1. Properties of Stars

3. Learning goals:
Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.
Define arc second, parsec.
Define brightness, apparent magnitude, absolute magnitude.
Describe the methods used to determine the temperature, luminosity, and radius of a star.

4. Distances Questions: Which stars are the brightest?
Which stars are putting out the most watts? (luminosity = energy per second)
NEED TO KNOW:
Distances
The most fundamental and accurate (within a certain range) means of finding distances is measuring the parallaxes of stars.

5. You already know about the parallax effect:
Demonstrating parallax
Parallax of Stars
Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

6. Define arc second How many degrees in a circle?
How many arc minutes in a degree?
How many arc seconds in an arc minute?
How many arc seconds in a degree?
How many arc seconds in a circle?
__?__ radians = 360 degrees
1 radian = 57.3 degrees
How many arc seconds in 1 radian?
360, 60, 60, 3600; 1,296,000; 2 pi; 206,265 arc sec/rad

7. PARSEC: Parallax ARc SECond
A star having a parallax of 1 arc second is 1 parsec away
1 parsec (pc) = 3.26 light years
1 kiloparsec (1 kpc) = 1000 pc; 1 megaparsec (1 Mpc) = 1,000,000 pc
Baseline is 1 Astronomical Unit
Small angle formula for distance in AU’s:
Define arc second, parsec

8. Measured Parallax of Stars 6.7 22 667 2170 ly
Works accurately for stars within about 200 pc (Hipparchos satellite)
Biggest problem: measuring the miniscule shift of a star against more distant stars
6.7 22
667
2170 ly
Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

9. Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

10. Using SIMBAD to find the parallaxes of the stars of Exercise 2
41 Cygni data (partial)
Parallax = 4.24 ± 0.16 mas or ± arc seconds
Distance = 1/parallax = 1/ = 236 pc or ~770 ly

11. Inverse square law for light
p. 494

12.   Brightness How the star looks to US HERE ON EARTH.
times farther away 
100 Watt
1000 Watt
1 Watts
10 times farther away 
2 x farther away, 1/4 as bright
3 x farther away, 1/9 as bright
Define brightness, apparent magnitude, absolute magnitude

13. Every 5 magnitudes difference means 100 x difference in brightness
Apparent Magnitude
Every 5 magnitudes difference means 100 x difference in brightness
One magnitude difference is times in brightness.
( = 100)
Define brightness, apparent magnitude, absolute magnitude

14. When you see only “magnitude,” that means APPARENT magnitude.
The magnitude (m) of star A is 1, the magnitude (m) of star B is 6. How many times brighter is A than B?
a) b) c) d) 1000
m of star C is 12, m of star D is 2: How many times brighter is star D than star C? (Or, equally stated, how many times dimmer is star C than star D?)
a) b) c) d) 10,000
The Sun is the brightest star in the sky, with an apparent magnitude of about Sirius is next in line, with an apparent magnitude of -1.5; how many times brighter is the Sun than Sirius?
a) b) c) 100, d) 10,000,000,000

15. Using SIMBAD to find the apparent magnitudes of the stars of Exercise 2
41 Cygni data (partial)
V = apparent magnitude through “visual” filter
Think of it as mv . IR UV

16. Absolute Magnitude
Absolute magnitude is the apparent magnitude a star would have if its distance = 10 parsecs.
Relates luminosities by “placing” stars on common scale.
Smaller the absolute magnitude number, the more luminous the star.
41 Cygni
dpc = 236 parsecs
mv = 4.016
What does the answer tell you?
Define brightness, apparent magnitude, absolute magnitude

17. Define brightness, apparent and absolute magnitude

18. Luminosity Classes Supergiant I Bright-Giant II Giant III Sub-Giant IV
Main Sequence Star (dwarf) V
We estimate the luminosity of a star by measuring how broad the absorption lines are in its spectrum.
At a given temperature, the less luminous stars have atoms colliding a lot more than in the giant stars.
Describe the methods used to determine temperature, luminosity, radius

19. High
Luminosity
Low
Low
High
Temperature

20. Using SIMBAD to find the parallaxes of the stars of Exercise 2
41 Cygni data (partial)
F5 Iab

21. The H-R Diagram

22. Relationship between absolute magnitude and luminosity - bring in the Sun!

23. Luminosity Depends on Size (radius, R) Temperature
Explain
Describe the methods used to determine temperature, luminosity, radius

24. The H-R Diagram