E1 - galactic disk and spiral arms

• MW has a flat 2D distribution of stars with a thin and a thick disk
• the thin disk contains blue, young, high metallicity stars
• the thick disk contains red, old, lower metallicity stars
• there are patches of blue stars, ie. star forming regions
• from the pitch angle, it is classified as a late type spiral (b/c)
• the sun is part of a minor spiral arm, Orion-Cygnus
• there is also a halo of old globular clusters
• it has a barred nucleus with a Super Massive Black Hole (SMBH) at the centre

image: Graham and Spitler (2009)

• the disk is well described by a Sersic index $n=1$
• the bulge is described by $n=3$
• the 'wiggles' are a signature of the spirals

• young blue stars (A or earlier) have a scale height of $<300$ pc

• $95\mathrm{\%}$ of the stars in the solar neighbourhood have solar metallicity

• these are called 'population I' stars, and form the disk

• this component has a radius $\approx 12$ kpc

• old red stars (G, K) have a scale height of about $1$ kpc

• their environment are poor in gas with low star formation

• these are called 'population II' stars, and form the thick disk

• this component has a radius $\approx 8$ kpc