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