ASTR Fall Semester Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture04]

University-wide, Gustav-motivated Calendar Modifications

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!

Course Syllabus

Chapter 17: The Nature of Stars

Describe a Population of Stars

Individual Stars… Location in Space –Coordinate (angular) position on the sky [Right ascension & Declination] –Distance from Earth [use Stellar Parallax] Motion through Space –Motion across the sky [“proper” motion] –Motion toward/away from us (radial velocity) [use Doppler Effect]

Google Earth/Sky

Stellar Parallax (§17-1) Understand Figs. 17-1, 17-2, and eyes+thumb illustrations. Star ‘A’ exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star ‘B’. –Which star is farther from us? –How much farther away? If parallax angle (p) is measured in arcseconds and distance is measured in ‘parsecs’ (see §1-7 and Fig. 1-14), then... – d = 1/p

Stellar Parallax (§17-1) Understand Figs. 17-1, 17-2, and eyes+thumb illustrations. Star ‘A’ exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star ‘B’. –Which star is farther from us? –How much farther away? If parallax angle (p) is measured in arcseconds and distance is measured in ‘parsecs’ (see §1-7 and Fig. 1-14), then... – d = 1/p

March sky image

September sky image

Stellar Parallax (§17-1) Understand Figs. 17-1, 17-2, and eyes+thumb illustrations. Star ‘A’ exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star ‘B’. –Which star is farther from us? –How much farther away? If parallax angle (p) is measured in ‘arcseconds’ and distance is measured in ‘parsecs’ (see §1-7 and Fig. 1-14), then... – d = 1/p

Individual Stars… Location in Space –Coordinate (angular) position on the sky [Right ascension & Declination] –Distance from Earth [use Stellar Parallax] Motion through Space –Motion across the sky [“proper” motion] –Motion toward/away from us (radial velocity) [use Doppler Effect; §5-9]

Motion Across the Sky (“proper” motion)

Prominent and Obscured Objects

NOTE: Transient Events (in time) also occur

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

Stars of different brightness

Stars of different colors

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

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 “1 st magnitude” stars Successively fainter stars were catalogued as 2 nd magnitude, 3 rd magnitude, etc. Faintest stars (visible to the “naked eye”) were catalogued by Greek astronomers as 6 th 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.

Stars of different brightness

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. –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 1/10 th its original distance, it becomes 100 times brighter

Apparent Brightness varies with Distance

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

Color-Temperature Relationship

More About: Continuous Spectra from Hot Dense Gases ( or Solids ) Kirchhoff’s 1 st 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)

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.