The Milky Way Galaxy Chapter 12:
The Milky Way (Galaxy) Almost everything we see in the night sky belongs to the Milky Way. From outside, our Milky Way might look very much like our cosmic neighbor, the Andromeda Galaxy, which is half again bigger. We see most of the Milky Way as a faint band of light across the sky.
Early Studies of the Galaxy The shape of the Milky Way was believed to resemble a grindstone, with the Sun close to the center First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations.
Determining the Structure of the Galaxy Galactic Center Galactic Plane The structure of our galaxy is hard to determine because: 1) We are inside (We can see the trees but not the forest) 2) Distance measurements are difficult (as always) 3) Our view towards the center is obscured by gas and dust
Strategies to explore the structure of the Milky Way galaxy • Select bright objects that you can see throughout the Milky Way and trace their directions and distances. • Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. • Trace the orbital velocities of objects in different directions relative to our position.
Measuring Distances: The Cepheid Method The more luminous a Cepheid variable, the longer its pulsation period. Instability Strip Observing the period yields a measure of its luminosity and thus its distance!
The Period-Luminosity Relation Massive stars after the main-sequence → Large and luminous (supergiants) → Pulsate more slowly as they cross the instability strip → Longer period of brightness variation → Luminosity Period
Exploring the Galaxy Using Star Clusters • Open clusters lie in the disk of the galaxy. • Globular clusters are scattered over the entire sky, but are strongly concentrated toward Sagittarius. 8
Globular Clusters • Dense clusters of ~50,000 –1,000,000 stars Globular Cluster M80 • Old (~11 billion years), lower-main-sequence stars • Approx. 200 globular clusters in our Milky Way
Locating the Center of the Milky Way Galaxy Harlow Shapley: Distribution of globular clusters is not centered on the sun, but on a location which is heavily obscured from direct (visual) observation 10
The Structure of the Milky Way Galaxy Spiral Arms Open Clusters, O/B Associations
Observing Neutral Hydrogen:The 21-cm (radio) line Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation. Equal magnetic fields repel→ Higher energy Magnetic field due to proton spin Opposite magnetic fields attract→ Lower energy 21-cm line (photon) Magnetic field due to electron spin
The 21-cm (radio) photon emitted from the ground state of hydrogen atom Enables observation of cold hydrogen throughout the galaxy.
21-cm view of the Milky Way Color key:Red=high H density —Blue=low H density
Infrared view of the Milky Way Distribution of cool stars Infrared photons are not strongly absorbed and provide a clear view throughout the Milky Way Distribution of dust grains Cool stars emits mostly near-infrared radiation (just beyond the red) Interstellar dust emits far-infrared radiation (longer wavelength) 15
Orbital motions in the Milky Way Disk stars: Nearly circular orbits in the disk of the galaxy Halo stars: Highly elliptical orbits; randomly oriented
Orbital motions in the Milky Way, continued Differential Rotation • Sun orbits around galactic center at 220 km/s • One orbit takes ~ 240 million years • Stars closer to the galactic center orbit faster • Stars farther out orbit more slowly
Mass determination from orbital velocity: aAU3 ____ MA + MB = (MA and MB in units of solar masses) Py2 The more mass there is inside the orbit, the faster the star has to orbit around the galactic center. Combined mass: M = 11 billion Msun M = 25 billion Msun M = 100 billion Msun M = 400 billion Msun
The Mass of the Milky Way If all mass was concentrated at the center, the rotation curve would follow a version of Kepler’s 3rd law. Faster than expected. Dark matter? Rotation Curve = Plot of orbital velocity versus radius.
The Mass of the Milky Way, continued Total mass in the disk of the Milky Way: ~ 200 billion solar masses Additional mass in an extended halo: ~ 2 trillion solar masses Most of the mass is not emitting any radiation: Dark matter!
Stellar Populations Population I: Young bluish stars: metal rich; located in spiral arms (extreme) anddisk (intermediate) Population II: Old reddish stars: metal poor; located in the halo (extreme) and nuclear bulge (intermediate) 21
The abundance of the elements in the universe Linear Scale Logarithmic Scale Elements heavier than He are very rare.
Metals in Stars Population II:Absorption lines almost exclusively from Hydrogen Population I:Many absorption lines also from heavier elements (“metals”) Young stars contain more metals than older stars. → The gases forming the Milky Way consisted exclusively of hydrogen and helium? Heavier elements (“metals”) were later produced in stars?
Metals in Stars Population I:Many absorption lines also from heavier elements (“metals”) Population II:Absorption lines almost exclusively from Hydrogen Young stars contain more metals than older stars. → The gases forming the Milky Way consisted exclusively of hydrogen and helium? Heavier elements (“metals”) were later produced in stars?
The History of theMilky Way The traditional theory Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II) → Rotating cloud collapses into a disk-like structure → Later populations of stars (pop. I) are restricted to the disk of the galaxy
Modifications of the traditional theory Ages of stellar population may pose a problem to the traditional theory of the history of the Milky Way Possible solution: Later accumulation of gas, possibly due to mergers with smaller galaxies. Recently discovered ring of stars around the Milky Way may be the remnant of such a merger.
Exploring the structure of the Milky Way with O/B Associations Perseus arm Orion-Cygnus arm Sun Sagittarius arm O/B Associations trace out 3 spiral arms near the sun. Distances to O/B Associations by spectroscopic parallax.
Radio observations 21-cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy. Distances to hydrogen clouds determined through radial-velocity measurements (Doppler effect!) Clear indication of spiral arm structure
The spiral pattern of the Milky Way revealed Stars and neutral hydrogen Dust Sun Bar Ring Based on radio and optical studies Based on far-infrared observations
Star formation in spiral arms Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves, initiating star formation.
Theories of Spiral Structure • Density Wave TheoryWaves of compression (like sound waves) that move around the galaxy; gas clouds run into the compressed regions, creating a moving traffic jam. • Self-Sustaining Star FormationStrong stellar winds and supernova explosions create chains of star-forming events; differential rotation drag the growing clumps of new stars around the galaxy. • Collisions/MergersGalaxies do collide or merge, and in the process gravitational influence on one another can trigger and accelerate many events including star formation.
Density Wave Theory Spiral arms are basically stationary shock waves Shocks initiate star formation Stars and gas clouds orbit around the galactic center and cross spiral arms Star formation self-sustaining through O/B ionization fronts and supernova shock waves
The Nature of Spiral Arms Spiral arms appear bright (newly formed, massive stars!) against the dark sky background, but dark (gas and dust in dense, star-forming clouds) against the bright background of the large galaxy Chance coincidence of small spiral galaxy in front of a large background galaxy
Self-Sustaining Star Formation Star forming regions get elongated due to differential rotation Star formation is self-sustaining due to ionization fronts and supernova shocks
Grand-design spiral galaxies Grand-design spirals have two dominant spiral arms. Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms Explained byself-sustaining star formation Explained bythe density wave theory NGC 300 M 100
Example: The Whirlpool Galaxy Grand-design galaxyM 51(Whirlpool Galaxy) Self-sustaining star forming regions along the spiral arm patterns are clearly visible
Spiral structure summary The density waves may be responsible for the overall spiral structure while self-sustaining star formation may modify the spiral arms and produce branches and spurs. Collisions between galaxies may initially set up a density wave pattern.
The Galactic Center Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Extinction by 30 magnitudes →Only 1 out of 1012 optical photons makes its way from the GC towards Earth! galactic center Wide-angle optical view of the GC region
Radio view of the galactic center Many supernova remnants; shells and filaments Arc Sgr A Sgr A Sgr A*: The galactic center The galactic center contains a supermassive black hole!
The mass of the black hole at the galactic center The orbital motions of individual stars near the galactic center indicate that the mass of the central black hole is~ 2.6 million solar masses.
X-Ray view of the galactic center Galactic center region contains many black-hole and neutron-star X-ray binaries Supermassive black hole in the galactic center is unusually faint in X rays, compared to those in other galaxies It is currently dormant — hot accretion disk not fully developed due to lack of infalling matter → Chandra X ray image of Sgr A*
26,000 ly 80,000 ly Schematic Milky Way Extreme Population II Old, Red (cool),Very Metal-Poor. Globular clusters Extreme Population I Young, Blue (hot),Very Metal-Rich. Spiral Arms Cepheid Variable Spiral Tracers include: OB associations Emission nebulae High-mass variables Open clusters Hydrogen clouds (21-cm) Molecular clouds (CO) Spherical Component halo population stars, including nuclear bulge Disk Component Sun disk population stars, gas, and dust Intermediate Population I Metal-Rich. Intermediate Population II Old, Red (cool),Metal-Poor. Nuclear Bulge Supermassive black hole at the galactic center (Dark matter in a large spherical halo?)
How do we figure…? Size ~ 80,000 – 100,000 light years for the disk • Use Cepheid variables to estimate distances to globular clusters and their distribution. Mass ~ 100 – 400 billion solar masses. (x10 for dark matter?) • Observe the motions of stars/gas clouds around the center. Gravity Mass inside the star’s orbit Shape ~ Barred spiral • Use spiral tracers: OB associations, hydrogen clouds, etc. Age ~ 13 billion years old Open clusters < 7 billion years • Estimate the ages of clusters: Globular clusters ~ 11 – 13 billion years Formation ~ Small to large via collisions/mergers (cannibalism) • Build computer models with the observed data and known laws of physics.
Finally … Dust: – obscuration– star formation Cannibalism (collisions/mergers): – galaxy formation– spiral structure Supermassive black hole: – every large galaxy’s center