1. Stars and Light

2. Light is used as a tool for...
determining the composition of a star
determining the temperature of a star
determining other properties of the star

3. Using light to determine temperature
Remember that light comes in different wavelengths and frequencies

4. Using light to determine temperature
The wavelength of visible light indicates its colour.

5. Using light to determine temperature
All objects emit their own light with different wavelengths
The hotter the object, the lower the wavelength of maximum intensity, called λmax

6. Using light to determine temperature
All objects emit their own light with different wavelengths
The hotter the object, the lower the wavelength of maximum intensity, called λmax
λmax = x 10-3 / T where T is the temp. in Kelvin (K)
(to get T in Kelvin, take T in Celcius and add 273)

7. Using light to determine temperature
Using this, we can determine the temperatures of distant stars, as long as we know the wavelength that they emit with the greatest intensity.
Betelgeuse, red, top left
Rigel, blue, bottom right
Temperatures: Rigel=9076 K
Betelgeuse=3488 K

9. Using light to determine temperature
Note that light can be emitted in non-visible wavelengths as well.
If a star cools, then the peak λ will be in the infrared range, and a large portion will be in the radio range.
If the star is hot, the peak may be in the UV, X-ray, or gamma ray range

10. Using light to determine composition
One of the ways that light interacts with matter is that it may be absorbed by the outer electrons in an atom.
This can only occur with certain  of light, which differs for each element.
For instance, when white light (which contains all visible ) is shone through hydrogen gas, the light that gets through is:

11. Using light to determine composition
The missing bands are from light that has been absorbed. Thus, this is called the absorption spectra for hydrogen.
Each element has a unique absorption spectra, so we can think of it like a fingerprint.
By analyzing the spectra of light emitted by a star, and looking for the “fingerprints”, we can tell which gases are present in the star!