Classifying Stellar Spectra                               Printable form

 

Stellar spectra tell us the temperatures and the luminosities of the stars. In this exercise, you will be looking at the data that lead people to classify stars based on their spectra and doing some of the classification yourself. Definitely read about spectral types in your text before you start.

 

Most stars produce absorption spectra, dark lines on a background of rainbow   colors, like the ones shown below. In this picture, each of the colored horizontal bars is due to a different star. The stars names, e.g. HD 12993, are on the right. HD stands for the Henry Draper catalog and the number is the number for that star within the catalog. These spectra are arranged in order from hotter to colder going from top to bottom

 

The colors correspond to wavelength of the light, with longer wavelengths for the red light and shorter for the blue.

http://voyager2.dvc.edu/faculty/kcastle/Classifying_stellar_spectra_files/image002.png

Stellar Spectral Types: OBAFGKM        

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

 

The spectral classes are specified by the letters O, B, A, F, G, K, M, L, T going hotter to colder. Each letter is subdivided by assigning a number 0 through 9 following the letter and going from hotter to colder. Text Box: Stellar Spectral Types: OBAFGKM 
Credit & Copyright: KPNO 0.9m Telescope, AURA, NOAO, NSF  APOD May 30, 2001
helium lines domin 

So B0 is colder than O9 and hotter than B1. Obviously not every type is shown.

 

Originally only black and white photos of the spectra  (using film) were available and people decided on the spectral type from the absorption (dark) lines.   The color images are nice, but the spectra can be analyzed more precisely by plotting the intensity of the light at each wavelength as here. There is a figure with many plots of stars’ spectra below. The mount of light is on the y-axis and the wavelength is on the x-axis.

 

The hottest star is on top. The stars get colder going down. The plots have been shifted so that they don’t overlap. Short wavelength is on the left, the same way as for the color photo. The places where the lines jut down are where the dark spectral lines occur, where there is less light.

 

Please notice the way that plots of the hotter stars’ light rise toward shorter wavelengths.  This is because the colored background of the spectrum is due to a black body curve, like the ones shown in your textbook and like the ones in the figure below.  The shapes of these black body curves all look the same because it is a logarithmic plot, i.e. the axes are in powers of 10.  The ploits in the text  are linear on both axes.  Regardless of the axes, the hotter the object, the shorter the wavelength at the peak of the curve.

http://voyager2.dvc.edu/faculty/kcastle/Classifying_stellar_spectra_files/image009.png

Black bodies (and other dense objects) emit light at all wavelengths. But our plots include only wavelengths from 350nm to 750nm. So the hottest stars have the peak of their black body curves off the plot on the short wavelength side. This is quite obvious in the hottest three stars plotted below. The last spectrum plotted is so cool that the peak of the curve is off the plot on the long wavelength side. In between, the peak of the curve moves toward longer wavelength as you go to cooler stars.

http://voyager2.dvc.edu/faculty/kcastle/Classifying_stellar_spectra_files/image007.png

Please print out the spectra for the stars that you were assigned and put them in order, hottest to coldest based on the peak of the curve.  There is a quiz in WebCT that assigns your  stars by number. The plots of stellar spectra are found from links at the end of this lab.

 

To decide on the spectral type, we look at the absorption lines and apply specific criteria.

 

Each spectral line always appears at the same wavelength (except for Doppler shift which is not happening here). Not every star shows the same spectral lines.  Although most stars are made of same materials, temperature is the main factor determining which absorption lines are seen and how strong they are.

 

Use the Example spectra to see where the lines are. Two spectral traces are shown, so that features of both hot and cold stars can be identified.

 

The symbols show where the lines are and ALL the lines from the same symbol will show up at once (e.g. all the hydrogen lines are either present or not).

 

At the end of this lab are links to sets of reference spectral plots. Each of these plots is identified by a number in the sequence (irrelevant), then a letter and Arabic number for the spectral type and a Roman numeral specifying the luminosity class.  For example, if you open O/B III the first plot will be labeled 62 O6.5 III. The number 62 is irrelevant, O6.5 tells the temperature of the star and III is the luminosity class, giant.

 

When people first developed spectral types they identified specific lines that were sensitive to temperature and that could be seen in the wavelength region available with photographic film. The wavelength region available in our plots is larger, so the spectra look somewhat different.

 

The next figure is a flow chart of how to decide upon a spectral type, letter and number. Use the chart to decide to within a letter.  Look at the reference spectra to see how the appearance changes as the stars get cooler.

image010.png

Once you have decided upon an approximate spectral type, look at reference spectra with the same letter, but look at the different luminosity class (I, III, V) examples. Then decide which luminosity class fits your star best. Be prepared to change the spectral type a little from what you (previously) thought was best. 

 

How should you decide among the luminosity classes?

 

Main Sequence stars, luminosity class V, have wider line and more blended lines.   Supergiants, luminosity class I, have very narrow lines.  Often there are more lines seen in supergiants, since the very weak lines are smeared out and invisible in main sequence stars.

 

The blended features in G and K stars (like the G band at 4300Å and the blend between 5000Å and 5300Å) will be many individual lines in luminosity class I.

 

Luminosity class I stars, especially O, B and M stars, have some emission (bright) lines. They show up as parts of the plot that jut upward (often on the short wavelength side of an absorption line).

 

Just do your best and make a decision. Don’t spend your life on this.

 

Finally, use the underlying black body curve to calculate a temperature. This is a form of Wien’s Law. Find the peak of the black body curve for each of your stars. If the peak is off the paper on the short or long wavelength side, just say that it is not available and tell whether it is off the paper on the long or the short side. If you can find the wavelength for the peak, calculate the temperature from:

 

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

 

The answers are in the thousands. Don’t change your spectral type from the lines based on this. The lines affect where you choose the peak and that alters this result (especially for the A and B stars). There is a table of temperature and spectral type below that you may find interesting as a check.

 

Your star assignments are on the WebCT quiz. Links to the stellar spectra are below.

 

Comparison spectra

O/B  V

O/B III

KMII, OI

A/F V

A/F III

B,A I

G/K V

G,K III

F,G I

M V, and  FG IV

MIII, BFG II

K,M I

 

 Unknown spectra

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For EACH star tell

1) Star number

 

2) Wavelength at the peak of the Planck curve. IF  the peak is off the plot, say so and tell whether  the peak will be off the plot on the long or the short end 

 

3) A couple of sentences about  how you decided on the spectral type and luminosity class. Use the flow chart to guide your work and to guide your comments.

 

4) Spectral Type, Letter and number

 

5) Luminosity class, I, II, III, IV, or V

 

6) Temperature calculated from 2 if you have a value

For the lab as a whole, of course you will write an objective and a conclusion.

 

The results CAN be emailed, preferably without the plots.

Temperatures for various Spectral Types,

from Allen’s Astrophysical Quantities, Fourth Edition , Cox, Arthur ed.

Spectral Type

Temperature,

Main Sequence  V

Giant, III

Supergiants, I

O5

42000

 

 

O9

34000

 

32000

B0

30000

 

 

B2

20900

 

17600

B5

15200

 

13600

B8

11400

 

11100

A0

9790

 

9980

A2

9000

 

9380

A5

8180

 

8610

F0

7300

 

7460

F2

7000

 

7030

F5

6650

 

6370

F8

6250

 

5750

G0

5940

 

5370

G2

5790

 

5190

G5

5560

5050

4930

G8

5310

4800

4700

K0

5150

4660

4550

K2

4830

4390

4310

K5

4410

4050

3990

M0

3840

3690

3620

M2

3520

3540

3370

M5

3170

3380

2880