This is a load of stuff copied almost verbatim out of the encyclopaedia ----a (Sorry about the copyright)
"The surface brightness of ellipticals [This includes spirals] at optical wavelengths decreases monotonically outwards from a maximum value at the centre, following a common mathematical law of the form:
I = I0 * (r/a + 1)^-2
I=intensity I0=Central intensity r=radius a=scale factor"
"From studies of other galaxies it can be shown that spiral arms generally follow a logarithmic spiral form such that:
log r = a - b*theta
where theta is a position angle measured from the centre to the outermost part of the arm. [I have no clear idea what this means -Ben] The range in pitch angles for galaxies is from about 50 deg to approx. 85 deg... [blah blah blah] pitch is constant for any given galaxy... [blah blah] For Milky way theta is approx. 75 degrees"
And most entertainingly:
"There are five optically identified spiral arms in the part of the Milky Way wherein the solar system is located, see table 5b."
And here's table 5b!
|Number||Name||Galactic longitudes (in deg)||Distance|
|+I||Perseus||103-190||10,000 ly at 120 deg|
|0||Orion||60-250||1,500 ly at 180 deg|
|-I||Sagittarius||275-30||5,000 ly at 330 deg|
|-II||Norma||300-330||10,000 ly at 330 deg|
|-III||3kpc Arm||330-30||20,000 ly at 0 deg|
I'm not sure what all of that means.
Also (yes, there's more); The MW is 72 000 ly in diameter, the sun is c. 30 000 ly from the bright centre of the universe. The MW weighs approx 1 000 bn Solar masses, with a black hole weighing about 4 000 000 Solar masses at the centre. The central bulge is approx. spherical, and the thickness of the disk is approx 0.2 of the diameter.
Hope this helps me, or someone. If anybody can tell me quite how that description of the spirals works, I will be grateful.
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