Sky Surveys - Teacher's Guide to Specific Sections
Introduction
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.
POSS
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
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Click on the image for a larger view |
Supernova #2
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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.
2MASS
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
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.
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