1. Chapter 17 Measuring the Stars
Acknowledgements: These lecture notes have been adapted from lecture materials developed by Dr. Bruton of the Dept. of Physics and Astronomy at S.F.A.
Measuring the Stars

2. Properties of Stars
Distance
Speed
Radius
Brightness
Temperature

3. 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.

6. Stellar Distances
Parsec - the distance from our Sun at which the angle between the Earth and the Sun subtends an angle of one arc second
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).

7. Measuring A Star’s Brightness
Inverse-Square Law - the apparent brightness of a star decreases with increasing distance from Earth

9. Power = (Area)x(Intensity)
Luminosity - the amount of Energy per unit time (power) a star radiates
Stefan-Boltzmann Law - a star of temperature T radiates an amount of energy each second equal to sT4 per square meter - This is the intensity.
Power = (Area)x(Intensity)

11. 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

13. Measuring A Star’s Temperature
A star’s surface temperature can be determined from its color using Wien’s Law.
Reddish  coolest star
Orange-ish
Yellowish
White
Bluish  hottest star

16. 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

19. 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?

20. 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?

21. 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?

22. 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!

23. 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

24. 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

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

27. 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.

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

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

31. End of Chapter 17...