Spiral Galaxy란 무엇인가?
A spiral galaxy is a flattened, rotating disc of hundreds of billions of stars, threaded with gas and dust and marked by bright arms winding out from a central bulge. Roughly two thirds of large galaxies are this shape, and the Milky Way is one of them.
The arms should not exist, and working out why they do took decades. A galactic disc does not rotate rigidly: the inner regions complete an orbit far faster than the outer ones. Any pattern made of a fixed set of stars would therefore be wound into a tight, unrecognisable coil within a couple of rotations — and galaxies have completed dozens. This is the winding problem, and its resolution, worked out by Bertil Lindblad and developed by C.C. Lin and Frank Shu, is that the arms are not made of particular stars at all. They are a density wave: a pattern of where orbits crowd together, which stays put while stars stream through it, the way a traffic jam stays in one place on a motorway while individual cars pass through it.
Passing through matters. Gas entering an arm is compressed, and compression is what tips a molecular cloud into collapse — so star formation happens preferentially in the arms. Massive blue stars are born there and die there, within a few million years, never travelling far from where they formed. That is the entire reason arms are visible: not because they contain more stars, but because they contain the brightest ones. Take the blue light away and the arms nearly vanish.
This model is built on the orbits rather than on the spiral. Every one of tens of thousands of points is on its own ellipse, evaluated afresh each frame, and the orientation of those ellipses advances slowly with radius. Where the ellipses crowd, arms appear — and they stay put while the stars move through them, because that is what a density wave is. Stop the clock and the arms are still there. The angular speed also falls as roughly 1/r, the flat rotation curve that is the single most important piece of evidence for dark matter.