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Light
Astronomy 315Professor Lee
CarknerLecture 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 108 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 = AT4
is the Boltzmann constant (5.67 X 10-8 W/m2 K4)
A is the surface area of the object (in m2) 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