Sometime around the mid 1800's new tools began to be added to astronomer's tool kit. Each of these changed the reach and power of astronomy
 

• theory of light = Radiation
• theory of interaction of Matter and Light = Blackbody radiation
• theory of atomic structure (Quantum Mechanics!!!!!)
• 
  

Once we learned to "read" those messages we can start telling the stories.

This lecture we will review the properties of light as it relates to Astronomy and Astrophysics.

This is only optical or visible light. Light of a specific kind.

Visible light can be broken up into its component rainbow of colors via a prism
 
 

For years people argued about the wave-like or particle like nature of light.

• Sound
• Ocean waves
• Traffic

• Bullets
• Billiard Balls
• Car

Waves have two important properties

Wavelength = distance between peaks (Wavelength = m)

frequency = number of peaks passing a point in one second (f = 1/sec)
 
 

A prism bends light in way that SHORT wavelengths get bent more than LONG ones.

We measure the wavelength of light in Angstroms
 
 

In the middle of the 1800s it was discovered that visible light was an ELECTROMAGNETIC wave.

Electromagnetism <-> electric charges (electrons), electric fields, magnetic fields, electric currents.
 
 

By shaking an electric charge fast enough you can produce any size (wavelength) electromagnetic wave you want.
 

Thus comes the ELECTROMAGNETIC SPECTRUM - the spectrum of all possible wavelengths of "light".
 
 

As we will see later on, shorter wavelengths correspond to higher energies in the light.

Notice that the atmosphere only lets Radio, Optical and some Infrared light in. The rest is screened

For most human history astronomy meant optical astronomy. Only in the last 50 years have new wavelength "windows" been opened.

Radio was first. Then infrared and X-rays etc. Now the sky is being mapped in Gamma-rays.
 
 

Objects look very different at different wavelength. Each window has a different "story" to tell.
 
 

Ever notice that things that are farther away look dimmer?

Car lights a long way away appear faint.

This comes about because the "brightness" (B) of a light source decreases with distance (D) the same way gravity does.

If I_o is the intrinsic amount of light an object is pumping out and D is the distance to the object then its apparent brightness is
 
 

This relation is very important for astronomers because sometimes we actually know the intrinsic amount of light celestial objects emit. (Lets call it the intrinsic brightness).

So if we know I_o from physics and we observe B at the Earth then we can find D the distance to the object.

Remember that determination of astronomical distances is always one of the most difficult problems.

Any object which we know I _o is called a Standard Candle .

Standard Candles:

• Some kinds of Pulsating Stars (Cepheid Variables)
• Some kinds of Supernova
• Some kinds of Galaxies

There are different ways for matter to emit light.

That means different physical processes.

Each process will produce a different spectral signature

Here is one important kind of SED: the Blackbody Curve
 
 

"Blackbody radiation" is "thermal radiation".

It is the light emitted simply because of the random jiggly motions of atoms and molecules due to heat.

The hotter an object is the more jiggly motions the atoms experience.

Technical Point - The term blackbody refers to the way something dark, like a black piece of construction paper, will absorb and emit light. A blackbody is an object that is in thermal balance with its radiation.

Why are blackbodies important? Because big collections of stuff like stars, planets and people all have a temperature and so they all emit radiation like a blackbody.

Here is the kicker. The "peak" wavelength of a blackbody (the wavelength at which most of its light is emitted) depends on its temperature.
 
 

Now you know what "red hot" means.
 
 

The Universe was hot. Radiation and matter where strongly coupled. As the Universe cooled the radiation and matter stopped interacting. This blackbody radiation became a fossil existing everywhere and everywhere cooling as the Universe expanded. Now it corresponds to a temperature of just about 3 degrees.