Classifying Stellar Spectra

In this week's exercise, you will learn how astronomers extract information from the spectrum of a star, and how astronomers group stars together in classes based upon their spectra.

• Two ways of representing spectra
• What do the spectra tell us?
• Your chance to classify stars
Please read the instructions below before starting to do the CLEA worksheet!

Two ways of representing spectra

When astronomers measure the optical spectrum of a star, they pass the star's light through a narrow slit, then send it through a prism or diffraction grating, and finally focus the dispersed light into an image. The result is something like this:

If one uses a detector which records color (such as color film), then the picture above would show a bright band of different colors with dark lines superimposed on it.

Another way to represent the same information is with a cross-section through the spectrum, graphing the amount of light visible at each wavelength versus the wavelength:

The same information is visible in both pictures. You should become comfortable switching between the two types of representation.

To practice, visit the Harvard Mini-Spectroscopy page

http://mo-www.harvard.edu/Java/MiniSpectroscopy.html

and play with their applet. It allows you to modify a 1-D graphical spectrum and see the equivalent changes in a 2-D "photographic" version.

What do spectra tell us?

People have long known that the stars are far, far away; in the nineteeth century, astronomers finally measured the distances to a few nearby stars with reasonable accuracy. The results were so large -- thousand of trillions of miles -- that most people figured we'd never be able to visit them or learn much about them. After all, we can't go to a star, grab a sample, and bring it back to earth; all we can do is look at light from the star. In fact, at least one prominent philosopher and scientist went on the record as saying that we'd never be able to figure out their compositions.

Of all objects, the planets are those which appear to us under the least varied aspect. We see how we may determine their forms, their distances, their bulk, and their motions, but we can never known anything of their chemical or mineralogical structure; and, much less, that of organized beings living on their surface ...
Auguste Comte, The Positive Philosophy, Book II, Chapter 1 (1842)

(Comte refers to the planets in the quotation above; he believes that we can learn even less about the stars)

But, it turns out, light from the star encodes a wealth of information about the physical state of its outer atmosphere. Light is produced in the inner regions of a star and works its way out to the "surface" -- which is really a part of the gaseous atmosphere called the photosphere. Photons produced in the photosphere have a good chance to escape outwards into space and, eventually, reach us. As photons fly through the outermost layers of the stellar atmosphere, however, they may be absorbed by atoms or ions in those outer layers. The absorption lines produced by these outermost layers of the star tell us a lot about the chemical compositition, temperature, and other features of the star.

The important thing to remember is that light may be absorbed by a particular atom if two conditions are met:

• the atom (or ion) is present in the outer atmosphere
• the temperature of the atmosphere is just right to put the atom (or ion) into the right energy state to absorb a visible photon

So, if you don't see absorption lines from some particular element -- say, iron -- in a star's spectrum, then it can mean two very different things:

1. there is no iron in the outer layers of that star

or

2. there is iron, but the temperature isn't right for the iron to absorb light at visible wavelengths

Astronomers have been examining stellar spectra for over one hundred years. There are a lot of stars! In order to manage them more easily, astronomers have grouped stars into spectral classes based upon the appearance of their spectra. Stars in the same class share similar temperatures and masses.

Your job tonight is to practice classifying stars. It's really quite simple: you are given a set of "standard spectra", which show the appearance of a star of class B0, B5, A0, A5, etc. All you have to do is compare the spectrum of an unknown star to the standards: the closest match yields the class of the unknown.

You will use part of the CLEA lab The Classification of Stellar Spectra for this exercise. Log into the computers in the lab, double-click on the "clea" icon, then double-click on the "clea-spe" item. After you've logged into the program, run the "Classify Spectra" option.

Go through the attached portions of the CLEA exercise. Fill in answers to all the questions. On the page labelled Data Table: Practice Spectral Classification, you must give a type and reasons for the following stars:

• HD 37767
• HZ 948
• Feige 41
• HD 6111
• HD 242936
• HD 5351
• HD 27685
• HD 21619
• SAO 81292 (watch out, this one has some emission lines)

On the page labelled Optional Exercise: Absorption Lines for Spectral Classes, you must provide wavelengths and elemental identifications for the important lines in each of the following spectra classes:

• O5: very hot, bluish-white stars; hotter even than Rigel
• A1: stars similar to Sirius
• G6: stars like our own Sun
• M5: very cool, red stars; cooler even than Betelgeuse