Penn AstroLab 3:
The Andromeda Galaxy

Last Updated 22 Apr 1999

Name:__________________________Date project began: ____________________

Section #:_______ Instructor:___________________

Project Goals

  1. To learn how to search the NASA Extragalactic Database.
  2. To find out about our bigger neighbor in the Local Group of galaxies.
  3. To study the Andromeda galaxy at visual, infrared and X-ray wavelengths.

Background Information

The Great Spiral Galaxy in Andromeda is the companion galaxy to our Milky Way in the Local Group of galaxies. Designated as M31 in the Messier Catalogue of objects, the Andromeda Galaxy is believed to appear similar to our own Milky Way (believed, in the sense that we do not really know what the Milky Way looks like from "outside"), and is actually somewhat more massive.

M31 is actually bright enough to be visible to the naked eye, if viewed from a dark site. In deep photographs, M31 is quite spectacular.


Start running a web browser on a computer. From Lab 1: The Pleiades you should alread be familiar with how to access SkyView:

You may wish to review that procedure.

In this lab, we will be introducing the use of the NASA Extragalactic Database (NED). This is a powerful resource for finding out about various extragalactic objects. Like the name implies, it does not contain data on stars and such within our own Milky Way galaxy, such as the Pleiades. You can access the homepage of NED at

As usual, 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!


  1. The first task is to obtain the celestial coordinates of M31. We will use NED for this purpose. First, access the NED Search by Name form. Then, enter "M31" as the object name, choose "J2000.0" as the equinox, then click on the "Submit Query" button. (If you have trouble, try this to perform the search directly. Try it for yourself though, to learn the ropes of NED!

    Enter the coordinates you find for M31 (in equinox J2000.0) below:

    RA = ________ h ________ m __________ s (J2000.0)
    DEC = ________ ° ________ ' __________ " (J2000.0)

  2. In the form returned by NED, there is a wealth of other information about M31. Take a moment to look this over.

    (a) What is the galactic latitude and galactic longitude of M31?

    (b) What is the radial velocity of M31 with respect to the Sun? Is it moving toward or away from us?

    (c) What is the morphological (Hubble) type of M31? What does this mean?

    Note that if you wish, you can print this out and attach it to your lab notebook.

  3. Take the position you obtained from NED above, and use it to bring up the Digital Sky Survey image from the SkyView Basic Form. Be sure to designate the "2000" equinox, to highlight "Digitized Sky Survey" in the catalogue menu, and to choose a wide field of view, such as "3.00 degrees". If you have difficulty, try this.

    This image will be in false color, with intensity coded as color. If you want a more "realistic" greyscale version, go to the advanced form, choose "B/W Linear" as the Color Table, "Hist. Eq." (for example) as the Brightness Scaling. Be sure to select a reasonable image size (3.00 degrees, 600 x 600 pixels are good).

    Note that in fact SkyView has the NED name resolver built-in, and you could get the same result by typing "m31" in the name/position box instead of the position. However, it is one of the goals of this exercise that you learn how to use NED and SkyView independently.

    Sketch what you see:

    You should see two companion galaxies to M31 in the image! These are M32 (just south of M31) and NGC205 (NW of M31). Be sure to indicate these on your sketch. Remember that on sky images, it is conventional to have North up and East to the left.

  4. Look at the image(s) retrieved from SkyView. Estimate the angular size of M31. (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.)

  5. A recent determination of the distance to M31 gave a value of 778 ± 21 kilo-parsecs (kpc) ( Stanek & Gamavich 1998). Using the angular size determined above, estimate the physical size (in parsecs) of the disk of M31.

  6. It is now time to look at the Andromeda Galaxy in other wavelengths. We will first try the infrared, using data from the InfraRed Astronomical Satellite (IRAS). Return to the SkyView Basic Form if it is still on your screen (or launch a new one and re-enter the position of M31 or just "m31") and now select "IRAS 100 micron" from the menu of catalogues (be sure the Digitized Sky Survey is de-selected). Keep the other toggles the same, such as a 3-degree field of view. Submit the request and wait for the image to return (this should be much faster than the visual image). If you have problems, try this.

    Sketch what you see:

    Emission at 100 microns wavelength is largely due to warm dust in the interstellar molecular clouds within the disk of the galaxy. You may wish to refer to your textbook for help in reviewing the properties of dust and the interstellar medium. Interpret what you see in terms of this:

  7. Now select "IRAS 25 micron" in the catalogue menu, and re-submit. (In case of difficulty, you can do this directly.) Compare what you see at 25 microns to the 100 micron and optical images.

  8. We can also observe M31 at X-ray wavelengths. Note that only the most energetic phenomena, such as white dwarfs, neutron stars, black holes and other "extreme" objects are likely to produce X-rays. Thus the X-ray image should appear very different to the visual and infrared images.

    Select "PSPC 1-deg intensity" in the catalogue menu, and re-submit. This will return X-ray data obtained by the Einstein satellite mission. (In case of difficulty, you can do this directly.) What are you seeing in the X-ray image? How does this compare to the other wavebands you have looked at? (Note: do not get confused by the blue ellipse and circle, this is just due to background counts in the telescope and just delineate the regions of sky where the telescope looked. The real sources are the small points!)

    Try to put this in the context of the above discussion regarding energetics. Might these X-ray sources have counterparts in our own galaxy?

Further Explorations

These are some optional related activities you can pursue on-line or off:

© Steven T. Myers 1998