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Connecting the Properties of Stars: HR Diagrams, Variable Stars and Distance
text: Chapter 12 (sections 12.6 - 12.8)
Are Surface Temperature and Luminosity related?
Plot data of surface temperature and luminosity from each star.
If no relationship: graph of luminosity vs. temperature is random:
We observe
This tells us that they are related.
Hertzsprung and Russell independently discovered this in early 1900's
call this graph a Hertzsprung-Russell Diagram (H-R diagram)
Figure 12.16 in the text shows an H-R diagram with logarithmic axes. Link to Figure 12.16
This is NOT a map !!
About 90% of the stars will be on the Main Sequence.
The Names of the regions imply that we have SIZE information on this diagram
recall luminosity depends on temperature and sizetwo stars with the same temperature - the brighter star is bigger
so if we look at a vertical line on the figure (same temperature) brighter stars must be bigger
so we can go straight up from Main Sequence stars to Giants and then Supergiants
A careful examination of sizes indicates that size increases from the lower left corner up to the top right corner. Figure 12.18 in the text shows this. Link to Figure 12.18.
Some HR diagrams will use spectral class instead of Temperature on the horizontal axis.
Some HR diagrams will use absolute magnitude instead of luminosity on the vertical axis.
HR diagram applet showing sizes and colors: http://instruct1.cit.cornell.edu/courses/astro101/java/evolve/evolve.htm
On the Main Sequence, we find that mass and luminosity are also related
bright stars in the upper left have higher massdim stars in the lower right have lower mass
This relationship arises from
high mass stars have stronger gravityneed higher temperatures to have higher gas pressures to offset the higher gravity
these higher temperatures result in higher luminosity
Another way to identify the luminosity of starsuse Roman Numerals:
I = high luminosityV = low luminosity (main sequence stars)
determined by width of spectral lines
lines are very narrow in the most luminous stars and wider in main sequence starscaused by density differences (luminous stars have lower densities)
(remember that rotation and magnetic fields also broadens lines)
Sun is a G2V star
stars whose luminosity changes significantlysome are irregular
some are periodic with periods of several days to several hundred days
often from the star's size regularly changing - pulsatingmost variable stars, when plotted on H-R diagram end up just to the left of the red giant region
this region is called the "instability strip"
Figure 12.23 in the text, shows this region on a H-R diagram. Link to Figure 12.23.
Since Brightness depends on distance in a known manner
and Luminosity does not depend on distance,
use these to find distance.
1. Find two stars that you think are identical in luminosity. (not always easy to do)
2. Measure the brightness of both stars (look up into the sky!)
3. Determine the distance to the second star from the known change in brightness with distance (inverse square law).
Use this system to estimate distances to many other stars.
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