Experiment 5: Spectral classification of stars

Objective :

To get familiar with the spectra of different stars

Introduction :

In this experiment we will get familiar with the spectra of different stars. You will be given a set of five spectra at random corresponding to five different stars from a large sample of spectral data given below. You will identify the spectral lines in each of these data sets and correlate it with the reference sample of lines. On the basis of the observed spectral lines you will make a broad classification of these stars according to the Harvard Classification scheme OBAFGKM.

Theory :

In this project you will classify five different stars according to the Harvard Classification Scheme


The description of different classes is given below:

The 'O' type stars are very hot and bluish in color. Their surface temperature ranges from 40000 K to 20000 K. Due to the high temperature they show strong absorption and sometimes emission lines from different ions such as HeII, CIII, NIII, OIII, SiIV and SiV as well as from neutral Helium atom HeI. The HI Balmer lines are visible but weak. An example of this star which can be seen by naked eye is Meissa which lies in the constellation Orion.

The 'B' type stars are also very hot with surface temperature ranging from 20000 K to 10000 K. They are also blue in color. Their spectra show neutral Helium lines, which are strongest at 'B2'. One also sees lines from ions such as OII, SiII, MgII. the HI Balmer lines are relatively strong. An example of a 'B' type star is Rigel, which is the brightest star in constellation Orion.

The 'A' type stars are white in color and have surface temperature ranging from 10000 K to 7500 K. These have very strong Hydrogen HI lines.The lines of ions such as MgII, SiII. The CaII can be observed but they are weak. An example of an 'A' type star is Sirius in constellation Canis Major, which is the brightest star in night sky.

The 'F' type stars have surface temperature in the range 7500 K to 6000 K and have a yellow-white color. The HI lines are getting weaker but still relatively strong. CaII and FeII lines start gettign stronger. Neutral metal lines (FeII) and CaI also becoming visible. An example is Canopus in constellation Carina. It is the second brightest star in the night sky.

The 'G' type are yellow stars with surface temperature toughly in the range 6000 K to 4500 K. Here the HI lines are weak. CaII lines continue to become stronger. FeII lines strong. FeI, CaI lines becoming stronger. Sun is a 'G2' type star. Another example is Alpha Centauri A in constellation Centaurus.

The 'K' type are orange stars with surface temperature in the range 4500 K to 3600 K. The HI lines are now very weak. The spectrum is dominated by metal absorption lines. CaII lines very strong. The neutral metal lines FeI, SiI visible. The molecular bands of TiO become visible by 'K5'. An example is Aldebaran in the constellation Taurus.

The 'M' type stars are cool red stars with surface temperature less than 3600 K. Here the spectra is dominated by TiO bands and neutral metal (FeI) lines. The HI lines are absent. CaI lines are also very strong. An example is Betelgeuse in the constellation Orion.

The different classes are also shown graphically.

A spectral plot of Sun along with identification of several lines is shown in the following figure :

A useful link for stellar classification is http://en.wikipedia.org/wiki/Stellar_classification

Examples of stellar spectra corresponding to each of these classes is available at, http://nedwww.ipac.caltech.edu/level5/ASS_Atlas/frames.html


The spectral graphs given below are generated from spectral data of various stars. A sample of these data set is given below.


You may download these files and plot them using your favorite graphing program to get some familarity with data. This step is not essential since we directly give the plots of these data sets below. In each data file the first column is wavelength in Angstroms and the second column is flux. Ignore the remaining columns. An example of this plot is given below. The stellar spectra is shown in red.

You will be expected to correlate the absorbtion lines with those provided in the reference list of various atomic spectral lines. These reference lines are also shown (in green) in the plot for easy identification.


  • Step 1: Randomly select five stars from the set given below. We have given the higher resolution spectrum for each star.

  • Step 2: Classify the spectral data of these stars according to the Harvard Classification scheme. Give reasons for your classification.

  • Step 3: Check your classification according to the code provided. The code gives the actual classification of each star. You are only expected to do a broad classification according to the alphabets OBAFGKM. Hence please ignore the remaining details provide in the code.


Acknowledgement : A Stellar Spectral Flux Library: 1150 - 25000 A (Pickles 1998)