Spiral Arms

Bands of Dust Within the Milky Way are dark structures. Dark nebulae Coal Sack in Australia Interstellar dust blocks star light. Much of the Milky Way is blocked by dust. skychasers.net/paulanderson

Hydrogen Line Hydrogen is the most abundant element in space. Atoms of hydrogen (H I) emit radio waves from magnetic interactions. Proton and electron spins 21 cm wavelength emitted photon proton electron

Radio Map Radio telescopes can use 21 cm HI to peer through dust. Emissions show the strong emissions from the disk. Disk 100 pc thick Fountains outside the disk

Dust in Motion Doppler shifts measure the radial velocity of dust clouds. A: larger orbit than Sun B: same orbit as Sun C, D: smaller orbit than Sun Swinburne University

Galactic Map Radio maps of dust show concentrated bands. Partially blocked by galactic core These are spiral arms. Constellation names Sun in Orion Arm wikipedia (R. Hurt)

Local Sources Strong radio and x-ray sources line up with the spiral arms. Supernova remnants New star formation D. Marques

Solar Neighborhood Most stars we see are in a small part of the Orion Arm.

Density Waves Spiral arms are not stable. Patterns of orbits coincide Ellipses become spirals These are called density waves and help in star formation.