A spiral galaxy's arms are traffic jams that stars drive through
If the arms of a spiral galaxy were fixed chains of stars, the galaxy's uneven rotation would wind them into a tight coil within a few turns. They survive because an arm is really a slow-moving wave of crowding. Stars and gas pass through it, and the squeeze lights up newborn blue stars.
Edwin Hubble described the type in his 1936 book The Realm of the Nebulae. A typical spiral has a flat, spinning disc of stars, gas and dust, roughly five times wider than it is thick. At the centre sits a bulge of mostly older stars, usually around a supermassive black hole such as our own Sagittarius A*. Surrounding it all is a faint, dust-free halo of ancient stars and globular clusters. Some halo stars were stolen from small galaxies being swallowed; the Milky Way is absorbing the Sagittarius Dwarf right now. A few, like Kapteyn's Star, happen to pass near the Sun and stand out for their unusually fast motion across our sky.
About two-thirds of spirals, including the Milky Way, have a bar of stars across the middle, with arms trailing from its ends. That share seems to have grown over cosmic time, from around 10 percent eight billion years ago to a quarter 2.5 billion years ago. Together with irregular galaxies, spirals make up roughly 60 percent of galaxies today, mostly in quiet regions away from crowded cluster cores.
Bertil Lindblad spotted the winding problem in 1925: inner stars circle faster than outer ones, so a straight arm would soon curl up. In 1964 C. C. Lin and Frank Shu proposed that stars follow slightly oval orbits whose orientations shift gradually with distance from the centre, bunching up into spiral patterns. Gas entering these dense zones collapses into stars. The massive blue ones burn out quickly, leaving a darker trail behind each wave and making the arms stand out. A rival idea has supernova shock waves seeding fresh stars, with rotation smearing them into spirals, and both processes may operate.
Spirals formed surprisingly early. BX442, a neat two-armed grand design spiral, is about eleven billion years old, and BRI 1335-0417's light took 12.4 billion years to reach us.
Source: Spiral galaxy
