# Galaxies Lecture 01

• Galaxy Facts
• Stars from the most massive component (95% of mass)
• They sometimes have satellite galaxies
• Dwarf galaxies are the most common
• They come isolated or in groups
• Groups contain tens of galaxies, clusters contain thousands.
• Star Clusters vs. Galaxies
• Clusters are a few pc across; galaxies are larger than 1 kpc.
• Clusters have ${\displaystyle 10^{5}}$ stars; galaxies have over ${\displaystyle 10^{7}}$.
• Galaxies are always self-gravitating
• Galaxies contain gas, dust, and cosmic rays
• Dynamics of Stars
• The stellar mass of galaxies completely determines the dynamics.
• Calculating the collision time of stars:
{\displaystyle {\begin{aligned}\ell _{mfp}&={1 \over n\sigma }\\t_{col}&={1 \over n\sigma v}\\&={1 \over (.25pc^{-3})(\pi (2R_{\odot })^{2})(20{km \over s})}\\&\sim 2.5\cdot 10^{18}yrs\\\end{aligned}}\,\!}
• Stars never collide (collision time greater than Hubble time).
• Dynamics of Gas
• Calculating collision time of gas:
{\displaystyle {\begin{aligned}t_{col}&={1 \over n\sigma v}\\&={1 \over (1cm^{-3})(\pi (10^{11})^{2})(1{km \over s})}\\&=500yrs\\\end{aligned}}\,\!}
• Gas collides often on timescales of galactic dynamics
• Energy Changes
• Gas energy changes via heating and cooling
• Stellar energy changes via changes in gravitational potential (ex. galaxy collisions, ejection of stars from a region)