The Milky Way Galaxy

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The Milky Way Galaxy

• Our home in the Universe

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Overview

• Galaxies groupings of matter within empty
  Universe
• 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
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Finding the center of the galaxy
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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
  plane.

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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
  clusters.

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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
  determined.

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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.

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Physical characteristics of the Milky Way

• Disk shaped with central bulge
• Spherical halo of stars and globular clusters
• Spiral arm pattern
• About 1011 M?
• REALLY BIG!
• 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
  location

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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

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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
  bulge

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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)

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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)

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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?

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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.

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

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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.

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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.
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Density Wave model
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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.
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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

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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

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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
  measured.
• These measurements can be used to find out the
  mass and the mass distribution of our galaxy.

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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

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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
  Galaxy.

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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.

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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
  physics.
• 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.