Transcript ASTR 1101-001 Spring 2008 - Louisiana State University

ASTR 1102-002
2008 Fall Semester
Joel E. Tohline, Alumni Professor
Office: 247 Nicholson Hall
[Slides from Lecture05]
Gustav’s Effect on this Course
• Fall Holiday has been cancelled, which means
our class will meet on Thursday, 9 October.
(This makes up for one class day lost to Gustav
last week.)
• We will hold an additional makeup class on
Saturday, 20 September! (This will account for
the second class day lost to Gustav last week.)
• Date of Exam #1 has been changed to Tuesday,
23 September!
Chapter 17: The Nature of Stars
Individual Stars…
• Location in Space
– Coordinate (angular) position on the sky
– Distance from Earth
• Motion through Space
– Motion across the sky (“proper” motion)
– Motion toward/away from us (radial velocity)
• Intrinsic properties
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–
–
–
Brightness (luminosity/magnitude)
Color (surface temperature)
Mass
Age
Astronomers’ Magnitude System
• Ancient, Greek astronomers made
catalogues of all the (visible) stars in the
sky
– Name
– Position on the sky (angular coordinates)
– Any observed motion?
– Brightness on the sky (hereafter, apparent
brightness)  The Greeks defined a
“magnitude” system to quantify the (apparent)
brightness of each star.
Astronomers’ Magnitude System
• The brightest stars were labeled “1st magnitude” stars
• Successively fainter stars were catalogued as 2nd
magnitude, 3rd magnitude, etc.
• Faintest stars (visible to the “naked eye”) were
catalogued by Greek astronomers as 6th magnitude
stars.
• Astronomers continue to use this “magnitude” system,
extending it to much fainter objects (that are visible
through telescopes but were not bright enough to be
seen by Greek astronomers).
• The Sun can also be put on this “magnitude” system.
Apparent magnitudes (m)
Stars of different brightness
Catalog of Stars
Data drawn from two textbook appendices:
Appendix 4 = “The Nearest Stars”
Appendix 5 = “The Visually Brightest Stars”
Apparent brightness due to…
• Each star’s intrinsic brightness
• Each star’s distance from us
• A star of a given intrinsic brightness will
appear to get fainter and fainter if you
move it farther and farther away from us
Concept of Apparent Brightness
• 10 stars that are identical in every respect (all having, for
example, the same intrinsic brightness) will appear to
have different brightness in the night sky if they are all at
different distances from us.
• Apparent brightness varies as the “inverse square” of the
distance.
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–
Move a star twice as far away, it becomes 4 times fainter
Move a star 3 times farther away, it becomes 9 times fainter
Move a star 10 times farther away, it becomes 100 times fainter
Move a star to half its original distance, it becomes 4 times
brighter
– Move a star to 1/10th its original distance, it becomes 100 times
brighter
Apparent Brightness varies with
Distance
Determining Intrinsic Brightness
• Measure apparent magnitude, “m”.
• Measure distance to star using stellar parallax.
• “Move” star to a standard distance from us (10
parsecs) and adjust its magnitude accordingly
 absolute magnitude, “M”.
– The absolute magnitude of each star in our catalog
tells us “how bright the star would appear to be if it
was 10 parsecs away from us.”
– “M” is therefore a measure of each star’s intrinsic
brightness
Intrinsic Brightness Distribution
of Stars in our Galaxy
Catalog of Stars
Data drawn from two textbook appendices:
Appendix 4 = “The Nearest Stars”
Appendix 5 = “The Visually Brightest Stars”
Individual Stars…
• Location in Space
– Coordinate (angular) position on the sky
– Distance from Earth
• Motion through Space
– Motion across the sky (“proper” motion)
– Motion toward/away from us (radial velocity)
• Intrinsic properties
–
–
–
–
Brightness (luminosity/magnitude)
Color (surface temperature)
Mass
Age
Continuous Spectra
from Hot Dense Gases (or Solids)
• Kirchhoff’s 1st Law: Hot dense gas produces a
continuous spectrum (a complete rainbow of colors)
• A plot of light intensity versus wavelength always has the
same general appearance (blackbody function):
– Very little light at very short wavelengths
– Very little light at very long wavelengths
– Intensity of light peaks at some intermediate wavelength
• But the color that marks the brightest intensity varies
with gas temperature:
– Hot objects are “bluer”
– Cold objects are “redder”
Continuous Spectra
from Hot Dense Gases (or Solids)
• Kirchhoff’s 1st Law: Hot dense gas produces a
continuous spectrum (a complete rainbow of colors)
• A plot of light intensity versus wavelength always has the
same general appearance (blackbody function):
– Very little light at very short wavelengths
– Very little light at very long wavelengths
– Intensity of light peaks at some intermediate wavelength
• But the color that marks the brightest intensity varies
with gas temperature:
– Hot objects are “bluer”
– Cold objects are “redder”
The Sun’s Continuous Spectrum
(Textbook Figure 5-12)
Continuous Spectra
from Hot Dense Gases (or Solids)
• Kirchhoff’s 1st Law: Hot dense gas produces a
continuous spectrum (a complete rainbow of colors)
• A plot of light intensity versus wavelength always has the
same general appearance (blackbody function):
– Very little light at very short wavelengths
– Very little light at very long wavelengths
– Intensity of light peaks at some intermediate wavelength
• But the color that marks the brightest intensity varies
with gas temperature:
– Hot objects are “bluer”
– Cold objects are “redder”
Color-Temperature Relationship
Wien’s Law for Blackbody Spectra
• As the textbook points out (§5-4), there is
a mathematical equation that shows
precisely how the wavelength (color) of
maximum intensity varies with gas
temperature.
Color Filters: U, B, V
Individual Stars…
• Location in Space
– Coordinate (angular) position on the sky
– Distance from Earth
• Motion through Space
– Motion across the sky (“proper” motion)
– Motion toward/away from us (radial velocity)
• Intrinsic properties
–
–
–
–
Brightness (luminosity/magnitude)
Color (surface temperature)
Mass
Age
Intrinsic Brightness vs. Color
Hertzsprung-Russell (H-R) diagram
Individual Stars…
• Location in Space
– Coordinate (angular) position on the sky
– Distance from Earth
• Motion through Space
– Motion across the sky (“proper” motion)
– Motion toward/away from us (radial velocity)
• Intrinsic properties
–
–
–
–
Brightness (luminosity/magnitude)
Color (surface temperature)
Mass
Age
Measuring Stellar Masses
• Astronomers determine the mass of a star by examining
how strong the gravitational field is around that star.
(Isaac Newton’s law of universal gravitation; §4-7)
• By studying the motion of planets around our Sun,
astronomers have determined that the Sun has a mass
of 2 x 1030 kilograms.
• We cannot measure the mass of individual, isolated
stars.
• We have an opportunity to measure the mass of a star if
it resides in a binary star system.
– Fortunately, most stars are in binary systems!
– The Sun is unusual in this respect because it does not have a
companion star about which it orbits.
Measuring Stellar Masses
Intrinsic Brightness vs. Stellar Mass