What is a stellar spectrum?
Good question! The sun and other stars give off huge amounts of light, and even though we see stars as being only a single color, they actually give off light of many different colors, or wavelengths. A stellar spectrum is a way of showing how much light in each wavelength a star gives off. On each of these spectra, the horizontal axis represents wavelength, and so each vertical line corresponds to a different wavelength. If a vertical line is bright, the star is giving off a lot of light in that wavelength. If it is dimmer, it is giving off just a little light in that wavelength.
Why are there so many vertical
lines through the spectra?
As you can see, stellar spectra are not smooth as you might expect, but they are littered with varying light and dark lines. For some certain wavelengths, the star is giving off light, but it is being blocked, so light of that color has a hard time reaching us. For each one of these wavelengths, there is a dark vertical line going through the spectrum. Because the light for that wavelength is being absorbed, these are known as absorption lines.
What good are stellar spectra?
You may notice that for different stars, absorption lines show up in different places along the spectrum. This is because the locations of absorption lines depend on the chemical and physical properties of that particular star, and different stars have very different properties. So, by studying very precisely where absorption lines appear and the relative strength of the lines, we can indirectly learn about the chemical and physical nature of the star.
If these are supposed to
be different colors, why are they black and white?
The computer program which was used to manipulate these spectra was not designed to display color. Just looking at a color is not a very precise way of measuring a wavelength, instead the computer program displays wavelength along the X axis so that there was no need to keep track of the color of each line. If these images were in color, however, they would range from about 5200Å (blue-green) to about 6400Å (yellow).
Spectrum of Regulus taken by
Michael Dulude
Spectrum of Alpheratz taken by
Jonathan Blair
Spectrum of Betelgeuse taken by Jeremy Sepinsky
Spectrum of Sirius taken by Tereasa Ferguson
Solar Spectrum taken by John Bochanski
Spectrum of Procyon taken by Alicia White
Another spectrum of Sirius, by Winston Grier
Spectrum of Arcturus taken by Christopher Pilman
Eclipsing Binary Light Curve
For this assignment, we used an astronomical technique known as photometry, rather that spectrometry. In photometry, we measure the brightness of star light through different colored filters. Measurments are made over an extended period of time since we are interested in how much light is given off at different times. For the most part, stars give off constant amounts of light. They do not get brighter or dimmer by significant amounts on a regular basis. However, there are exceptions, and when a star's light output changes over time, it is known as a variable star.
One particular type of variable star is an eclipsing binary. It is called binary because what we see as one star is actually two stars orbiting around each other. It is called eclipsing because as the stars move back and forth, one will move in front of the other. When one star is blocking its companion, the other star is said to be eclipsed. Since part of the eclipsed star is obscured, not as much light reaches us from the binary system as it did before. When the star is uneclipsed again, the amount of light reaching us goes back up. This continues with a regular period, and so we can expect the light output of the star to go up and down in a predictable manner.
This image (called a light curve) is a composite plot of the students' measurements of the light output of one particular star, W Ursa Majoris, affectionately known as WUMa. WUMa has a period of about eight hours, and in that time there are two eclipses, one for each star passing in front of the other. By studying the shape of this curve, we can determine a great deal about the two stars which make up the binary system. The data in the plot below was collected with two different color filters, indicated by the different symbols.
