1. Star Properties
(Power Point 12)

2. Student Learning Objectives
Describe and differentiate between the various mechanisms responsible for the categories into which they (“stars”) are divided.
Classify stars
Use star properties to determine star characteristics
Recall how star properties are related

3. What is the parallax method for measuring distance?
Stellar parallax is a change in an object’s apparent position caused by a change in the observer’s position.
d = 1 p
Stock Image

4. The apparent change in position is actually very slight.
Because of parallax, the nearby star appears to be in a different part of the sky in January as in June. The apparent change in position is actually very slight; on the diagram it has been EXAGGERATED.
The apparent change in position is actually very slight.
The change has been EXAGGERATED here.
Image Credit: frankswebspace.org

5. Practice
Parallax angles are very small. We can use these angles to calculate the parsecs to nearby stars pc = 3.26 LY
Vega: parallax angle = 0.12 arcseconds
Polaris: parallax angle = 0.01 arcseconds
Sirius: parallax angle = arcseconds

6. How are stellar properties related?
Brightness refers to the number of photons.
Intrinsic to Star - depends on luminosity
Apparent on Earth - depends on distance
Magnitudes are a measure of brightness, and brightness is the direct result of energy output.
B = 2.512Dmagnitude
B = L
4pd2

7. Practice
If two stars are the same size, what would make one star appear fainter than the other?
B = L .
4pd2
What is the difference in brightness between a magnitude –2 star and a magnitude +1 star?
B = 2.512Dmagnitude

8. Magnitudes & Distance
Absolute Magnitude is the magnitude a star would have at a distance of 10 parsecs.
Animation by Patrick M. Len

9. Practice
Rank these stars from brightest to dimmest, as viewed from Earth.
Rank these stars from brightest to dimmest, if viewed at 10 parsecs.
Apparent
magnitude
Absolute magnitude
Sun
−27 +5
Canopus −1 −3
Vega
+0.5
Kapteyn’s Star +9
+11

10. A star’s total energy output is called luminosity, while flux is the energy per m2 from the star.
Image Credit: astro.unl.edu

11. Practice Star A: Large Surface Area at 6,000 K
Star B: Small Surface Area at 6,000K
Which would have a greater luminosity?
Which would have a greater flux?
Which would be brighter if both were the same distance from us?

12. Mass-Luminosity Relation
The more mass a star has, the more Watts it produces.
L = M3.5
Mass → Temperature → Energy → Brightness/Magnitude
Image Credit: wordpress.com

13. Practice
The mass of our Sun is taken as 1. How does each factor (2x, 3x, etc.) of mass change the luminosity?
Star with 2 x Msun
Star with 3 x Msun
Star with 4 x Msun

14. What does a star’s spectral class indicate?
Spectral class is determined from a star’s atomic spectrum.
All stars have similar compositions
Differences are primarily due to differences in temperature
Openstax: phys.libretexts.org

15. Type Color Temperature
Blue-Violet
30,000+ K B
Blue
18,000 K A
Blue-White
10,000 K F
White
7,000 K G
Yellow
5,500 K K
Orange
4,000 K M
Orange-Red
3,000 K L
Red
2,000 K T
Dark Red
700-1,300 K

16. Wondrous Variety of Planets & Stars
Author Unknown

17. Practice What is the spectral class of our star, the Sun?
Rank these star types from hottest to coolest, and indicate the color range: A, G, M, O, F

18. The diagram is a graph of stars in a cluster.
What is an H-R diagram?
The diagram is a graph of stars in a cluster.
The position of a star on the graph indicates its stellar properties.
Stage of stellar evolution
Mass
Energy
Openstax Image

19. Source: Universe Today

20. Practice
Our star, the Sun, is a main-sequence star. Stars spend most of their lifetime as a main-sequence star. What does it mean to be a main-sequence star?
What spectral type do we expect to find at the upper end of the main-sequence? Are these high or low mass stars?

21. Luminosity Class
Stars are further divided into luminosity classes on the HR diagram.
I, II, III, IV
Giants V
Main-Sequence
VI, VII
Dwarfs
Openstax Image

22. Practice What is the luminosity class of our Sun?
What is the luminosity class of the following types of stars?
B type star
M type star
Star in the Giant stage

23. Why are binary stars important?
The mass of a binary system can be determined from the orbital period.
(m1 + m2)P2 = a3

24. Stellar separation can be measured visually.
Visual Binaries
Stellar separation can be measured visually.
Source: SFA Observatory

25. Spectroscopic Binaries
Doppler shifts from orbital periods are observed.
Video Credit: Ohio State University

26. Eclipsing Binaries
Orbital periods and relative sizes are indicated in light curve.
Kepler - HATP7 Light Curve

27. Practice List the three ways astronomers detect binary star systems.
What star property can be determined from the periods in a binary star system? Why is knowing this quantity important?
Compare and contrast our Sun to other stars.