Physics 162
Galaxies & Cosmology

H. E. Smith Spring 2001

  Physics 162 - Galaxy Classification Lab
                         Due: Thursday, April 26

In this exercise you will apply the Hubble Classification scheme for galaxies to selected galaxies on prints from the National Geographic -- Palomar Observatory Sky Survey (POSS). Begin by reading The Hubble Atlas of Galaxies by Allan Sandage, comparing carefully the galaxy images and the classification criteria as described by Sandage. Attached are two lists of galaxies with millimeter coordinates on the POSS prints. The first is a list of well known comparison galaxies whose classifications are given to allow you to ``calibrate" your eye and the POSS prints. I suggest that you look up these galaxies on the POSS prints and classify them yourself before you look at the catalogued classification. The second list contains coordinates locating bright (V < 10.5) galaxies on the POSS prints for you to classify. Use the eyepieces provided to study each of the program objects, write down a description of the galaxy and a class under the Hubble classification scheme as described by Sandage. Also, attempt assigning a luminosity class for the spiral galaxies only. Pay closest attention to the Hubble type as this counts for most of the credit.

You are encouraged to discuss the classification schemes with one another. I request, however, that you inspect the POSS prints individually and also that the detailed classification submitted for credit be solely your own work. Submit your classifications and brief galaxy descriptions (neatly transcribed, concentrating on classification criteria) on the sheets provided, by 4:00 pm April 26.

The National Geographic - Palomar Observatory Sky Survey (POSS)

During the 1950s a major survey of the northern sky was undertaken at Palomar Observatory with the support of the National Geographic Society. This survey, made with the 48" Schmidt telescope, covers the sky from the north celestial pole to --33 declination on blue sensitive (Kodak 103a-O) and red sensitive (103a-E) photographic plates. The original 14"x 14" glass plates were printed through transfer positives as negative contact prints. Each print represents a 6° x 6° area of the sky with a scale of 67 arcseconds per mm. The blue passband is comparable to the classical photographic band, mpg, and the red is centered around H (6563Å). The blue (``O'') plates go more that one half magnitude fainter than the red (``E'') plates with limiting magnitudes of approximately mb = 21.1 and mr = 20.5 respectively.

The plates are catalogued as pairs in 6° declination strips around the sky in order of right ascension. A data block in the upper left (northeast) corner contains the exposure date, the 1950.0 celestial coordinates (Right Ascension = ; Declination = ) of the plate center, and a print number preceeded with an ``O`` or ``E'' for the emulsion type. For example, the prints catalogued as +06° 19h 36m are prints O/E 171 with plate center = 19h 40m 39s and = +06° 13' 22". Further information about the survey and prints is given by Minkowski and Abell (1963) and some applications described by Lund and Dixon (1973). King and Raff (1977) give an empirical calibration of the aparent magnitude as a function of image diameter for the prints.

More recently Palomar has undertaken a second generation sky survey using higher resolution, more sensitive photographic emulsions - the POSS-II. This survey is similar to the original POSS, but uses three colors: blue (IIIaJ), red (IIIaF), similar in passband to the old POSS, and near-infrared (IV-N) at about 8000Å. These plates are being digitally processed at the Space Telescope Science Institute and are partially available online

The POSS prints are housed in the CASS Marlar Library. The prints are in atlas cases in the Library Atlas Room along with copies of the Hubble Atlas, the van den Bergh Luminosity Classification, and eyepieces for use in inspecting the prints. Please handle the prints with great care: make certain that your hands are clean, be certain that the prints are laid flat while you are working with them, do not place other articles on them or write on them or on top of them, and return them carefully to the box from which they came. You will need a mm ruler to find the galaxy positions, which are to be measured from the lower left corner of the plate. Susan Rappoport, CASS Librarian, will be available when the atlas room is open, M-F 9:30--6:30.

There is an alternative way to access the POSS via the Web at the Space Telescope Science Institute Digitized Sky Survey (DSS) where you can pull up digitized images. The J2000 coordinates of the appropriate galaxies may be found on one of the pages below.

I strongly recommend that you begin your exercise with the actual POSS prints; in the case of galaxies the highly nonlinear response of photographic emulsions provide a substantial advantage in the ability to see the high-surface brightness nuclear regions simultaneously with the faint outer regions. You will find that you need to download the image (GIF or FITS format; if you don't know about FITS or are using a PC or Mac, download GIF) from the DSS and use your favorite image display tool to play around with the gamma of the image extensively in order to get a classifiable image. Use the POSS-II fields where available - the galaxies show up much more readily. For most of the classification galaxies a field of a few arcminutes will be sufficient, but the comparison galaxies below were selected to give you a tour of nearby bright objects and may be pretty big images (e.g. the major axis of M31 is about a degree).


Minkowski, R. and Abell, G. 1963 in Basic Astronomical Data, K. Strand, ed. (Chicago: University of Chicago Press).
Lund, J. and Dixon, R. 1973, PASP, 85, 230.
King, I. and Raff, M. 1977, PASP, 89, 120.

For galaxy positions:

Click here if your data set is Lab #1 (first plate on your exercise is -24° 00h 52m (300,067)).
Click here if your data set is Lab #2 (first plate on your exercise is -24° 03h 28m (106,026)).
Click here if your data set is Lab #3 (first plate on your exercise is +36° 02h 20m (054,306)).

Comparison Galaxies
Galaxy Position POSS Coordinates Classification
  Plate (X,Y)  
NGC 205 00h 40m 22s +41° 41' 26" +42° 00h 30m 148,115 E7
M31 00h 42m 44s +41° 16' 09" +42° 00h 30m 123,092 Sb I-II
M32 00h 42m 42s +40° 51' 52" +42° 00h 30m 123,071 E0
M33 01h 33m 51s +30° 39' 37" +30° 01h 18m 085,171 Sc(s) II-III
M81 09h 55m 33s +69° 03' 55" +72° 10h 12m 305,062 Sb(r) I-II
M82 09h 55m 52s +69° 40' 47" +72° 10h 12m 300,096 Irr
NGC 3351 10h 43m 58s +11° 42' 14" +12° 12h 48m 326,199 SBb(r) II
M87 13h 30m 49s +12° 23' 28" +12° 12h 24m 179,238 E0
NGC 4594 12h 39m 59s -11° 37' 23" -12° 12h 24m 062,236 Sab
M101 14h 03m 12s +54° 20' 55" +54° 13h 56m 158,230 Sc(s) I
NGC 6946 20h 34m 52s +60° 09' 14" +60° 20h 32m 173,155 Sc II