Like the Sun, stars and gas in the disk are roughly 74% hydrogen, 24% helium,
and 2% of heavier elements.
Stars in the halo have no more than a tenth of the Sun's level of
heavier elements; the most metal-poor have only 10-4 of the Sun's
abundance of iron, carbon, etc.
But none are completely free of heavier elements; these are NOT the first stars
formed after the Big Bang.
The Milky Way formed from smaller fragments that were drawn together by their gravity. Globular clusters were probably made in these small dense fragments. As their stars aged they "polluted" the gaalxy with gas containing elements heavier than helium, made by nuclear fusion. This gas fell onto the disk and into the bulge, mixing with the gas that made subsequent generations of stars.
The terms "Population I" (disk of the Milky Way: metal-rich, still making young stars) and "Population II" (halo: metal-poor, only old stars) are not good for describing the bulge (metal-rich but old stars).
The center of the Milky Way is not "active" like the galaxy nuclei in Chapter
19. It is very bright only because it contains a lot of stars
-- maybe 1/10 of all the Milky Way's stars lie within 2000 light years of
The Milky Way has a central black hole; applying Kepler's laws to the motion of the stars shows that its mass is 4 x 106 MSun. Comparing the Doppler shift from the stars' spectra to the rate at which they move in angle across the sky tells us the distance to the center: about 25,000 light years.