Let There Be Light!

Before we launch into a discussion of astronomical instruments we must first take a brief look at light (a more detailed discussion will follow in a future lecture).

Fork or Spoon - Wave or Particle?

Depending on the context in which we work, light may be considered to be either a wave or a particle. This duality represents one of the deepest mysteries of modern physics. Happily for us we don't have to resolve this mystery and for the time being we will discuss the wave properties of light.

Waves of What?

If light is a wave then just what is waving? We now consider light to be a continuously moving combination of electric and magnetic fields. This crucial understanding was developed in the last century by the British physicist James Clerk Maxwell. Thus, we consider light to be a traveling electric and magnetic disturbance that is commonly called an electromagnetic wave. Visible light - what our eyes respond to is just one form of an electromagnetic wave. All electromagnetic waves travel with the same velocity:

Wavelength and Frequency

Color is one of the most important properties that our eyes respond to. From a physical point of view, color depends on the wavelength or the frequency of the light. Thus, it is either wavelength or frequency that distinguishes one electromagnetic wave from another.

• Wavelength: is the distance between successive wave crests on an electromagnetic wave. The Greek letter lambda is commonly used to represent wavelength.
• Frequency: is the number of wave crests that pass by each second. The higher the frequency the more rapidly the electromagnetic wave is vibrating.

The Electromagnetic Spectrum

An electromagnetic wave can have, literally any wavelength. When the wavelengths are arranged along a continuum we then we have what is known as a spectrum. Visible light waves are "tiny". Wavelengths for visible light are measured in units of nanometers (billionths of a meter) or in Angstroms (ten-billionths of a meter). The visible spectrum ranges from 400 nm (blue-violet) to 700 nm (deep red). Even smaller than visible light are (in descending order):

• Ultraviolet Light (UV): 400 nm - 10 nm
• X ray: 10 nm - 0.01 nm
• Gamma ray: less than 0.01 nm

• Infrared (IR): 700 nm - 0.1 cm
• Microwave: 0.1 cm - 1m
• Radio Wave: longer than 1 m

The instruments that astronomers use are optimized for specific parts of the electromagnetic spectrum.

But Soft! What Light Through Yonder Window Breaks?

Romeo and Juliet; Act 2; Scene 2

For most wavelengths, the atmosphere is opaque. Astronomers are able to see through the atmosphere at only select wavelength regions or windows. The following figure shows just how bad it is!

If we want to observe the universe in other wavelength regions we have no choice but to get above the atmosphere. Since the beginning of this century astronomy has been done:

• on high mountain tops
• from balloons
• from rockets
• from orbiting satellites
• (eventually) from the moon

The major "absorbers" of light in the atmosphere are:

molecule	region blocked
H2O (water vapour)	infrared, short radio
O2 (Oxygen molecule)	short radio
CO2 (Carbon Dioxide)	infrared
ozone	completely blocks UV and shorter
variable transparency due to dust and cloud	visible

The most energetic events in the universe will produce gamma rays. Click here to see a quicktime movie simulation of a netron star being swallowed by a black hole. If this happens within 1000 ly of earth we are toast!	A Chandra image of the center of our galaxy, 25 Kly away as it would appear in X-ray	Image of our own star - the Sun taken in the Extreme Ultraviolet (19.5 nm) showing some very energetic processes on the Solar surface.	The night time sky in all its splendor as seen by our eyes - the visible wavelength region.	A beautiful infrared image of the globular cluster M3	Star formation occuring in the V110 Orionis region A cool cloud of gas and Hydrogen is starting to collapse into a star.	A color coded image of radio emission from of a star forming region in Cygnus the swan.
gamma ray	x-ray	ultraviolet	visible	infrared	microwave	radio

A Case Study: The M17 Star Forming Region

as Seen in Different Spectral Regions

 

Spectral Window	Image
X-ray spectral window : showing very energetic (high temperature) parts of the star forming region. In this case the blue area represents very hot gases (about 1.5 million degrees C) flowing away from newly formed stars. This image was taken by the Chandra X-ray Space Telescope.
Image of M17 (omega nebula) as it appears at visible wavelengths. This is an Earth-based image produced by the Anglo-Australian Observatories.
Infrared image of the star forming region emphasizing hot, dusty areas
Microwave image of the star forming region - bright central condensation is a maser

Each wavelength region tells you something different about M17. The shorter wavelength regions emphasize very hot parts of M17 while the long wavelength regions emphasize large structures, magnetic fields and masers. We will discuss this region and others like it in a future lecture on star formation. If you want to find out more about the M17 star forming region follow this link. ...

Time toTake a Quiz!

If you are ready, click on "Big Al"  to take a 20 question multiple choice quiz. It should take about 10 minutes and provide you with an evaluation of your comprehension of the past few lectures.  Don't forget to "register" the quiz so that I have a record that you have completed it.

Seeds Chp 6; 100-104
Kaufmann Chp 5;80-100