The Milky Way

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					             The Milky Way
•   Center, Shape
•   Globular cluster system
•   Galactic coordinates
•   Size of the Milky Way
Milky Way from Iowa
Milky Way from Australia
Milky Way from Australia
  How do we locate the center of
        the Milky Way?

• Can’t see center directly with visible light
  because of obscuring clouds in the plane of
  the Galaxy

• Look above the plane of the galaxy
             Globular clusters
•   Compact, spherical group of stars
•   Up to several 100,000 stars
•   All stars formed together, same age
•   Form a halo around the Milky Way
Globular cluster system
       Globular cluster system
• Centered on the center of the Milky Way
• Extends far above and below the plane

• By observing globular clusters, we can
  determine the direction to the center of the
  Milky Way (and, later, our distance from
  the center).
Globular clusters in Sagittarius
Galactic coordinates
Globular clusters are found primarily in what
             part of the Milky Way?

 1. Halo
 2. Disk
 3. Central 1 kpc
 4. Bulge
 5. Disk and Bulge
        Size of the Milky Way

• How do we find the size of the Galactic
  halo or the distance to the Galactic center?

• Star counts, as described in the text book,
  are a bad way.
Pulsating stars
      Use known luminosity to
        determine distance
Flux versus luminosity relation
 Flux A LuminosityA  DistanceB 
                    Distance 
                               
 Flux B LuminosityB          A 

We can figure out the luminosity of a
pulsating star by timing the pulsations.
Since, we can measure its flux, we can
then find the distance to the star.
Size of Milky Way
      Luminosities of components
• With distances, can do star counts, correct for
  dust absorption and estimate luminosities
  –   Disk: 19109 L
  –   Bulge: 2109 L
  –   Halo: 2109 L
  –   Total: 23109 L

• Average star is dimmer than the Sun, total
  number stars is ~200 billion.
           Review Questions
• What are globular clusters? How are they
  distributed in the Galaxy?
• What are Galactic coordinates?
• Why do some stars pulsate? Why are
  pulsating stars useful in measuring distances?
• What is the size and shape of the Milky Way?
    Orbits of stars in the Milky Way

•   Stellar orbits in disk and halo
•   Finding the mass from the orbit
•   Mass of the Milky Way
•   Rotation curves
•   Dark matter
   What keeps the planets in orbit
         around the Sun?

• The force of gravity from the Sun

• To orbit, a planet at a particular distance
  from the Sun must have a particular orbital
 Orbits of stars in the Milky Way

• The orbit of a star is determined by the total
  mass lying inside the orbit
• By measuring the speed of the star’s orbit
  and its distance from the center, we can
  figure out the total mass lying inside the
  orbit of the star
Stellar Orbits in the Galaxy
              • Stars in the disk all orbit the
                Galactic center:
                  • in the same direction
                  • in the same plane (like planets do)
                  • they “bobble” up and down
                      • this is due to gravitational pull
                        from the disk
                      • this gives the disk its thickness
              • Stars in the bulge and halo all
                orbit the Galactic center:
                  • in different directions
                  • at various inclinations to the disk
                  • they have higher velocities
                      • they are not slowed by disk as
                        they plunge through it
                      • nearby example: Barnard’s Star
     The distance between the Sun and the
         Galactic center is closest to:

1.   800 pc
2.   8 kpc
3.   8,000 kpc
4.   8 Mpc
5.   Variable, between 1 and 10 kpc
               Mass of the Galaxy

We can use Kepler’s Third Law to estimate the
 mass of the Milky Way inside the Sun’s orbit
  • Sun’s distance from center of Milky Way:
    8,500 pc = 1.8 x 109 AU
  • Period of Sun’s orbit around the center of the
    Milky Way: 230 million years (2.3 x 108 yr)
Simplified form of Kepler’s 3rd law
     using convenient units
       M1  M 2  2
       Where M in solar masses
             a in AU
             P in Earth years
Mass of the Milky Way within the
           Sun’s orbit

  M1  2 
           1.8 10
                                    
                                   9 3
                                           110     11

      P    2.3 10 8
                                    2

Where M in solar masses, a in AU, P in Earth years

• Mass within Sun’s orbit is 1011 M
• Total mass of MW Galaxy is ~ 1012 M
• Total number of stars in MW Galaxy  2 x 1011
         Kepler’s 3rd Law applied to Binary Stars

                       4       2
                                a P
                                 3   2

                    G(m1  m2 )
• G is gravitational constant
• G = 6.67·10-11 m3/kg-s2 in SI units
• m1, m2 are masses (kg)
• P is binary period (sec)
• A is semi-major axis (m)
        Kepler’s 3rd Law applied to Galaxy

 4 2
         r P
          3   2
                      Where M(r) is mass inside r (kg)
GM ( r )
        Change from P to velocity = v
                     4         4r 
                          2                       2
  P                        r 
      v             GM (r )     v 

                 GM (r )
Rotation curves
Rotation curve of the Milky Way
Rotation curve of Milky Way
Mass of the Milky Way

              density  R-2
               Dark Matter
• Dark – it doesn’t produce light (any kind)
• Does have mass, produces gravity
• Nature is unknown
• Might be normal matter in a form that
  doesn’t emit much light – very small and
  dim star, little black holes
• More likely it is elementary particles other
  than normal matter
     What properties of the sun could be
      used to measure the total mass
      enclosed within the sun's orbit?

1.   mass and orbital speed
2.   mass and distance from the center
3.   mass and age
4.   orbital speed and distance from the center
       The Milky Way in various

• Milky Way in infrared, radio
• The 21 cm line of Hydrogen
Milky Way from Australia
           Scattering of light
• Blue light is scattered more
• Red light is transmitted more

• Stars seen through dust appear redder than
  they really are
• If we want to try to see through dust, what
  kind of light should we use?
Electromagnetic spectrum
              ---------- radio ------------
Milky Way in optical light
Milky Way in infrared light
Milky Way in radio waves
Hydrogen emits 21 cm radio waves
Same effect in other
atoms is used to do
magnetic resonance
imaging (MRI)
                Spiral arms

• Tracers of spiral structure
• Objects found in spiral arms
• How are the spiral arms formed?
Tracing spiral arms
  Spiral arms can be traced from the
positions of clouds of atomic hydrogen

To do this yourself,
read 19.5-19.6
21 map of spiral arms
        Tracers of spiral arms

• Young stars and related objects also trace
  spiral arms

• Emission nebulae = H II regions
• Molecular clouds
• Clusters of young (O and B) stars
Spiral arms
 Why can't we see visible radiation from
   the central region of the galaxy?

1. No visible light is emitted by the central
   region of the galaxy.
2. Interstellar dust blocks our view.
3. Too many stars are in the way.
4. Gravity curves the light away from the
   earth and Sun.
So what causes spiral arms?
Density waves
          Spiral arms are patterns
• According to the density-wave theory, spiral arms are
  created by density waves that sweep around the Galaxy
• The gravitational field of this spiral pattern causes stars
  and gas to slow down near the arm
• This compresses the interstellar clouds, triggering the
  formation of stars
• The entire arm pattern rotates around the Milky Way
  once every 500 million years
Density waves
Density waves
Which of the following objects are not found
 primarily in the spiral arms of the Galaxy?

 1.   white dwarf stars
 2.   HII regions
 3.   supernovas
 4.   O and B stars
               Review Questions

• How can we measure the mass of the Milky Way?
• Why do stars behind dust clouds appear red?
• Why is the sky blue?
• Why are wavelengths of light outside the visible useful
  in studying the Milky Way?
• How is the 21 cm line of Hydrogen produced?
• Describe the spiral arms of the Milky Way and what
  causes them.
The nature of the dark matter in the Milky
                 Way is:

1.   Definitely in the form of brown dwarfs
2.   Probably cold, dark hydrogen molecules
3.   Likely super-massive black holes
4.   Definitely cold gas, unknown composition
5.   Not known
      From the rotation curve of a hypothetical
     galaxy as shown in the figure, one could infer

1.    A concentration of
      dark matter inside
      2 kpc
2.    A spherical
      distribution of
      dark matter
3.    Dark matter only
      outside 2 kpc
4.    Uniformly
      distributed dark
      matter in the disk
5.    No dark matter is
      needed to explain
      this plot
          The Galactic Center

• Center of the Milky Way
• Stars orbiting the black hole
• X-ray flares from the black hole
Fish eye MW
Radio image,
 central 3 ly

• Center is
  the red
  ellipse at
  the center
• Called
  Sgr A*
 Infrared image,
   central 3 ly
• Sgr A* does not
• There are about
  1,000,000 stars
  in the area
  covered by this
• Stars are only
  1000 AU apart.
Stellar Orbits in the Galactic Center
  Mass of Sgr A* can be measured
        using stellar orbits
• Fastest moving star moves at 2% of the
  speed of light, 5000 km/s
• Mass is about 3 million solar masses
• Emits radio and X-rays
• Almost certainly a black hole
X-ray image,
 central 3 ly

Sgr A* is the
bright object in
the center of
the image.

Makes flares
in X-rays.
  The best estimate of the mass of the black
  hole at the galactic center was made using:

1. Observations of nearby star orbits in IR
2. Radio observations of accretion disk
3. Orbit of stars in spiral arms [Kepler’s 3rd
4. X-ray observations of coronal gas
5. Dark matter studies using rotation curves
           Review Questions
• What properties of a star’s orbit around the
  Galaxy enable one to measure the mass
  inside its orbit?
• What is the shape of the rotation curve of the
  Milky Way and why is was it unexpected?
• What lies at the center of the Milky Way?

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