The Milky Way Galaxy

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The Milky Way Galaxy
The infinitude of creation is great enough to
make a world, or a Milky Way of worlds, look in
comparison with it what a flower or an insect
does in comparison with the Earth.
Immanuel Kant (1724 1804) German philosopher
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WHAT DO YOU THINK?

1. How big is the Milky Way Galaxy?
2. Where is our solar system located?
3. Is there really a HUGE black hole at the center
   (and why werent we sucked into it in 2012 ??)

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Our View of the Milky Way
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Our view

• Clearly a disk shaped, with us inside
• Optical view blocked by dust cold gas
• Infer we probably look like other galaxies
• Spiral?
• Elliptical?
• Neither?

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M51 (enhanced)
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A spiral galaxy edge on
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Some other spirals
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Our view

• Infer we probably look like other galaxies
• gt Milky Way is a Spiral Galaxy!
• We see new, young stars, gas, dust
• We see the pancake shape across our sky
  indicating a disk.
• We can use other wavelengths to confirm structure
  of spiral arms.

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Drawings not photos of our galaxy!
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• Milky Way Structure
• Disk
• Bulge
• Halo

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Regions of the Milky Way Galaxy
diameter of disk 100,000 l.y. (30,000 pc)
radius of disk 50,000 l.y. (15,000 pc)
thickness of disk 1,000 l.y. (300 pc)
number of stars 200 Billion
Sun is in disk, 28,000 l.y. out from center
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Mapping the Galaxy

• Using Optical light
• Cepheid variables in globular clusters
• Distribution around galactic center defines
  location of our solar system
• Using Radio Waves
• Cool Hydrogen Gas emits 21-cm light
• Map out spiral arms

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Mapping the Galaxy

• Using IR light
• Warm clouds of gas/dust locates star formation
  sites in spiral arms
• Using X-rays
• Highest energy emissions from center of galaxy
• Supermassive Black Hole

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Views of the Milky Way
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Mapping the Galaxy- Optical

• Using Optical light
• Cepheid variables in globular clusters
• Distribution around galactic center defines
  location of our solar system

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Cepheid Variable Stars in Globular Clusters
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• Henrietta Leavitt, along with Harlow Shapely,
  used variable stars to determine the direction
  distance to the center

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Globular Clusters

• Not centered around Sun
• Centered about 30,000 light years away
• Towards Sagittarius

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Mapping the Galaxy

• Using Radio Waves
• Cool Hydrogen Gas emits 21-cm light
• Map out spiral arms

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Structure of Milky Way Galaxy

• Disk
• younger generation of stars
• contains gas and dust
• location of open clusters
• Bulge
• mixture of both young old stars
• Halo
• older generation of stars
• contains no gas or dust
• location of globular clusters

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The StarGasStar Cycle
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Stellar Orbits in the Galaxy

• Stars in the disk all orbit the Galactic center
• in the same direction
• in the same plane (like planets orbit our sun)
• they bobble up and down
• due to gravitational pull from the disk
• this gives the disk its thickness

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Stellar Orbits in the Galaxy

• Stars in the bulge and halo all orbit the
  Galactic center
• in different directions
• at various angles to the disk
• they have higher velocities
• not slowed by disk as they plunge through it

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Spiral Arms

• Galactic disk does not appear solid
• Spiral arms are not fixed strings of stars which
  revolve like the fins of a fan.

M 51
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Spiral Arms

• Caused by compression waves which move around the
  disk.
• Increase density of matter at crests
• density waves revolve at different speed than
  individual stars orbit Galactic center
• Note how the spiral arms appear bluer compared to
  the bulge or the gaps between the arms.

M 51
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Does the Milky Way have a Bar?
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Mapping the Galaxy

• Using IR light
• Warm clouds of gas/dust locates star formation
  sites in spiral arms

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Where the new stars are.
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Where the new stars are.
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Views of the Center!
Infrared
Visual
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The Center in Radio X-Rays
Although dark in visual light, there are bright
radio, IR, and X-ray sources at the center of the
Galaxy, known as Sgr A.
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Radio Image of Center of Milky Way SNR
Supernova Remnant Sgr A brightest radio source
in Sagittarius
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11 years of observationin IR 6 stars orbiting
unseen central mass 4 Million x Mass of Sun
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Mapping the Center - IR
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Mapping the Center in IR

• Use IR telescopes to measure orbits of
  fast-moving stars near the Galactic center.
• One star passed within 1 light-day of Sgr A
• using Keplers Laws, mass 2.6 million M?
• What can be so small, yet be so massive?

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Mapping the Galaxy in X-Rays

• Using X-rays
• Highest energy emissions from center of galaxy
• Confirms Supermassive Black Hole

Chandra image of Sgr A
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Mapping the Galaxy in X-Rays

• Rapid flare rise/drop time (lt 10 min)
• Tiny emission region only 20 times the size of
  event horizon of 2.6 million M? black hole.
• Observations consistent w/ supermassive black
  hole at the center of our Galaxy.
• Energy from flare probably came from a
  comet-sized lump of mattertorn apart before
  falling beneath the event horizon!

Chandra image of Sgr A
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Missing Mass?

• The edges of the galaxy orbit center too fast
  to stay attached using gravity from mass we
  see.
• So there must be more mass we dont see pulling
  as well?
• Dark Matter (Missing Mass)

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Summary of Key Ideas
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Discovering the Milky Way

• A century ago, astronomers were divided on
  whether all stars and nebulae are part of the
  Milky Way Galaxy.
• The ShapleyCurtis debate was the first major
  public discussion between astronomers as to
  whether the Milky Way contains all the stars in
  the universe.
• Cepheid variable stars are important in
  determining the distance to other galaxies.
• Edwin Hubble proved that there are other galaxies
  far outside of the Milky Way.

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The Structure of Our Galaxy

• Our Galaxy has a disk about 100,000 light-years
  diameter and about 2000 light-years thick, with a
  high concentration of interstellar dust and gas.
  It contains around 200 billion stars.
• Interstellar dust obscures our view into the
  plane of the galactic disk at visual wavelengths.
  However, hydrogen clouds can be detected beyond
  this dust by the 21-cm radio waves emitted by
  changes in the relative spins of electrons and
  protons in the clouds.

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The Structure of Our Galaxy

• The center, or galactic nucleus, has been studied
  at gamma-ray, X-ray, infrared, and radio
  wavelengths, which pass readily through
  intervening interstellar dust and H II regions
  that illuminate the spiral arms. These
  observations have revealed the dynamic nature of
  the galactic nucleus, but much about it remains
  unexplained.
• A supermassive black hole of about 4 x 106 Msun
  exists in the galactic nucleus.
• The galactic nucleus of the Milky Way is
  surrounded by a flattened sphere of stars, called
  nuclear bulge, through which a bar of stars and
  gas extend. The entire Galaxy is surrounded by a
  halo of matter that includes a spherical
  distribution of globular clusters and field
  stars, as well as large amounts of dark matter.

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The Structure of Our Galaxy

• A disk with at least four bright arms of stars,
  gas, and dust spirals out from the ends of the
  bar in the galactic nuclear bulge.
• Young OB associations, H II regions, and
  molecular clouds in the galactic disk outline
  huge spiral arms where stars are forming.
• The Sun is located about 26,000 light-years from
  the galactic nucleus, between two major spiral
  arms. The Sun moves in its orbit at a speed of
  about 828,000 km/h and takes about 230 million
  years to complete one orbit around the center of
  the Galaxy.

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Mysteries at the Galactic Fringe

• From studies of the rotation of the Galaxy,
  astronomers estimate that its total mass is about
  1 x 1012 Msun. Much of this mass is still
  undetectable.

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Key Terms
dark matter (missing mass) disk (of a
galaxy) distance modulus galactic
cannibalism galactic nucleus galaxy halo (of a
galaxy) microlensing Milky Way Galaxy missing
mass nebula (plural nebulae)
nuclear bulge rotation curve (of a
galaxy) Sagittarius A ShapleyCurtis debate spin
(of an electron or proton) spiral arm synchrotron
radiation 21-cm radio radiation
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WHAT DID YOU THINK?

• What is the shape of the Milky Way Galaxy?
• The Milky Way is a barred spiral galaxy. A bar of
  stars, gas, and dust runs through its central
  region. It has at least four spiral arms and is
  surrounded by a spherical halo of stars and dark
  matter.

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WHAT DID YOU THINK?

• Where is our solar system located in the Milky
  Way Galaxy?
• The solar system is between the Sagittarius and
  Perseus spiral arms, about 26,000 light-years
  from the center of the Galaxy.

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WHAT DID YOU THINK?

• Is the Sun moving through the Milky Way Galaxy
  and, if so, how fast?
• Yes. The Sun orbits the center of the Milky Way
  Galaxy at a speed of 828,000 km/h.