The Milky Way Galaxy
A Grand Design

posted Apr 4, 2001, jeb

Outline

• Messier Objects
• Structure of The Milky Way
• Types of Stars
• Star Clusters
• Milky Way Facts
• Measuring the Milky Way
• Orbital Motion
• Evolution
• Spiral Galaxy?
• Mass of the Milky Way Galaxy
• Center of the Milky Way Galaxy
• Supermassive Black Hole
• Summary

Messier Objects

Many fuzzy objects (nebula) are visible in the night sky, and are included in the catalog of Messier Objects, which the 18th century French astronomer Charles Messier compiled to aid him in his search for comets.

The above image of fuzzy objects shows several components of the Milky Way's matter superimposed on an image of the Lagoon Nebula.

• A - North American Nebula - emission nebula
• B - Jewel Box open cluster
• C - Globular Cluster 47 Tucanae
• D - Cygnus Loop - supernova remnant



Nebulae
• emission nebula - glowing gas and dust
  • Orion nebula
• absorption nebula - obscures vision
  • Horsehead nebula
• reflection nebula - reflects starlight, no emitted radiation
  • Pleiades
• planetary nebula - formed by matter ejected by red giant stars - emitted light
  • Ring nebula
  • Dumbbell nebula
• supernova remnants - remains from violent explosion of a star
  . -> often leaves neutron star (pulsar)
  • Crab nebula
• spiral "nebula" - outside the Milky Way Galaxy
  . -> other galaxies
  • Andromeda


Astronomers often refer to objects by their number in the Messier Catalog
For example:
• The Andromeda Galaxy is M31
• The Triangulum Galaxy is M33
• The Whirlpool Galaxy is M51
• The Sunflower Galaxy is M63
• The Blackeye Galaxy is M64
• Bode's Galaxy is M81
• The Cigar Galaxy is M82
• The Southern Pinwheel Galaxy is M83
• The Pinwheel Galaxy is M101
• The Spindle Galaxy is M102
• The Sombrero Galaxy is M104

Structure of the Milky Way

In visible light we see an accumulation of stars, but the structure is not so clear

In infrared, or far-infrared the structure of a disk and a bulge starts to be revealed

Subdivisions


• Galactic Bulge
  • Radius of 6,000 light years
  • Composed of old stars (population II) and young stars (population I)
  
  
  
• Galactic Disk
  • Radius of 60,000 light years
  • old stars (population II) and young stars (population I), gas, and dust
  • Spiral arms
  
  
  
• Galactic Halo
  • Radius of 65,000 light years
  • Includes globular clusters
    • which contain only old stars (population II)
  
  
  
• Dark Halo
  • Radius of 200,000 or 300,000 light years
  • Contains 80%-95% of the mass of the galaxy (dark matter)
  What is this Dark matter?
  • More on this later!

Types of Stars

• younger, metal-rich stars
• found in galactic disk

Population II Stars

• older, metal-poor stars
• found in galactic halo

Nuclear Bulge

• contains old stars (population II)
  • and new stars, closer in to center

Galactic Disk

• contains newer stars (population I - like the Sun)
  • and older stars

Galactic Halo

• contains only old stars (population II)

Star Clusters

1. Galactic (or Open) Clusters
   Hyades cluster
   • Population I stars
   • young
     (like Pleiades)
   
   
   
2. Globular Clusters
   M3
   • Population II stars
   • about 10 - 12 billion years old
   • beautifully symmetrical collections of 10,000 - 1,000,000 stars
   • composed of old stars (Population II) (more than 10 billion years old)
   • all seen in the same general region of the sky
   • 1910's Harlow Shapley tried to explain why they are in the same general region of our sky
     • realized these clusters form spherical halo around point
     • Shapley guesses (he turns out to be right) that the globular clusters are surrounding the center of our galaxy.

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   Milky Way Facts

   Diameter of M.W. Disk	120,000 light years
   Radius of Central Bulge	6,000 light years
   Thickness of M.W. Disk	1,000 light years
   Distance (Earth to Galactic Nucleus)	30,000 light years
   Period for Solar System (sun, earth, etc.) to circle M.W.	225,000,000 years
   Number of Stars in the Milky Way	100,000,000,000 (1011)
   Total Mass of Milky Way	7.2 X 1011MSUN or 1012MSUN

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   Measuring the Milky Way

   How can we measure the distance to objects in the sky?

   Stellar Parallax:
   1/2 angle through which a star's position shifts as earth orbits the sun.
   (actually this only works in determining stellar distances for nearby stars.)
   Nearest Stars:
   • Alpha Centauri complex (triple-star system)
     • Proxima Centauri at 1.3 pc (4.3 ly)
       • 0.76 arc-secs
   • Barnard's Star
     • 1.8 pc (6.0 ly)
       • 0.55 arc-secs

   Since stellar parallax fails beyond 100 parsecs or so, we need other methods

   Standard Candle

   If we know how bright something really is (known as Luminosity or Absolute Brightness) then we can determine from how bright it appears (Apparent Brightness) how far away it is
   

   This follows simply from the inverse square law:
   
   
   

   Pulsating variable stars
   • RR Lyrae variables
     • characteristic light curve
     • periods of 0.5 to 1 day
   • Cepheid variables
     • characteristic light curve
     • periods of 1 to 100 days
   The pulsation period of each of these stars depends on luminosity
   
   Therefore, a measurement of the pulsation period immediately tells us the star's luminosity
   we can see its apparent brightness
   and from the inverse square law, we can determine the distance
   
   This is a standard candle, which allows us to measure distances out to about 15 million parsecs (15 Mpc)

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   Orbital motion of the stars in the Milky Way

   In the disk of the Milky Way, stars and other matter are rotating around the center in a regular pattern, as revealed by Doppler effects
   
   In the galactic halo and the galactic bulge, the motion is largely random
   Summarizing the properties of the Galactic Disk, Halo, and Bulge
   
   

   TABLE 23.1 Overall Properties of the Galactic Disk, Halo, and Bulge

   GALACTIC DISK GALACTIC HALO GALACTIC BULGE Highly flattened Roughly spherical—mildly flattened Somewhat flattened and elongated in the plane of the disk ("football shaped") Contains both young and old stars Contains old stars only Contains both young and old stars; more old stars at greater distances from the center Contains gas and dust Contains no gas and dust Contains gas and dust, especially in the inner regions Site of ongoing star formation No star formation during the last 10 billion years Ongoing star formation in the inner regions Gas and stars move in circular orbits in the Galactic plane Stars have random orbits in three dimensions Stars have largely random orbits but with some net rotation about the Galactic center Spiral arms No obvious substructure Ring of gas and dust near center; Galactic nucleus Overall white coloration, with blue spiral arms Reddish in color Yellow-white

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   Tracing the Milky Way's history

   • The oldest stars (in the globular clusters) orbit the galaxy in elongated elliptical orbits.
   • Galaxy must have been born from huge gas cloud
   • Where are the first stars without heavy metals?

   Very important clue on evolution of galaxy:

   The lower the metal abundance (pop I vs. pop II), the farther the objects are found from the plane of the disk
   (remember, low metal abundance (pop II) means old)
   • stars are born with elemental abundance's of gases of birth
   • stars inherit orbital motion abut galaxy of parent cloud
   • massive stars evolve quickly and spew heavy elements into interstellar medium

   Milky Way has formed as it is through Evolution

   • Huge cloud of primordial gas collapsed into galaxy
   • Broke apart into small clouds -> formed stars
   • First stars (population II) formed in globular clusters more than 10 billion years ago.
   • Galaxy continued to Evolve
   • Sun emerged 5 billion years ago.

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   Is the Milky Way a Spiral Galaxy?

   We observe many spiral galaxies.
   • (NOTE: stars exist between arms - bright (O & B) stars are found "only" in the arms)
   How can we look for spiral structure? - or how do we map out the arms?
   1. galactic (or open) clusters
      • (found in spiral arms)
   2. O & B stars
      • bright, short lived
   3. HI 21 cm line
      • HI is the astronomer's name for hydrogen atoms
      • when the electron "flips its spin", it emits a 21 cm radio wave
      • (radio frequency passes through dust)
   4. HII regions
      • HII ionized Hydrogen
      • (found preferentially in spiral arms)
   5. Molecular clouds ( CO)
   Methods 1, 2, and 4 use visible light, and are limited by dust blockage
   Conclusion:
   There is evidence for arms
   Artist's conception, based on data accumulated

   A Puzzle of the Arms

   Why don't they wrap around the MW?
   • 225,000,000 years for the Sun to circle the Milky Way
   • age of sun is about 4,500,000,000 years
   
   Therefore - the sun has been around the Milky Way 20 times.
   
   If the arms had been around this many times they would have wound up.
   • differential rotation
   
   
   But - arms are formed by new stars
   • (which fade away in 10⁷-10⁸ years)
   
   
   How can arms be formed?

   Remember

   Why don't the arms wrap up around the galaxy?
   Sun takes 225,000,000 years to orbit
   Arms fade away (O & B stars) in about 10,000,000 years and new arms form.

   Theories of Arm Formation

   1. Density wave theory
      Density waves move through galaxy compressing matter as it passes and setting off star formation.
   2. self-propagating star formation (or Supernovae chain reaction)
      One supernova sends out wave which creates new stars and subsequent supernovae.

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   The Mass of the Milky Way Galaxy

   Dark Matter
   • rotation curve for the Milky Way Galaxy
   • This rotation curve means our Galaxy is much larger than the luminous region
     • the luminous region is surrounded by an extensive, invisible dark halo
     • the dark halo contains most of the mass of the Milky Way, in the form of invisible Dark Matter
   • candidates for Dark Matter
     • black holes
     • brown dwarfs
       • low-mass prestellar objects, never stars
     • white dwarfs
       • relic low mass stellar objects, no longer generating energy
     • faint, low-mass red dwarfs
       • Hubble Space Telescope did not see these red dwarfs, so they are ruled out
   • Search for Stellar Dark Matter
     gravitational lensing

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   Center of the Milky Way

   Direction from Earth known from globular clusters
   • Sagittarius
     • clouds of gas and dust obscure view
     • cannot see into center with visible light
       • largest optical telescopes can see about 1/10 the distance to the center
     • can use infrared light, radio, x-rays, gamma-rays
   Radio
   • 1930's Karl Jansky
     • detected static on his receiver at approximately the same time each day (just 4 minutes earlier each day)
     • this turned out to be radio noise from the center of the galaxy which his "directional" antenna pointed at once a day.
   • Very intense radio emission coming from center
     • For example, image produced by very large array ( VLA) in New Mexico (27 radio telescopes)
   Infrared
   • IRAS (Infrared Astronomical Satellite)
   • very intense source from center of MW
   • brightness = 80,000,000 suns (bright)
   • size = 10 light years (compact)
   gamma-rays
   • intense (-rays from matter/anti-matter annihilation
   • corresponds to ten billion tons of anti-matter annihilation per second
   X-rays
   • again - very intense emission
   Also - stars are found to be more closely packed.
   • Distance between about 1/300 the distance between the Sun and nearby stars.

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   Black Hole at the center of the Milky Way?

   Many Astronomers believe this is the only explanation for the data.
   What is a black hole?
   • A region of space in which matter has collapsed to the point where nothing can escape - not even light!
   Many astronomers believe the mass equivalent to about 3,000,000 suns has collapsed to the center of the MW.
   Then as matter continues to be pulled in, large amounts of radiation will be emitted.
   Artist's conception of the Milky Way Galaxy at six scales, each increasing in resolution by factor of 10

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   Review the Doppler effect

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   Summary

   • There are many types of nebula in the Milky Way.
   • The Milky Way is subdivided into a nuclear bulge, a disk, a halo, and a corona.
   • Globular clusters inhabit the halo.
   • Population I and population II stars appear in different regions.
   • Dark matter makes up a large fraction of the matter in the MW.
   • We cannot see the center of the MW with visible light, but infrared, radio, x-rays, and (-rays show much.
   • It is likely that a giant (3,000,000 solar masses) black hole lives at the center.
   • The Milky Way is a spiral galaxy.
   • Solar System (sun and planets) orbits MW every 225,000,000 years.
   • Arms of spiral galaxy are bright due to brightest stars (O & B).
   • Milky Way was formed by collapse of large cloud of gas and dust.