Chapter 23; the Milky Way galaxy Milky Way anatomy Orbits, spiral arms, and galactic rotation/ Variable stars as a tool Cool phenomena at the core

Recall some basic stuff. New Stuff Galactic Halo: site of globular clusters, & miscellaneous old (halo) stars. Gas poor region, lacking in heavy elements Bulge: very dense place, site of vigorous star formation. Population I stars: young disk stars. Population II stars: old halo stars.

Traffic news: galactic motions Dynamics of the Milky Way galaxy. Things that approach us are blueshifted. Things that move away from us are redshifted. Implication: large scale well ordered differential movement in galactic disk. One complete rotation ~ 225 million years. Orbital speed ~ 220 km/s

Galactic motions continued.... Behavior not so orderly in the halo and central bulge regions. Motions tends to be quite random in these regions. Arcturus is a well known example of a halo star just passing on through our neighborhood (region around Sun in disk). It’s orbit actually takes it far above and below the galactic plane.

Galaxy evolution. Remind you of anything?

Spiral arms and Density waves Contain: open clusters, OB & T Tauri associations, lots of gas, dust. Inner parts rotate faster than outer parts. (remind you of anything? Conservation of....) maintenance of spiral structure dependent on density wave theory. Density waves: coiled waves of gas compression moving thru the galactic disk much as sound waves do. Wave itself remains intact despite distances.

More wave stuff.. Drivers star formation (Fig , page 616) cumulative motions over time. Inter-galactic interactions.

Pretty Spirals. NGC 3184 Andromeda NGC 2997

Variable stars as measuring tools Beginning of 20th century saw a laborious effort in cataloging variable stars. A variable star is one whose luminosity changes with time, whether erratically or more regularly. Some examples: binary stars (eclipsing binaries), novae, & type I supernovae (cataclysmic variables stars - sudden large increases in brightness), & intrinsic variables (Stars where variability in brightness is an intrinsic trait). Intrinsic variable star types: Pulsating variables: RR Lyrae and Cepheid variables.

Cepheids and RR Lyrae variables

Period-Luminosity relationship. Discovered by Henrietta Leavitt, Harvard Univ., Laborious effort comparing many images in time for thousands of stars. 1. Long period variables have large luminosities, short period variables have low luminosities. 2. Apparent brightness ~ Luminosity/ distance => distance Notice anything about the luminosity of the RR Lyrae vs. the Cepheids?

Galactic mass and creatures of the (galactic core) deep Kepler’s law: total mass(solar mass) = orbit radius(A.U.) 3 /period(yrs) solar masses, 100 billion solar masses! Problem: galaxy’s mass is spread out, hence mass calculated is mass within sun’s radius of galactic bulge.

Mass continued Problem is a little more complicated. Mass of the galaxy on a larger scale is based on Why not look at orbital motion and mass of gas as a function of increasing radii ideal case if r = 15 kpc Curve corresponding to unseen matter

Even more on mass Must be more mass past 15 kpc! 8 kpc 15 kpc, 2 x solar masses 40 to 50 kpc, 6 x solar masses ? Dark Matter center

Dark matter Dark Halo region surrounds luminous halo region. Dark matter not recognized at any electromagnetic wavelength (radio to gamma). Known only by its gravitational effects. Possible reasons/sources:black holes, brown dwarfs (low mass pre-stellar objects that never reach fusion stage), low mass red dwarfs. MACHOS: Massive Compact Halo Objects – red and brown dwarfs Detection method – gravitational lensing

Gravitational lensing

Gravitational lensing, the evidence

Galactic center Visible light limit; 1/10 th into center IR – 50,000 stars per (pc) 3 density ~ million X our neighborhood! Radio; 400 pc rotating ring of 30,000 solar masses. Surrounds a bright radio source. Contains a large black hole! 3 million solar masses.

Chapter 24: Normal Galaxies Hubble developed a classification scheme based on visual characteristics of galaxies. Known as the Hubble Classification Scheme. Ellipticals SO galaxies Spirals Barred Spirals Irregulars

Pretty Pictures of: Spirals S and letters a,b,c according to size of central bulge which is correlated to tightness of spiral arm windings. Some features

Spirals and their features Large numbers of reddish old stars in bulges and halos. Halos contain globular clusters. Comprised mostly of A-G type stars in disk with O-B association stars in spiral arms. Appearance is whitish- blue as a result. Rich amounts of gas and dust. Star formation occurring. Barred spirals differ by appearance of an elongated bar of stellar and interstellar matter passing through the bulge. All other characteristics the same as for spirals. Milky Way may be a SBb or SBc! Go back and check these out <= !

Ellipticals & SOs Facts sheet Ellipticals range from E0 very circular looking to E7 most elliptical/elongated looking. There can be giant (few million pcs across, trillions of stars) to dwarfs (~ 1 kpc across, a million stars) in size. Lack spiral arms. Little to no cool gas/dust. No evidence of star formation, mostly old, reddish low mass stars. Chaotic/disordered movement of stars, almost no rotational movement seen. X-ray pictures note presence of very hot (~several million degrees) gas distributed throughout. Some have very faint disks. SOs & SBOs: thin disk, flattened bulge, no spiral arms and no gas.

Ellipticals and SOs are pretty too! NGC 1201, Type SO

Throwaway Pile: Irregular Galaxies If they didn’t fit the mold of the other schemes, Hubble just stuck them in this category. Tend to be rich in interstellar matter and young hot-blue stars. Irregular, often chaotic looking structure. Irr1s looks like misshapen spirals. Irr2s much rarer, explosive looking. Magellanic Clouds: satellite Irr galaxies around the Milky Way. ~ 50 kpcs away. The LMC (Large Magellanic cloud) contains ~ 6 billion solar masses of material. Both contain lots of gas, dust and blue type young stars, globulars and old stars.

Who says Alley cats can’t be pretty too! Local alley cats

Magellanic Clouds

Galactic distributions Current distance measurements allow us to get a big picture of the bigger picture. Clustering is the key word. Galaxies like to hang around each other in small clusters, then clusters themselves like to hang together, then,… you get the picture!

The Local Group: us, Andromeda, etc..

Us and others