M5 Galactic Dynamics and Spiral Structure

Topic

The central focus of this model is the galaxy viewed as a rotating system.

The speed at which stars and gas orbit the galactic center remains virtually constant even at great distances from that center—rather than decreasing as it would if only visible mass (stars and gas) were driving the rotation. This discrepancy between observation and expectation reveals the presence of additional, non-luminous mass; its gravitational pull is what actually sustains the rotational speeds observed in the galaxy's outermost regions.

However, the spiral arms visible in the galaxy's disk are not a fixed set of stars rotating in unison; instead, they are the visible trace of a density wave propagating through the disk at a different rate than the rotation of the material itself. Individual stars move in and out of the arm over time rather than remaining trapped within it forever.

By compressing gas as it passes, this density wave regulates—arm by arm—where and at what rate new stars form within the galactic disk. Thus, from the rotating galaxy with which this text begins to the regulation of star formation with which it concludes—taking in the non-luminous mass sustaining outer rotation and the spiral arms as density waves—galactic dynamics reveal that what appears at first glance to be a fixed structure (a galaxy rotating as a solid block) is actually a constantly shifting pattern that even the stars tracing it cannot follow indefinitely.

ID:4930