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The Milky Way Galaxy


Finding the center of the galaxy ... only infrared and radio can penetrate dust (see COBE photo of galaxy in infrared) ... Our galaxy has 4 major spiral arms. ... – PowerPoint PPT presentation

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Title: The Milky Way Galaxy

The Milky Way Galaxy
  • Our home in the Universe

  • Galaxies groupings of matter within empty
  • contain stars, dust, gas
  • formed in early Universe
  • many types of galaxies
  • groups of galaxies form clusters
  • groups of clusters form superclusters
  • some galaxies collide
  • some galaxies have very active (energetic) cores.
    Black hole at center?

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Finding the center of the galaxy
Finding the center of the galaxy
Finding the center of the galaxy
  • The interstellar dust is concentrated in the
    plane of the galaxy, and therefore obscures our
    view of stars within the galactic plane.
  • Interstellar extinction
  • Like a motorist determines his position on a
    foggy day by looking for tall buildings that
    extend above the fog and are visible, astronomers
    determine the location of the galactic center
    from our view point by looking at stars (Globular
    Clusters) that are lie outside the galactic

Finding the center of the galaxy
  • Astronomers use globular clusters (a class of
    star clusters associated with the galaxy but lie
    outside) in order to locate the galactic center.
  • However, one needs to know the distance to these
    clusters in order to find the center.
  • Astronomers use pulsating variable stars (Cepheid
    variables and RR Lyrae stars) that are found in
    globular clusters to find the distance to these

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Finding the center of the galaxy
  • In early 90s Henrietta Levitt discovered that the
    pulsation period of a Cepheid is directly related
    to it Luminosity.
  • Shortly after Harlow Shapley discovered an
    identical relation in RR Lyrae stars.
  • So, by measuring the pulsation period, we can
    know the luminosity, and from observations we can
    determine the apparent brightness of the star.
    Once luminosity and apparent brightness and
    luminosity are known, the distance can be

Finding the center of the galaxy
  • Using the distance to globular clusters,
    astronomers have determined the galactic center
    to be approx. 8pc away from us.
  • Before, these calculations (the first such
    attempts were made by Shapely in 1920) it was
    believed that the Sun (and thus the Earth) was at
    the center off the Galaxy.
  • Just as Copernicus Galileo dislodged Earth from
    is long held special place at the center of the
    solar system, Shapley managed to show that Earth
    is nowhere near the galactic center.

Physical characteristics of the Milky Way
  • Disk shaped with central bulge
  • Spherical halo of stars and globular clusters
  • Spiral arm pattern
  • About 1011 M?
  • If Milky Way were size of Earth, solar system
    would be size of a cookie
  • Suns location 2/3 from edge to center
  • 240 million year revolution period at Suns

Physical characteristics of the Milky Way
Milky Way in infrared(IR) light a) far IR from
IRAS spacecraft. b) near IR from COBE
observations. The light in b) come from stars in
the plane of the galaxy
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Things in the Milky Way
  • Star clusters
  • globular
  • open (Pleiades)
  • associations
  • Gas (form dark or emission nebulas)
  • Dust (form dark or reflection nebulas)
  • Stars
  • Population I - young stars
  • Population II - old stars

Star classification Young vs. old
  • Population I
  • younger
  • bluer
  • located in spiral arms in disk
  • Population II
  • older
  • redder
  • located in globular clusters (halo) and galactic

Star clusters
  • Globular
  • 105 to 106 stars, large, located in halo and
    nucleus (bulge) of galaxy, older stars (Pop II),
    no gas or dust
  • Open
  • few hundred stars, small, located in spiral arms,
    young (Pop I) stars, no gas or dust
  • Associations
  • very large, lie along spiral arms, very young
    blue stars, still contain gas and dust

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Interstellar MatterGas and Dust
  • Dust
  • extremely sparse
  • very efficient at blocking visible light
  • blocks most of Milky Way from view in disk
  • zone of avoidance is region of sky along
    galactic disk where our view is blocked by dust
  • only infrared and radio can penetrate dust (see
    COBE photo of galaxy in infrared)

NGC 4565 How we would see the milky way if we
could see it edge on. Gas dust can be seen in
the plane of the galaxy
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Interstellar MatterGas
  • Gas
  • 90 hydrogen, 10 helium (by number)
  • can have molecules
  • cold gas detected by absorption spectrum
  • emission nebula emission spectrum, gas heated by
    nearby hot stars. Typically look red in pictures
    (hydrogen emission lines in the red part of
    spectrum). Called HII region.

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Interstellar MatterDust
  • Dust
  • very small (nano- to micro- meter sizes)
  • consists of silicate core with icy surface
  • blocks light (makes distance objects dimmer and
    seem more distant), causes reddening of light
    from objects behind it by scattering blue light
    more than red light
  • reflection nebula reflect starlight. Typically
    look blue in pictures (reflecting light from hot
    blue star)

Spiral Arms
  • How do we see them?
  • Mapped by looking for gas and dust concentrations
    using optical and radio information
  • 21 cm spin flip radio emission line
  • What are they?
  • Higher density regions - more stars per unit
    volume than other regions of Milky Way
  • Why are they?
  • Good question! Density waves?

21 centimeter (radio) line of hydrogen
  • Special radiation emitted by cold hydrogen allows
    us to map out cold hydrogen clouds (otherwise
    they are invisible)
  • detected at radio wavelengths of 21 cm
  • spin-flip transition
  • When the spin of the proton and the electron in a
    hydrogen atom are in the same direction they are
    in a higher energy state.
  • Then an electron can flip its spin and get to a
    lower energy state, and in the process emit
    radiation(photon) with a wavelength of 21cm.

21-cm radio emission from interstellar hydrogen
The entire sky at 21 cm We see that most of the
21cm emission, and therefore, abundance of
hydrogen gas in the galactic plane.
  • Our galaxy has 4 major spiral arms.
  • Our Sun lies in one of the shorter spiral arms -
    the Orion arm

Why are there spiral arms?
  • Density waves - these denser regions are
    created when fast moving interstellar gas dust
    moves into slow moving density waves or spiral
    arms. Then, this fast moving gas and dust gets
    compressed into a nebula.
  • This compression begins the process by which new
    stars are formed.

A density wave on a highway
This process is similar to how a slow moving
truck causes a traffic jam on a highway, creating
a region of higher density of cars.
Density Wave model
Importance of spiral arms
  • Star formation definitely occurs in spiral arms
  • dust clouds seen going in on one side, stars
    going out on the other
  • spiral arms are home to youngest stars (including
    short-lived hot blue stars)

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A spiral galaxy M83
Visible light
We see evidence of star formations in the spiral
arms in other spiral galaxies.
21-cm radio wavelength.
The Galactic Center
  • In the very middle of our galaxy, something wild
    is happening
  • very powerful radio source (other radiations
    absorbed by bulge and disk) at very center only
    about 10AU in size
  • towards constellation of Sagittarius
  • Million solar mass black hole?
  • Streams of antimatter also detected coming out in
    jets from center of galaxy - only black hole can
    do this

Formation of the Milky Way
  • Large cloud of swirling gas collapses to form
    disk and bulge (similar to Solar System)
  • localized collapses form early stars which enrich
    interstellar medium with heavy elements
  • Pop II stars formed early
  • Collapse complete, disk formed, spiral arms form,
    Pop I stars form
  • munching of smaller galaxies occurs
  • Eventually all gas and dust used up and star
    formation will cease

The Question of Dark Matter
  • The spiral arms of our Galaxy suggest that the
    disk rotates
  • The stars, dust and the gas is orbiting the
    galactic center.
  • The rotation rate of our galaxy has been
  • These measurements can be used to find out the
    mass and the mass distribution of our galaxy.

The Question of Dark Matter
  • Recent calculations indicate that the mass of our
    Galaxy to exceed 1012 M?.
  • However, observations from all forms of EM
    radiation (from radio waves to gamma rays) tells
    us that all the stars, dust and gas in the galaxy
    only account for 10 of the mass of the galaxy.
  • Then, what makes up the balance 90 of the mass
    of our galaxy?
  • Missing matter or Dark Matter problem

The Question of Dark Matter
  • This material, which seems not to emit any type
    of EM radiation (not even 21cm radio waves), but
    which is by far the predominant constituent of
    our Galaxy is called Dark Matter.
  • The only way we know of its presence is due to
    the gravitational effects this matter exerts, and
    the corresponding effect on the rotation of our

The Question of Dark Matter
  • The nature of this mysterious dark matter has
    been and still is one of the leading research
    problems in Astrophysics. Current proposals are
  • MACHOs massive compact halo objects. Less than
    1M?, dim objects like brown dwarfs, white dwarfs,
    or black holes.
  • Non zero Neutrino masses Latest research in
    elementary particle physics sees signs that the
    elementary particle, the neutrino may have a
    small but a non zero mass as contrary to the
    predictions of the Standard model.

The Question of Dark Matter
  • Non zero Neutrino masses Latest research in
    elementary particle physics sees signs that the
    elementary particle - the neutrino may have a
    small but a non zero mass. This is contrary to
    the predictions of the Standard model of particle
  • WIMPs Weakly interacting massive particles. This
    is a new class of subatomic particles predicted
    by a new theory (not yet confirmed by
    experiment). These particles have a mass but does
    not emit or absorb any kind of EM radiation.