Transcript Light - Helios

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 (l)
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 = lf

Frequency is directly related to energy
High frequency = high energy
Low frequency = low energy
Color

This is called visible light
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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

The radiation is a
continuum of wavelengths
called a spectrum

We can describe the
spectrum as a curve on the
intensity versus
wavelength diagram
Spectrum
Peak Wavelength and Temperature

A higher temperature
produces a spectrum that
peaks at shorter
wavelengths
Wien’s Law: lmax =
3,000,000/T
Where T is in Kelvin and l is
in nanometers
Intensity and Temperature
A higher temperature means more total
energy emitted
Stefan-Boltzmann law: P = seAT4

s 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
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Microwave and radio have long
wavelengths and low energy
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Next Time
Read Chapter 5.1-5.8