A short course in
The Milky Way and the ISM
Dr. Maura McLaughlinWest Virginia [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
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
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 the center.
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 CenterThe 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
ClustersTwo types of clusters of stars:
1) Open clusters = young clusters of recently formed stars; within the
disk of the Galaxy
2) Globular clusters = old, centrally concentrated
clusters; mostly in a halo around the galaxy
Globular Cluster M13
Open cluster NGC 1983
Globular Clusters
• Dense clusters of 50,000 – a million stars
• Approx. 200 globular clusters in our Milky Way
• Old (~ 11 billion years), lower-main-sequence stars
Globular Cluster M80
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 M31the Andromeda Galaxy in 1923 using the100” 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
Milky Way Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Our local group is a poor cluster:
> 30 galaxies
1 Mpc diameter
Of bright galaxies,
14 elliptical 3 spiral 4 irregular
Most of galaxies are dwarf ellipticals.
Our Galaxy Cluster: The Local Group
Milky Way Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Largest members are:
Milky Way
Andromeda (M31)
Triangulum (M33)
Andromeda is the largestbut we think MW may bethe most massive.
Our Galaxy Cluster: The Local Group
Milky Way Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Largest members are:
Milky Way
Andromeda (M31)
Triangulum (M33)
Our Galaxy Cluster: The Local Group
Milky Way Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Largest members are:
Milky Way
Andromeda (M31)
Triangulum (M33)
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 nomenclatureNumbers 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 nomenclatureNGC means New General Catalog of nebulae and star clusters, compiled by John Dreyer in 1888.
Contains 8000 objects.
The Structure of the Milky WayDisk 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.
Magnetic field due to electron spin
Opposite magnetic
fields attract => Lower energy
Equal magnetic
fields repel => Higher energy
Magnetic field due to proton spin
Infrared View of the Milky Way
Interstellar dust (absorbing
optical light) emits mostly infrared.
Near-infrared image
Infrared emission is not strongly absorbed and provides a clear view throughout the
Milky Way
Nuclear bulge
Galactic plane
Infrared View of the Milky Way
Near-infrared image
Nuclear bulge
Galactic plane
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 WayWe 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 = 4 billion Msun
M = 25 billion Msun
M = 100 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
massesAdditional mass in an extended
halo:
Total: Approx. 1 trillion solar
massesMost 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
PopulationsPopulation 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
Metal Abundances in the Universe
Logarithmic Scale
All elements heavier than He are very
rare.
Linear Scale
Metals in StarsAbsorption lines almost exclusively from Hydrogen:
Population II
Many absorption lines also from heavier elements (metals):
Population IAt 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
WayQuasi-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
Distances to O/B Associations determined using Cepheid
variables.
O/B Associations trace out 3 spiral arms near the Sun.
Sagittarius
arm
Orion-Cygnus
arm
Perseus arm
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 self-sustaining through
O/B ionization fronts and supernova shock
waves.
The Galactic Center (1)
Wide-angle optical view of the GC region
galactic center
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!
Radio View of the Galactic Center
The galactic center contains a supermassive black hole of approx. 4
million solar masses.
Sgr A*: The center of our galaxy
Many supernova remnants; shells and
filaments.
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 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
becausea) dense interstellar clouds are the birth
places of starsb) dark clouds alter and absorb the light from
stars behind them
The Interstellar Medium (ISM)
Structure of the ISM
• HI clouds (molecular clouds)
• Hot intercloud medium:
The ISM is 99% interstellar gas and comprises 10-15% of the visible mass of MW. It occurs in two main types of clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc
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.
• 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 dispersionPulsars 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 = ne0
D
∫ dl
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??
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