Copyright © 2015, W. W. Norton & Company Prepared by Lisa M. Will, San Diego City College Lecture Slides CHAPTER 15: Our Galaxy: The Milky Way Understanding Our Universe S ECOND E DITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal

Our Galaxy: The Milky Way  Describe the Milky Way’s structure.  Explain the history of star formation and chemical composition in the Milky Way.  Discuss how the Milky Way teaches us about galaxy formation.

Structure  The Milky Way is visible as a band of light across our sky.  This is similar to our view of some distant galaxies.  Comparing these observations will teach us about the Milky Way.

Structure (Cont.)

Structure: Vast Clouds  The dark regions visible in the Milky Way are now known to be vast clouds of interstellar gas and dust, obscuring the light from the stars behind them.

Structure: Vast Clouds (Cont.)

Structure: Interstellar Dust  Interstellar dust blocks shorter wavelengths more efficiently: interstellar reddening.  Long wavelengths (infrared and radio) penetrate dust.

Structure: Neutral Hydrogen  Neutral hydrogen emits radio waves with = 21 cm.  Light of this wavelength penetrates the dust.  Radio and infrared surveys have helped us map the structure of the Milky Way.

 Observations of the distribution of interstellar hydrogen and young stars indicate that the Milky Way galaxy has a spiral structure. Structure: Neutral Hydrogen (Cont.)

 Surveys of the motions of stars near the center of the Milky Way indicate that the central region is not spherical, but is rather an elongated bulge.  Milky Way is a barred spiral galaxy. Structure: Spiral Galaxy

 Globular clusters are tightly gravitationallly bound collections of up to a million stars.  Because globular clusters reside within a spherical halo around the Milky Way, astronomers use them to map out the galaxy. Size: Globular Clusters

 Main sequence turnoffs show globular clusters contain very old stars.  Globular clusters contain RR Lyrae and Cepheid variable stars that can be used as standard candles. Size: Globular Cluster Stars

 Globular clusters are very bright, so they can help find distances in our galaxy and to other galaxies.  They were used to determine the size of the Milky Way and that the Sun is not at the center. Size: Distance Between Galaxies

 The distance to the galactic center cannot be directly measured because it is blocked by interstellar dust.  The gravitational center of the globular clusters is the gravitational center of the entire Milky Way: 27,000 ly from our solar system. Size: Galactic Center

 We now have a sense of the size of the Milky Way galaxy.  The disk is about 100,000 ly across.  The solar system exists within the disk.  The center of the Galaxy forms a bulge.  The halo extends across 300,000 ly. Size: Milky Way Disk

 We find globular clusters in the galactic halo.  Globular clusters are amongst the oldest objects known – up to 13 billion years old!  We see no young globular clusters. Composition & Age

Composition & Age: Open Clusters  Open clusters are loosely bound collections of dozens to thousands of stars found in the disk of a spiral galaxy.  Open clusters have a wide range of ages, from far younger to a little older than our Sun.

Composition & Age: Open Clusters (Cont.)

Composition & Age: Galactic Halo  All open clusters are much younger than globular clusters.  Difference in ages is a clue!  Stars in the halo must have formed first.

Class Question Where in the Milky Way galaxy is the solar System located? A. Bulge B. Disk C. Halo

Composition & Age: Hydrogen and Helium Stars  Stars are mostly hydrogen and helium.  Heavier elements must have been formed in stars.  The more massive elements found in a star, the more prior star formation took place.

Composition & Age: Heavy Elements in Stars  We observe younger stars to have more heavy elements than older stars.  Residing in the disk of our galaxy, the Sun has about a 2% concentration of heavy elements, a relatively high amount.  Even globular cluster stars have a small amount of heavy elements, but much less than disk stars. => There must have been at least one generation of stars that existed prior to globular clusters.

Composition & Age: Star Formation  Higher heavy element amounts correspond to more abundant star formation in the inner disk, but other effects may be present.  Galactic fountain model: gas is ejected from the disk by hot young stars and supernovae and then falls back onto the disk.

 Molecular clouds in the plane of the Milky Way form a thin disk.  Newly formed stars are also concentrated this thin disk.  Older stars make a thicker disk, above and below the plane of the Galaxy. Structure: Halo Formation

 Molecular clouds anchor Milky Way’s magnetic field.  Cosmic rays— high-energy, charged particles moving at near the speed of light—can be trapped by the magnetic field. Structure: Cosmic Rays

 The Doppler shifts of the 21-cm line tell us the motions of neutral hydrogen in the Milky Way.  Redshifts and blueshifts appear on opposite sides of the galactic center. Mass & Dark Matter: Doppler Velocities

Mass & Dark Matter: Pattern of Rotation  This is the pattern of the rotation velocity of a disk.  The observed velocities allow us to measure our galaxy’s rotation curve and determine structure within the disk.

Mass & Dark Matter: Rotation Velocity  The rotation curve for the Milky Way is similar to those we saw for other spiral galaxies.  The Milky Way is mostly dark matter. Visible matter dominates the inner part of our galaxy, and dark matter dominates its outer parts.

Supermassive Black Hole  X-ray, infrared, and radio observations reveal the center of the Milky Way.  The images show hundreds of thousands of stars, as well as wisps and loops of material distributed throughout the region.

Supermassive Black Hole (Cont.)

Supermassive Black Hole (Cont.) (this image is from Dr. Stolovy’s research combining Spitzer and Hubble data in infrared wavelengths)

Supermassive Black Hole (Cont.)

 The stars near the galactic center have orbital periods on order of a decade.  Using Kepler’s third law with these stars, we estimate the black hole at the galactic center has a mass of about 4 million M ⊙. => supermassive! Supermassive Black Hole: Galactic Center

 Gamma ray bubbles extend 25,000 ly above and below the galactic plane.  The bubbles may have formed material fell into the supermassive black hole, releasing enough energy to power the bubbles. Supermassive Black Hole: Gamma Ray Bubbles

Galaxy Formation  The Milky Way offers clues to galaxy formation.  The stars in globular clusters in the halo have some heavy elements, so at least one prior generation of stars must have existed.  Halo objects were formed before interstellar gas was all concentrated into the disk.  Later star formation has been concentrated in the disk.

 Galaxies do not exist in isolation.  The Milky Way is one of three large galaxies amongst a few dozen dwarf galaxies. => Local Group Galaxy Formation: The Milky Way

 The Milky Way probably formed by mergers of many smaller proto-galaxies in a dark matter “cloud.”  Several of these are still orbiting the Milky Way as satellite galaxies. Galaxy Formation: Dwarf Companion Galaxies

Chapter Summary  Our solar system is in the disk of the Milky Way, a barred spiral galaxy that is 100,000 light-years across.  There is a supermassive black hole at the center of the galaxy, as well as an extended dark matter halo.  The Milky Way offers many clues about the way galaxies form.

Astronomy in Action Size of Active Galactic Nuclei Click the image to launch the Astronomy in Action Video (Requires an active Internet connection)

Nebraska Applet Milky Way Rotational Velocity Explorer Click the image to launch the Nebraska Applet (Requires an active Internet connection)

Nebraska Applet Milky Way Habitability Explorer Click the image to launch the Nebraska Applet (Requires an active Internet connection)

Copyright © 2015, W. W. Norton & Company wwnpag.es/uou2 Prepared by Lisa M. Will, San Diego City College This concludes the Lecture slides for CHAPTER 15: Our Galaxy: The Milky Way Understanding Our Universe S ECOND E DITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal