The thick disk

About 4 per cent of the MW's stars belong to a thicker disk, which is aligned with the (thin) disk, but has a scale height of $z_h\approx 1{\hbox{\rm kpc}}$. Its existence has been discovered only recently, in the 1980s. The difference with the thin disk is not just the kinematics of its stars: it turns out that thick disk stars are on average older than the stars of the thin disk, and have a lower metallicity [Fe/H]$\sim$-0.5, i.e. they have less Fe (and other metals as well, but we can usually measure Fe accurately) for a given amount of hydrogen (we'll see later what the notation [Fe/H] means).