•Arms
NOT
obvious if you look at:
–Old
objects like the sun
•Arms
ARE obvious if you look at:
–Maps
of gas clouds
•21
cm Hydrogen
•Radio
maps of CO
–Far
infrared observations of dust
–Young
stars
•O,
B stars
•“HII”
ionized hydrogen regions surrounding O,B stars
–
–Clouds
somehow form in arms , then dissipate between them
–Short
lived objects only get a short distance from their places of birth
•O
stars, Lifetime = few million years,
at 250 km/s Þ500 pc
–