ASTR 1346 1st Edition Lecture 15 Outline of Last Lecture I Galaxies II Hydrogen Clouds III Parts of Our Galaxy A Galactic Center 1 Synchrotron Radiation B Galactic Bulge C Galactic Disk Outline of Current Lecture I Parts of Our Galaxy A Galactic Halo II Galactic Spiral Arms III Orbital Motion IV Rotation Curve V Probable Candidates of Dark Matter Current Lecture l A The Galactic Halo is a region around the disk and bulge that is roughly sphericalmildly flattened It contains old metal poor stars Population 2 stars which is why the halo looks red and it contains about 150 globular clusters There is no gas or dust here so there is no new star formation Formation ceased about more than 10 billion years ago These stars are also called RR Lyrae Stars old stars this is the oldest part of our galaxy The orbits of objects here are random and in 3 dimension ll Our Milky Way has at least 4 spiral arms 2 major arms are designated the Perseus arm and the Scutum Centaurus arm We are in the Orion Spur which is located between the Perseus and Sagittarius arms lll These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The orbital motion is differential rotation Objects move in any direction in the center and the bulge In the disk objects all go around the center in uniform motions In the halo objects move in any direction not necessarily around the center lV The speed of the Sun around the galactic center is 230kn s and our Solar System s orbital period around the Milky Way is 230 million years We have gone around the Galaxy about 20 times When objects leave our Galaxy they speed up instead of slowing down suggesting there is unseen mass dark matter Over 90 of our Galaxy is composed of Dark Matter V A probable candidate of dark matter is a black hole but few of these fit the role Another candidate is MACHOs Massive Compact Halo Objects which consist of brown dwarfs white dwarfs faint low mass red dwarfs Another candidate are WIMPs Weakly Interacting Massive Particles