M31


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. 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). This telescope is situated on Kitt Peak in southern
Arizona, and is shared between CWRU and the national community served
by Kitt Peak National Observatory. In order to provide color
information, we use photographic plates coated with different
emulsions, which are sensitive to different regions of the spectrum,
in conjunction with filters which only let pass part of the full range
of wavelengths. Three plates having three different combinations of
emulsion and filter were digitized for computer use by undergraduate
Vanessa Harvey during the summer 1996 Research Experience for
Undergraduates (REU) program, funded by the National Science
Foundation. The digitization was carried out with Kitt Peak's PDS
plate densitometer, a high resolution, photometrically stable,
scanner. The three frames approximate the primary colors red, green
and blue, from which it was possible to create this estimate of a true
color picture.

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. At the very center
of the Andromeda Galaxy is a brilliant point of light, which is a very
tightly packed star cluster, but this is not visible in this saturated
image.

The entire galaxy is rotating in space, with the lower portions
approaching while the upper parts recede. The rotation is not
completely smooth, showing `bumps' where the spiral arms occur, which
are probably due to the spiral density wave that maintains the
arms. By applying gravitational theory to this rotation, we can
`weigh' M31, and when we do it seems that there may be ten times as
much material as we can see in the visible portions of the galaxy,
distributed in a huge dark halo.  Photographic plates are hardly used
in astronomy any more, due to their very low efficiency (a few percent
at best) compared to electronic detectors such as charge-coupled
devices (CCDs) which can collect more than 50% of the light falling on
them. Plates are also less accurate for measuring light intensities,
but they can still be of benefit for imaging large areas at one time.

Location: 00h 40m 00s +41deg 00min (1950.0), constellation of
Andromeda (oddly enough).

Distance: approximately 2.2 million light years. 
Size: over 65000 light years across. 

Minimum credit line: Bill Schoening, Vanessa Harvey/REU
program/AURA/NOAO/NSF (for details see Copyright Statement)