1. Lecture 12- Stars: Distances and Magnitudes
Smaller parallax > greater distance
Largest parallax = 0.75 arcseconds
1 arcsecond = dime at 1.3 miles

2. Units of Stellar Distance
1 parallax-second = 1 parsec = 3.08E+16m =206,265 au
If 1 au were 1cm, Alpha Centauri at a distance of 1.5 miles
The Light Year = (2.9979E+08 m/sec)*(3.156E+07 sec) = 9.461E+15 m
1 parsec = 3.26 ly

3. Sources of Parallax Measurements
Pre-1990: ground-based telescopes, ~ 1000 stars measured
Hipparchus: 118,000 stars measured
See book for breakdown on precision
Hipparcos web page:

4. What distances do we have for the nearest stars?
Appendix 12 of book
Hipparcos table 361
What do these numbers mean? What do they tell us about the nature of stars?

5. What are the distances to the stars in Cygnus?

6. Distances to the Stars of Cygnus
990pc
3.5 pc
22 parsecs
17pc
125 pc
467 pc
52 pc

7. This is a real question. I want some answers.
What does this very basic information (distances and brightnesses of naked eye stars) tell us about the nature of stars?
This is a real question. I want some answers.

8. Brightnesses of Stars: The Magnitude System
How to quantitatively describe the brightness of stars?

9. Magnitudes, Apparent and Absolute
Apparent magnitude is the brightness of an object as it appears to you
System due to Hipparchos (2nd century BC)
Nowadays system made more precise
Magnitude changes are “logarithmic”, each magnitude means factor of in brightness
Brightness in magnitudes in Table 16.1
Apparent magnitudes of brightest stars in Appendix 13

10. Absolute Magnitude: a measure of the intrinsic brilliance of a star
Pick a star (any star)
Imagine moving it to a distance of 10 parsecs
The apparent magnitude it would have is its absolute magnitude
The absolute magnitude is a distance-independent quantity
Look at Appendix 13 and think about the meaning of the absolute magnitudes

11. Why such a big deal about absolute magnitudes?
The difference between the apparent magnitude (m) and the absolute magnitude (M) is a measure of the distance to an object
(m-M)
Distance (pc) 10 1
15.8 2
25.1 5
100.0
1000 20
100,000

12. If you know the absolute magnitude M of a star (or other astronomical object) and you measure its apparent magnitude m, you then know its distance. This difference (m-M) is called the distance modulus