![[M31]](androb00.jpg)
M31/M32/M110
Downloadable versions (see
NOAO Conditions of Use):
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).
Minimum credit line: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF
(for details see Conditions of Use)
400 x 400 25 kb color JPEG
1024 x 1024 184 kb color JPEG
2048 x 2048 656 kb color JPEG
8192 x 8192 14.9 Mb color JPEG
8192 x 8192 65.6 Mb 8-bit color TIFF
8192 x 8192 196.8 Mb 24-bit color TIFF
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.
Distance: approximately 2.2 million light years.
Size: over 65000 light years across.
Return to:
galaxies page,
spiral galaxies page.
Comments by e-mail to nsharp@noao.edu
