Laboratory exercises in astronomy spectral classification answers

Science Lab-What Are Stars Made Of?

What Are Stars Made Of?

Description:

When astronomers first started to study stars, they noticed that stars appeared to have different intrinsic colors. Later, with more sophisticated instrumentation, these color differences were verified to be distinct differences in the amount of light emitted at each wavelength – meaning the difference in color was real, and not perceived. More recently (in the early 20th century), astronomers used spectrographs to separate the light from stars even further, and they noticed that the stars gave off absorption spectra. Different color stars gave off different spectra. It took decades for astronomers to put together the meaning of all these different pieces of data, but eventually they were able to determine what caused the different spectra and what stars are made of.

In this exercise, students will look at stellar spectra and blackbody curves of stars to determine the stars’ temperatures. They will classify some spectra, using a comparison set of spectra. They will also note the peak wavelength of the spectra they classify.

Introduction:

Understanding what information can be gained from observing the light emitted from objects in space is vital to astronomy. One very important phenomenon of light that will be used in this exercise is the spectrum – this is light from an object that has been spread out by wavelength, similar to the way light can be spread out by wavelength when it passes through a prism or dust particles embedded in raindrops. There are two very distinct properties of a spectrum that will be important in this exercise: energy distribution and absorption lines.

The energy distribution curve is sometimes called a blackbody curve because the shape of an energy distribution curve for a star is much light that of a theoretical blackbody. A theoretical blackbody is a substance that absorbs all light incident upon it and re-emits that light at all wavelengths with the peak wavelength of that emission corresponding to the temperature of the blackbody. You will be examining blackbody curves for several stars to determine the peak wavelength of that curve, and, thus, the temperature of the star that emitted that light.

Absorption lines are dark lines where much less light is observed. These occur at discrete wavelengths within the spectrum of a star. The reason that much less light is present is because that light was absorbed by an intervening gas. In the case of a star, the intervening gas is the atmosphere of the star. So, the absorption lines in the spectrum of a star’s light are due to materials found in the atmosphere of the star.

1. Download and save or print out comparison spectra. The Astrophysics Data System (ADS) is a resource that houses electronically scanned refereed publications in astronomy. Find the article written by Jacoby, Hunter and Christian in 1984, published in the Astrophysical Journal Supplement (ApJS). The article is titled “A Library of Stellar Spectra.” The spectra that you are interested in begin on page 259 of this article. The ADS is found at http://adswww.harvard.edu/

2. Describe the differences of the main spectral types. Considering only the letters that denote the spectral class (O, B, A, F, G, K, M), determine the characteristics that separate one class from another based upon their spectra. Be sure to notice both peak wavelength and absorption lines.

3. Go to the ELODIE website to view stellar spectra. ( http://atlas.obs-hp.fr/elodie/ ) Look up and view the spectra of the stars from the list below. Identify the spectral type of each star by comparing the spectrum on ELODIE with the library from the article found in step 1. Be sure to note both the peak wavelength of the curve and any absorption lines you notice.

HD 191984

HD 194244

HD 206778

HD 209747

HD 210418

HD 211976

HD 212754

HD 212943

4. Use the WWT to find each star whose spectrum you identified. Use the research button to find out the correct spectral type for the star by checking the spectral type on SIMBAD.

OPTIONAL Step 5. Write a paper about the determination of spectral classes of stars. Write about what you have learned about the spectra of stars and how the spectra are related to the chemical composition of stars. Include anything you learn about the history of stellar spectral classification.