theres no place like home

1
11 - The Milky Way Galaxy

• (theres no place like home)

2
William Herschel maps out the distribution of
stars and gets
The sun
The universe of Herschel
3
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?
4
Henrietta Leavitt the Cepheid P-L
Relationshipsee original paper here
Light curve of a Cepheid variable
Large Small Magellaic Clouds
Period versus magnitude of Cepheids in SMC
5
Globular cluster with variable stars
H. Shapley maps distribution of Globular Star
Clusters using Cepheids (wheres the mass
centered?) We are NOT at the center. What
happened?
6
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
7
The Shapley-Curtis Debate (1920)
Curtis
Shapley
MWG
MWG
The debate solved nothing! Questions in science
are not resolved by debates, but by observations
experiments
8
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 Shapleys
• Galaxy proper motions undetected
• Zones of avoidance in other systems

9
1923 - Hubble Measures Distance to M 31 using
Cepheid Variables
Debate OVER!
Discovery of Cepheids in M 31
100-inch Hooker Telescope, Mt. Wilson
Edwin Hubble
10
Shapleys MWG was too big for a couple of reasons
Ignoring the extinction due to dust will result
in deriving a photometric distance that is too
large by a factor of 10A/5!
Star cluster is really HERE
Region with dust absorption A mags
But the extinction makes it fainter, so we would
incorrectly think that it is HERE based on
brightness measurements
Trumpler - 1929
11
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 IIs, but
used Ls of Type Is.
12
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 Shapleys MWG was too big, and his
distances to the spiral nebulae too small
13
Stellar Photometric Distances
For an apparent (observed) magnitude m, absolute
magnitude M, and distance d in parsecs Without
dust m M5logd-5 and so d 10(m-M5)/5
pc (reminder m Md10pc) With dust m
M5logd-5A and d 10(m-M-A5)/5 pc where A is
the extinction by dust in magnitudes (Note
sometimes astronomers use the distance modulus
m-M 5logd-5 to express the distance to some
objects)
14
(No Transcript)
15
(No Transcript)
16
MWG in 21-cm H line
17
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
18
(No Transcript)
19
Problem to get the right answer, need to know
how stars speeds change with distance from
center
20
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 arent doing this! - DARK MATTER!
21
NGC 253 (in nearby Sculptor Group)
22
What MWG might look like as seen from above,
based on recent data from the Spitzer Space
Telescope (infrared)
23
It is the hot, young massive stars that trace out
the spiral structure
Galaxy Evolution Explorer (GALEX) sees UV light
24
The Galactic Nucleus
Near-IR image from 2MASS (2 Micron All Sky Survey)
25
The center of the Galaxy in radio waves
26
X-ray images from the Chandra X-ray
Observatory LOTS of hot gas!
27
Black Hole in the Center - Sgr A