reading question 5

Structure of the Milky Way Galaxy -has three main structures Disk -contains very young stars formed by stellar density waves (spiral arms) -surrounds the galactic bulge -contains interstellar medium such as dust and gas -contains the younger open star clusters Bulge -tightly packed center of galaxy -last to form from the collapsing gas cloud -orbits of stars less organized -does not contain majority of mass like the sun in our solar system -contains a giant black hole Halo -First structure formed by collapsing cloud -contains much older stars (red) -fewer heavier elements because they formed before star-gas cycle began -no interstellar medium (gas or dust) -contains mostly globular clusters (very old) -less organized star orbits Dark Matter -we can use the sun?s orbit of the galaxy to determine the mass that falls inside its orbit -then we use more distant stars in the galaxy -orbital speeds remain constant -but more mass appears to be in the bulge so the stars should rotate like our solar system -outer stars should rotate slower -instead mass appears to be evenly spread, we just can?t see it all -this mass gives off faint if any light and interacts with gravity Star-Gas-Star cycle -universe born with only hydrogen and helium -all heavy elements were made by stars Star Formation: made from gravity and molecular hydrogen gas Nuclear fusion: stars make new elements from fusion Returning gas: supernova/stellar winds return gas with new elements Hot bubbles: return gas cools and blends into hydrogen clouds Atomic-hydrogen clouds: interstellar gas clouds fill galactic disk Molecular clouds: gas in disk cools, forms molecules and collapse to make stars