To get a full picture of the sky, astronomers conduct sky surveys in a number of different wavebands. Traditionally, this has involved photographic plates on optical telescopes such as the Palomar Sky Survey. After the advent of radio astronomy, sky surveys were carried out in radio bands. In the 1970's IR satellites such as IRAS began to survey the heavens, and X-Ray wavelengths were added to the mix in the 1980's. Finally, the 1990's brought gamma-rays into the picture with GRO.
The universe looks different at different wavelengths. The visible light of the sky is domiated by starlight. However, when we look at radio wavelengths for example, we see synchrotron radiation emitted by fast-moving electrons in cosmic magnetic fields. In the infrared bands, we see thermal emission from cooler dusty regions in our galaxy, as well as dust-enshrouded star-forming regions.
We will start our exploration of the sky by looking at stars. In particular, we will look at a star cluster.
Start running a web browser on a computer. Go to the SkyView site
and open up the Basic Form. This will take you to a form where you can enter coordinates, select a catalogue, and get back a map of the corresponding region of the sky. Note that the connection to this NASA site at the Goddard Space Flight Center can be slow during the daytime, and you may wish to conduct this exercise in the evening.
Familiarize yourself with the controls. The most important ones for our purposes are the box where you enter the coordinates, the menu of catalogues, the button designating the size of the field of view, and the toggle for the map grid.
We recommend that you keep a "laboratory notebook" for these exercises. In addition to recording your work on and answers to the questions, you can make notes related to these activities and questions that you have concerning the material presented. Also, if you have access to a printer, you can include printouts of the images and any maps generated here. If you are conversant with HTML and the web, you can also keep an "on-line" lab notebook!
Part of this exercise is to understand the relationship between the Earth and the celestial coordiate system of Right Ascension (RA) and Declination (Dec), in particular the difference between our usual Solar Time and the sidereal time of the clockwork heavens. For a review of these topics, see your course lecture notes, your textbook, or the AstroLab Lecture Note - Celestial Navigation prepared for this purpose.
Our target has coordinates (RA, Dec) of
Enter these into the coordinate box of the form (make sure the equinox is set to "2000"). You might try cutting and pasting from the above text directly into the box, if you know how to do that. If not, just type it in. Set the Image Size to "3.00" degrees. Make sure the "Digitized Sky Survey" catalog is selected (highlighted by clicking on it with the cursor). Then click the "Submit" button and wait for an image to come back (it should appear in another window which will be opened by the browser). Note that this may take a while, as the survey database is large and it has to fetch the information for the bit of sky you requested. If you have trouble making it work, try clicking here for direct access. You should try and do it yourself to get the hang of the SkyView interface.
Examine the image returned by SkyView. It will be a "false-color" map of the requested region of the sky. Since we chose the Digitized Sky Survey as our catalogue, the colors on our image correspond to the two filters (roughly blue and red) that the sky survey was taken in. Thus, the colors sort of correspond to real colors. But for the other (non-visible wavelength) surveys, don't assume this! After studying the image, describe what you see:
Note that if you wish, you can print this out and attach it to your lab notebook. The group of stars you are looking at is called the Pleiades, or "Seven Sisters" from Greek mythology. It is also known by its designation "M45" from the Messier Catalogue. This group of stars is what is known as an open cluster. The stars are all part of a physical grouping, and are actually located in space neighboring each other (and not just superposed by chance viewing). Furthermore, they were born from the same parent gas cloud at about the same time.
Estimate the angular size of the Pleiades. (Since you specifiy the field-of-view when you submit your image request, you know how big the image frame is. You can also toggle the coordinate grid on if you wish and then re-submit the request for the image.)
The distance to the Pleiades is estimated to be about 125 parsecs. Using the angular size determined above, estimate the physical size (in parsecs) of the cluster.
Examine the image more carefully. Are there any features in the map that might be artificial (that is, not really on the sky, but due to artifacts in the image processing)? If so, what might be the cause of this?
Examine the image more carefully still. It might be helpful to zoom in by reducing the image size to 1 degree and re-submitting. Is there noticable nebulosity, or fuzziness, surrounding the stars of the Pleiades? What color is it? What might be the cause of this?
Now we will check other wavebands to see if we can find corresponding features. First, we will look in the infrared. De-select the sky survey, and then select "IRAS 25 micron" to get data from the archive of the InfraRed Astronomical Satellite at a wavelength of 25 microns. Make sure the image size is set back to 3.00 degrees, the same as in the sky survey, and that the center of the image is the same. Again, if you have trouble doing it yourself, click here to get it directly. Note that in this case, the colors correspond to intensity, not visible color. How does this infrared image compare to the visual image of the Pleiades? What might the infrared emission be tracing?
Try looking in the other IRAS bands (12 micron, 60 micron, and 100 micron). How does the appearance of this cluster change with wavelength in the IR?
Given the right ascension of the Pleiades (3h 47m), and your knowledge of how the Sun moves along the ecliptic, you can estimate when the Pleiades are visible in the sky. On approximately what day of year are the Pleiades on the local meridian at midnight? (If you need help, see the Celestial Navigation notes.)
Given the above, and today's date, at what time (Eastern Time) will the Pleiades be on our meridian? When will it rise and set? Be sure to show your calculations.
These are some optional related activities you can pursue on-line or off:
Find a sky atlas in the library or in the lab room. Use the coordinates to locate the Pleiades on the appropriate chart in the atlas. What constellation is the Pleiades located in?
Look up this object in Burnham's Celestial Handbook. Note the interesting details presented there, particularly the historical anecdotes and observational description. What is the mythological significance of the name "Pleiades"?
Using an atlas or nightly sky chart (eg. Sky and Telescope), find some celestial landmarks for finding the Pleiades at night. Describe how a non-astronomer might find the Pleiades in the sky, using well-known constellations and compass directions at some given time.
Do an on-line WWW search for "Pleiades". Here are some interesting things I found when I tried it:
© Steven T. Myers 1998