PALOMAR SKY SURVEY PRINTS: INTERSTELLAR MATTER
Dec 8 , 1999
PLEASE BE CAREFUL when using these photographic prints.
Do NOT touch with pen or pencil.
Do NOT write on paper that lies over the prints.
The 48-inch telescope on Mt. Palomar in California was designed with a large field of view so that the entire sky visible from Palomar could be photographed in a reasonable time (some 10 years). The photographs were made on glass plates, and contact paper prints (14 inches square) were made of them. The Palomar prints come in pairs, one obtained from a blue-sensitive photographic plate, the other from a red-sensitive plate. Each print covers an angle of 6.5 degrees x 6.5 degrees in the sky. The plate scale is 1.12 minutes of arc per millimeter, or about 0.5 degree (the diameter of the moon) per inch. In the upper left-hand corner of each print is a small box with identifying information: the color sensitivity (O or E), the date that the photographs were taken, and the position of the particular region in the sky.
The prints are negatives. Stars therefore show up black; the brighter the star, the larger the star image on the print. For nebulae and galaxies, this means that the brighter they appear in the sky, the darker
they appear on the prints. (This is, of course, opposite to most of the photographs in the text, in which stars, nebulae, and galaxies appear white.)
These prints are important research tools actually used by astronomers to discover new objects and to study the various kinds of stars, interstellar matter, and galaxies that make up our universe.
The four prints to be studied are O,E 1099 and O,E 754. The former prints cover a region immediately to the north (to the top) of the latter prints. Both regions are part of the constellation Cygnus, a part of the sky through which the Milky Way runs. There is an overlap of about 1inch between the two sets of prints.
I realize that the reading will be out of context somewhat (the reading for each section is on top of the various pages that follow), but try to extract the important stuff out (how the nebula looks, why it looks that way, what excites the fluorescence mechanism, and how/where the nebula fits in the context of stellar life and evolution). It's also important to read the captions to all associated pictures.
A) RED and BLUE on the Palomar Prints
First decide which of the prints (O or E) is blue-sensitive (400 nm) and which is the red-sensitive (640 nm). One simple way to do this is by using the fact that the very bright star on the prints is Deneb ( = a Cygni), an A2 supergiant that is one of the 20 brightest-appearing stars in the sky. (Its absolute magnitude of -7 also makes it one of the most intrinsically luminous stars in the Galaxy.) Using the known spectral class of Deneb, determine the wavelength at which Deneb is brightest.
Now draw a blackbody curve for Deneb, labeling the wavelength of Deneb's maximum intensity and the two wavelengths of the prints’ maximum sensitivity.
With this result in mind, compare the two prints (O and E 754) and decide which print is the blue-sensitive one and which is the red. Be convincing.
Using prints E and O 1099, find a reasonably bright
a) blue star
b) red star
c) yellow star
List positions (in the form of LxT, where L = distance from the left edge of the print in centimeters and T = distance from the top edge of the print in centimeters) of these two stars picked below.
(Make sure you know which edges of the print are top and left before making measurements!)
In each case, explain the reason for how you knew it was the color star that you claimed.
B)H II REGIONS (or EMISSION NEBULAE): Kaufmann, section 20-2 and box 20-1
H II regions (pronounced "H two" regions; the "two," II, means that the hydrogen is ionized) are concentrations of matter where the hydrogen is completely ionized by the ultraviolet energy radiated from nearby hot blue stars. As the text emphasizes, the strongest visible bright line emitted is the Balmer H-alpha line of hydrogen (the 3 à 2 transition), at a wavelength of 656 nm, in the red part of the visible spectrum. Do the H II regions in fact show up darker on the print that you chose as the red-sensitive one?
AGE DETERMINATION OF H II REGIONS: method 1
Since H II regions only surround young O and B stars, these nebulae must be young also. Radial velocity data (from the Doppler shift in the spectrum) show that these regions are slowly expanding about their central stars. Since this expansion will cause a decrease in the gas density, the intensity (i.e., how black they are on the print) of H II regions should also decrease as they get older. Which of the H II regions (on print 754) would you judge to be the youngest by this criterion? Identify by position.
List at least four assumptions you implicitly made in using this criterion to judge the relative ages of the H II regions; one example is given:
1) the HII regions initially (at birth) had the same hydrogen gas density
AGE DETERMINATION OF H II REGIONS: method 2
Another indication of age is the structure of the dust clouds that are often associated with H II regions. If the dust (which shows up "white" on the negative) shows distinct striations, the dust patterns must be rather young, since the clouds' expansion causes the striations to diffuse gradually with age. According to this age criterion, which H II region on print 754 would you judge to be youngest? Again identify by position.
List some assumptions you implicitly made in using this criterion to judge the relative ages of the H II regions.
Did you pick the same nebulae with both criteria above? If not, why do you think not?
Look for an HII region called the Pelican Nebula on print 754.
Hint: the pelican is standing upright and is about 1 inch tall
C) PLANETARY NEBULAE: Kaufmann section 22-3; box 22-1
Planetary nebulae are slowly expanding gas shells surrounding very hot white dwarfs. The radiation from the shell is very similar to that we receive from an H II region: the Balmer H-alpha line is again very
strong in the red part of the spectrum; the planetary nebula should therefore show up better on a red-sensitive photograph. Try to find one on print E1099 (Hint: it's less than 1/8 inch in size, but it does look like the picture in the book on page 401 or 405, except quite round or like the picture of M97 on the Messier object chart by my office.)
You are responsible for including all the important steps of the method (diagram, trig, etc.)
2) Once you've found the nebula on the red print, try to find it on the blue print; it won't be easy!
3) The expansion speed of the nebula is about 25 km/sec; where does the information come from?
Determine how long ago in years the white dwarf ejected the gas shell.
4) Is the age you've just calculated most likely an underestimate or an overestimate? Why?
(Hint: What assumption(s) did you make in the previous calculation?)
D) INTERSTELLAR DUST: Kaufmann section 20-2
Absorption due to interstellar dust shows up on these negative prints as a white spot, cloud, or streak. The "white" indicates the "absence" of stars in that particular region of the sky. Of course, the stars are not really absent there, it's just that much of their light has been absorbed by the dust. Print 754 shows this effect particularly well.
Which wavelengths, red or blue, are more greatly absorbed (and re-emitted, of course; i.e., scattered) by the dust? Is this the print on which the dust shows up better? Explain.
Stars between the dust clouds and earth are unaffected by the dust while stars behind the clouds may be dimmed to such an extent that none (or very little) of their light can shine through. Therefore, all of the stars seen "on" such a dust cloud are between us and the cloud. With this in mind, can you think of a way to estimate the relative distances to the dust clouds on the print? Describe your method clearly.
Of the two dust clouds located at 3x25 cm and 27x30 cm on E754, which do you judge to be nearer?
Why? Can you be quantitative?
Give three assumptions that are implicit in the method you used above.
On what sections of the print (754) is the dust least dense? positions?
In these dust free regions, we are looking farthest into space. Estimate the number of stars in a square portion of print 754 (say « in x « in) in a region where the dust is very dense and in a region where the dust is least dense.
position of a "dusty" region & star count?
position of a non-dusty region & star count?
No galaxies can be seen on either print. Is this expected or unexpected? Why?
E) MYSTERY NEBULAE: Kaufmann section 20-2 and Figure 20-4
You have learned about several different types of nebulae in this course. Look at the nebulae on prints O/E-754 at positions: 27-28 x 17-18 cm. By determining the color of the nebulae and the stars inside, you should be able to determine the type of nebula they are and the types of nebulae that they are not. Explain.
F) SUPERNOVA FILAMENTS: Kaufmann section 20-8
Filaments of gas are very prominent on print E1099. They are red due to the hydrogen emission at 656 nm, just like the H II regions previously studied (in A) are, except that there are no hot blue stars near enough to ionize the gas. So what could be exciting or ionizing the gas in the filaments? One suggestion is that the gas is ionized by the shock wave released during a long-ago supernova explosion. As the shock wave passes through the interstellar medium, it collisionally excites the hydrogen gas to fluorescence.
Notice that the filaments on E 1099 are slightly curved. Where do you judge the apparent center from which these filaments have apparently been blasted to be?
Behind the dust clouds in the center of print 754 there is a radio and x-ray emitting source (Cygnus X-3), perhaps the remnant of a supernova.
Choose one of the filaments on print E1099. If the filaments lie at a distance of 3000 light years from earth, determine the radius of curvature of the filament arc in light years. (Use the same method that
you used in determining the size of the planetary nebula.)
Draw a large, well-labeled diagram of the prints, showing the curvature that you see, the apparent center, etc.; show all work.
How long ago (roughly) did this supernova explode?