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



You learned that stellar spectral types correspond to the stellar surface temperatures, which are plotted on the x-axis of the H-R Diagram. O stars are the hottest and bluest, M stars the coolest and reddest, with stars like the Sun, a G star, at intermediate temperature.  Astronomers also have a system for specifying luminosity classes, which correspond to stellar luminosities. These are a rougher classification scheme than the spectral types, and are more approximately represented on the y-axis of the H-R Diagram (Figure 1). luminosity classes on the Hertzsprung-Russell diagram

A star's luminosity correlates with its radius. Given two stars with the same temperature, the bigger one will be more luminous. Supergiants like Betelgeuse are bigger than the Earth's orbit! The luminosity classes are specified with Roman numerals:

• Supergiants are denoted classes I and II.

• Giants are denoted class III.

• Subgiants are denoted class IV.

• Dwarfs are denoted class V.

The Sun is a dwarf star, so it is a G V star. All Main Sequence stars are dwarfs (class V), with the exception of some extreme, most luminous blue stars, which are blue supergiants (class I).

The luminosity classes are associated with different age stellar populations, because most stars leave the Main Sequence and increase in luminosity near the ends of their lives.  Stars just leaving the Main Sequence are subgiants (class IV), and red giants (class III) have clearly left the Main Sequence. Thus, both luminosity class III and IV stars are associated with older stellar populations. On the other hand, massive stars maintain the same, extremely high luminosity, for most of their short lives. Therefore, supergiants (classes I and II) are all massive stars and are associated with young populations. Betelgeuse, in the constellation Orion, is clearly red even to the naked eye, and is an example of a short-lived, massive star that has become a supergiant. Note that almost all of the red stars visible to the naked eye are red giants (class III) or red supergiants (class I), because red dwarfs (class V) are too faint to be seen.

In this activity, you will work in groups to to see how different stellar luminosity classes trace different stellar populations in our Milky Way Galaxy, and how the disk of our Galaxy is related to them.


  1. You GSI will hand out materials to four groups. Each group will receive a list of bright stars corresponding to one of the luminosity classes, a star chart, and two laser pointers. Note the luminosity class of your list here:
  2. Identify the constellations containing these stars, and list the constellations here:

  3. Identify these constellations on your star chart so you can easily find them in the sky.  Mark the listed stars on the star chart.
  4. The GSI will set the sky to a level where its surface brightness is fairly high, but where the brighter stars can be seen. The GSI will ask each group to identify their constellations on the dome, by luminosity class. Notice where the stars of each luminosity class appear on the sky.
  5. Once all the groups have identified their constellations throughout the sky, your GSI will turn on the Milky Way projectors.
  6. Which luminosity class best traces the Milky Way?

  7. Which is the worst?

Concluding questions

  1. Explain why the luminosity class(es) that best traced the Milky Way did so, and why the others did not.

  2. If you look up a list of the brightest stars in a text book or online, you'll see they come from all different luminosity classes and different intrinsic luminosities.  Why?

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

  4. The list of stars shows two columns, l and b, which are the Galactic Longitude and Galactic Latitude, respectively. Plot the positions of the luminosity class I and II stars using their Galactic Longitude and Latitude. Using different symbol or color, plot the positions of the luminosity class III stars on the same graph.  Repeat for each of luminosity classes IV and V.  You should end up with one graph with four different symbols or colors.  Be sure to hand this in with the rest of the activity. Which luminosity class(es) seem to best trace out the Milky Way structure based on these plots. Why?

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

Last modified: 10/31/12 by MSO

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