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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.