Stellar Classification Lab
Purpose:
Understand some of the factors important to classifying stellar spectra and analyze spectra collected with a simulated telescope. Understand how continuum and absorption lines are used in classifying stars and be able to calculate the temperature of a star based on the continuum of the spectrum.
Introduction:
The spectral type of a star is a way of sorting stars that allows the astronomer to figure out not only the temperature of the star, but also its luminosity (expressed often as the absolute magnitude of the star) and its color. These properties, in turn, can help in determining the distance, mass, and many other physical quantities associated withthe star, its surrounding environment, and its past history. Knowledge of spectral classification is fundamental to understanding how astronomers put together a description of the nature of stars and a theory of their evolution.
In this lab you will use standard spectra to develop a classification scheme for spectra; then compare it to the established classification pattern developed by astronomers. You will use the established pattern to classify unknown spectra and spectra of stars you collect with a simulated telescope.
Procedure:
Your instructor will give each pair of students a set of stellar spectra and will ask you to come up with a way of ordering them based on their characteristics. You will be asked to explain how you developed the scheme and then will compare it to the order for the standard spectra used by astronomers.
You will then practice identifying main sequence stars using the standard set of 13 representative spectra to identify the classification of each spectrum you are asked to classify and to identify strength of spectral lines and temperature of each star.
Finally you will collect stellar spectra using a simulated telescope and classify the spectra you collect using the tools embedded in the program.
Part I:
Your instructor will give you a set of 13 standard stellar spectra. Astronomers use such lists to decide the classification of a star.
As you look at the spectra try to find similarities and differences. Use similarities and differences to devise a method to sort the spectra and put them in some kind of order.
Once you have sorted them to your satisfaction, use the answer sheet to list the factors you took into account to sort the standard spectra. For example you might have chosen a particular dark line and sort by its width on each of the 13 standards. A secondary characteristic might have been how much the entire spectrum seems to “dip” or “rise” at one end or the other. Whatever you chose, write out your criteria in as detailed a way possible so that someone else could use your factors to classify spectra.
Include your list of sorted spectra in order on the answer sheet. The numbers are randomly generated and have no meaning except as a reference. Before going on, check with your instructor to get the key to see how these spectra are ordered by astronomers. Compare these to your order on the answer sheet.
Part II:
Your instructor will demonstrate the CLEA program that you will be using to classify spectra. Spend a few minutes exploring the program to see what its capabilities are.
When you feel confident that you know some of the things the program will do for you, open the Classify Spectra part of the program. You can load in the list of standard spectra for main sequence stars. You should see that these are the same as the ones you just sorted.
Practice classifying using the stars assigned by your instructor and fill in on the table on the answer sheet.
Wein’s law provides a way to find the temperature of a star by knowing its most intense wavelength using the equation:
maxx
where maxis the wavelength with maximum intensity and T is temperature in degrees Kelvin. Rearrange the equation to solve for T, then calculate the temperature of the F5 star as outlined below.
Look at the spectrum for the F5 standard star and find the point on the graph where the intensity is closest to 1. Record this wavelength on your answer sheet, then calculate the temperature of the star using the equation on the answer sheet. Compare your value to the expected temperatures for F type stars from this website:
Part III:
You are now ready to take spectra of your own using the simulated telescope. Begin to by thinking about what you may expect. Some guiding questions are on Part III of the answer sheet.
Take spectra for 3 more stars. Copy and save them your report and mark them as indicted.
Time saving notes and hints:
1. The telescope is pointed in a direction in space, but the earth continues to turn, bringing new stars into view. To remain focused on a particular star you must tell the instrument to track the portion of sky you want to view.
2. Each spectrum graph should be saved (print screen key, followed by paste) or printed before you navigate away from it. Otherwise you will have to collect it again to be able to print it out.
3. The standard spectra are like markers on a ruler, you can estimate where your star fits between them. For example, in the B class we have B0, B1, B2, B3, B4, B5, B6, B7, B8, B9 and then A0. The 0 stars in each class are the hottest, and the 9 stars the coolest.
4. Signal to noise ratio is just a measure of how much of the light you are receiving is coming from the object you want to collect data for and how much (noise) comes from background radiation. Higher signal to noise ratios mean better data. You can stop the collection at any point and resume it to get a better S/N if your data is sketchy.
5. You can request time on bigger telescopes that collect data much more quickly. In the interests of making the exercise “real” you will often be denied time, but keep trying. Save dimmer stars to collect when you have the larger telescope.
6. You can look at either a tracing of the spectroscopic data, or the same data in grey scale. Or, you can look at both simultaneously. Try to compare the two different ways of looking at spectra, there are advantages and disadvantages to each.
7. The difference button is very useful when you want to see how your star compares to the standard. No differences at all will indicate you may actually be looking at the standard star!
8. In addition to main sequence you can access other stellar spectra standards. If your spectrum seems odd it may worth checking out some of the more detailed standards for stars near the spectral class of the star you are studying.
9. By running the mouse over the intensity curve you can get the numerical value for intensity at any point with maximum intensity 1.0 and minimum (no photons collected) of 0.
10. You can access absorption line frequencies by double clicking on low points in the spectral curves after you have opened the spectral line table. You can identify which of the elements is responsible for the absorption and which particular absorption line you are looking at.