The Milky Way•Almost everything we see in thenight sky belongs to the MilkyWay.•We see the Milky Way as a faintband of light across the sky.•From outside, our Milky Waymight very much look likeour cosmic neighbor, theAndromeda galaxy.
First Studies of the Galaxy•First attempt to unveil the structureof the Galaxy was by WilliamHerschel (1785), based on opticalobservations(by counting # of faintstars he could seen in differentdirections).•The shape of theMilky was believed toresemble agrindstone, with thesun close to thecenter.
Determining the Structureof the Milky WayThe structure of our Milky Way is hardto determine because:• We are inside it• Distance measurements are difficult•Our view towards the center is obscuredby gas and dust
Strategies to Explore theStructure of our Milky Way1. Select types of objects with known brightness(via spectroscopy, characteristic light curve, etc.)that you can see throughout the Milky Way andplot their directions and distances.2. Observe objects at wavelengths other than visible(to circumvent the problem of optical obscuration)and catalogue their directions and distances.3. Trace the orbital velocities of objects in differentdirections relative to our position.
Measuring Distances:The Cepheid MethodInstability StripThe more luminous a cepheid variable,the longer its pulsation period.Observingthe periodyields ameasure ofits luminosityand thus itsdistance!
The Cepheid MethodAllows one tomeasure thedistances toyoung “open”star clustersthroughoutthe MilkyWay.
Exploring the Galaxy UsingClusters of StarsTwo types of clusters of stars:1) Open clusters = young clustersof recently formed stars; within thedisk of the galaxyOpen clusters handcPersei2) Globular clusters = old, centrallyconcentrated clusters of stars; spreadthroughout the galaxy & into its “halo”Globular ClusterM 19
Locating the Center of the Milky Way•Shapley founddistribution ofglobular clustersisnotcenteredon the Sun,•Instead, they arecentered on alocation which isheavily obscuredfrom direct (visual)observation.
The Structure of the Milky Way75,000 light yearsDiskNuclear bulgeHaloSunGlobular clustersOpen clusters, O/Bassociations
Infrared View of the Milky WayInterstellar dust(absorbing optical light)emits mostly infrared.