Chapter 23: Our Galaxy Our location in the galaxy Structure of the galaxy Dark matter Spiral arm formation Our own supermassive black hole

Our place in the galaxy: Early views

The problem: Interstellar Extinction

The solution: Globular clusters and variable stars

Variable stars

Observing the Milky Way

Structure of the Milky Way

Star orbits in the Milky Way

Stellar populations

Spiral arms

21 cm radiation

The sky at 21 centimeters

Mapping the galaxy

The Milky Way

Rotation of the Milky Way

Sun’s orbital motion

Rotation curves

Dark matter

What is dark matter? We don’t know. This is actually one of the most important unanswered questions in modern astronomy. A few ideas: –MACHOS –neutrinos –WIMPS

MACHOs: MAssive Compact Halo Objects Dim, dark objects with masses < 1 M  –Brown dwarfs –White dwarfs –Black holes Search for MACHOs used gravitational lensing. Low mass MACHOs ( M  ) not significant part of dark matter halo. MACHOs of 0.5 M  account for about half of dark matter. –What else is out there?

MACHO detection via gravitational lensing

Other dark matter candidates Neutrinos –If neutrinos are sufficiently massive and enough are present in the galaxy then it is possible they could be the missing dark matter. –Neutrino oscillations require that neutrinos have mass, but it is very small (~ m electron ). –Neutrinos must be part of dark matter, but it is unclear how much. WIMPs –Weakly Interacting Massive Particles –Predicted by some theories but not yet confirmed experimentally.

Formation of spiral arms

Density waves

Sagittarius A*