Figure: The Andromeda Galaxy (M31) with long slits

This picture shows M31 (NGC 224) and its small companions M32 (NGC 221), lower center, and NGC 205 (sometimes designated M110), to the upper right.

To illustrate the concept of long-slit spectroscopy, two long-slits have been added over the galaxy image. In long-slit spectroscopy, only the stellar light from the major and minor axes of the galaxy can pass through the long-slits. In this way, astronomers record only the light of stars located in the disk plane and perpendicular to it. By studying their spectra, astronomers can find out their velocity distribution (see next figure), providing crucial information about the existence of central black holes in that galaxy.

M31 is a large spiral galaxy, very similar in appearance to, and slightly larger than, our own Galaxy, and our closest normal-galaxy companion (the very close Magellanic clouds are classified as irregular galaxies). In fact, from a distant vantage point, Andromeda and the Galaxy would appear as a pair, a binary or double galaxy system, if it were not for the rather smaller, though still significant, spiral galaxy M33. As our nearest neighbor, Andromeda is extremely large on the sky. This picture extends for over two and a quarter degrees, or more than four times the width of the full moon, and still does not include the full extent of M31. M31 is visible to the naked eye, although we can only see the bright inner bulge, and it has therefore been known since at least the year 964AD, when Persian astronomer Al-Sufi described it as a `little cloud'. We can see that the western (right) side of M31 is closer to us, by the fact that the dark dust lanes belonging to the inner spiral arms show up in silhouette against the nucleus on that side only.

The image was made by combining three separate frames derived from photographic plates taken in 1979 at the Burrell Schmidt telescope of the Warner and Swasey Observatory of Case Western Reserve University (CWRU).

Credit for the M31 image: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF