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The Hidden Galaxy (IC 342)
Will Niedbala

What if I told you that one of the night sky's brightest objects is not even visible to the naked eye? Such is the case with IC 342, The Hidden Galaxy. Though the object is only a mere 11.4 million light years away, and is one of the largest galaxies observable in our sky, it is "hidden" by the galactic disk of the Milky Way. This means that, in order to see IC 342 (which is also known as Caldwell 5), we must look through the bulk of our own galaxy, causing most of the light emitted from it to be cancelled out by light from other stars, or absorbed by dust in the Milky Way.

Though unobservable with the naked eye, this galaxy is quite stunning when viewed through a telescope. It was first viewed by amateur astronomer William Frederick Denning, in 1895, while he was searching for comets in the constellation Camelopardalis. Because of its location in the night sky, and the extinction of its light by the Milky Way's dust, astronomers saw it as being closer to us than it actually is, and primarily classified it as a member of the Local Group, a cluster of galaxies that includes the Milky Way. After further research, however, the galaxy has been reclassified as a member of the Maffei 1 group, the closest cluster of galaxies outside our own.

The extinction of light by dust from the Milky Way, as mentioned in the first paragraph, continues to play a role in IC 342's research, as new estimates as to its actual surface brightness, distance, and size come out fairly frequently. Though we may never know for certain, current data from the National Optical Astronomy Observatory suggests a distance of about 3.5 megaparsecs, or 11.4 million light years.

We ourselves can easily calculate the physical size of the galaxy given this distance and the image at the top of this web page. Each pixel in that image corresponds to a tiny fraction of the sky. These are what we call arcseconds, and every pixel in that image is equivalent to roughly 1.32 of them. If we take a measurement of how many pixels the galaxy in the image is composed of, and multiply that number by 1.32, we can calculate the exact size of the galaxy in the night sky, what we call angular size. This angular size can be multiplied by the distance to get an actual, physical major axis size of 57,633 light years, and a minor axis size of 41,507 light years. Given that the Milky Way is 100,000 light years across, we see that IC 342 is roughly half the size of our galaxy. A current measurement for the size of the galaxy is 70,000 lightyears, giving a decent range of values, and showing how much uncertainty is apparent in this calculation.

Even if the galaxy is hard to study with the naked eye, pictures of it, such as the one above, can give a wealth of information about the galaxy. The image above is actually a composition of multiple images taken over the course of a night, which allows more light to be captured, ensuring that we can see the object clearly. As seen in that image, and the one directly below, this galaxy can be classified as a "barred spiral." This classification comes from the fact that the galaxy is composed of several "arms" that can be seen spiralling out from the center, or core. Looking closely at this core, we see that it is slightly elongated, looking more like a rectangle, or bar, than a circle (hence the term barred spiral).

Also in these images are several distinct regions of color. These colors are not only cool to look at, but also give information as to what is happening inside the galaxy. The core and inner regions of Caldwell 5 appear yellow or orange, indicating that the stars located there are older and cooler than the rest. The blue regions, found primarily in the spiral arms of the galaxy, indicate that these are areas of active star formation. This is because blue stars are younger and hotter than the rest, having only recently began their lives. There are many hundreds of bright stars peppering the viewing field, but almost none of them are actually in IC 342. These are stars that form the galactic disk of our own Milky Way.

Figure 1. IC 342 as taken by the Sedona Stargazer Observatory camera

The distinct regions visible in Caldwell 5, along with the fact that we can see it head on, make it a very promising galaxy to study star formation. However, even though it looks as though it's facing us directly, this galaxy is actually tilted slightly away from us. We can calculate the amount that the galaxy is skewed, or the inclination angle, by simply taking the cosine of the ratio of the axes. This calculation gives us an angle of 43.93 degrees; showing that even though the galaxy appears to be face-on, looks can be very decieving.


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"Hiding out behind the Milky Way." WISE - Multimedia Gallery. 7 Apr. 2010. <>.

Wikipedia, "IC 342".

Wikipedia, "Milky Way".

Right Ascension (J2000) 03h 46m 49s
Declination (J2000) 68d 05m 46s
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B(300s x 10), V(300s x 8), and R(300s x 5); C(60s x 7)
Image dimension 1092x736 pixels; 23.8x16.1 arcminutes
Date/time observed March 16, 2017, 3:33:10 UST

In order to create the color image at the top of the page, I took the images described in the table, above. I used several types of calibration to cancel out excess light from the image. First was the bias calibration, which eliminated noise inherent in the camera. Next was a dark frame calibration, which eliminated thermal noise from the heat of the camera. The last calibration was a flat field calibration, which divided out noise caused by dust or sensitivity issues.I then aligned the calibrated images, and combined the images from each filter, so I had one image each for blue, clear, red and green. In the combination process, I used the combination method, "median." At this point I had a single image composed of all four filters. I heavily weighted the blue filter, because it was the filter with the least amount of light. This weighting resulted in the noise being very prominent in the bottom right of the image. This noise is speculated to be from a light source close to the camera, which is why it wasn't caught by the calibration.



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