a barred spiral galaxy, Andrew Butler
M91 is located 60
million light-years from Earth. It is an object that astronomers classify
as a "galaxy."
Galaxies are immense groupings of stars that
contain anywhere from millions to hundreds of billions of stars.
galaxies, there are almost no stars. The earth itself resides in a galaxy
called the Milky Way
Galaxy. All the stars you can see at night reside
in the Milky Way, plus a lot more stars you can't see with
eye, either because they're too far away or not bright enough. For the
same reason, we can see
only a few galaxies with our naked eyes. However,
with large telescopes, astronomers have been able to see billions of other
galaxies! Not only this, but astronomers have also found many types of
galaxies, with a wide variety of shapes, sizes, and properties. In general,
galaxies are divided into three categories: spirals,
irregulars. Spiral galaxies, as their name suggests, have various spiral
arm patterns. They are characterized by the size of their central bulges,
which are dense groupings of stars, and how tightly their
arms are wound.
Some spirals, like M91, even have a prominent bar going straight through
their bulges, from whose ends the arms appear to begin. These kinds of
galaxies are called barred spirals and are
characterized in ways similar
to those of regular spirals. Two characteristics of barred spirals that
make them distinct from regular spirals are that their arms can start
either from a ring located around the bulge or from
the bulge itself.
Galaxies that appear to be somewhere in between these two characteristics
are given their
own designation. M91 is such a galaxy. It has a medium-sized
bulge and intermediately wound spiral arms
that appear to start from an
area in between the bulge and the ring, so it is given the designation
The SB stands for the fact that it’s a barred spiral, the
b stands for it being in between a and c (large bulge
with tightly wound
spiral arms and small bulge with loosely wound spiral arms, respectively),
and the rs stands
for it being in between r and s (spiral arms starting
from a ring and spiral arms starting at the bulge,
Amazingly, it has
only been a couple hundred years since humans have known about galaxies.
century French comet hunter, Charles Messier, discovered many
fuzzy, faint objects in the night sky
previously unknown to mankind, a
lot of which we now know are galaxies. He catalogued these objects and
compiled them into what is currently known as the Messier catalog. On
one particular night, March 18, 1781,
he discovered eight nebulous objects
in the same region of the sky, which we now know are galaxies in the
Galaxy Cluster. He catalogued the last object he found as M91 (the M stands
Unfortunately, he recorded an incorrect position for it.
For many decades, M91 eluded the eyes of
astronomers. Finally, almost
200 years later, in 1969, amateur astronomer William C. Williams found
Today, we know that
M91 is moving away from us at a speed of about 400 km/s, which is about
mph! This particular speed is peculiar because the Virgo Cluster
is moving away from us at a speed of 1100
km/s, so M91 must be traveling
toward us at 700 km/s relative to the other galaxies in the Virgo Cluster.
and Colors in the image above:
The middle of the
galaxy is orange-yellow. This indicates that there are a lot of older,
redder, and low-mass
stars near the bulge. There doesn't seem to be many
dust lanes in that area, so this means that new stars probably won't be
forming very much in that area of the galaxy anymore (dust and gas compress
stars). The spiral arms are definitely emitting blue light, which
is produced by blue stars. Dust lanes appear
as dark lines blocking the
light of these stars. This indicates the presence of a lot more dust than
bulge. Since blue stars don't live very long, this means that there
has been a lot of recent star formation in
the spiral arms. The other
galaxy visible to the lower left of M91 shows these characteristics as
(orange-yellow bulge, blue spiral arms). Since elliptical galaxies
have used up most of their gas and dust,
and thus don't contain many young
blue stars, this other galaxy must also be a spiral.
The above image taken
at near infrared wavelengths shows the distribution of K and M stars in
emit most of their light in the near infrared. Since K and
M stars live a very long time, M91 must be relatively
old and star formation
must have occurred in its early history, not just its recent history.
The above image shows
the Doppler shift of the carbon monoxide (CO) emission in M91, which has
wavelength in the radio part of the electromagnetic spectrum. The CO
emission in this image is produced
when a CO molecule drops down into
a lower energy level because of a change in rotational inertia. Since
molecular clouds emit at this wavelength, the image shows the distribution
of molecular clouds in M91. The
darker spots show where the emission is
redshifted (moving away from us) relative to the center of the galaxy
and the lighter areas show where it is blueshifted (moving toward us)
relative to the center of the galaxy.
Since the top half of the central
area is blueshifted and the bottom half is redshifted, M91 must be rotating
A light profile maps
the light of the galaxy. In other words, it shows how bright the galaxy
appears at various
radii from its center. Using a light profile, the scale
length of a spiral galaxy can be determined. The scale
length is the distance
over which the light emitted by the galaxy decreases by 1/e (e
≈ 2.7), or approximately
For the major (apparently
"taller") axis of M91, the natural log fit was:
y = -0.036x + 7.072
(y = mx + b)
where y is the natural
log of the brightness, m is the negative inverse of the scale length in
pixels, x is the
radius from the center of M91 in pixels, and b is the
natural log of the central surface brightness. The radius
in pixels at first because the image dimensions were measured in pixels.
The radius will be converted to kpc later (1 kpc = 3260 light-years).
The uncertainties on m and b were:
Δm = ±
1.235 × 10^-3 pixels and Δb = ± 0.110
Using the values
in the equation, the scale length for the major axis of M91 was about
28.0 ± 0.97 pixels.
For the minor (apparently
"shorter") axis, the fit was:
y = -0.052x + 7.715
(y = mx + b)
Δm = ±
7.81 × 10^-4 pixels and Δb = ± 0.048
Thus, the scale length
for the minor axis of M91 was about 19.3 ± 0.29 pixels.
To find the linear,
actual size of the scale length of M91, the scale of the image, 1.18 arcseconds/pixel,
the distance to M91 had to be used. Approximating M91's distance as
18 Mpc (1 pc = 3.26 light-years), the distance to the Virgo Cluster, the
scale lengths came about to be:
Major axis = 2.8
± 0.1 kpc
Minor axis = 2.0
± 0.03 kpc
A larger scale length
means that the galaxy has a slower decrease in brightness than a galaxy
smaller scale length. M91's scale lengths are somewhat average.
Using these quantities,
the angle at which the galaxy is inclined relative to the plane perpendicular
to our line
of sight can be calculated. For M91, it came out to be:
Elmegreen, D.M. 1998,
Galaxies and Galactic Structure (New Jersey: Prentice Hall).
Jarrett, T. H. et
2MASS Large Galaxy Atlas", 2003, Astronomical Journal 125, 525
Knight, J D. Messier
Catalog M91 - M100.
M. L. 2003, Astronomy: A Physical Perspective, 2nd ed. (Cambridge: Cambridge
University Press). Sections 14.5.3, 16.5, 17.1.
Yoshiaki et al. "The
Virgo High-Resolution CO Survey: I. CO Atlas." Publications of
Society of Japan 55.1 (2003): 17-58.
for the Exploration and Development of Space: M
research has made use of the NASA/IPAC
Extragalactic Database (NED) which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract with the
National Aeronautics and
is in the constellation Coma Berenices. North is up and East is
to the left.
||The image is 9.1 by 10 arcminutes,
which is about 160,000 by 170,000 light-years at the distance of
M91 (60 million light-years).
|Dates of observation
2005 March 8
a dark, bias, and flat to calibrate the data and remove noise.
Combined all the images in one filter together using the “combine”
function (median combine) in Maxim. Used the B filter image for
the blue component, the V filter image for the green component,
the R filter image for the red component, and the C filter image
for the luminance component. Assigned in following proportion:
Lum = 20%, R = 6, G = 2, B = 8.5. Then used gamma function value
of 0.75 to show both the bright and faint areas of the galaxy
with a minimum pixel value of 15 and maximum pixel value of 600.