ASTR 1346 1st Edition Exam 3 Study Guide Lectures 14 20 Lecture 14 April 7 The Milky Way Galaxy What is a Galaxy A galaxy is a huge collection of stellar and interstellar matter stars gas neutron stars black holes etc isolated in space and held together by its own gravity What is our Galaxy Our galaxy is called the Milky Way because it looks like a River of Milk from horizon to horizon Our Solar System is in one of the spiral arms of our Galaxy What are hydrogen clouds Interstellar dust obscures our view into plane of disk at visual wavelengths Despite this hydrogen clouds can be detected by 21 cm radio waves emitted by changes in relative spins of electrons and protons Parallel spins are higher energy configuration and opposite spins are lower energy configuration What is the Galactic Center The Galactic Center is an active crowded place The stars are held in orbit by a 4 million solar mass black hole This is located near the constellation Sagittarius Define Synchrotron Radiation Synchrotron Radiation are high speed electrons spiraling around the magnetic field What is the Galactic Bulge The Galactic Bulge is an oval region near the Galaxy s center It is 20 000ly across and 13 000ly thick Old and young stars mingle here giving off a yellow white color to the bulge with the younger stars toward the center More gas and dust are here which indicates vigorous star formation What is the Galactic Disk The Galactic Disk is the flattest part of our Galaxy It s 100 000ly across and 2 000ly thick Spiral arms and dusty GMC s exist here There is ongoing star formation because of gas and dust It mainly contains young stars that re metal rich Population 1 stars this and open clusters and associations of stars gives the disk a blue white color Orbits of objects are circular uniform but different speeds around the center Lecture 15 April 9 Orbital Motion and Dark Matter What is the Galactic Halo The Galactic Halo is a region around the disk and bulge that is roughly spherical mildly 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 What are Spiral Arms 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 What is orbital motion 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 What is our rotation curve 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 What are some probable candidates of dark matter 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 Lecture 16 April 14 Galaxies What are Hubble s 4 different types of galaxies In 1924 American astronomer Edwin Hubble categorized galaxies according to appearance He separated them into 4 different types Spirals Barred Spirals Ellipticals and Irregulars What is a spiral galaxy Spirals are classified by the relative strength of the central bulge and the tightness of spiral arms There are three different classes of spirals Sa Sb and Sc Sa indicates a strong bulge and tight indistinct arms Sb is an intermediate type Sc indicates small bulge and loose welldefined arms The diameter of the spirals is between 15 000 and 150 000ly The structure has a nucleus bulge disk and halo What is a barred spiral galaxy Barred Spirals are like ordinary spirals except for their strong stellar bar The bar rotates as a unit and spiral arms emerge from the ends of the bar Our Milky Way Galaxy is a Barred Spiral Galaxy Lecture 17 April 16 Galaxies What are ellipticals Ellipticals show little internal structure They have an elliptical shape no disks spiral arms or dust lanes and their brightest stars are red They range from E0 circular E1 slightly flattened up to E7 flattest Ellipticals are a few thousand to a few million light years in diameter There is a halo of old stars with little cool gas or dust What are lenticulars Lenticulars are intermediate They re between E7 ellipticals and Sa spirals They re classified by SO if no bar and SB if with bar They have a thin disk and flattened bulge with no gas or spiral arms What are irregulars Irregulars have the shortest range of sizes with an irregular and chaotic structure They have lots of star formations and blue stars and are separated into 2 different types Irr 1 are smaller than spirals but larger than dwarf ellipticals Irr 2 are rarer and have more of an explosive appearance What is a group galaxy A group galaxy is a poor cluster with a few dozen bright galaxies There is either regular spherical or irregular asymmetrical Regular is usually rich and contains mostly elliptical and lenticular galaxies Irregular is usually poor and contains more spiral and irregular galaxies There are more than 3 000 galaxy cluster that have been cataloged to date The local group contains 3 spiral galaxies Milky Way M31 M33 It has 40 45 galaxies elliptical irregular and poor What is a cluster Clusters are rich clusters that have hundreds to thousands of bright galaxies Rich clusters contains thousands of galaxies with one or more giant elliptical galaxies in the center ex Virgo About 2 500 mostly dwarf galaxies are centered on two bright ellipticals Coma Cluster is rich and regular What is a supercluster Superclusters have clusters of cluster and contains dozens of individual clusters Superclusters are the largest coherent structures seen in the Universe The local cluster is roughly centered on the Virgo Cluster and contains tens of thousands of galaxies