Transcript PowerPoint File

Lecture 12- Stars: Distances and
Magnitudes
Smaller parallax > greater distance
Largest parallax = 0.75 arcseconds
1 arcsecond = dime at 1.3 miles
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
Sources of Parallax Measurements
• Pre-1990: groundbased telescopes, ~
1000 stars measured
• Hipparchus: 118,000
stars measured
• See book for
breakdown on
precision
Hipparcos web page:
http://astro.estec.esa.nl/Hipparcos/table361.html
http://astro.estec.esa.nl/Hipparcos/table365-new.html
What distances do we have for
the nearest stars?
•
Appendix 12 of book
•
Hipparcos table 361
• http://astro.estec.esa.nl/Hipparcos/table361.
html
What do these numbers mean? What do they tell us
about the nature of stars?
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
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.
Brightnesses of Stars: The
Magnitude System
How to quantitatively describe the
brightness of stars?
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 2.512 in brightness
• Brightness in magnitudes in Table 16.1
• Apparent magnitudes of brightest stars in
Appendix 13
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
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)
0
1
2
5
10
20
Distance (pc)
10
15.8
25.1
100.0
1000
100,000
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