Tracing the Spiral Arms
	• 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 500 pc