(there’s no place like home) The Milky Way Galaxy

William Herschel maps out the distribution of stars and gets: The sun The “universe” of Herschel

William Parsons, 3rd Earl of Rosse, builds the “Leviathan of Parsonstown” - draws “spiral nebulae” What were they? Stars & planetary systems forming in our own “universe”? Separate “island universes”?

Henrietta Leavitt- discovered the period luminosity relationship

Cepheid Variable Stars

These stars are aging stars. They lie in a region of the HR diagram called the Instability Strip. A star becomes a Cepheid variable star (unstable to oscillations) several times before it dies. These stars are more massive than the sun.

Once you measure the period of variability you can then just read off the luminosity. Example. A star with a 20 day period has a luminosity 10,000 times that of the sun. Luminosity-Period Relation

Cepheid Variable Stars Cepheids pulsate at different rates By measuring the period, we can get the distance Period tells you the luminosity More luminous stars pulsate slower Brightness easy to measure Distance can be found L = 4  d 2 B Because these stars are so bright, you can see them at vast distances 1 kly – 100 Mly

Henrietta Leavitt & the Cepheid P-L Relationship see original paper: herehere Light curve of a Cepheid variable Large & Small Magellaic CloudsPeriod versus magnitude of Cepheids in SMC

H. Shapley maps distribution of Globular Star Clusters using “Cepheids” (“where’s the mass centered?”) We are NOT at the center. What happened? Globular cluster with variable stars

The Shapley-Curtis Debate (1920) MWG Shapley Curtis The debate solved nothing! Questions in science are not resolved by debates, but by observations & experiments

“Dust Happened” At visible wavelengths, the center of our galaxy suffers ~ 30 mag of extinction by dust!! Even with big modern telescopes, we cannot see very far in the plane of our galaxy at visible wavelengths

Nature of the Spiral Nebulae and the Great Debate Shapley  Novae brightnesses incompatible with M31 being as big as MWG  Rotation of M101 Curtis  Novae indicate a smaller MWG than Shapley’s  Galaxy proper motions undetected  Zones of avoidance in other systems

Hubble Measures Distance to M 31 using Cepheid Variables 100-inch Hooker Telescope, Mt. Wilson Edwin Hubble Debate OVER! Discovery of Cepheids in M 31

Star cluster is really HERE But the extinction makes it fainter, so we would incorrectly think that it is HERE based on brightness measurements Region with dust absorption: A mags Ignoring the extinction due to dust will result in deriving a photometric distance that is too large by a factor of 10 A/5 ! Trumpler Shapley’s MWG was too big for a couple of reasons:

Using RR Lyrae stars & Type II Cepheids, thinking they are Type I, will make the distances appear larger For a given apparent brightness, a higher L star must be more distant. Observed Type II’s, but used L’s of Type I’s.

Other problems: S Andromedae, a “nova” in the Andromeda Galaxy, was actually a supernova -with much higher L and hence distance Proper motions in galaxies “measured” would require speeds greater than light if they were distant - these measurements turned out to be wrong! Summary: Shapley’s MWG was too big, and his distances to the spiral nebulae too small

MWG in 21-cm H line

Along a given line of sight, clouds with highest Doppler shift in 21-cm line are closest to center of MWG Others will be closer or further

Problem: to get the right answer, need to know how star’s speeds change with distance from center:

Rotation Curve of MWG - v is almost constant or slightly increasing with distance from center! Most other galaxies have similar rotations curves. Dilemma!! This requires that mass is distributed far out in these galaxies, but images seem to show that stars aren’t doing this! - DARK MATTER!

NGC 253 (in nearby Sculptor Group)

What MWG might look like as seen from above, based on recent data from the Spitzer Space Telescope (infrared) Spitzer Space Telescope

It is the hot, young massive stars that trace out the spiral structure Galaxy Evolution Explorer (GALEX) Galaxy Evolution Explorer (GALEX) sees UV light

The Galactic Nucleus Near-IR image from 2MASS (2 Micron All Sky Survey)

The center of the Galaxy in radio waves

X-ray images from the Chandra X-ray Observatory Chandra X-ray Observatory LOTS of hot gas!

Black Hole in the Center - Sgr A*