Stellar populations

Population I disk population Population II

extreme

Population I older

Population I intermediate

Population II halo

Population II

members gas A-type stars stars of galactic nucleus high-velocity stars with z-velocities \\>30 km/sec subdwarfs

young stars associated with the present spiral structure strong-line stars planetary nebulae (planetary nebula) long-period variables with periods \\<250 days and spectral types earlier than M5e globular clusters (globular cluster)

supergiants Me dwarfs novae (nova) RR Lyrae stars (RR Lyrae star) with periods

\\>0.4 days

Cepheids (Cepheid variable) RR Lyrae stars with periods

\\<0.4 days

T Tauri stars (T Tauri star) weak-line stars

galactic clusters of Trumpler's class I

average height over galactic plane (parsecs) 120 160 400 700 2,000

average velocity perpendicular to galactic plane z(km/sec) 8 10 17 25 75

axial ratio of spheroidal distribution 100 ? 25? 5 2

concentration toward centre little little strong? strong strong

distribution extremely patchy; spiral arms patchy; spiral arms smooth? smooth smooth

age (10⁹ years) 0.1 0.1–1.5 1.5–5.0 5.0–6.0 6

total mass

(10⁹ suns) 2 5 47 (combined disk and intermediate Population II) 16