1. … how I wonder what you are.
Stars
… how I wonder what you are.

2. Goals Stars are Suns. Are they: What categories can we place them in?
Near? Far?
Brighter? Dimmer?
Hotter? Cooler?
Heavier? Lighter?
Larger? Smaller?
What categories can we place them in?

3. Angular Size Linear size = how big something really is
Meters, inches, light years, feet
Angular size = how big something looks
Degrees, arcminutes, arcseconds, milliarcseconds
Circle = 360 degrees
1 degree = 60 arcmin
1 arcmin = 60 arcsec
1 arcsec = 1000 mas

5. Distance One proof of a heliocentric Universe is stellar parallax.
Tycho Brahe saw no parallax.
Copernicus: stars too far away.
Nearest star: Proxima Centauri
Parallax angle = 0.76 arcsec
Tycho’s precision = 1 arcmin

6. The Parsec
What is the distance of an object with a parallax angle of 1 arcsec?
Distance = 206,265 AU
This distance is 1 parsec (pc)
1 pc = 206,265 AU = 3.3 ly
1 lightyear = distance light travels in one year.

7. Concept Test
If Star A has a parallax of 2 arcseconds, and Star B has a parallax of 0.25 arcseconds:
Star A is closer to us than Star B. Both are farther from us than 1 pc.
Star A is closer to us than Star B. Both are closer to us than 1 pc.
Star A is closer to us than 1 pc. Star B is farther than 1 pc.
Star B is closer to us than 1 pc. Star A is farther than 1 pc.
Star B is closer to us than Star A. Both are farther away than 1 pc.

8. Distances Closest star: Proxima Centauri parallax = 0.76 arcsec
Distance = 1.3 pc or 4.3 lightyears

9. Terms Brightness = How intense is the light I see from where I am.
Magnitude is numerical term for this.
Luminosity = how much light is the thing really giving off.

10. Magnitude Scale The SMALLER the number the BRIGHTER the star!
Every difference of 1 magnitude = 2.5x brighter or dimmer.
Difference of 2 magnitudes = 2.5x2.5 = 6.3x brighter or dimmer

11. Magnitude vs. Brightness
For Feb 24, 2012
Sirius = -1
Jupiter mag = -2
Venus mag = -4
Mars mag = -1
Mag. Difference
Factors of 2.5
Brightness Diff. 1
2.51 = 2.5
2.5 2
2.52 = 2.5 X 2.5
6.3 3
2.53 = 2.5 X 2.5 X 2.5 16 4
2.54 = 2.5 X 2.5 X 2.5 X 2.5 40 5
2.55 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5
100 6
2.56 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X 2.5
250

12. Star light, star bright
Sirius is magnitude -1.5 Polaris is magnitude 2.5
Is Sirius really more luminous than Polaris?
No, Sirius is just closer.

13. Apparent and Absolute
Apparent Magnitude = brightness (magnitude) of a star as seen from Earth.  m
Depends on star’s total energy radiated (Luminosity) and distance
Absolute Magnitude = brightness (magnitude) of a star as seen from a distance of 10 pc.  M
Only depends on a star’s luminosity

14. example Our Sun: Polaris:
distance = 4.8 x 10-6 pc
So: M = 4.8
Polaris:
m = 2.5,
distance = 132 pc
So: M = -3.1
Polaris is 1500 times more luminous than the Sun!

15. Concept Test Star Distance m M A 5 pc 1.0 B 10 pc 2.5 C 20 pc -1.0
The most likely absolute magnitudes (M) for each is:
A = 2.5, B = -2.5, C = 2.5
A = 2.5, B = 2.5, C = -2.5
A = -2.5, B = 2.5, C = 2.5
A = 2.5, B = 2.5, C = 2.5
None of the above.

16. Stellar Temperatures Hot How hot are stars?
Stellar Spectra
Hot
Cool
How hot are stars?
Thermal radiation and temperature.
Different stars have different colors, different stars are temperatures.
Different temp, different trace compositions

17. Spectral Classification

18. Stellar Masses How massive are stars?
Kepler’s Laws – devised for the planets.
Apply to any object that orbits another object.
Kepler’s Third Law relates:
Period: “how long it takes to orbit something”
Semimajor axis: “how far you are away from that something”
Mass: “how much gravity is pulling you around in orbit”
Where M is the Total Mass.
Can calculate the mass of stars this way.

19. Binary Stars Most stars in the sky are in multiple systems.
Binaries, triplets, quadruplets, etc….
Sirius
Alcor and Mizar
Tatooine
The Sun is in the minority by being single.

20. 0.005 arcsec

21. Stellar Masses How massive are stars?
Most stars have masses calculated this way.
Find:
The more massive the star, the more luminous it is.
The more massive the star, the hotter it is.

22. Stellar Radii How big are stars?
50 mas
How big are stars?
We see stars have different luminosities and different temperatures.
Stars have different sizes.
If you know:
Distance
Angular size
Learn real size.

23. Betelgeuse Angular size = 50 mas Parallax = 7.6 mas = 0.0076 arcsec
Apparent mag = 0.6
Distance = 1/parallax = 132 pc
True size = distance * angular size = 1400 Rsol
Model solar system: 97 yards in radius.
Absolute Mag = m – 5log(d/10pc) = -5
Our sun M ~5, Betelgeuse = 10,000x luminosity

24. Angular versus Linear
Supergiants, Giants and Dwarfs

25. H-R Diagram Can order the stars by:
Temperature (or spectral type)
Luminosity (or absolute magnitude).
And see where other qualities fall:
Mass
Radius

27. Luminosity Class I V III The roman numerals.
Stars at same temp can have different luminosities.
Corresponds to different classes: MS, giant, supergiant. I V
III

28. Concept Test Which star is: Of Main Seq. Stars? Star Spec Type m M A
F0 V
0.0 B
G2 V
10.0
4.4 C
K5 III
-2.0 D
F7 I
-1.0
-5.0 E
K3 V
5.0
6.5
Which star is:
Hottest?
Coolest?
Faintest as seen from Earth?
Most luminous?
Of Main Seq. Stars?
Most massive?
Most like the Sun?

29. Homework #10
For Monday March 5:
Read Bennett 9, Do quiz 9