26 January 2016
THE GRAND SPIRAL
I'm still waiting for Astro-Physics to come up with a fix for the "Y2K+16" bug that appeared in their mount control software after New Year's. I was ready to do some imaging of the Horsehead Nebula in Orion on the night of January 2. Unfortunately, when I instructed the telescope mount to slew to the target, it took off in the opposite direction. Luckily I was paying attention and stopped it before the telescope crashed into the pier. But for now, SOCO is out of action until we can download the revised mount control software. I've missed the entire imaging window for January— hopefully they'll come up with the fix before the February imaging window.
With no new imagery being acquired, I decided to go back and re-process some imagery acquired back in 2014. Some of the imagery from that period had problems associated with the focuser (this was before I got the Optec TCF-S3i focuser), but there were some image sets with good enough quality to work on. This time, instead of taking hours to manually process the imagery, I could use SuperSIAM to automatically process it. Much easier!
One of the image sets I chose to work on was acquired in late September and early October of 2014. Over three nights, I acquired images to make a mosaic containing M 31, the famous Andromeda Galaxy in the constellation Andromeda. Using the Optec NextGEN Ultra Widefield 0.7X Telecompressor with my imaging camera gave me a horizontal field-of-view (FOV) of approximately 1.6 degrees. This could take in the central portion of the galaxy, but not its edges. So, I acquired one set of images centered on the galaxy and then four additional sets of imagery, each of the four overlapping one corner of the central set of images. After processing, the resulting five color composite images were mosaiced in Photoshop to produce a single large image covering the entire galaxy.
This mosaic is presented in Figure 1. M 31 is undoubtedly the grandest, most spectacular galaxy we can observe. In imagery like mine, it just seems to hang there in space, a real physical object in three dimensions. M 31 is larger than our Milky Way Galaxy, although measurements indicate that it is less massive. Located approximately 2.6 million LY away, M 31 is a close neighbor to the Milky Way. M 31 and our Milky Way Galaxy are approaching each other at a rate of approximately 110 km/sec. This means that, in around 4 billion years, the two galaxies will collide. Astrophysicists believe that the result of this collision will be the formation of a huge elliptical galaxy, tentatively named "Milkomeda". The fate of our Sun and Solar System as a result of this collision is unknown— we may be incorporated into the new elliptical galaxy, or we may be ejected from it during its formation to become a lonely wanderer in intergalactic space.
Figure 1. M 31, the Andromeda Galaxy.
While M 31 is classified as a spiral galaxy, its spiral structure is not as readily apparent as in other spiral galaxies, like M 33. The dark dust bands within the galaxy divide M 31 into several concentric rings. It is believed that this structure is the result of the disruptive effects of interactions with neighboring satellite galaxies, in particular, the two small elliptical galaxies (M 32 and M 110) visible in my image. Also, if you look carefully at M 31 in my image, you will note that it is not perfectly flat along its galactic plane but rather exhibits a somewhat bent "S" shape as you get away from the bright galactic central region. This may again be the effect of interactions with satellite galaxies, but possibly also the result of gravitational interactions with M 33, which lies around 800,000 LY away from M 31.
Figure 2 shows what astrophysicists call the Local Group of galaxies. The three largest members of this group are our Milky Way Galaxy, M 31 (Andromeda Galaxy), and M 33 (Triangulum Galaxy). In addition to these three large spiral galaxies, the Local Group contains over 50 smaller galaxies, most of which are small elliptical or irregular galaxies. M 31 has around 20 small companion galaxies that are gravitationally bound to it. The best-known are M 32 and M 110, which can be seen in Figure 1. M 32 is the more compact, brighter of the two located visually below the plane of M 31. M 110 is the more extended elliptical galaxy riding visually above the plane of M 31. It is thought that interactions between M 31 and these two companion galaxies (particularly M 32) have produced the disruption of the spiral arms of M 31.
Figure 2. The Local Group of galaxies, containing our Milky Way Galaxy and the Andromeda Galaxy.
Figure 3 shows my image of another pair of small companion galaxies to M 31, NGC 147 and NGC 185. This image is also a mosaic of two images acquired on the 28th and 29th of September, 2014. NGC 147 and NGC 185 are located in the sky around 6 degrees north of M 31 in the neighboring constellation Cassiopeia, and are separated by around 1 degree. As shown in Figure 2, NGC 147 and NGC 185 are physically located between M 31 and our Milky Way Galaxy. NGC 185 is the brighter of the two (visual magnitude 10.1) and lies at a distance of around 2.05 million LY from us. NGC 147 has a visual magnitude of 10.5 and lies at a distance of around 2.53 million LY. A recent study suggest that these two small galaxies form a stable binary system with an orbit around M 31 that does not result in their plunging into the larger spiral galaxy in the forseeable future.
Figure 3. NGC 185 (left) and NGC 147 (right), small companion galaxies of the Andromeda Galaxy.
The Andromeda Galaxy is one of the most frequently imaged astronomical objects. A Google search on the Internet will turn up numerous images by professional and amatuer astronomers. There is often a wide variation in the color representation of the galaxy in the images— some versions tend toward the blue-ish, while others tend toward the red-ish depending on the particular color balance used in their production. So, what is the "correct" color for the Andromeda Galaxy? As I have stated numerous times on this website, my image processing procedures are designed to produce "true-color" representations of astronomical objects. That is, the objects exhibit the colors that you would really see if your eyes were sensitive enough to adequately respond to the weak light coming from them. Not everyone has this goal— some astro-imagers prefer a more artistic representation that is subjectively pleasing to them— and that's perfectly fine. Coming from a physical science background, I'm inclined toward a more realistic, objective representation.
So, having created the "true-color" representation of M 31 shown in Figure 1, how do I know that it really has the "correct" color? There are a number of ways. For example, I can go to an online database such as Simbad and determine what the spectral classes (and spectral fluxes in various wave bands) of various stars are in an image. The different spectral classes have characteristic colors, which I can compare to the colors of the corresponding stars in my image. There are even sources (like Mitchell Charity's "What color are the stars?" webpage) that give the average Red-Green-Blue (RGB) triad values for the various spectral classes of stars. If the star colors are "off" in my image, then I can probably assume that the color of the object (galaxy, nebula) is also "off".
Along these lines, Mitchell Charity presents RGB triad values for several astronomical objects based on spectral studies by various researchers. For galaxies, these values represent the integration of the light from all components of the galaxy (stars, HII regions, dust clouds, etc.). For example, the RGB triad for a "typical" spiral galaxy would be (R,G,B) = (255,225,199). To give you an idea of what this color looks like, I've made the background color of this web page this color. Using Photoshop, I can determine the average RGB triad values for the interior portion of M 31 in Figure 1— they turn out to be (R,G,B) = (255,224,207), after normalizing the brightest to 255. This is pretty close to Charity's "typical" value. Based on this analysis, I can conclude that the "creamy brown" color of M 31 in my image is probably reasonably close to what it should be.
Another way to check the color of imaged objects is to compare them to published images in sources where extensive color calibration has been performed. For larger objects (like M 31), a good source is the Cambridge Photographic Star Atlas (Axel Mellinger and Ronald Stoyan, Cambridge University Press, 2011). The images of the night sky used to produce this publication were subjected to detailed, sophisticated calibration to bring out the true colors of the stars and other objects in them. M 31 appears in several of the plates in this publication, and in them it exhibits a color similar to that in Figure 1. This is also true for other objects that I've imaged— the large emission nebula NGC 1499 (California Nebula) appearing in Plate 18 of the Photographic Star Atlas exhibits a deep red color similar to that in my image of this object. An interactive online version of the all-sky imagery contained in the Photographic Star Atlas can be found on Axel Mellinger's website.
One thing that my image of M 31 doesn't adequately show is the halo of blue-white stars found along the outer edge of the galaxy. This feature is hinted at by the blue-ish collection of stars near the upper right edge of the galaxy in Figure 1. The brightest knot, which has the designation NGC 206, is a huge collection of bright young supergiant stars. Unfortunately, the dimness of the individual stars in this halo (due to their great distance) results in most of their blue light being attenuated by interstellar and atmospheric scattering and absorption.
With a visual magnitude of 3.4, M 31 can easily be seen with the naked eye under clear sky conditions even in the presence of some light pollution, as in the suburbs of many towns. Under dark sky conditions, it is readily visible as a small glowing patch of light. To the ancients, who observed under pristine sky conditions with no light pollution, the Andromeda Galaxy would have been a fixture of the night sky during the months it was above the horizon. Thus, it is pointless to ask who "discovered" the Andromeda Galaxy. What we may ask is, "Who was the first to point out the existence of the Andromeda Galaxy?".
This honor is usually ascribed to the Persian scholar Abdal-Rahman Al Sufi. Around the year 964 AD, Al Sufi wrote of a "small cloud" in the constellation Andromeda. Some speculate that other Persian astronomers may have noted its existence as early as 905 AD. This was centuries before it was first observed through a telescope by the German Simon Marius (1612), and included as number 31 in his list of "nebulous objects" by Charles Messier (1764). But, can we find an accounting of this object that significantly pre-dates these?
It seems that the answer is "yes", and this occurrence takes us back close to the start of civilization itself. Two cuneiform tablets residing in the British Museum, upon translation in the late 19th Century, revealed themselves to be essentially guidebooks to the stars and constellations (the forerunners to our modern astronomical guiebooks). These tablets are collectively called the MUL.APIN. The first tablet contains a list of constellations, while the second tablet contains methods for calculating astronomical phenomena. The MUL.APIN is thought to have originated in Babylon in the Second Millenium BCE. The Dutch Assyriologist and astronomer Teije de Jong suggests a more definite date for the creation of the MUL.APIN (1300 ± 150 BCE) based on a study of dates in the rising star lists contained in the second tablet.
As described by John H. Rogers (1998, "Origins of the ancient constellations: I. The Mesopotamian traditions", Journal of the British Atronomical Association, Vol. 108, No. 1, p. 9-28), the first tablet of the MUL.APIN provides three lists containing a total of 71 constellations ("stars") divided up according to their location in the sky (northern sky, equatorial sky, and southern sky). In the northern sky list, there are two entries associated with parts of our modern-day constellation of Andromeda. The first is named MUL.LU-LIM and is identified as the constellation "the Stag". It is described as:
The star that stands beside it [the constellation of the She-Goat, i.e., our present Lyra]: the Stag, the messenger of the Stars.
The stars of MUL.LU-LIM are thought to be the stars of the eastern part of Andromeda and possibly Cassiopeia. The description of MUL.LU-LIM also contains the following:
The dusky stars which stand by the breast of the Stag.
This entry is thought to represent the current stars 18, 31 and 32 Andromedae. It is thought not to be a description of a separate constellation but rather a descriptor that is a prelude to identifying the next constellation in the list, the Rainbow (MUL.DINGER-TIR-AN-NA). Since the stars 18, 31 and 32 Andromedae are in the vicinity of the Andromeda Galaxy, it has been proposed that "the Rainbow" is actually the Andromeda Galaxy. If this is correct, then a reference to the Andromeda Galaxy goes all the way back to Bronze Age Mesopotomia.
I had hoped to acquire a new set of images of M 31 last December. These images would have included longer exposures in the Blue spectral band to bring out the halo of blue-white stars around the edge of the galaxy that I didn't get with the 2014 imagery. Unfortunately, persistent clouds around the end of last year prevented me from getting the new imagery. So, that activity will be put off until the end of the coming summer. The sky will always give you another chance to get something you missed— assuming you're still here the next time it comes around!
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