Download X-ray radiation

Lecture 30:
The Milky Way
topics:
structure of our Galaxy
 components of our Galaxy (stars and gas)
 how we observe different components

Our Galaxy
Edge-on View
The structure of our Galaxy
not so easy to determine the structure
of our Galaxy from the inside because
of the effects of dust in our disk
 initially thought that the Sun was at the
center of the Galaxy

Herschel’s Map of the Galaxy
Period-Luminosity relation
Globular cluster with RR-Lyrae
The real center of the Galaxy
globular clusters can be seen above the
“smog” of the disk
 period-luminosity relation for RR-Lyrae
stars used to find distances to globular
clusters
 GC’s in a huge sphere, but the center is
located several kpc away from the Sun
– this is the true center of the Galaxy

Verified by Infra-red Observations
dust extinction inversely proportional to
wavelength – longer wavelengths less
obscured
 dust emits light at far infra-red
wavelengths (30-300 microns)
 infra-red observations give us a direct
view of the structure of our Galaxy

The Infra-red view
A Census of Our Galaxy: stars

stars – about 50 billion solar masses

disk stars (about 80 percent)


bulge stars (about 20 percent)


young, metal-rich, blue
mixture of young and old, red and blue
halo stars (less than 1 percent)

uniformly old and red, metal-poor
A Census of Our Galaxy: gas

molecular hydrogen (H2)


atomic (neutral) hydrogen (HI)


about 5 billion solar masses
density
about 5 billion solar masses
ionized hydrogen (HII)

very small amount of mass, but fills most
of the volume of the disk
Gas in the interstellar medium
temperature density 3
(atoms/cm )
molecular cloud
cores
H2
60 K
10,000
molecular clouds H2
30 K
300
atomic clouds
atomic H
100 K
100
diffuse atomic
gas
atomic H
10,000 K
1
hot bubbles
ionized H
106 K
0.01
Chemical Composition
70 percent hydrogen
 28 percent helium
 2 percent “heavy elements” (metals)

Observing neutral Hydrogen
neutral hydrogen emits radiation in the
radio due to magnetic spin-flip
transitions
 often called ’21-cm’ transition

spin-flip
transition
Magnetic Resonance
Imaging
Map of Neutral Hydrogen in
our Galaxy
observing molecular hydrogen
molecular hydrogen is too cold to
produce emission lines
 we can use Carbon Monoxide (CO) or
other molecules as a tracer
 these molecules produce emission lines
in the radio

Molecular clouds
in Orion
Observing hot ionized gas
supernovae and massive stars produce
“bubbles” of hot gas
 as the bubbles expand outwards, they
produce shock waves, which cause

X-ray radiation
X-ray emission from Tycho’s SN
Observing hot ionized gas
because of the high temperatures, the
gas gets ionized.
 when electrons fall back from their
excited state, they produce emission
lines at optical wavelengths

Cygnus Loop
Observing hot ionized gas
shock waves from SN also act as
subatomic particle accelerators
 charged particles (electrons) moving in
a magnetic field produce radiowavelength synchrotron radiation

Cassiopeia A
Radio
X-ray
Radio
CO
FIR
NIR
vis.
X-ray
gamma
ray
Summary

structure of our Galaxy


components of our Galaxy


disk, bulge, halo
stars, atomic gas, molecular gas, ionized
gas
how we can observe these components
different physics producing radiation
 characteristic wavelengths of each process

atomic
gas
molecular
clouds
hot
bubbles
star
formation
supernovae and
stellar winds
heavy
elements
Star formation
Reflection Nebula
Horsehead
Nebula