# Light Astronomy 315 Professor Lee Carkner Lecture 4.

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Light Astronomy 315 Professor Lee Carkner Lecture 4

Scale Exercise  What is scale for solar system (0.0016 ly)?  measure ball diameter =  real/model = scale  scale = 0.0016/2 =  What is the model value for the distance to Sirius (9 ly)?  real/scale = model  9 /0.0008= 11250 cm =  Distance to other end of Science Building  Change scale so that ball equals 9 ly  new scale = 9/2 =  Find size of galaxy in model (100000ly)  100000/4.5 = 22222 cm =  Distance to Old Main

Disturbing the Universe   Can’t visit directly or send probes  Would take ~100000 years to get to nearest star   Can do some simulations in the lab  But how do we know if they are right?

Light   What is light?   How do these properties give us information about the object that emitted the light?

What is Light?   EM radiation can be thought of in two different ways:   As a stream of photons (particle)  Light is both a particle and a wave  We use what ever formulation is most useful

Properties of Light  When we examine a light emitting object, what do we want to know?  Energy   Photon Flux   How much total energy is emitted by an object depends on how much energy each photon has and how many of them are emitted

Wavelength  Each photon has a wavelength   Energy is inversely related to the wavelength ( )  Long wavelength =  Short wavelength =  We will often measure wavelength in meters or nanometers (1 billionth of a meter, or 1X10 -9 m)

Waves

Speed and Frequency  c = 3 X 10 8 m/s = 186,000 miles/s  We can use this speed to write the frequency: c = f   Frequency is directly related to energy  High frequency = high energy  Low frequency = low energy

Color   This is called visible light   Short wavelength, high energy = blue  Long wavelength, low energy = red

A Spectrum

Star Colors  Stars come in 4 basic colors

How is Light Produced?  Every object in the universe emits blackbody radiation that depends on its temperature   Given in degrees Kelvin   Room temp = 300 K  Higher T means more radiation

Spectrum   The radiation is a continuum of wavelengths called a spectrum   We can describe the spectrum as a curve on the intensity versus wavelength diagram

Peak Wavelength and Temperature   A higher temperature produces a spectrum that peaks at shorter wavelengths  Wien’s Law: max = 3,000,000/T  Where T is in Kelvin and is in nanometers

Intensity and Temperature  A higher temperature means more total energy emitted  Stefan-Boltzmann law: P =  AT 4    is the Boltzmann constant (5.67 X 10 -8 W/m 2 K 4 )   A is the surface area of the object (in m 2 )  T is the temperature in Kelvin

Using Radiation Laws  Wien’s Law   If you can find the peak wavelength you can find the temperature   Stefan-Boltzmann law  Hot objects emit more energy then cool objects   The intrinsic brightness of a star depends on both its temperature and size

Alberio  This is the double star Alberio  Two stars orbiting around each other  Both are the same distance from Earth  Size of star image proportional to brightness  What is the relative temperature and size of the stars?

The Electromagnetic Spectrum  Light can have a wide range of wavelengths   This corresponds to a wide range in energies   Today we call the range of wavelengths the electromagnetic spectrum

The EM Spectrum

The EM Spectrum and You  You see in visible light, feel infrared as heat and get a sunburn from ultraviolet   Microwave and radio have long wavelengths and low energy 

Next Time  Read Chapter 5.1-5.8

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