# How do you find the distance to the galaxies having equal diameter?

by iVenky
Tags: diameter, distance, equal, galaxies
 P: 103 I am reading Richard Feynman's book Vol I. There he tells us how people usually used to find out the distance to some distant objects. I understand triangulation for objects that are near to the earth or for stars that are near to our solar system. But how did they use the same principle to find out the distance to those galaxies(which have the same diameter of our galaxy) that are far away from the earth ? Here's the excerpt- "Knowing the size of our own galaxy, we have a key to the measurement of still larger distances- the distances to others galaxies. Figure 5-7 is a photograph of a galaxy, which has much the same shape as our own. Probably it is the same size, too. (Other evidence supports the idea that galaxies are all about the same size.) If it is the same size as ours, we can tell its distance. We measure the angle it subtends in the sky; we know its diameter and we compute its distance -triangulation again" Can someone help me?
 Mentor P: 7,318
P: 708
How do you find the distance to the galaxies having equal diameter?

 Quote by iVenky But how did they use the same principle to find out the distance to those galaxies(which have the same diameter of our galaxy) that are far away from the earth ?
Take a pen and a piece of paper. Draw an elongated equilateral triangle with vertex A being the observer, and vertices B and C marking the edges of the observed galaxy.
Simple measurements allow us to name the angle at vertex A - it is the angular size of an object on the celestial sphere. For example, for the Andromeda galaxy it's over 3 degrees.
By assuming that the observed galaxy is of the same size, we are given the length of the distance BC - the same as the size of Milky Way, so ~100 000 ly.

This gives us enough information to calculate the distance using simple trigonometric relations. In the case of our Andromeda example, this nets a result of ~2 million ly, which is not a bad approximation for the ~2,5 million ly of actual distance.
 P: 30 Wait a second here. Assuming the size of a galaxy is a big assumption. This method could give extremely inaccurate results. All galaxies the same size? I think not my friend. Size of galaxies will most probably vary as much as the sizes of the stars. Some can be many magnitudes larger then our own. So in all probability galaxies will follow suit. If this is so then judging distant in this methods would be grossly inaccurate. No?
Mentor
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 Quote by solar71 Wait a second here. Assuming the size of a galaxy is a big assumption.
We don't assume a galaxy is a certain size without good reason.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,517 A lot of hand waving, especially for someone like Feynman.
 P: 30 "Probably it is the same size, too. (Other evidence supports the idea that galaxies are all about the same size.)" This is what I was talking about "PROBABLY" it is the same size." that doesn't sound very scientific to me. All about the same size... I don't think so.
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 Quote by solar71 "Probably it is the same size, too. (Other evidence supports the idea that galaxies are all about the same size.)" This is what I was talking about "PROBABLY" it is the same size." that doesn't sound very scientific to me. All about the same size... I don't think so.
You're missing the point.
Of course galaxies are not all exactly of the same size. The exercise with calculating distances with the equal size assumption shows that it is possible to get ballpark results by using simple assumptions. It doesn't matter if a galaxy is in actuality half the size or five times the size of ours, you'll get the distance that is accurate to within one order of magnitude, where otherwise you'd have nothing.

You can then try to check or narrow the result by trying to measure the actual size. You can do it e.g., by using standard candles to find out the distance, at which point calculating the size is trivial.
You'll find out then, that the initial assumption of equal sizes was a very good one, especially when comparing galaxies of similar visible macrostructure, that is, spiral galaxies with spiral galaxies etc. - just as it was explained in the quote from Feynman in the OP.

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