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This post was edited by MartinTimothy at 2011-12-19 04:50|
Crop the image to 1359 x 1037 pixels, centered on the brightest part of the galaxy, extending to the cluster of emission nebulae, the red bits in the disc, lower right and to similar emission nebulae upper left.
We want to find out more…
Pythagoras told us, the square on the hypotenuse, is the sum of the square of the other two sides, we want to know the measurement of the hypotenuse, which is the straight line opposite the right angle, here represented by the square corners, provided courtesy of Microsoft Picture Manager.
1359 squared is 1846881, 1037 squared is 1075369, add the upper and lower and get 2922250, find the square root is 1709, thus the Galaxy subtends an angle of 1709 pixels, rounded out to 1700.
We want to know, because there are two more distant spiral galaxies, located below right of center in the same shot, fortunately both are nearly upright, which makes it easier to guage their width, the lower and smaller of the two, fits quite neatly in a square box 14 X 14 pixels, the larger is little longer than fourteen pixels, call it 16, then for the sake of simplicity we add another pixel to get 17.
Dividing that by the angle of 1700, subtended by the much closer NGC 253, and find that the more distant galaxy is around one hundred times smaller, we are gonna say it is similarly one hundred times more distant.
| So far we are getting a handle on the scale of the universe, exactly the same principle applies locally. |
Space based telescopes, give one hundred and thirty degrees, as the longitudinal dimension of the Milky Way, Sky Catalogue 2000.0 tells us that, NGC 253 is 25.1 arc minutes, or .4163 of one degree long, we divide that by the one thirty degrees of the Milky Way, to get 310.78, which means NGC 253 is that many times further away, than the Milky Way galaxy.
Radio data from Sagittarius A*, recognized as the heart of the MW galaxy, has returned a distance of 24250 light years, NGC 253 is 310.78 times further away, multiply the distance to A* by 310.78, and get 7,536,415 LY, as the distance to NGC 253!
We got the figures, by finding that NGC 253 is 1700 pixels across, the larger of the two more distant galaxies is one hundred times smaller at seventeen pixels, and one hundred times further away at 753,641,500 light years, multiply by 365.24 x 24 x60 x60 x 186,000 for miles!
The smaller of the two, is somewhere around fourteen pixels across, however since it was less than that we do some more calculus, as each pixel is taken away, so does the distance increase, should it become eight and one half pixels, the distance would be 1578.83 million light years.
Since it was not that small, we will say half way between seventeen and eight and a half, so add four and a quarter pixels, for an image half as wide again, of twelve and three quarter pixels, a fair total... then for a place, that is similarly halfway between the upper and lower limits, go to a distance half way between, at 1130.46 million light years.
|How can you say that all spiral galaxies are the same size??|
Reply: The MW galaxy, has the same overall profile as many similar disc galaxies, including NGC 253, ie an active center, and emission nebulae spaced similar distances apart, less active and smaller galaxies, have less active profiles, while spherical galaxies fall into a different category altogether, neither classes of object should be mistaken for anything else.
In the case of the two smaller more distant galaxies, in the above example, each pixel of image width reduction adds around 44.3, and 91 million light years in linear distances respectively, indeed there are other edge on spirals visible on the same plate, including one at upper left center, which does appear to have an image width of around eight and one half pixels.
In this model, it is 1,578.83 million light years distant, where each pixel in image width reduction adds 188.2 million ly, which is why we assume galaxies are at a given mean, using the best estimates of the MW, which say it is one hundred thousand light years across, as a yard stick.
We would be speaking in terms of thousandths of a pixel in image width, were we to locate the true size of distant galaxies!!
There is a critter called a Dwarf Spiral, that can trip you up, however all calculations should be accompanied by redshift data, which will expose any of those, that are trying to sneak onto the main list.
Another is Malin 1, named after its discoverer David Malin, who revolutionized astronomical photography in the 1970's and 80's, which does not fit into this scheme, however Malin 1 objects are very rare and seldom encountered, while redshift data would instantly expose them for what they are.
This pic shows the relationships in the Milky Way family
The MW is surrounded by star clouds, which formed in emission nebulae at the tidal boundary of the galaxy, the gravity source can not keep the disc stable outwardly for ever, and bits will spin off and go their own way at that place, this is the physical force that makes all similar disc galaxies the same size, give or take.