ASTR 1102-002 2008 Fall Semester [Slides from Lecture06] 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 Motion through 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 Motion through 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 Motion through 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 Motion through 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