Why it’s important
Components of the Galaxy
The Galactic centre
The dark halo
It can be studied in unique detail
It’s a highly typical galaxy
Few galaxies L>L*
Most light from galaxies with L~L*
No accident that L~L*?
We live at edge of disk
Disadvantage: structure obscured by “dust”
Advantage: can study motions of nearby stars
COBE Near IR View
Radius stellar disk 12 kpc=37000 light years
Distance Sun to centre 8kpc=24000 l.y.
Half-mass radius ~40kpc?
Thickness stellar disk ~400pc=1200 l.y.
Stellar mass ~5£1010 M¯
Gas mass ~5£109 M¯
At Sun surface density ~1020 atoms / cm2
Gas layer ~300 light years (3 1020cm) thick, so n~0.3 atoms / cm3
Density of air ~1020 atoms / cm3
So squashed to density air layer ~1 cm thick
Can see ~1kpc; when squashed could see only ~ 10cm through it
Stars form at rate few / yr
Spitzer space telescope (IR)
Stars born on nearly circular orbits
Stars have random velocities
Spiral structure increases random velocities over time
Derive age of solar neighbourhood: 12.2Gyr
Local Standard of Rest (LSR) on circular orbit around GC
Shifts stars radially
Sun may have shifted ~2kpc