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The Milky Way Galaxy. The Milky Way Galaxy. Panoramic View. 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. . Studies of Galactic Structure.

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The Milky Way Galaxy

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The Milky Way Galaxy

Panoramic View


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.


Studies of Galactic Structure

The first serious study of the Milky Way was carried out

by William and Caroline Herschel in the late 1700’s.

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.


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.


We now know that this model is wrong because the

Herschels did not know about interstellar dust.


In the early 1900’s, a Dutch astronomer, J.C. Kapteyn,

repeated the Herschel’s 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.


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.


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.


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.



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.


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


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


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.


In the Milky Way Galaxy,

stars of all ages are uniformly distributed through the


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.


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

Population I

Population II


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.

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.

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.


21 cm


21 cmRadiation

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.


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


We can use this information and

Newton’s form of Kepler’s Third


to estimate the mass of the galaxy. Interior to the orbit

of the sun, this mass is about 9  1010 M.


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


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 => 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



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.

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.


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


This mechanism is

thought to be

responsible for the

flocculent spiral



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??

Radio image of

Galactic center.

Stellar motions near

the galactic center