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Messier 100 (NGC 4321)

Seung Yong Lee

Ghost of Jupiter


Galaxy is a system of stars, gas, and dust, held together by the force of gravitational attraction. (Burstein et al.) It varies in shape and size, depending on the magnitude of influence of the gravitational field. There are three types of galaxies, each being named by its shape. The spiral galaxies has a S shape, while the elliptical galaxy has no overall structure. The irregular galaxies does not have a particular shape.  The galaxy that you are looking above is called the Messier 100 or NGC 4321. It is a spiral galaxy with 76 kly in diameter, which is about 80 percent the size of our Milky Way galaxy. This galaxy, originally discovered by Pierre Mechain on March 15, 1781, is part of the Virgo Cluster and located in between the Epsilon Virginis and Beta Leonis (Plotner: 2015). This galaxy is 56 million light years away from us. We know this from measuring the pulsation of stars called Cepheids, which are massive and luminous yellow supergiant stars having high intrinsic brightness, whose values can be used as indicators to find the distance (Burstein et Al.).

Interpretation of colors and structures 

Messier 100 or NGC 4321 is remarkable for its majestic spiral arms. It has pink spots which are identified as star forming regions. The blue color results from the fact that hot massive stars that are newly formed emit more blue light than red light. The spiral shape of the galaxy results from the rotating system of dust and gas, which makes part most of the innermost layer, caused by the force of gravity (Burstein et. al). Some places are brigher than others. The reason why this happens is because of the light emitted from the star formation and the concentration of brighter stars. This explains why the center bulge is the most yellow and brighter region. Also, we can notice that there are some other galaxies. These are Dwarf Ellipticals or Lenticular Galaxies in the Virgo Cluster. 

Calculation for the size and the inclination of the galaxy

Using Maxim, a computer software, we can find the diameter by finding the coordinates of the image and then using the Pythagorean theorem to determine the distance. The coordinates for M100 were (y1,x1; 520px, 245px) and (y2,x2; 597px, 479px). So using trigonometry, we get the following result.

= 246.343px.

This value only indicates the angular size of the galaxy not the real physical length. So we need to use the small angle formula to find the linear size of the galaxy. But before that we need to adjust the pixel scale of our telescope. The pixel scale for our CCD, which is the image sensor of the telescope, is 0.59 arcsec per pixel, and twice that for 2 x 2 binning. Calculating all this, we have a result of 290.68 arcsec or 4.83 arcmin. Now, we can use the small angle formula the linear size of the galaxy.

= l/d

is the angular size in radians, l is the physical size, and d is the distance. So

l = d x

l = 4.83 arcmin or 0.001409 radians x 16.568

Mpc = 0.023348 Mpc or 76164.6 light years.

Using this similar process, we can also calculate the inclination of the galaxy with respect to the "plane" (i) of the sky. First we used trigonometry to find the major and minor axis. Then we used the following equation, where "a" is the major axis length and "b" is the minor axis length.

cos(i) = b/a

i = cos-1(b/a)

So, using the values from trigonometry, we can calculate the inclination of the galaxy.

i = cos -1(245.26 px / 246.34px)

i= 0.0901975

This result indicates that M100 have an inclination that is close to 0. This means that the galaxy is almost face-on to our galaxy. We have to consider that the result could have some margin of error depending on the slight variation in the coordinates in Maxim. Because it is manually done, there could be a slight change in value.


Messier 100 is a spiral galaxy with a magnificent arm that is composed of many high-mass stars, dust, and gasses. And it is a shorter galaxy than our own Milkyway galaxy. Nevertheless, it has remarkable feautres that makes one wonder about the beauty of the universe.


Burstein, D., Blumenthal, G., Greeley, R., Smith, B., Voss, H., & Wegner, G. (2002). 21st century astronomy. New York: Norton.

Nasa/IPAC Extragalactic Database. has been used to find basic quantitative information about the galaxy.

Plotner, Tammy. Messier 100. Universe Today, 5 Jan. 2010. Web. 4 May 2015. <>.

Right Ascension (J2000) 12:22:54
Declination (J2000) 15:49:19
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B (5min x 10), V(5min x 4), and R (5min x 4); C (5min x 6)
Image dimension 1092x736 pixels; 23.8x16.1 arcminutes
Date/time observed

March 15, 2015, 00:03 UT

Descriptions We took all the images from a 16 inch telescope at the Calvin Rehoboth Robotic Observatory. All images blue, red, green, and clear has been calibrated using dark, bias, and flat field frames. The calibrated images were all combined and aligned. Then, all the calibrated, combined of each image were combined. The color ratio of the image was 1.2 :12.5 :1.5, and it was given a luminosity weighting of 25 percent. Finally, the saturation was adjusted to 150 percent between the range of 400 and 6000.



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