The Milky Way Galaxy

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The Milky Way Galaxy
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The Milky Way Galaxy
Panoramic View
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We now know that our Milky Way is a highly
flattened system of stars, about 25,000pc across,
with the sun about 2/3 of the way from the
center.
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Studies of Galactic Structure
The first serious study of the Milky Way was
carried out by William and Caroline Herschel in
the late 1700s. They attempted to find the
structure of the Milky Way Galaxy by counting the
numbers of stars they could see through their
telescope in different directions in the sky.
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The model of the Galaxy they derived is known as
the Grindstone model they believed the galaxy
was slightly flattened with the sun near the
center.
Sun
We now know that this model is wrong because
the Herschels did not know about interstellar
dust.
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In the early 1900s, a Dutch astronomer, J.C.
Kapteyn, repeated the Herschels study
photographically. His model, now known as the
Kapteyn Universe, was equally wrong. It
situated the sun near the center of the Milky Way
Galaxy.
Again, Kapteyn was unaware of the presence
of interstellar dust.
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Modern Studies of the Milky Way Galaxy
The first real progress in determining the
structure of the Milky Way came about in 1908
when Henrietta Leavitt discovered that Cepheid
Variable stars (and their lower-luminosity
counterparts, the RR-Lyrae stars) obey a
Period-Luminosity Relationship.
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Harlow Shapley discovered RR Lyrae variable
stars in Globular Clusters associated with the
Milky Way. He used the Period-Luminosity
relationship to calculate the distances to these
globular clusters.
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Shapley reasoned that the center of the Globular
Cluster system should coincide with the actual
center of our Milky Way Galaxy. This gave him a
way to find the position of the center of our
galaxy, even though it is obscured by dust as
seen from the Earth.
Sun
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Shapley found that the true size of our Milky Way
Galaxy is much larger than found by both the
Herschels and Kapteyn. Those earlier studies had
been flawed because they did not take into
account absorption of light by interstellar dust.
The distance to the Galactic center is now known
to be about 8400pc (8.4 kpc), from the sun, and
the diameter of the disk of the galaxy is about
25 kpc.
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Where is most of the interstellar dust in the
Milky Way Galaxy found?
In the halo
In the nuclear bulge
In the nuclear bulge and halo
In the disk
In the disk and nuclear bulge
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Components of the Milky Way
The Disk Component The disk is flattened like a
pancake and contains gas, dust, young
middle-aged stars and open star clusters.
In our Galaxy, the disk is organized into Spiral
Arms
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Components of the Milky Way
The Spherical Component Includes the Halo and
the Nuclear Bulge.
The Halo Contains old stars, globular clusters
and little gas and dust. The Nuclear Bulge
Contains a mixture of old and young stars,
along with gas and dust.
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In the Milky Way Galaxy,
stars of all ages are uniformly distributed
through the galaxy
stars of different ages are found in different
parts of the galaxy, with young stars in the halo
and old stars in the disk

stars of different ages are found in different
parts of the galaxy, with young stars in the disk
and old stars in the halo.

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Astronomers identify two populations of stars in
our Galaxy, called Population I and Population II

• Population I Are young and middle-aged,
  metal-rich
• stars found in the disk of the Galaxy.

• Population II Old and metal-poor stars found in
  the
• halo and nuclear bulge of the galaxy

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Population I
Population II
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The Formation of the Galaxy

• The galaxy probably began as a roughly spherical
  
• rotating gas cloud composed entirely of
  hydrogen
• and helium, shortly after the Big Bang.

• Gravity caused this cloud to collapse, primarily
  
• along the rotational axis, leading to
  flattening of
• the galaxy. Some early star formation
  occurred,
• and some of these stars became supernovae and
• enriched the interstellar medium with metals.

• The flattening continued, forming the disk of
  the
• galaxy. The older (first generation) of stars
  retain
• their spherical distribution. Young,
  metal-rich
• stars are found only in the disk.

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Our Galaxy is a Spiral Galaxy. How do we know
this?
The spiral structure of our galaxy can be
observed using

• Optical Tracers These are young stars (O and
  B-type stars), OB Associations and young open
  clusters. These
• objects should not have moved far from their
  places of birth, and so should still trace out
  the spiral arms.

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Local Spiral Arms from optical tracers
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Spiral Arms in our Galaxy can also be observed
using

• Radio Tracers Clouds of neutral hydrogen
  concentrate
• along spiral arms. They emit 21 cm
  radiation.

Visible
21 cm
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21 cm Radiation
is emitted by Neutral Hydrogen in interstellar
clouds
When a Hydrogen atom flips from Parallel to
Anti-Parallel, it emits a radio photon with a
wavelength of 21 cm.
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Spiral Arms in the Galaxy (21 cm radiation)
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Rotation of the Galaxy
Our Sun is in orbit around the center of the
galaxy. The orbital velocity is about 230 km/s.
It takes about 200 million years for our sun to
orbit the galaxy (Galactic year).
We can use this information and Newtons form of
Keplers Third law
to estimate the mass of the galaxy. Interior to
the orbit of the sun, this mass is about 9 ? 1010
M?.
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Rotation of the Galaxy
We can observe the velocities of objects at
different positions in our galaxy and form the
Rotation Curve of our galaxy
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Rotation of the Galaxy
The astonishing thing about this rotation curve
is that it continues to be flat well beyond the
position of the sun, implying that there is a
large amount of matter in the outer parts of the
Galaxy.
The rotation curve gt total mass of the Galaxy
? 1 ? 1012M?.
However, this matter cannot be accounted for in
terms of stars, gas or dust. It is not,
apparently, luminous. Thus it is known as Dark
Matter or Missing Matter. It may account for
90 or more of the mass in our Galaxy!!
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The Origin of Spiral Structure
When we look at external spiral galaxies we find
that these galaxies come in two types.

• Grand Design spiral galaxies in which spiral
  arms
• are well developed and can be traced over
  nearly
• 360o.

• Flocculent spiral galaxies in which the spiral
  arms
• appear to be made of small spiral segments.

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Grand Design Galaxies
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Flocculent Spirals
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What causes the spiral arms?
Density Wave Theory Spiral arms are
compression or density waves which rotate
around the disk of the galaxy. These density
waves cause the compression of molecular
clouds, leading to star formation.
Density waves are thought to be responsible for
the Grand Design spiral galaxies.
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What causes spiral arms?
Self-Sustaining Star Formation In this scenario,
star formation triggers nearby interstellar
clouds to contract, leading to more star
formation. This self-sustained star formation
leads to clumps of new stars which are drawn
out into spiral arms by the differential rotation
of the galaxy.
This mechanism is thought to be responsible for
the flocculent spiral galaxies.
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The Galactic Nucleus

• Is very complex and not well understood the
  nucleus
• is hidden behind thick clouds of dust.
  Observations can
• only be made in the Radio and the Infrared.
  The nuclear
• region is characterized by
• High Star Densities
• Clouds of neutral hydrogen organized into two
• expanding arms
• A powerful radio source, Sagittarius A at the
  center
• emitting synchrotron radiation and thermal
  radiation
• The core of Sagittarius A (A) is associated
  with high
• velocity gas clouds. ? a 106 M? Black
  Hole??

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Radio image of Galactic center.
Stellar motions near the galactic center
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