University of Michigan - Department of Astronomy


updated: 04/19/2000

Nebulae in Cygnus

Now see that mind that searched and made
All Nature's hidden secrets clear
Lie prostrate prisoner of night.
-- Boethius



In this Exercise we will study a region in the constellation of Cygnus that contains many gaseous nebulae of different types. Since the region is so large, we will use wide-field photographs from the Palomar Observatory Sky Survey (POSS) to perform the investigation. The POSS is a set of photographs covering the entire sky north of -27 declination. For each 36 square degree field in the northern sky, the 48-inch Schmidt telescope on Mt. Palomar in California was used to obtain two photographs, one sensitive to the blue region of the spectrum, the other sensitive to red. The main reason why two sets of photographs were taken will become very clear in this lab; you will notice how different the sky looks in different wavelength regions.

The two fields we will investigate are adjacent to each other in the north-south direction. You will notice that the very bright star Deneb appears near the top of one set of prints and near the bottom of the other set. We will refer to the prints by the approximate coordinates (declination and right ascension) of their centers. The northern set is centered near DEC = 48o, RA = 20h 24m, and the other set is centered at DEC = 42o, RA = 20h 30m. More accurate coordinates can be found in the gray box located in the upper left hand corner (the north-east corner) of each print. Also listed in this area you will find the letter "O" or "E." These letters specify the blue and red-sensitive photographic emulsions, respectively. Kodak's 103-O emulsion is most sensitive to the blue region of the optical spectrum between 330 nm and 530 nm, while the 103-E emulsion is most sensitive to red light in the range 600 nm to 680 nm. While using any POSS prints, you should be aware that the brightest stars in the sky often produce "ghost images" that can be found diametrically opposed to the star's image relative to the center of the photograph. The ghost images appear as oval rings and simply should be ignored.

One last thing before we begin: these POSS prints are expensive and difficult to replace. Please be very careful with them, and NEVER attempt to draw on the plastic coverings on the prints. Thank you!

Directions and Questions

The 48o, 20h 24m Prints

Use Figure 1 for this section.

  1. We will first examine the O and E prints of the 48o, 20h 24m field to investigate the colors, and hence surface temperatures, of several stars. Figure 1 is a chart that represents the field of this photograph, and on it are marked 10 bright stars. Compare the relative intensities of these ten stars on the O and E prints and determine their approximate colors according to the scheme below. The exposures of the two photographs were scaled such that a star with the same color as the Sun will appear to have the same brightness on the two prints. Simply list which stars appear blue, neutral (the color of the Sun), or red.

    Stars that appear brighter on the blue print:

    Stars that appear brighter on the red print:

    Stars that appear to be of equal brightness on both prints:

    Indicate which of these stars are hotter than the Sun, about the same temperature as the Sun, or cooler than the Sun. How do you know this? (Hint: Consider Wien's Law from the spectroscopy lab).





  2. On which plate is the background stellar density higher? Why?





  3. Now we will move on and investigate the nebulosity on these prints. First, notice how much more apparent the nebulosity is on the red print compared to the blue print. This is due to the fact that the gas gives off an emission-line spectrum with a major contributor being the hydrogen alpha line at 656.3 nm. There are also weaker emission lines at the blue end of the spectrum, which accounts for the fact that the nebulae can be seen on the blue-sensitive prints.

    On Figure 1, roughly sketch several of the emission-line regions that are prominent on the E print. Nebulae which appear filamentary tend to be expanding shells of gas from a supernova event which have come in contact with the surrounding interstellar medium. The shock wave that results heats the gas and causes it to glow. This is an example of what is known as collisional excitation of a gas. Label these types of nebulae with the letter F on Figure 1.

  4. Nebulae which appear more confined tend to be heated and glow as a result of photoionization by strong ultraviolet emission from the hot stars embedded in the gas.

    i) Mark these regions with the symbol HII on Figure 1.

    ii) In as many cases as possible, locate the stars that are responsible for ionizing the gas in these nebulae. Label these stars O on your chart.

    iii) By examining their relative intensities on the O and E prints, determine the colors of these stars. ______________

    iv) Compared to the Sun, what surface temperatures do these stars have?


  5. At least one prominent planetary nebula is also located on these prints. A planetary nebula looks like a small, dark, circular emission region which appears on the red (E) print but is not really visible on the blue (O) print.

    i) Scan the E print carefully and locate the planetary nebula. Keep in mind that a typical planetary nebula has an apparent diameter of about 2 arcminutes or roughly 2mm on the POSS prints. Also, the edge of the planetary nebula is well defined, unlike the stars. Mark its location with a symbol PN on Figure 1.

    ii) Using a ruler, measure the distance from a nearby numbered reference star to the planetary nebula. Indicate the direction of the nebula from the reference star as well as the distance in millimeters. The point of this is to be able to reproduce this measurement on the O print, so you can measure the position angle from any other star on the print, so long as you can find the same star on the O print.

    Reference star number:

    Distance to planetary nebula:

    Position angle of PN with respect to the reference star:

    iii) Using information from part (ii), find the planetary nebula on the O print. Can you see the central star of the planetary nebula on either print? What does this imply to you?



  6. You should notice that there are a number of regions in this area where there appear to be fewer background stars. This is actually the result of interstellar obscuration of star light by cool clouds of gas and dust in the interstellar medium between us and the more distant stars.

    i) Locate and outline some of the more prominent regions of heavy obscuration on Figure 1. Label these regions with the symbol D.

    ii) On which print, O or E, does the obscuration appear to have the greatest contrast against the surrounding stars? (Remember that you are looking for contrast in the number of stars, so ignore the fact that the nebular H-alpha emission is sometimes also obscured.)


    iii) Can you think of a reason that the obscuration would be more prominent on one print than the other?




    iv) What do you think is primarily responsible for this obscuration?




  7. A striking example of yet another type of nebula can be found in this region of Cygnus. Unlike the emission nebulae that appear brighter on the red print than on the blue print because of strong emission in the line, this is a region where dust is reflecting the light of a bright star located within the nebula. This type of object, known as a reflection nebula, will appear on the blue print, but is not really noticeable on the red print. Scan the O print and locate one reflection nebula. It will be around a bright star and will be fairly uneven, unlike the diffraction circles which appear around all the bright stars.

    i) Sketch it on Figure 1 and label the nebula with the symbol R.

    ii) What color star is illuminating this cloud of gas and dust?

    iii) Reflection nebulae and obscuration are both caused by dust, so they are related phenomena. Then why does a reflection nebula appear blue, while general obscuration makes sources appear redder than they would be otherwise? In your answer, include a diagram of the geometries of these phenomena.









    The 42o, 20h 30m Prints

    Use Figure 2 for this section.

    Now we will examine the 42o, 20h 30m prints. As we already mentioned, the bright star in the upper left hand corner is Deneb. Near the opposite corner of the print is the star Sadr (aka Gamma Cygni), surrounded by the Gamma Cygni nebulosity. On the eastern (left) edge of the E print is the famous Pelican Nebula. The Pelican's shoulder, wing, and eye and bill can all be seen. Running from Deneb down the center of the field is a region of heavy obscuration known as the Great Rift in Cygnus. Behind this rift is a strong radio source, around which many of the filaments in the previous pair of prints are distributed. Distances in this region range from roughly 0.6 kpc (1 kpc = 1000 pc) for the Pelican Nebula, to about 1.5 kpc for the Gamma Cygni nebulosity.

  8. Locate four prominent regions in this field where gas and dust clouds obscure the light of more distant stars.

    i) Roughly sketch the outlines of these regions on Figure 2.

    ii) Since we usually only see stars in front of these clouds, the more distant the cloud, the more stars there are that will be seen in front of it. By examining the relative numbers of stars in front of these clouds, estimate their relative distances. Number the obscuring regions from 1 to 4, from the nearest to the farthest, respectively.

  9. Near the star Sadr in the lower right hand corner of the field there are two regions of particularly strong emission separated by a rift of obscuration. Propose an explanation for why the obscuration appears on both the E and O prints. (Hint: Before we saw obscuration of background stars. What is being obscured here?)






Summarize what you have learned about the different types of nebulae:

  1. Filamentary nebulae
  2. H II regions (localized emission nebulae)
  3. Planetary nebulae
  4. Reflection nebulae
  5. Obscuring clouds

Use a separate page for your work, and write about one paragraph per type. In your summaries, make sure you include the following points:

figure 1



  figure 2