The Milky Way Galaxy
A Spiral in Space

April 5, 2017, 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 - 47 Tucanae - globular cluster
• 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
  • Eskimo nebula
  • Cat's Key nebula
• supernova remnants - remains from violent explosion of a star
  . -> often leaves neutron star (pulsar)
  • Crab nebula
• spiral "nebula" - now known to reside outside the Milky Way Galaxy
  . -> other galaxies (this was not realized at the time of Messier)
  • 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 the late 18th Century, long before distances to stars were known, William Herschel tried to estimate the size and shape of our Galaxy
  • Note - it was not even known then that there were galaxies outside the Milky Way
• He recorded all the stars, and created a map of the Galaxy.
  • He did not realize most of the stars were obscured from his view
  • He concluded the Galaxy was a flattened, disk-shaped, with the Sun near the center
  • He greatly under estimated of the size of the Galaxy.


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

Subdivisions


• Galactic Bulge
  • 6 kpc (about 20,000 light-years) across
    4 kpc (about 12,000 light-years) perpendicular to the plane
  • Composed of old stars (population II) and young stars (population I)
  • shaped like football, about half as wide as long
  
  
  
• Galactic Disk
  • 30 kpc (about 100,000 light-years) diameter
    "only" 300 p.c. (about 1,000 light-years) thickness
  • old stars (population II) and young stars (population I), gas, and dust
  • Spiral arms
  
  
  
• Galactic Halo
  • Spherical shape
  • Includes globular clusters
    • which contain only old stars (population II)
  • Radius of about 65,000 light years
  • Absence of gas or dust
  
  
  
• 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
• and in the galactic disk

Galactic Bulge

• thickening near center of galaxy
• 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 mostly old stars (population II)

Star Clusters

1. Galactic (or Open) Clusters
   Hyades cluster
   • Population I stars
   • young
     (like Pleiades)
   
   
   
2. Globular Clusters
   M80
   • 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.

Milky Way Facts

Measuring the Milky Way

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 25 million parsecs (25 Mpc, or Megaparsecs)

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

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

• First stars were formed in a very large region of space, with matter spread out in irregular patterns
• First stars (population II) formed more than 10 billion years ago, and collapsed into globular clusters.
• Huge region of clouds of primordial gas and early stars collapsed into large cloud of matter
• Galaxy continued to Evolve, as slowly spinning cloud collapsed into disk and dense core
• Leaving behind stars formed early as a halo
• Disk rich in gas and dust formed, from which the Sun emerged about 5 billion years ago.

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. Distribution of gas based on HI 21 cm line Doppler measurements
   • 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



So

Why don't the arms wrap up around the galaxy?
Sun takes 225,000,000 years to orbit
• However, arms fade away
  • brightest stars (O & B stars) burn out (and explode as supernovae) in about 10,000,000 years
  • longer living stars (like the G star known as our Sun) and emission nebulae drift away from the bright region of star formation over many millions of years

Theories of Arm Formation

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


• So stars are formed in specific regions that appear as arms
  • and over time the regions fade as stars burn out, or drift away

The Mass of the Milky Way Galaxy

Weighing the galaxy
• The orbital speed of a star or gas cloud around the Galaxy is determined by mass inside the orbit
  Calculation of the mass inside the Sun's orbit
  Kepler's Law: total mass = (orbital size)³ / (orbital period)²
  • orbital size in AU
  • orbital period in years (225,000,000)
  orbital size = 8000 pc x 206,000 AU / pc
  total mass ( within 8 kpc) = 9 x 10¹⁰ solar masses
  or 90 billion times the mass of the Sun
  
  
• Luminous matter of Milky Way extends to about 15 kpc
  luminous matter (stars and gas) with 15 kpc directly measured to be about 6 x 10¹⁰solar masses
  or 60 billion times the mass of the Sun
  Calculation of mass within region of spiral structure and globular clusters (within about 15 kpc)
  total mass (within 15 kpc) = 2 x 10¹¹ solar masses
  or 200 billion times the mass of the Sun
  
  
• rotation curve for the Milky Way Galaxy
  • this implies our Galaxy is much larger than the luminous region
  Calculation of mass beyond visible structure (within about 40 kpc)
  total mass (within 40 kpc) = 6 x 10¹¹ solar masses
  or 600 billion times the mass of the Sun
  
  
• 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




TABLE Mass of the Milky Way Galaxy

Mass in solar mass units Visible matter All matter within orbit of Sun (8 kpc) ? 90 x 109 within luminous region (15 kpc) 60 x 109 200 x 109 larger volume (40 kpc) ? 600 x 109




Dark Matter
Our galaxy is much larger than the size of the luminous portion (globular clusters and disk)
Dark matter halo dwarfs the inner halo of stars and gas
• estimate 2/3 of mass within the luminous region is dark matter
Most of the mass of the Galaxy is invisible dark matter, which we do not understand


• candidates for explanation of Dark Matter
  • Stellar Dark Matter
    • black holes
      rare but could make up some of dark matter
    • 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 examined a globular cluster with extreme sensitiviy
        and did not see the red dwarfs, so they are ruled out
  • exotic subatomic particles
    • WIMPs (Weakly interacting massive particles)
    • direct detection of WIMPs are being searched for in underground detectors
    • the Large Hadron Collider is attempting to produce WIMPs in proton-proton collisions
  • modified law of gravity at large distances
  
  
• There may be more than one species of dark matter
  • for example, there may be many brown dwarfs and low mass stars (invisible) within inner parts of galaxies
  • while the dark matter further out might be composed of exotic particles
  
  
• Lensing Search for Stellar Dark Matter
  gravitational lensing
  brightening of a star during a lensing event
  The objects observed cannot account for most of the dark matter

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, very large array ( VLA) in New Mexico (27 radio telescopes)
    • produces detailed images
    • region of Galactic Center in radio spectrum
      
    • apparent ring of rotating matter of a few parsecs

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.

Black Hole at the center of the Milky Way?

Stars near the Galactic Center are found to be in orbits consistent with those expected when orbiting an mass of about 4 million solar masses.

Many Astronomers believe a supermassive black hole 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 4,000,000 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
• 50 kpc to 0.5 pc scale
  • 50 kpc
  • 5 kpc
  • 500 pc
  • 50 pc
  • 5 pc
  • 0.5 pc

Review the Doppler effect

Summary

• There are many types of nebulae in the Milky Way,
  • but the spiral nebulae are galaxies beyond the Milky Way.
• The Milky Way is a spiral galaxy.
• The Milky Way is subdivided into a galactic bulge, a disk and a halo.
• 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 Milky Way.
• We cannot see the center of the Milky Way with visible light, but infrared, radio, x-rays, and gamma-rays reveal its features.
• It is likely that a giant (4,000,000 solar masses) black hole lives at the Milky Way's center.
• Solar System (sun and planets) orbits Milky Way every 225,000,000 years.
• Arms of spiral galaxies are bright and bluish largely due to brightest stars (O & B), as well as emission nebulae.
• The Milky Way was formed by the collapse of a large region of gas and dust.