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1. A star?s color, temperature, size, brightness and distance are all related!
2. The Beginnings Late 1800?s, early 1900?s ? how light is produced by atoms is being intensely studied by?
Gustav Kirchoff & Robert Bunsen
Max Planck?Josef Stefan...
Ludwig Boltzmann?Albert Einstein
3. Black Bodies In 1862, Kirchoff coins the phrase ?black body? to describe an imaginary object that would perfectly absorb any light (of any wavelength) that hit it.
No light transmitted through, no light reflected off, just totally absorbed.
4. a perfect absorber of light would also be a perfect emitter
amount of light energy given off each second (its brightness or luminosity) and the color of its light are related to the object?s temperature.
5. Molten lava and hot iron are two good examples of black bodies, but?
a star is an excellent black body emitter.
6. Max Planck, a German physicist, was able to make theoretical predictions of how much light of each color or wavelength would be given off by a perfect black body at any given temperature.
These predictions or models are today called Planck Curves.
8. What 2 characteristics of the curves change as the temperature increases?
9. Can we draw some conclusions? Hotter stars should be brighter than cooler stars.
Hotter stars should emit more of their light at shorter wavelengths (bluer light)
Cooler stars should emit more of their light at longer wavelengths (redder light).
All stars emit some energy at all wavelengths!
10. In 1879, Josef Stefan discovered that the luminosity of a star was proportional to the temperature raised to the 4th power.
In 1884, Stefan?s observations were confirmed when Ludwig Boltzmann derived Stefan?s equation from simpler thermodynamic equations.
11. Stefan-Boltzmann Law Today, we honor both scientists by naming the equation after them?the Stefan-Boltzmann Law:
At the surface of the star, the energy that?s given off per square meter (Watts / m2) called the luminous flux is...
W / m2 = 5.67 x 10-8 T4
12. At 100 K (cold enough to freeze you solid in just seconds), a black body would emit only 5.67 W/m2.
At 10x hotter, 1000 K, the same black body would emit 104 times as much light energy, or 56,700 W/m2.
13. If the temperature of a star were to suddenly double, how much brighter would the star become?
If the temperature of a star somehow fell to 1/3 of what it was, how much fainter would the star become?
14. In 1893, Wilhelm Wien (pronounce ?vine?) discovered by experiment the relationship between the ?main? color of light given off by a hot object and its temperature.
This ?main? color is the peak wavelength, called ?max , at the top of the Planck Curve.
16. Wien?s Law Wien?s Law says that the peak wavelength is proportional to the inverse of the temperature:
?max = 2.9 x 106 T = 2.9 x 106
T must be in Kelvin, and ?max in nanometers.
17. What is the peak wavelength of our sun, with a T = 5750 K?
What is the peak wavelength of a star with a surface temperature of 3500 K?
18. A reddish star has a peak wavelength of 650 nm. What is the star?s temperature?
A star has a peak wavelength in the ultra-violet of 300 nm. What is the star?s temperature?
19. We now have a ?color thermometer? that we can use to determine the temperature of any astronomical object, just by examining the light the object gives off.
We know that different classes of objects are at different temperatures and give off different peak wavelengths.
20. What kinds of objects? Clouds of cold hydrogen gas (nebulae) emit radio waves
21. Warmer clouds of molecules where stars form emit microwaves and IR.
22. Protostars emit IR.
23. Sun-like stars emit mostly visible light, while hotter stars peak in the UV.
24. Neutron stars and black holes peak in the X-ray.
25. Star cores emit gamma rays.
26. Where would the peak wavelength be for
a lightning bolt
the coals from a campfire