Hubble Diagram - Teacher's Guide
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Sky Surveys
 Teacher's Guide
     - Introduction
     - POSS
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Sky Surveys - Teacher's Guide to Specific Sections


The concept of mapmaking should be familiar to most students. Discuss the need for maps to help us find objects in the night sky as well, especially objects that are not visible to the naked eye. You may wish to look at some simple star charts with your students that show the familiar constellations and bright stars. You can find such star charts in many books and magazines about astronomy.


The Palomar Sky Survey (POSS) was carried out in the 1950's with the 48-inch Schmidt Telescope on Mount Palomar in southern California. Data were recorded on photographic plates. A second Palomar Sky Survey (POSS II) was carried out in the 1980's with better emulsions. Scientists are still working on converting POSS II to digital format; therefore, the students will be looking at data from the first POSS.

Images can be retrieved from the POSS by either entering the object name, such as Pal 3 for the Palomar 3 globular cluster, or by entering the right ascension (RA) and declination (Dec) of the object. If you enter the object name, another web site with a search engine called SIMBAD will look up the coordinates for the students. This process is automatic and requires no work on the part of the students.

Be sure that the students are retrieving .gif files instead of .fits files. FITS files are a format used by astronomers, and the files require special viewers. The default for the field of view should be adequate for this project.

The images from the POSS and the SDSS are oriented differently. If you want to orient them the same, take the SDSS image and rotate it 90 degrees counterclockwise. Next, flip it across the vertical axis. This technique only works for objects near the celestial equator (0 degrees declination). A lot of the Early Data Release lies in this part of the sky, including all four supernovae.

For the first supernova, the picture has been rotated and flipped for the students. When they call up the plates on their own, they will need to perform the transformation on their own. The supernovae are subtle, and you have to look very close to find them. Shown below are the fields with the supernovae marked.

Supernova #1

Click on the image for a larger view

Supernova #2

Click on the image for a larger view

The supernovae are very difficult to find. If you have an image viewing program that allows you to zoom in on an area, rotate and flip the object, the supernovae will be much easier to find. Let students enjoy the search, but don't take too much time on this section.


You may wish to discuss infrared light here. Remind students that all objects give off electromagnetic waves due to their temperature. If the object is too cool to emit visible light, it will emit longer wavelength light such as infrared. Night vision scopes work by detecting infrared light. Since some objects are too cool to give off visible light, you can see some objects in the infrared that cannot be seen at other wavelengths.

Another advantage of infrared light is that, unlike visible light, infrared light is not scattered by dust clouds. Therefore, dust is transparent to infrared light but opaque to visible light. Several of the images have dust clouds that are not visible in the infrared images, most notably NGC 4753.


ROSAT, the x-ray survey, reveals a different class of objects. Some objects are so hot they give off a lot of energy as x-rays instead of as visible light. Objects that look ordinary in the visible spectrum may be giving off vast amounts of energy. However, since most of the energy is x-rays we may not notice how bright these objects are in a normal photograph.

ROSAT cataloged about 60,000 x-ray sources. Each energy of x-rays is assigned a different color. The objects will not look like objects familiar to the students. Frequently, the x-ray sources are located within galaxies or quasars. A black hole pulling in gas is an excellent source of x-rays. Cygnus X-1, a star being ripped apart by a black hole, gives off strong x-rays. If it were not for the x-ray emission, this ordinary star may have escaped notice.

Students will look at the x-ray sources and try to find the object that is giving off the x-rays. Once they get the RA and Dec of an x-ray source, they can look up those coordinates in the SDSS database and see what may lie there.

Students will frequently discover that active galaxies and quasars correspond to x-ray sources. They may occasionally find an x-ray source with no visible counterpart. They did not necessarily make a mistake. Sometimes the source is too faint in the visible part of the spectrum to find. The SDSS uses a good-sized telescope, so this will be rare in our data, but it wouldn't hurt to mention the possibility to students.