Often called Population I stars, the relatively younger stars of the galactic disk can be further subdivided into four distinctive groups: very young spiral arm; young thin disk; intermediate-age disk; and older thick disk and nucleus. As mentioned previously, the spiral arms include most of the galaxy's interstellar gas and dust, young stars, and stellar associations, including: O and B stars; supergiants; Cepheid variables; pre-main sequence, T-Tauri stars and Herbig-Haro objects (e.g., R Coronae Australis/CD-37 13027); and some A stars. Less than a hundred million years old, these stars are rich in metals (as rich as, but ranging up to twice, Sol's abundance) and have highly circular galactic orbits within 1,000 ly of the galactic plane. While often extremely bright when not obscured by dust, these stars probably total substantially less than one percent of all Milky Way or nearby stars.
Young thin disk stars lie within 1,500 ly of the galactic plane and have galactic orbits of low eccentricity. Around one billion years or more in age, they include many A and F stars, AFGK giants, some GKM main-sequence dwarfs, and white dwarfs. While they have a mean metals abundance near Sol's (1.0), some may be twice as rich. Totalling as much as nine percent of all stars in the solar neighborhood, they include Sirius2 (A0-1Vm and A2-5VII -- also DA2-5) and Vega (A0Va).
Intermediate-age disk stars include our Sun (G2V), most G and some K and M dwarfs, some subgiants and red giants, and planetary nebulae. Many are around five billion years old and have a metals content ranging from 0.5 to 1.0 of Sol's (with a mean around 0.8). These stars lie within 3,000 ly of the galactic plane, with moderately eccentric galactic orbits. For example, Sol is traveling at seven kilometers per second northward out of the plane and may eventually rise 200-250 ly above it after 15 million years, while the Alpha Centauri3 (G2V, K1V, and M5.5Ve flare star) system may eventually travel about 800 ly out with an upward velocity that is three times faster. As much as 84 percent of the stars in the solar neighborhood are included in this group.
Most thick disk and many nucleus stars are old, but probably less than 10 billion years old. They include many K and M dwarfs, white dwarfs, some subgiants and red giants, moderately metals-poor stars, long-period variables, and RR Lyrae variables with periods less than 12 hours. Most thick disk stars lie within 5,000 ly of the galactic plane (thick disk mean of 3,500 ly) and have considerably eccentric orbits. Their metals abundance ranges from 0.2 to 0.5 of Sol's (with a thick disk mean of 0.3). Thick disk stars may comprise as much as four percent of nearby stars, including Lalande 2115 (M2.1V) which is moving perpendicular to the galactic plane at a fast velocity of 47 km/sec.
