Download Slides from Lecture06

ASTR 1102-002
2008 Fall Semester
Joel E. Tohline, Alumni Professor
Office: 247 Nicholson Hall
[Slides from Lecture06]
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
–
–
–
–
Brightness (luminosity/magnitude)
Color (surface temperature)
Mass
Age
Apparent magnitudes (m)
Catalog of Stars
Data drawn from two textbook appendices:
Appendix 4 = “The Nearest Stars”
Appendix 5 = “The Visually Brightest Stars”
Stars of different brightness
Intrinsic Brightness Distribution
of Stars in our Galaxy
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