Distances to Stars: Parsecs and Light Years

1
Distances to Stars Parsecs and Light Years
2
The Stars are other Suns
Lamor che move il Sole E le altre stelle
Dante, end Of Paradiso

• Orthe Sun is the closest star

Think about them Demo of Starry Night
3
Basic Questions about Stars

• How far away are they?
• How hot are they?
• How massive are they
• What are they made of?
• Do they have planets too?
• How do they change with time (stellar evolution)

16 Cygni
4
Most Basic Method of Stellar Distance
Determination

• Trigonometric parallax an ordinary surveying
  technique

Figure 16.1 of textbook, and Section 16.2
DEMO
5
The concept of parallax and distance measurement
6
How big is the angular shift?
s
Th
r
If Th ltlt 45 degrees, Th (in radians) s/r 90
degrees make a right angle 60 arcminutes made one
degree 60 arcseconds make one arcminute
7
The values of stellar parallax

• Smaller parallax gt greater distance
• Largest parallax 0.75 arcseconds
• 1 arcsecond dime at 1.3 miles

8
Alpha Centauri, the closest star
9
How close/far is Alpha Centauri, and what units
do we use to describe its distance?

• 1 parallax-second 1 parsec 3.08E16m 206,265
  au. Alpha Centauri is 1.33 parsecs
• If 1 au were 1cm, Alpha Centauri at a distance of
  1.5 miles
• The Light Year (2.9979E08 m/sec)(3.156E07
  sec) 9.461E15 m. Alpha Centauri is 4.3 light
  years away
• 1 parsec 3.26 ly

10
Think back to descriptions of solar system
distances (light minutes to light days) and
compare the situation for nearby stars to get an
idea of the immensity of interstellar space
11
Sources of Parallax Measurements

• Pre-1990 ground-based telescopes, 1000 stars
  measured
• Hipparchus 118,000 stars measured
• See book for breakdown on precision (section 16.2
  of book)

12
What distances do we have for the nearest stars?

• Appendix 11 of book
• Abbreviated Version

13
Look at Appendix 12 (distances to brightest
stars) and think about the significance of those
data
14
What are the distances to the stars in Cygnus?
15
Distances to the Stars of Cygnus
990pc
3.5 pc
22 parsecs
17pc
467 pc
125 pc
52 pc