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Fun question for people who like to calculate things...


tenbyfifty
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How close do you reckon a face on spiral galaxy the size of our own milky way would have to be in order to fill the square of Pegasus? Do you think a galaxy could be this close without interacting with our own?

I say spiral galaxy because at that size it would look amazing.

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As far as im aware gravity acts over ALL distances but gets weaker the further away an object is. So technically all galaxies are interacting with our own at all times. If memory serves correctly we are heading towards M31 at a fair old clip and are in for a right old time in a few billion years

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Hey Guys,

Might be wrong but mybe I am right. Gotta be a first time.

I don't know how big the square of pegasus is and i dont know how big the milky way/our galaxy is but my OU course has taught me that

angular size = 57 x (actual size / distance)

so if some one puts in the appropriate figures

size of square of pegasus in degrees = 57 x (size of our galaxy / distance)

the 57 is to do with radians or something. maybe someone can come up with the distance in this one now.

Anthony

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According to Starry Night Pro the square of Peg is 20 degrees from corner to corner. This is a nice number

so I think we should use it. The disk of the Milky Way galaxy is about 100,000 light years in diameter, another nice number.

Therefore our distance according to anthony's formula

should be (57 x 100,000)/20 which is (drum roll) 285,000 light years. I think that's about a tenth of

the distance to M31, n'est pas?

This means that at some point in the future some lucky buggers are going to get an amazing view of M31 even it

ain't face on. Maybe it will be by then? Do galaxies roll as well as rotate?

How much do cryogenics cost these days? :laugh:

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See my answer in "Sir Patrick's and didn't understand it..." thread.

Any chance you could tell us how get that number Astroman? Anthony has a formula but it seems to give

a substantially larger figure asssuming the square is 20 degrees across and the milky way is 100,000 lys across.

No idea which is correct :?

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Sure. I used the width of Pegasus-15º30'. The formula for angular distance is a=s/d, where a=Angular size in radians, s=size of object in ly and d=distance. a=15º30' in radians is pi/8=.392857. Size of MW is ~110,000ly. Solve for distance = .392857X110,000=43,213 ly.

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Ah, you are correct, tenbyfifty. An easy formula for determining radians is as follows; convert degrees , minutes and seconds of arc to decimal. (Take minutes, multiply by 60 for. Take the decimals of that number and multiply by 60 and add to the previous decimals.) Multiply by pi and divide by 180. This turns out to be ~0.2705 for our illustration. Multiply that by 110,000 and you get 29,757 ly. Eek! Regardless, it'd be too close to retain anything like a face on spiral shape.

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Ok. My approach was to take the tangent of half the angle of the square of Pegasus. The tangent of 7.5 degrees is about .13 . This is the product of the height divided by the base, so with a height of 50,000 ly, the base must be roughly 380,000 ly.

A little mental scale modelling will show you that a galaxy with a diameter of 100,000 light years will span much more than 15 degrees at a distance of 43,000 or 29,000 ly. Still, at 380,000 ly I imagine the tidal forces would cause the two galaxies to be a little rude to each other.

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This thread has reminded me of more fundamental question about M31 headed this way which is, without further ado,

why is it headed this way?

I thought everything in the universe was moving outward and away from everything else, and how do you measure the direction

of a galaxy's movement over the short period that humans have been measuring such things? The changes in wavelengths must be tiny if that's what they're measuring.

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