University of Michigan - Department of Astronomy

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Finding the Milky Way with the Brightest Stars

Continuous as the stars that shine
And twinkle on the Milky Way

--William Wordsworth



In class and in the spectroscopy lab, you've seen that the surface temperatures of the stars mostly control the stellar spectra: O stars are the hottest and bluest, M stars the coolest and reddest, with stars such as the Sun (a G star), at intermediate temperature.  Additionally, you learned that the temperature also determines the luminosity of a star: blue stars are hotter and therefore brighter than red stars.  But, if that were the end of the story, you wouldn't see any red stars in the night sky!  The luminosity also depends on the radius of a star: a type M star could be as bright as a type O because it is bigger than the type O star.

Fortunately, the spectra tell us more than just temperature. If you look very carefully, you will find that some of the absorption lines in normal stellar spectra vary from star to star, even though the overall spectral type is the same. For example, stars with lines of elements such as Magnesium and Strontium indicate the stars are bigger, sometimes much bigger than the Sun. 

luminosity classes on the Hertzsprung-Russell diagramThe specific spectral features that reveal these luminosity differences define what astronomers call luminosity classes for stars. The luminosity classes are listed by Roman numerals:

• Supergiants are denoted classes I and II.

• Giants are denoted class III.

• Subgiants are denoted class IV.

• Dwarfs are denoted class V.

Although the Sun is almost 300 times the Earth’s diameter, like most main sequence stars it is a dwarf! Supergiants are actually bigger than the Earth’s orbit! 

The luminosity classes are associated with different populations.  Red giants – luminosity class III stars – are associated with very old groups of stars. Supergiants are more usually associated with young stars, although some supergiants are very red, cool objects. Betelgeuse – the brightest star in Orion and clear red even to the naked eye – is an example of a fairly young star that has become a supergiant.


The class will break into four groups, and you'll use information about the luminosity classes of stars to predict where the disk of the Milky Way is located.  You'll then check that prediction in the planetarium.


Your GSI will break up the lab into four groups of students and hand out a list of bright stars, and star charts.

  1. Discuss with your group what sorts of stars may or may not be likely to be most readily associated with the Milky Way in the sky (keep in mind, all of these stars are in the Milky Way, but some may trace the bright band of the Milky Way as seen from Earth better than others).  Write a brief explanation of which group you chose to use and why.

  2. Identify all the stars of your chosen luminosity class from your list.  If, for example, you think the dwarfs trace the Milky Way best, identify these in your star list. Then, identify at least 6 and up to 12 constellations containing these stars (the constellation is often part of the star’s name). List the constellations here.

  3. On your charts, find these stars and mark them relative to prominent other constellations so you can easily find them in the sky.
  4. Once all the groups have a set of constellations, head down to the planetarium.  Your GSI will give your group a least one laser pointer and turn on the stars to a level where the brighter stars can be seen but where there is still considerable light in the dome.
  5. Identify at least two constellations visible that you believe harbors the Milky Way.  Point at one of them with the laser pointer.  Note if another group is already pointing at it, you'll have to find another constellation.  Your GSI may ask you to point at more than one constellation.
  6. What constellation(s) did you end up pointing at?

  7. Once all the groups have identified constellations throughout the sky, your GSI will turn the lights down very low to show all the naked-eye stars.
  8. Describe what you see.  Can you make out the Milky Way yet?  Are all the constellations people are pointing at in a line?

  9. Your GSI will turn on the projection of the Milky Way and identify some of the constellations in which it is found. Each group should be prepared to discuss at this point why they chose a particular set of stars as the most likely Milky Way tracers, and why some of those stars seem to do the job better than others.

Concluding questions

  1. Why is the structure of the Milky Way so difficult to discern from the brightest stars only?

  2. In your list of stars are two numbers, l and b, the so-called Galactic Longitude and Galactic Latitude, respectively. What do you think these denote?

  3. Plot the positions of the luminosity class I and II stars using their Galactic Longitude and Latitude.
  4. Using another symbol, plot the positions of the luminosity class III stars on the same graph.  Repeat this for the luminosity class IV and the luminosity class V stars as well.  You should end up with one graph with four different symbols.  Be sure to include it with your lab when you turn it in.
  5. Which luminosity class(es) seem to best trace out the Milky Way structure based on these plots. Why?

  6. What other properties of these bright stars might tell use we live in a flat disk-like structure?

Last modified: 10/13/05

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