1. Chapter 12
Surveying The Stars

2. Properties of Stars Distance Speed Radius Brightness (Intensity)
Temperature

3. Luminosity ( L ) - the amount of energy a star radiates per unit time = power (e.g. Lsun= 4 x 1026 Watts.)
Intensity ( I ) – Power per unit area (Power/Area) Intensity is measured in Watts/m2.
Stefan-Boltzmann Law - a star of temperature T radiates an amount of energy each second (Power) equal to sT4 per square meter ( Intensity )

4. Definitions I = Power/area = T4. L = Power output of the star.
I = (Luminosity)/(surface area of a sphere).
I = L/4d2

5. Measuring A Star’s “Brightness”
Inverse-Square Law - the apparent brightness (Intensity) of a star decreases with increasing distance from Earth

8. The Inverse Square Law for Light
area = 4d2 d

9. The Magnitude System
Apparent Magnitude - logarithmic scale of brightness for stars (e.g. the size of the dots on star charts)
Absolute Magnitude - the apparent magnitude that a star would have if it were 10 parsecs away from Earth

11. Measuring a Star’s Distance
Parallax - the apparent change in the position of a star due to the motion of the Earth
Nearby objects exhibit more parallax that remote ones.

12. Stellar
Parallax

13. Stellar Parallax

14. Stellar Distances
Parsec - the distance corresponding to a parallax angle of exactly 1” (1 arc second) and the baseline is 1A.U. (distance between the earth and sun)
1 parsec = 3.26 light years
1 arc second = 1/3600 degrees
Light-year - the distance that light travels in one year.
Proxima Centauri is 4.2 light years from Earth (24 trillion miles).

15. Stellar Motion

16. Measuring A Star’s Temperature
A star’s surface temperature can be determined from its color using Wien’s Law.
Red  coolest star
Orange
Yellow
White
Blue  hottest star

17. Hubble Space Telescope view through the Galaxy reveals that stars come in different colors

21. Stellar Spectroscopy
Stellar Spectroscopy - the study of the properties of stars by measuring absorption line strengths
Spectral Class - classification of star according to the appearance of their spectra
O B A F G K M

25. Binary Stars
Optical Double - two stars that just happen to lie in the same direction as seen from Earth
Visual Binary - two stars that are orbiting one another and can both be seen from Earth

26. Binary Stars
Spectroscopic Binary - two stars that are found to orbit one another through observations of the Doppler effect in their spectral lines
Eclipsing Binary - two stars that regularly eclipse one another causing a periodic variation in brightness
Light Curve - a plot of a variable star's apparent magnitude versus time

27. Sirius A and Sirius B at ten year intervals reveals a binary system

28. Apparent Brightness Of An Eclipsing Binary System

29. Spectral Lines of a Binary System – Alternating Doppler Shift

31. Compare these spectra. What do these spectra tell us about the star?
Spectrum of Hydrogen in Lab
Spectrum a Star
What do these spectra tell us about the star?

32. Compare these spectra. What do these spectra tell us about the star?
Spectrum of Hydrogen in Lab
Spectrum a Star
What do these spectra tell us about the star?

33. Compare these spectra. What do these spectra tell us about the star?
Spectrum of Hydrogen in Lab
Spectrum a Star…..Day 1
Spectrum a Star…..Day 2
Spectrum a Star…..Day 3
Spectrum a Star…..Day 4
What do these spectra tell us about the star?

34. Mystery Star Properties
1. The star appears as a point of light through a telescope.
2. The absorption lines appear split and move over a
4 day period.
3. The brightness of the star also varies over 4 days.
Question: Why do you think the brightness of the
star is varying?
Answer: This could be an eclipsing binary star
system that cannot be resolved by a
telescope!

35. The H-R Diagram
Hertzsprung-Russell Diagrams - plots of luminosity versus temperature known stars
Most stars on the H-R diagram lie along a diagonal curve called the main sequence.

38. Stellar Luminosity Classes
CLASS DESCRIPTION
Ia Bright supergiants
Ib Supergiants
II Bright giants
III Giants
IV Subgiants
V Main-sequence stars/dwarfs

39. Along the main sequence, more massive stars are brighter and hotter but have shorter lifetimes

40. Stellar Lifetimes
Star Spectral Mass Central Luminosity Estimated Type (Solar) Temp(K) (Solar Lum) Lifetime
(106 Years)
Rigel B8Ia ,
Sirius A1V ,000
-Centauri G2V ,000
Sun G2V ,000
P-Centauri M5V >1,000,000

41. Variable Stars
Stars that have a change in brightness over time are called variable stars.
Examples:
Eclipsing binary stars
Cepheid variables
RR Lyra variables

42. A Typical Light Curve For A Cepheid Variable Star.
~ 15 days

43. H-R diagram with the instability strip highlighted

44. Star Clusters
Open Clusters: Loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way.
Globular Clusters: Tightly bound, roughly spherical collection of hundreds of thousands , and sometimes millions, of stars spanning about 50 parsecs. Globular Clusters are distributed in the halos around the Milky Way and other galaxies.

45. The Pleiades

46. H-R Diagram for the Pleiades.B6
Main Sequence turn off – Pleiades ~ 100 million yrs old
Missing upper main sequence stars

47. Open Cluster
Globular Cluster
Star Clusters

48. H-R Diagram for the globular cluster Palomar 3.
Main sequence turnoff indicates age ~ billion years

49. Matching Questions (a) composition. (b) parallax shift. (c) motion.
1. The temperature of a star can be determined from its_____________.
2. The pattern of the absorption spectral lines for a star contains information about a star’s________________.
3. The Doppler shift of a star's spectral lines tells us something about the star’s_______________.
4. The distance of a star from Earth can be determined from the star’s_______________.
5. The radius of a star can be determined from its ________________.
(a) composition.
(b) parallax shift.
(c) motion.
(d) luminosity and temperature.
(e) color.

50. H-R Diagram Questions 1. What property is measured along
the horizontal axis?
2. … along the vertical axis?
3. Where are the red giants?
4. … the white dwarfs?
5. … the hottest stars?
6. … the coolest stars?
7. … the largest stars?
8. … the smallest stars?

51. H-R Diagram Questions 9. Where are O class stars? 10. … M class stars?
… G class stars?
12. Where is the Sun?
13. Where are the high-mass main-
sequence stars?
14. Where are the low-mass main-
15. Where are the oldest stars?
16. Which stars along the main-sequence
live the longest?

52. The temperature of a star can be determined by its
Composition
Parallax shift
Motion
Color

53. The star with the highest surface temperature is a:
Yellow Star
Blue Star
Red Star

54. Luminosity is measured in
Joules
Amps
Watts
KWH (kilowatt hours)

55. End of Chapter 12