Astronomers claim to know an awful lot about the Universe, even though they've never visited1 any of it nor dissected interesting specimens in the lab2. How do they do that?
Have you ever wondered how do astronomers know
Well, we could fill the rest of the newsletter with these types of questions. A hundred years ago, we would not have known most of these questions, much less the answers. But the technique, the key to unlocking all of these, and many more mysteries was already known.
The technique is spectroscopy: the use of a spectroscope for scientific studies. Attention beginners: don't he intimidated by the word. it should be no more intimidating than 'telescope' or 'microscope'. Spectrographs, spectroscopes, and spectrometers are just instruments that break light into its various colors and, in some manner, record those colors.
The idea that ordinary light is made up of colors has been around for a while. In the 1670s Isaac Newton discussed his experiments that showed that white light - ordinary sunlight - would break into all the colors, from blue to red, when passed through a glass prism. Of course, everyone had seen this happening when they saw rainbows but no one had figured it out. Newton also showed that you could use a second prism to put the colors back together into white light.
I often wish that MIRA could afford a color newsletter; we really miss this capability when we're introducing spectra2. But astronomers never use color sensors; they are not sensitive enough to capture the few fragments of light that trickle into our telescopes. So modern spectrographs record the light on sensitive electronic detectors. Figure 1 shows how the spectrum of the sun looks to the MIRA blue spectrograph. (How did we take a spectrum of something as bright as the sun with an instrument designed to capture light from very faint stars? Answer: we took a spectrum of the sky, which is just sunlight scattered by dust and aerosols in the atmosphere.)
Figure 1: A solar spectrum taken with the MIRA telescope.
If this were a color image, the pair of dark lines on the left would be surrounded by very violet light and, at the right, the spectrum would he a deep red colon In between violet and blue on the left and orange and then red on the right is green in the middle of the spectrum. This is very pretty, even in black and white (to us astronomers) but it is hard to do anything very quantitative with it so we like to use a plot of the same information.
Figure 2 shows a plot of the spectrum in Figure 1. In this case, a number representing the position along the color axis lets us discuss the position of features and the 'Flux' scale lets us refer to the relative strength of various feature. Now, instead of saying "the two dark hands at the left of the spectrum we can say "the two strong lines at diodes 250 and 275 are the strongest lines in the spectrum."
The fine details, called 'lines' from the days of photographic spectroscopy, are still a little difficult to make out because of the base line that starts and ends low with a strong bump in the middle.
Figure 2: A plot of the solar spectrum in Figure 1.
This bell shape is due to a combination of the halfway between the violet and the red (in middle, green part of the spectrum) and the spectrograph, which was tuned to be most sensitive -in the middle part of this spectrum. It is even easier to discuss the features of the spec-trum if we create a smooth match to the gross features of the spec-trum, Figure 3, and then divide Figure 2 by Figure 3. This result, Figure 4, now lets us concentrate on the features themselves.
Figure 3. A baseline, or continuum fit to the solar spectrum in Figure 2.
Now that we've isolated these features, what do they tell us? Well, just about everything you ever wanted to know about anything in the Universe and certainly they will let us answer the questions at the top of this article. How astronomers decode these dark lines in the spectra of stars, nebulae, and galaxies to arrive at those astounding discoveries you read about in the newspaper will be the topic of several of the next articles in this series, Reading the DNA of the Universe.
Figure 4. A flattened version of Figure2 formed by dividing Figure 2 by Figure 3.
1. Sending test pilots to moon almost counts, unless you're one of the astronomers who trained for years to become an astronaut and got passed over.
2. If you've been attending some of the MIRA lectures, you'll know that astronomers do get to dissect a few objects from very nearby, like pieces of meteorites, some of which even come from neighboring planets.
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Last updated 3/8/02 DMC