@Spectrum, Looking back at a few of your resent posts, I think you may be confusing the use/meaning of Amin as a KEY or a CHORD. (my apologies if thats not the case)
in the KEY of Amin the notes are ABCDEFG (no sharps or flats in this key)
these are the same as the notes in the key of C (just staring on C) CDEFGABC
...so yes there is no F#. And an F# playing in the key of A minor /C major would have "tension"
(as experienced when you try the F# during the intro of the song).
But there are
different types of A minor chords in DIFFERENT KEYS.
Amin in key of C:
AB
CD
EFGA (Rt 2 b3 4 5 b6 b7 8) = A Aeolian scale (aka Natural minor)
This A Minor chord is built on the 6th degree of C major key.
Amin in key of G:
AB
CD
EF#GA (Rt 2 b3 4 5 6 b7 8) = A Dorian scale
This A Minor chord is built on the 2nd degree of G major key.
Amin in key of F:
ABb
CD
EFGA (Rt b2 b3 4 5 b6 b7 8) = A Phrygian scale
This A Minor chord is built on the 3rd degree of F major key.
Keep in mind, in modern music (post 1700s? early baroque era, any music history buffs out there?)
the use of a KEY in a song is not global. Meaning a song can move from key to key within a song or even borrow chords/scale voicings from different keys during a section, part, or even a single beat of a song. So the use of a chord in one section of a song doesnt mean the whole song is in that key.
That being said:
- Milky Way is in the KEY of Amin,
- once the bass enters the F# is a "borrowed" chord from the key of G (which gives us the D9/F# harmony),
- and the bridge is in the key of C using the chords G and F:
G GABCDEFG (from G mixolydian scale)
F FGABCDEF (from F lydian scale)
All of that is just the "math", analysis, and the "how" it works. But the reason "why" it works is the tension in the intervals: Its ART... shades of color layered together forming a pleasing picture.
as someone very early in the thread said, "because it sounds good"
hope that helps!