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A short course in
The Milky Way and the ISM
Dr. Maura McLaughlin
West Virginia University
[email protected]
July 10 2008
Pulsar Search Collaboratory
Outline
1.
Introduction to the Milky Way
2. The Milky Way in the universe
3. Stellar populations in the Milky Way
4. Dynamics of the Milky Way
5. The interstellar medium
6. Dispersion, scattering and scintillation of radio signals
The Milky Way
Almost everything we see in the
night sky belongs to the Milky Way.
We see most of the Milky Way as a
faint band of light across the sky.
Greek word for milk is “galact” ->
galaxy!!
The Milky Way
How in the world do we know this?!?!
First Studies of the Galaxy
First attempt to unveil the
structure of the galaxy by
William Herschel (1785), based
on optical observations.
The shape of the Milky Way was
believed to resemble a
grindstone, with the Sun close to
First Studies of the Galaxy
First attempt to unveil the
structure of the galaxy by
William Herschel (1785), based
on optical observations.
Did not know about gas
and dust!!
Determining the Structure of
the Milky Way
Galactic Plane
Galactic Center
The structure of our Milky Way is hard to determine because:
1) We are inside.
2) Distance measurements are difficult.
3) Our view towards the center is
obscured by gas and dust.
Exploring the Galaxy
Using Star Clusters
Two types of clusters of stars:
1) Open clusters = young clusters of recently
formed stars; within the disk of the Galaxy
Open cluster NGC 1983
2) Globular clusters = old,
centrally concentrated clusters;
mostly in a halo around the galaxy
Globular Cluster M13
Globular Clusters
Globular Cluster
M80
• Dense clusters of 50,000 – a million stars
• Old (~ 11 billion years), lower-main-sequence stars
• Approx. 200 globular clusters in our Milky Way
Locating the Center of the Milky Way
In early 1900s, Shapley shows that the distribution of
globular clusters is not centered on the Sun!
Locating the Center of the Milky Way
Their distribution is
centered on a location
which is heavily obscured
from direct (visual)
observation.
Using Cepheid distances,
he measured the distance
to the center of the
distribution of 20,000
parsecs (too big!)
Hubble’s breakthrough
Hubble identified a Cepheid variable in M31
the Andromeda Galaxy in 1923 using the
100” telescope at Mount Wilson.
Distance to M31 is 780 kpc
Our Galaxy Cluster:
The Local Group
Milky Way
Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Our Galaxy Cluster:
The Local Group
Our local group is a poor cluster:
Milky Way
> 30 galaxies
1 Mpc diameter
Of bright galaxies,
14 elliptical
3 spiral
4Small
irregular
Magellanic Cloud
Large
Magellanic Cloud
Most of galaxies are
dwarf
ellipticals.
Andromeda Galaxy
Our Galaxy Cluster:
The Local Group
Largest members are:
Milky Way
Andromeda Galaxy
Milky Way
Andromeda (M31)
Triangulum (M33)
Andromeda is the largest
but we think MW may be
the most
massive.
Small
Magellanic Cloud
Large Magellanic Cloud
Our Galaxy Cluster:
The Local Group
Largest members are:
Milky Way
Andromeda Galaxy
Milky Way
Andromeda (M31)
Triangulum (M33)
Small Magellanic Cloud
Large Magellanic Cloud
Our Galaxy Cluster:
The Local Group
Largest members are:
Milky Way
Andromeda Galaxy
Milky Way
Andromeda (M31)
Triangulum (M33)
Small Magellanic Cloud
Large Magellanic Cloud
Magellanic Clouds: Local group dwarfs
Mergers of Galaxies
Milky Way and Andromeda are moving towards
each other at 500,000 km/hour and are
expected to merge in about 3 billion years.
About nomenclature
Numbers with “M” in front
of them are Messier
objects, cataloged by
Charles Messier between
1758 to 1782. These were
about 100 diffuse
structures often mistaken
for comets.
About nomenclature
NGC means New General
Catalog of nebulae and star
clusters, compiled by John
Dreyer in 1888.
Contains 8000 objects.
The Structure of the Milky Way
Disk contains stars, open star
clusters and lots of dust and gas.
Sun is in disk at 8.5 kpc from
center of Galaxy (D = 25 kpc).
Halo contains only 2% as many
stars as the disk, and very little
gas and dust. We can’t detect
halos of other galaxies.
Nuclear bulge has radius of 2 kpc
and contains little gas and dust.
Observing Neutral Hydrogen:
The 21-cm (radio) line (1)
Electrons in the ground state of neutral hydrogen have slightly
different energies, depending on their spin orientation.
Opposite magnetic
fields attract =>
Lower energy
Magnetic field
due to proton
spin
Magnetic field
due to electron
spin
Equal magnetic
fields repel =>
Higher energy
Observing Neutral Hydrogen:
The 21-cm (radio) line (2)
Observing Neutral Hydrogen:
The 21-cm (radio) line (3)
21 cm emission maps out spiral arms
Infrared View of the Milky Way
Near-infrared image
Galactic plane
Nuclear
bulge
Infrared emission is not
strongly absorbed and
provides a clear view
throughout the Milky Way
Interstellar dust
(absorbing optical
light) emits mostly
infrared.
Infrared View of the Milky Way
Near-infrared image
Galactic plane
Nuclear
bulge
Spitzer Space Telescope view of Milky Way
Orbital Motions in the Milky Way (1)
Disk stars:
Nearly circular
orbits in the disk
of the galaxy
Halo stars:
Highly elliptical
orbits; randomly
oriented
The mass of the Milky Way
We use binary star systems to find the
masses of stars.
We can measure orbits of stars in the galaxy
to find the mass of the galaxy.
Orbital Motions in the Milky Way (2)
Differential Rotation
Sun orbits around
galactic center at
220 km/s.
1 orbit takes approx.
240 million years.
We have completed
roughly 20 orbits.
Mass determination from
orbital velocity:
The more mass there is
inside the orbit, the
faster the Sun has to
orbit around the
Galactic center.
Combined mass:
M = 100 billion Msun
M = 25 billion Msun
M = 4 billion Msun
The Mass of the Milky Way
If all mass was concentrated in the
center, rotation curve would follow a
modified version of Kepler’s 3rd law.
Rotation Curve = orbital
velocity as function of radius.
The Mass of the Milky Way (2)
Total mass in the disk
of the Milky Way:
Approx. 200 billion
solar masses
Additional mass in an
extended halo:
Total: Approx. 1
trillion solar masses
Most of the mass is
not emitting any
radiation:
dark matter!
Possible dark matter sources
• Neutrinos
• Massive compact halo objects
– Brown dwarfs
– Black holes
• Gas
• Planets
• Other exotic objects
How old is the Galaxy?
Stellar Populations
Population I: Young stars: metal rich;
located in spiral arms and disk
Population II: Old stars: metal poor;
located in the halo (globular clusters)
and nuclear bulge
How old is the Galaxy?
Stellar Populations
Our Sun is an intermediate
Population 1 star.
Metal Abundances in the
Universe
All elements
heavier than He
are very rare.
Linear Scale
Logarithmic Scale
Metals in Stars
Absorption lines almost exclusively from Hydrogen:
Population II
Many absorption lines also from heavier elements (metals):
Population I
At the time of formation,
the gases forming the Milky
Way consisted exclusively of
hydrogen and helium. heavier
elements (“metals”) were
later only produced in stars.
=> Young stars
contain more metals
than older stars.
The History of
the Milky Way
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.
Oldest GCs are 13 billion
years old.
Exploring the structure of the Milky
Way with O/B Associations
O/B Associations trace out
3 spiral arms near the Sun.
Distances to O/B Associations
determined using Cepheid variables.
Star Formation in Spiral Arms (1)
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.
Density wave
theory.
Star Formation in
Spiral Arms (2)
Spiral arms are basically
stationary shock waves.
Stars and gas clouds orbit
around the galactic center
and cross spiral arms.
Shocks initiate star formation.
Star formation selfsustaining through O/B
ionization fronts and
supernova shock waves.
The Galactic Center (1)
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.
Sgr A*: The center of our galaxy
The galactic center contains a supermassive black
hole of approx. 4 million solar masses.
Measuring the Mass of the Black
Hole in the Center of the Milky Way
By following the orbits of
individual stars near the center
of the Milky Way, the mass of
the central black hole could be
determined to be ~ 4 million
solar masses.
Question
Which part of the Milky Way contains mostly
Population II stars and globular clusters ?
A) the disk
B) the halo
C) the bulge
D) the spiral arms
Question
Which part of the Milky Way contains mostly
luminous O and B stars?
A) the disk
B) the halo
C) the bulge
D) the spiral arms
Question
Stars with more metals are likely to be _______
than stars with fewer metals.
A) younger
B) older
Question
Which one of the following galaxies is not a member
of the local group?
A) Milky Way
B) Antenna
C) Andromeda
D) Triangulum
Question
The rapid rotation of the outer parts of the disk of
our galaxy shows that
A) the center of the galaxy is very massive
B) there are many young stars in the outer
parts
C) there is a lot of mass in the outer parts of
the Galaxy
D) the rotation of the Galaxy is Keplerian
The Interstellar Medium (ISM)
The space between the stars is not
completely empty, but filled with very
dilute gas and dust, producing some of
the most beautiful objects in the sky.
We are interested in the ISM because
a) dense interstellar clouds
are the birth places of stars
b) dark clouds alter and absorb
the light from stars behind them
Structure of the ISM
The ISM is 99% interstellar gas and comprises 10-15% of the
visible mass of MW. It occurs in two main types of clouds:
• HI clouds (molecular clouds)
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
• Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.
• HI clouds (molecular clouds)
• Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.
If we have ionized hydrogen then we will also have….
FREE ELECTRONS!
Pulse dispersion
Pulsars are dispersed in frequency
by free electrons in the interstellar
medium.
Photons with higher frequencies
travel faster through space and
arrive earlier than lower frequency
ones.
The total delay is proportional to
the distance to the pulsar.
Using a model for the interstellar
medium, we can use this property to
estimate distances to pulsars.
A bright single burst
We are always searching for
new radio signals in our data
Hot off the press:
- a new radio transient
- extragalactic origin
- note frequency dispersion
discriminates against RFI!
- D ~ 500 Mpc (1.7 Gly)
- origin unknown (NS-NS?)
Pulsar distances
DM 
D
 n dl
e
0
We measure DMs in pc cm-3.
Can use measured DMs to estimate
distances to pulsars!

Black and yellow points are at two
different frequencies. Which color
is the higher one??
Pulse scattering
Pulse scattering
Amount of
scattering  f? ?
Scattering vs DM
Pulsar scintillation
Twinkle twinkle little pulsar
Degree of ISS (interstellar
scintillation) will depend
on distance, medium and
velocity of pulsar.