Download The Milky Way Galaxy

The Milky Way
Announcements
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Assigned reading: Chapter 15
Please, follow this final part of the course
with great care
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It is the most difficult one, less intuitive one
Lots of new notions
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.
From the outside, our Milky
Way might look very much like
our cosmic neighbor, the
Andromeda galaxy.
The Milky Way, Our Galaxy
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Our Sun is part of a large systems of other stars called a “Galaxy”.
The name of our galaxy is “Milky Way” (the word “galaxy” in Greek means
“milky way”
What is a galaxy?
Galaxies are systems made of dark matter, stars, gas, dust
Galaxies are the building blocks of the Universe
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The bottom line about galaxies: made by baryonic, i.e. normal matter (stars, gas
and dust), and dark matter (unknown nature)
Let’s start by studying a special galaxy, our own: The Milky Way
The basic structures of the Milky Way Galaxy: bulge, disk, halo
What happens inside the Milky Way: the-star-gas-star cycle
The motions of the Milky Way: how they happen and where; What do they
mean, and what they are useful for?
The mysterious center of the MW: the super-massive black hole
How Galaxies are really made
Dark matter outweighs
visible matter by 10 to 1
It is the dominant source
of gravity in the Universe
The Universe is made
of Dark Matter
Visible Matter is only
the tip of the iceberg
Yet, even if we detect
Its presence, we still do
not know what Dark
Matter is made of!
The Nearest Bright Galaxy:
the Andromeda Galaxy
About 2 million light
years away.
Angular size (about 2
degrees) ---> the size of
the Milky Way.
Another island universe!
Often galaxies are found in clusters
The Big Questions
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What is the shape of the Milky Way galaxy?
How big is it? What is its shape?
Does it move? Why does it not collapse?
How do we know where we are in the Galaxy?
What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
How do we know the rotating structure of the
Galaxy?
How do we explore the Galaxy and
establish its shape and size?
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Since a galaxy is a system of stars, one way is to just count fainter and
fainter stars in all directions.
Once I reach the most distant stars (which appear as the faintest) at the
edge of the system, I find no more fainter stars.
By repeating this procedure along all directions, I can get an idea of the
shape of the stellar system.
Once I measure the distance to the stars, I also estimate the size.
Herschel and Kapteyn did just that and other things, and concluded that
the Galaxy was a flattened disk (correct), with the Sun at the center
(wrong), and with a radius of about 5 kpc (wrong, too little).
They got the wrong answers because they were looking through dust
and could only see the nearest stars.
It is as when we look around us in the middle of a think fog: all we can
see are the nearest things around us.
The Structure of the Milky Way
Galactic Plane
Galactic Center
The structure 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.
First Studies of the Galaxy
First attempt to unveil the
structure of our 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.
Dust – a hindrance to our study of the Milky Way
A view at visible wavelengths of the galactic plane.
Dust is generated in the late stages of low and high mass stars,
when carbon and silicon is dredged up from the cores and
ejected in stellar winds, planetary nebulae, and possibly
supernova remnants.
The blocking of visible light by dust is called dust extinction.
A Reminder About Scattering
If the dust is thick enough, visible
light is absorbed and scattered and
only the longer wavelengths get through.
In fact, too much dust can block light altogether,
especially UV, Optical and near-Infra-Red light.
This happens in the disk of galaxies
Radio
Microwave
longer wavelength
(redder)
Blocked by
Infrared Interstellar
Visible Dust
UV
X-ray
shorter wavelength
(more blue)
To study the structure of the Milky Way, we
need to measure distances to stars
There are well-tested methods for
measuring distances over short length
scales:
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Radar ranging - good for measuring distances
in the solar system (up to about 0.0001 light
years)
Parallax - good for measuring distances to a
few hundred light years
But what do we do about
objects too far away to use
radar ranging or parallax?
Standard Candles
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If we know an
object’s true
Luminosity
Brightness 
luminosity, we can
2
4  distance
measure its distance
by measuring its
apparent brightness.
An object that has a known luminosity
is called a standard candle.
Standard Candle #1 - Cepheid Variable
Stars
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Cepheid variable stars
have variable brightness
that is very regular.
The period of the
variation can be from
days to weeks – and it
seems to be a reliable
indication of the star’s
luminosity!
Important!! Go refresh
Capter 12, Section 4.
Standard Candels #2: Type Ia supernovae
Cepheid Variable Stars
Henrietta Leavitt
Henrietta Leavitt
(1868-1921).
Luminosity=4D2B
Exploring the Galaxy Using Very
Bright Objects: Clusters of Stars
Two types of star clusters:
1) Open clusters: young clusters of recently
formed stars; within the disk of the Galaxy
Globular Cluster M 19
Open clusters h
and c Persei
2) Globular clusters: old, centrally concentrated
clusters of stars; mostly in a halo around the
Galaxy
Globular Clusters
• Dense clusters
of 50,000 – 1
million stars
• Old (~ 11
billion years),
lower-mainsequence stars
• There are
approx. 200
globular
clusters in our
Milky Way
Globular Cluster M80
1920 Harlow Shapley
Observed that the globular star clusters were centered
about a point that was displaced from the Sun. Shapley
proposed that the point was the center of the Milky Way.
Harlow Shapley's diagram of the distances of
the globular clusters from the Sun.
Globular Clusters and
Understanding our Galaxy
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The globular clusters in the halo of the Milky Way
have told us two important things about our own
galaxy:
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The Sun is not at the center of the galaxy
The galaxy is a much larger system than it appeared
based on early observations
The Anatomy of (the light-emitting
part of) the Galaxy
Basic components. I.
Thin disk:
young stars (Pop I), gas, dust, metal rich
associations, young clusters
Bulge:
old and young stars (Pop I&II), glob.
clusters, some gas, SMBH
Spiral arms:
young stars (Pop I), gas, associations, open
clusters
Halo:
old stars (Pop II), metal poor, glob. clusters
Basic Components. II.
Globular
clusters
Halo
Key Parts:
- disk: supported by
rotation
- halo: supported by
motion pressure
- bulge: supported by
both rotation and
motion pressure
Bulge
Disk
Disk supported by
rotation
Halo supported by
random motion
Bulge supported by
random motion and
small rotation
Suggests formation scenario:
•Bulge old: rapid fragmentation and
collapse of gas
•Disk young: progressive collapse of
gas, dissipation and formation of
rotating disk, star formation
How massive is the Galaxy?
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To measure mass, we need orbital motions
Sun orbits Center of Galaxy at v~220 km/s and distance r~8.5
kpc
Circonference is 2 r; at that speed period of rotation is 240
million years
Thus, mass inside the Sun orbit is:
 MSun = a3 / p2 ~ 0.9 x 1011 Mo
Including the mass outside the Sun, one actually finds:
 MTot ~ 4 x 1011 Mo
However, mass in star (from star counting) is much less:
 MStar ~ 0.8 x 1010 Mo
Rotation of the disk
This is verified only if all the mass is at the center, like in the Solar system.
If there is mass at all radii, then the rotation curve does not decrease; it might stay flat or
even increase!
We measure the mass of galaxies using rotation curves
Dust – a hindrance to our study of the Milky Way
A view at visible wavelengths of the galactic plane.
Dust is generated in the late stages of low and high mass stars,
when carbon and silicon is dredged up from the cores and
ejected in stellar winds, planetary nebulae, and possibly
supernova remnants.
The blocking of visible light by dust is called dust extinction.
So, to examine our own galaxy, we must
use Radio, mm-wavelength, infrared, and Xray telescopes to peer through the
interstellar medium.
Very Large Array
Chandra X-ray Observatory
Infrared view of the sky
Radio Observations are key to
understanding the Disk.
Very Large Array
Interstellar hydrogen emits strongly at 21cm wavelengths.
A full sky image of hydrogen (21 cm emission)
By looking at the Doppler Shift of the 21 cm emission, we can reconstruct
the distribution of objects in the galaxy.
Radio observations help map the galactic disk
You are here
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Looking for 21-cm
wavelengths of light …
 emitted by interstellar
hydrogen
 as we look along the
disk of the Milky Way
(from inside), we see
21-cm photons
Doppler shifted
varying amounts
 this allows the
velocity and mass of
interstellar hydrogen
to be mapped
A Map of the Milky Way Based on
21-cm wavelength light mapping
Spiral Galaxy M83 observed in both
visible light and radio wavelengths.
The Nature of the Spiral Arms
The dominant
structures in the disk
are the spiral arms.
Spiral arms are density
waves that move at
different velocities from
the stars.
What is a density wave?
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
The gas and stars in the galaxy rotate at a different rate than
the spiral arms (density waves)
Survey Question
We find mostly hot, massive stars in the spiral arms
of galaxies because
1) hot, massive stars are preferentially
produced in the spiral arms
2) less massive stars live long enough to rotate out of
the spiral arms
3) supernovae destroy the less massive stars in the
spiral arms
4) there is too high a density in the spiral arms to
create low-mass stars
The Star-Gas-Star Cycle
There is a continuous reprocessing of gas in
the galaxy into stars.
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Stars form from dense gas in molecular clouds
Stars age and then give up their outer layers
(via solar wind, planetary nebula, or supernova)
The ejected gas eventually finds its way back
into an overly dense region and become part of
the next generation of stars.
This process is repeated as long as there is
enough hydrogen around to create new stars.
What characteristic of a star would imply
that many star generations preceded it?
1) a high hydrogen abundance
2) a high helium abundance
3) a high “metal” abundance
4) a low “metal” abundance
5) a low helium abundance
Chandra survey of
the Galactic center using X-ray
light
Red: 1-3 keV Green: 3-5 keV Blue: 5-8 keV
Wang et al. (2002)
X-ray Flare from Sgr A*
Baganoff et al. (2003)
Our Galactic Center
• More than 5000 km/s
at a mere 17 light
hours distance -about 3x the size of
our solar system.
•The dark object has an
implied mass of 4 million
times the mass of the
Sun within this distance.
MPE: www.mpe.mpg.de/www_ir/GC/gc.html
Super Massive Black Holes
(SMBH)
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We now believe that all galaxies harbor a SMBH at their center.
When the SMBH actively accretes gas it liberates an enormous.
amount of energy (AGN, Quasars).
Life would probably not be possible if our SMBH wakes up and
becomes a Quasar.
Astronomers discovered that the larger the mass in stars around a
SMBH, the larger the SMBH itself.
There must be an intimate connection between the formation of stars
in a galaxy and the formation of the SMBH, but nobody knows what
this is.
Maybe SMBH are something intrinsic, or even essential, to the
process of galaxy formation.
Three key things to keep in
mind about the Milky Way
1) The Milky Way is an ecosystem for
stars.
2) The Milky Way is mostly empty space …
but it is rather dusty.
3) The Milky Way barely moves at all on
the scale of a human lifetime.
Discussion questions
• 1) What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
• Radio and Infrared
• what velocity and what period?
– vel. = 220 km/s, period = 240 million years
• True/False - The same stars move along with
the spiral density waves all the time.
– False, the stars orbit the galactic center at a
different rate than the spiral density waves.
Survey Questions
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What is the shape of the Milky Way galaxy?
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How do we know where we are in the Galaxy?
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What wavelengths of radiation effectively
penetrate the dusty interstellar medium?
Survey Questions
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How do we measure the motion of the Milky
Way?
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How is the disk of the Milky Way supported?
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How is the bulge supported?
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How do we know there is a super-massive black
hole in the center of the Milky Way?