Date: December 8, 2010
Title: Basic Spectroscopy for Amateurs
Podcaster: Mark DeVito
Description: A brief discussion of basic spectroscopy.
Bio: Mark is an amateur astronomer in Fredericksburg, Virginia and just getting into amateur spectroscopy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Scott Udell, who’d like to give a shout-out to the students, staff, and faculty of the University of North Dakota’s Space Studies department. Hey there from a Class of ’89 alumnus who can remember when the classes were still part of aviation!
Welcome to the 365 Days of Astronomy podcast for December 8, 2010. My name is Mark, and I’m an amateur astronomer from Fredericksburg, VA in the US.
For this podcast, I decided to talk about an area of astronomy that I am just beginning to explore with greater depth, amateur spectroscopy. So, to help you, and me, we are going to start with the basics of light and move on to how amateurs can and are applying spectroscopy techniques.
The field of amateur spectroscopy truly is a niche within a niche. For me, heading into this sub-set of amateur astronomy made sense. I wanted something that was a bit off the beaten path and could produce real data and real science. In addition, as a volunteer at the National Air and Space Museum in Washington, DC I occasionally do demonstrations for the spectroscopy Discovery Station. Here we teach the general public what spectroscopy is and even show them some emission spectra using a visual spectroscope and gas tubes. At the Discovery Station I teach people that spectroscopy lets us “dissect “ the chemical makeup of stars and other celestial objects. It is for this reason too that I am so drawn to spectroscopy. I feel a connection with the object I am viewing because it is not just a faint fuzzy or image on the CCD. I know it and understand it. Not because I read about its composition but because I did it.
However, before we get started, I want to acknowledge the work of Keith Robinson, author of Spectroscopy: The Key to the Stars, whose book provided the basis for the information in this podcast.
Lets review some basics about light waves.
In 1864 Clerk Maxwell described the precise mathematical relationship between electric fields and magnetic fields. He determined that light waves have both electric and magnetic fields; however, even today, no one can explain what they truly are, only describe how they work. Electric and magnetic fields both have direction but only the electric field has an associated electric charge. Magnetic fields do not have a charge, but have North and South poles. Just as you probably learned in elementary school opposite poles and charges attract, same poles and charges repel. Because of the tight relationship between magnetic and electric fields, you cannot have one without the other.
You may recall from basic physics that if an electric charge moves, a subsequent magnetic field is produced and vice versa. If a magnetic field changes it produces an electric field. A good example of this is a dynamo where a coil of wire is turned in a magnetic field causing an electric field, which in turn causes an electric charge to travel down the wire.
Because light has both electric and magnetic fields, Maxwell determined that electric and magnetic fields could travel together in waves, therefore, oscillating electric and magnetic fields move outwards from the source in waves. Maxwell calculated the speed of these waves as equal to the speed of light and from that determined that light itself must be an electro-magnetic wave. There are several properties to electro-magnetic waves, which we must review as well. Many of them will be familiar to you I am sure.
First is the term wavelength. Like ripples on a pond emanating from the point where a pebble hit the water, electro-magnetic waves have peaks and troughs. The distance between those peaks and troughs is the wavelength. It is usually measured in meters; however, because we are talking very small portions of a meter, it is usually on the order of a few hundred nanometers. A nanometer is 1/1000 of a meter of 10-9 meters. Now, just to make things a bit more challenging, physicists use nanometers but astronomers use Angstroms, named after Anders Jonas Angstrom a Swedish physicist. One Angstrom is 10-10 meters. So, 10 Angstroms equal one nanometer.
The next common element is frequency. Frequency is defined as the number of times an electric or magnetic field oscillates in one second. This sounds rather complicated, so lets go back to our wave on the surface of a pond example. When our stone hits the water and sends out the ripples, the frequency is the number of ripples that pass a fixed point in one second.
So those are the basic terms we will need. So how does this all apply to spectroscopy. What is spectroscopy anyway? You may recall that Sir Isaac Newton played around with prisms and determined that white light is composed of all the colors in the rainbow, or spectrum. A rainbow occurs because different colors, which have different wavelengths, move at slightly different speeds through glass. For example, blue has a shorter wavelength than red.
In 1814 Joseph von Fraunhofer turned his prisms to sunlight and noticed that several hundred dark lines crossing the sun’s spectrum. He identified the wavelength of many of them but he did not know what caused them. We still refer to the sun’s spectral lines as Farunhofer lines.
So we have named some components of light, wavelength and frequency. We know that white light is composed of the colors of the rainbow and we know that there are these strange lines in the spectrum of the sun. So what does it mean? In order to answer this question we need one more piece of the puzzle. That’s where chemists Robert Bunson, of Bunson burner fame, and Gustov Kirchoff come in. In the mid-nineteenth these scientists discovered that chemical salts, when burned in a flame, produce spectra of bright lines. This was different than the continuous spectrum of produced by sunlight. In time it was determined that these bright line patterns were unique to the chemical burned in the flame. Like fingerprints on people, no two chemicals had the exact same pattern and thus could be used to identify the chemical from the spectra. Kirchoff produced three laws, known as Kirchoff’s Laws about spectra. Spectra only come in three types:
Continuous: This is the rainbow we see from a prism. A hot incandescent solid or dense gas produces it.
Emission: These are isolated bright lines at different wavelengths seen against a dark background. You may have seen an example of this in science class or possibly at a science museum. A hot incandescent thin or low-density gas produces these lines.
Absorption: This is a continuous spectrum with superimposed dark lines. It is the result of a relatively cool thin gas between the source and the observer. The dark lines at specific wavelengths identify the chemical composition of the gas.
So how do we see the spectra of a star? What does it mean? How can we put is to use? What type of equipment do you need?
To answer these questions join us for part two of this podcast. In part two I will be joined by Tom Field, an amateur astronomer and amateur spectroscopist from Seattle Washington.
End of podcast:
365 Days of Astronomy
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