1. Star Properties

2. Apparent Magnitude System of Hipparchus Group of brightest stars 1m
Stars about ½ as bright as 1m 2m
Stars about ½ as bright as 2m 3m •
Naked Eye Limit 6m

3. Apparent Magnitude
19th century photographers learn how eye responds to light (Pogson)
Doubling the brightness is not perceived as a doubling by the eye
Eye response is logarithmic
Ratio of 100 in brightness corresponds to a Difference of five magnitudes
Dm of 5  100X in light
Dm of 1  2.512X in light

4. Some Apparent Magnitudes
Sun
Full Moon
Venus at brightest
Sirius
Naked Eye Limit
Faintest Objects +30.0
Hubble

5. Learning the Brightness
Is a star bright...
Because it really is a bright star?
Because it is close to the Earth?
Stellar brightness depends on
Luminosity
Distance

6. Measuring Distance
Stellar Parallax
June
Sun
January

7. Stellar Parallax
June
January
Sun
Parallax
1 AU

8. Measuring Parallax
1 arcsec
1 AU
1 parsec

9. Stellar Parallax When p is measured in arcsec
and d is measured in parsecs
One parsec:
206,265 AU
3.26 light years

10. Stellar Parallax Nearest star to Sun (largest parallax)
a Cen p = 0.7 arcsec
Limit of accurate parallax
 200 pcs (angles of arcsec)
Hipparcos satellite (120,000 stars measured to arcsec)

11. Absolute Magnitude
The magnitude a star would have at 10 parsecs from the Sun.
The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same.
M, m, and d are related. Knowing two allows you to compute the third.

12. Putting the Pieces into Place
Ejnar Hertsprung
1911
Henry Norris Russell
1913

14. Luminosity Classes I Supergiants II Bright Giants III Giants
IV Subgiants
V Dwarfs

16. Luminosity Class implies Size
Consider the Sun and Capella
The Sun
G2V M=5
Capella
G2III M=0

17. Luminosity Class implies Size
Equal sized pieces of each star are equally bright
Capella is 100X brighter (5 magnitudes)
Capella must have 100X as much area
Surface area  radius2
Capella must be 10X larger than Sun.

18. Luminosity Class in the Spectrum
Supergiant A3
Giant A3
Dwarf

19. Sun G2V
Vega A1V
Betelgeuse M1I

20. Which of these stars is hottest?
Sun G2V
Vega A1V
Betelgeuse M1I
Can’t compare

21. Which of these stars is brightest?
Sun G2V
Vega A1V
Betelgeuse M1I
Can’t compare

22. Which of these stars is smallest?
Sun G2V
Vega A1V
Betelgeuse M1I
Can’t compare

23. Which of these stars is most distant?
Sun G2V
Vega A1V
Betelgeuse M1I
Can’t compare

24. Spectroscopic Parallax
Observe the spectrum and apparent magnitude of a star
Classify the spectrum
Main Sequence
Plot it on the H-R Diagram
Read off the M
From m and M compute distance

25. Color Index
12000 K B V * *
7000 K * *

26. Color Index Star Temperature mB mV . 1 12000 K 2.0 2.4
Color Index = mB - mV = B-V
B-V = = -0.4
B-V = = -0.1

27. Spectroscopic Parallax
Can now get distances to any object whose spectrum can be measured.
Limit  5000 pcs

28. Study Tools
Review 1
Review 2

29. The Advantage of Color Index
Measures temperature just like Spectral Type
Much easier to obtain
requires two measurements of brightness
spectral type requires getting the spectrum

30. Color-Magnitude Diagrams
Standard H-R Diagram
Color-Magnitude Diagram M
Spectral Type mV
B-V

31. Color-Magnitude Diagrams
Useful for star clusters
Can substitute mV for MV since you know all the stars are the same distance away.
Star Clusters
Open (galactic)
Globular

32. Structure of the Milky Way

33. Open Clusters Irregular shape Few tens to few hundred stars
In the plane of the galaxy
Young stars

34. Open clusters
M37
M16
M45

35. Color-Magnitude Diagram M45

36. Globular Clusters Spherical in shape Hundreds of thousands of stars
Halo distribution about galactic nucleus
Old stars

37. Globular Clusters
SFA Observatory M5 M3

38. Color-Magnitude Diagram M3