBetelgeuse = mBetelgeuse = 0.50
Which star looks brightest from Earth? Sun Which star is brightest?
Betelgeuse

13. Summary of Spectral Classes Keep in mind this classification is
for the star’s SURFACE,
only!!!!!

14. General properties of Stars
Brightness
Temperature
Mass
Is there a way
we can use the
general properties
of stars to get a
“snap shot” about
the stars????

15. THE H-R DIAGRAM Enjar Hertzsprung and Henry Norris Russell
graph of luminosity (absolute magnitude) vs temperature (spectral class)

16. Herzsprung Russell Diagram
H-R diagram plots
the luminosity
and temperature of stars
Luminosity
brightness
temperature

17. brightness
temperature

18. Luminosity
brightness
temperature

19. Mass - Luminosity Relation
brightness
mass

20. Most stars fall somewhere on the main-sequence
of the H-R diagram
Main-sequence stars are fusing hydrogen into helium in their cores like the Sun
Luminous main-sequence stars are hot (blue)
Less luminous ones are cooler (yellow or red)
Main Sequence

21. Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones
High-mass stars
Short lived stars
Low-mass stars
Long lived stars

23. End of Chapter 12
Now you know
the basics of star
measurements!

24. H-R Diagram Interpretation Graphing Activity

25. Absolute Magnitude _ Spectral Type -10 -5 5 10 15 0 1 2 3 4 5 6 7 8 95 10 _ 15
Spectral Type