Distances to Stars: Parsecs and Light
Years
The Stars are other
Suns
“L’amor che move il Sole
E le altre stelle”… Dante, end
Of Paradiso
Or…the Sun is the
closest star
Think about them:
Demo of “Starry
Night”
Basic Questions about Stars
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•
•
•
•
•
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
Most Basic Method of Stellar
Distance Determination
Trigonometric parallax: an
ordinary surveying technique
Figure 16.1 of textbook, and
Section 16.2
DEMO
The concept of parallax and distance
measurement
How big is the angular shift?
s
Th
r
If Th << 45 degrees, Th (in radians) = s/r
90 degrees make a right angle
60 arcminutes made one degree
60 arcseconds make one arcminute
The values of stellar parallax
Smaller parallax > greater distance
Largest parallax = 0.75 arcseconds
1 arcsecond = dime at 1.3 miles
Alpha Centauri, the closest star
How close/far is Alpha Centauri, and what
units do we use to describe its distance?
• 1 parallax-second = 1 parsec = 3.08E+16m
=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 = (2.9979E+08
m/sec)*(3.156E+07 sec) = 9.461E+15 m.
Alpha Centauri is 4.3 light years away
• 1 parsec = 3.26 ly
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
Sources of Parallax
Measurements
• Pre-1990: groundbased telescopes, ~
1000 stars measured
• Hipparchus: 118,000
stars measured
• See book for
breakdown on precision
(section 16.2 of book)
What distances do we have
for the nearest stars?
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•
Appendix 11 of book
Abbreviated Version
Star
Alpha Cent.
Sirius
Altair
Vega
Arcturus
Dist. (ly)
4.3
8.8
16.6
26
36
Dist. (pc)
1.3
2.7
5.1
8
11
Look at Appendix 12 (distances to brightest
stars) and think about the significance of those
data
What are the distances to the
stars in Cygnus?
Distances to the Stars of
Cygnus
990pc
3.5 pc
22
parsecs
17pc
125 pc
467 pc
52 pc