Classifying Stellar Spectra                               Printable form   alternate location Printable form

Stellar spectra tell us the temperatures and the luminosities of the stars. In your last homework, you plotted the apparently brightest and the nearest stars. In this exercise, you will be looking at the data that lead people to classify stars based on their spectra.

Most stars produce  ABSORPTION spectra, the ones with dark lines on a background of rainbow    colors.  As you can see the hydrogen and helium  lines dominate at high temperatures. At lower temperatures, there are LOTS of lines visible. These lines are from larger atoms, ones where the negatively charged electrons are further from the positively charge (attracting) nucleus. Since these electrons are far from the nucleus and there are lots of electrons in between them and the nucleus, it takes less energy to cause these electrons to absorb light. So we see the lines of these larger atoms in cooler stars.

Stellar Spectral Types: OBAFGKM
Credit & Copyright: KPNO 0.9m Telescope, AURA, NOAO, NSF  APOD May 30, 2001

Each band of color is a different star. The stars  have been put into order from hot at the top to cool at M5.  All of the photos include the same range of colors, but as you compare the spectrum of the M5 star with the spectrum of the O6.5 star, you can see that the blue color background is much brighter in the  O6.5 star than in the M5 star. This is because the  M5 star is so much cooler.  If you look at the red end of the picture, the O6.5 star doesn’t look very bright, but it is actually brighter than the red end of the M5 spectrum. The intensities have been adjusted to  keep the picture from being too bright at either end.  

The “F4 metal poor” star has little of the elements heavier than hydrogen.  To astronomers, anything except hydrogen and helium is a “metal”.  The temperature is very close to the temperature of the F5 star you have further up in the picture. If you look at the colors in the background of each, the balance is about the same. This indicates the same temperature. But the metal poor star has few lines.

The emission line stars are ones that have extended atmospheres.  The gas in these atmospheres is seen against the dark background because it extends so far from the star.  The emission lines are brighter places in the spectrum. In the M4.5 emission line star, there are some emission lines in the yellow part of the spectrum, to the blue (left here) of the g band of  iron (an absorption line about  30% of the way in from the right) . They are NOT the only ones in the sample that have emission lines.

As nice as the colored pictures are, it is both hard to get accurately-printed colors and unnecessary. A prism (triangular piece of glass) and a diffraction grating (like the plastic you used) spread out light  with a different position for each wavelength. So rather than showing lots of colored pictures, people are using plots of the amount of light as a function of wavelength. 

In the figure to the right, the amount of light has been measured at each wavelength and plotted.  The plots have been shifted vertically so that they do not overlap. Don’t worry, there are larger, clearer copies of this kind of plot in the reference files.

If the stars were at the same distance as one another, then we would see MUCH more energy from the hotter stars because the dense parts of the stars emit   with a black body spectrum. The black body

 spectrum was discussed in chapter 3 and plots of the amount of energy can be made. The figure below shows one form of the black body curves.The peak of the black body curve  depends on the temperature of the body. The peak of the curve is related to  the temperature of the body by the formula

Temperature (in Kelvin)

 =29,000,000/(peak wavelength in Ångstroms)

Decide the spectral type by comparing the plot to the plots for other stars. Use the shape of the curve to get the approximate temperature. Look at the hydrogen line strengths to decide  the spectral type for B A and hotter F stars. Use the Calcium h and k line to decide the type for  A5 to about G0. Use the iron g band (at about 4300Ångstroms) compared to the calcium h and k to decide in the range F5 to about G5. Use the molecular TiO band strength to get the spectral type in the M star range.    

To decide on the luminosity class for each star,  look FIRST at the  underlying black body curve and decide a temperature (or nearly) . Then look at the reference stars with different luminosities near the correct temperature. GENERALLY the giants (luminosity types I, II or III) have narrower lines than do the dwarf (V) stars.  In the hotter stars (B and O) there will be more lines visible in the giant and supergiant stars. This is because the lower surface pressure allows us to see skinny little lines that would be wiped out in a star with higher pressure.  Cool stars, like M stars, often have emission lines in the giants. The emission line is a part of the spectral plot that sticks up.  

It is not very easy to tell the luminosity class from the plots. Just do your best. The idea is to get into how it is done. It isn’t a matter of life or death.