• Position of sun: Use position of globular clusters (which are distributed symmetrically about center) on the sky to find sun is ~8 kpc from center
• Spiral density wave: Spiral arms are pattern rather than physical structure (hence don't wind up)
• Spiral tracers: Young stars, since they form in arms and die before leaving, make good tracers of spiral structure
• Dark matter: Rotation speed of stars in disk is constant past where visible matter ends; need unseen mass to explain motions

• Halo: Roughly spherical, sparsely populated, older metal-poor stars, globular clusters
• Disk: Gas and dust, star formation so young stars, more metal-rich stars
• Bulge: Center of galaxy, more metal-poor stars, heart is supermassive black hole (measure mass by stellar motions)

In a landmark study of more than 2,000 spiral galaxies from the largest galaxy census conducted by NASA's Hubble Space Telescope, astronomers found that so-called barred spiral galaxies were far less plentiful 7 billion years ago than they are today, in the local universe. The study's results confirm the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end.