29:50 Modern Astronomy
Fall 2002
Lecture 9 ...September 18, 2002
Starlight
Light
What is light? A: A wave of electricity and magnetism. Good discussion on p92
of textbook. We can specify waves by a number of properties; two of the most important
are wavelength 
and speed at which the wave propagates.
Two other important ones are the amplitude of the wave and the frequency.
Demonstration with wave machine.
For light waves the speed is special 
meters/sec =
186,000 miles per second.
The electromagnetic spectrum What is the wavelength of light to which
the human eye is sensitive? Answer: 0.00000040 - 0.00000070 meters. Bah! Use scientific
notation: 
meters.
A lot of other phenomena are identical to light except the wavelength is
different.
Transparency, Figure 6-5.
Longer wavelength: infrared (IR), microwave, radio waves.
shorter wavelength: ultraviolet (UV), X-rays, Gamma rays.
How is electromagnetic radiation produced? A; lots of ways. One important
way is blackbody radiation. Hot objects emit light. Hotter objects emit more light.
illustration/demonstration with light bulb.
To talk about production and characteristics of light, we need to introduce a defintion:
a spectrum. Spectrum is intensity of light versus (``is a function of'', in
fancy mathematical language) wavelength.
Graph of intensity of light versus wavelength.
Experiments in the golden days of yesteryear Many experiments on light in
19th century. Two important developments:
(1) The hotter the object, the brighter it glows.
(2) The hotter the object, the bluer it is.
Transparency with blackbody radiation.
The main results of 19th century physics regarding light production are summed up in Kirchoff's Laws, described in your book.
Systems of Temperature If we are going to talk about temperature, we need
a system. Common systems are:
Fahrenheit: lousy for scientific use.
Centigrade: 0 
C - 100 C
Kelvin: same degrees as Centigrade; but zero is absolute zero. All
molecular motion stops. ``Atoms would lie on the floor like marbles''.
Some temperature in the Kelvin system:
0K - ABSOLUTE ZERO!!!
77K - Temperature of liquid nitrogen
290K - temperature in this room
373K - water boils.
The relation between color and temperature: Wien's Law ``Say it with equations''
A blackbody radiator glows brightest (is most intense) at a wavelength (in meters)
. 
is related to the temperature (in Kelvins) by
Wien's Law:
Let's work out some examples. I just said that objects at a finite temperature emit light,
but that seems to be a lie. The tabletop is hotter than absolute zero, but it is not
glowing. What's up? Use Wien's law.
Temperature of tabletop is 
K. Figure out wavelength at which it is
brightest.
meters. This equals
10 microns. It is more than ten times longer than the longest wavelength that
the human eye is sensitive to. It is in the infrared part of the spectrum.
Now let's try the Sun.
The Solar Spectrum
Longrightarrow Spectrum of Sun. It looks like a blackbody in the sense of a steep
rise and a slow decline with wavelength. There is a peak at about 475 nanometers
= 
meters.
Let's use Wien's Law, 
K
Real hot! This gives us our first idea of what the temperatures of stars are like.
Now think about the color of Vega, and think about what that tells
you about the star.