1. Learning from Light

2. There are three different types of spectra…
Continuous spectrum
(“Thermal radiation”)
Emission spectrum
Absorption spectrum
hot
cool

3. “Thermal radiation”
Everything gives off light – electromagnetic radiation – due to its thermal energy.
Visible light cannot pass through the bag …
… but infrared light can!
(The bag is opaque to visible light, but transparent to infrared light.)
At temperatures we are used to – room temperature up to hundreds of degrees – that radiation is below the visible part of the spectrum  Infrared (IR)

4. The lowest energy part of the visible spectrum is RED
But as an object heats up, the energy of the light it gives off goes up – eventually into the visible part of the spectrum
The lowest energy part of the visible spectrum is RED
As it gets hotter still, it turns yellow, then white. WHY?

5. This is energy per unit surface area.
As an object gets hotter, it gives off more of the higher energy light
More of the blue
An even mix
More of the red
The hottest objects look BLUE!
This is energy per unit surface area.

6. Notice that the wavelength of the PEAK depends on temperature

7. Notice that the wavelength of the PEAK depends on temperature
This can be expressed mathematically as Wien’s Law
Or, solving for T:

8. Notice that the wavelength of the PEAK depends on temperature
This can be expressed mathematically as Wien’s Law
Or, solving for T:
What is the peak wavelength in the thermal spectrum of a star whose surface temperature is 15,000 K?

9. Notice that the wavelength of the PEAK depends on temperature
This can be expressed mathematically as Wien’s Law
Or, solving for T:
What is the peak wavelength in the thermal spectrum of a star whose surface temperature is 15,000 K?

10. Notice that the wavelength of the PEAK depends on temperature
This can be expressed mathematically as Wien’s Law
Or, solving for T:
The peak wavelength in the thermal spectrum of a star you’re studying is 350 nm. What is the surface temperature of the star?

11. Notice that the wavelength of the PEAK depends on temperature
This can be expressed mathematically as Wien’s Law
Or, solving for T:
The peak wavelength in the thermal spectrum of a star you’re studying is 350 nm. What is the surface temperature of the star?

12. Sound waves from a stationary train sound the same in all directions

13. Sound waves from a stationary train sound the same in all directions
The Doppler Effect
Sound waves from a stationary train sound the same in all directions
Sound waves from a moving train sound different depending on your location

14. It’s the same with light…
The Doppler Effect
It’s the same with light…
All lines shifted to longer wavelengths
All lines shifted to shorter wavelengths

15. Source at rest w.r.t. laboratory
The Doppler Effect
Source at rest w.r.t. laboratory
Source moving away slowly
Source moving away faster
Source moving toward us slowly
Source moving toward us faster

16. The Doppler Effect

17. The Doppler Effect
A yellow line in the helium spectrum has a wavelength of nm when measured in the lab. The same helium line has a wavelength of nm in the spectrum of a star.
Is the star moving towards us or away from us?
At what speed?

18. The Doppler Effect
A purple line in the spectrum of mercury has a wavelength of nm when measured in the lab. The same line in the spectrum of a nebula has a wavelength of nm. What is the nebula’s radial velocity with respect to the Earth?

19. The Doppler Effect allows us to measure an object’s radial motion only
“proper motion”

20. The Doppler Effect also allows us to measure a star’s rotation!
Rotating slowly
Rotating quickly
One side of the star is moving toward us, the other is moving away from us.
Each line is shifted both up AND down – they spread out.

21. Astro-Cash Cab!
Jack
Crystal
Emily
Isaiah
Amanuel

22. 1) A particular line in the spectrum of a galaxy has a wavelength of nm. The same line, when measured in a lab on Earth, has a wavelength of nm. The galaxy must be moving…
… towards us.
… away from us.
… can’t tell from this information.

23. 2) Which statement is always true?
a) A hotter object gives off more long wavelength photons than a cooler object.
b) A hotter object gives off more short wavelength photons than a cooler object.
c) The thermal photons given off by a hotter object have a higher peak wavelength than those given off by a cooler object.
d) The thermal photons given off by a hotter object have a higher peak energy than those given off by a cooler object.

24. 3) Which color star is hottest?
Red
Blue
Yellow
White

25. The Doppler Effect can tell us how quickly a star is rotating.
4) True or False ?
The Doppler Effect can tell us how quickly a star is rotating.