Light and Redshifting

## How Do Astronomers Know What They Know?

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#### Gathering Data

Microbiologists use microscopes.

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#### Gathering Data

Sentries use periscopes.

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#### Gathering Data

Astronomers use telescopes.

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Astronomers use data from light to tell them the following about the light source:

- Age 
- Chemistry 
- Temperature 
- Size 
- Distance 
- Velocity 
- ...and more!

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### What light do we EXPECT?

Molecules in stars ABSORB certain wavelengths of light depending on their CHEMISTRY. <small>(The frame below is interactive)</small>

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### What light do we EXPECT?

- What elements are the easiest to make? 
  - Hydrogen, Helium, etc. 
  - The easiest elements to make are the most abundant.

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### What light do we EXPECT?

- Stars are mostly Hydrogen. 
- Hydrogen's absorption spectrum is what we expect to see from stars. 
- DEMO: Diffraction gratings.

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### What light do we OBSERVE?

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### Observation

- The EXPECTED wavelength of Hydrogen Alpha (which is red) is OBSERVED to be redder for different galaxies in the night sky. 
- We call this phenomenon a COSMOLOGICAL REDSHIFT. 
- Edwin Hubble identified this phenomenon when observing nearby galaxies.

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### Background Info: Doppler Effect

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### Galaxies Redshift When Moving Away

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[https://science.dbuckley.dev/labs/hubble](https://science.dbuckley.dev/labs/hubble)

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### Hubble's Constant Lab in Canvas

1. Write the value of Hubble's Constant that you found.
2. Edwin Hubble's value in 1929 was 500 (km/s)/Mpc. The modern accepted value is about 67 (km/s)/Mpc.
3. ...next slides

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### Conclusions About the Universe (#3 on the Canvas Lab)

- Every galaxy is moving apart from each other. 
- The space between them is increasing. 
- The universe is expanding at a rate of about 67 (km/s)/Mpc.

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### Extra Video