Our Galaxy The Milky Way

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Our GalaxyThe Milky Way

• Chapter 23

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Milky Way Facts

• 100 400 billion stars
• 100 billion planets
• SBc galaxy
• 100,000 120,000 ly in diameter
• Sun is 27,000 ly from center

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Overview History

• Our Galaxy is a collection of stellar and
  interstellar matter stars, gas, dust, neutron
  stars, black holes held together by gravity.
• Our view of the Galaxy.

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History of Galactic ( Extragalactic) Astronomy

• 1610 - Galileo discovered the Milky Way is
  comprised of many stars
• 1755 - Immanuel Kant theorized that the galaxy
  has a planar structure, some nebulae might
  actually be entire other galaxies or island
  universes
• 1774 -1781 - Messier catalog compiled including
  Andromeda galaxy as M31
• 1781-1802 - William and Caroline Herschel
  conducted first all-sky survey and cataloged
  5000 nebulae, resolving some into their
  individual stars
• 1845 - William Parsons (Lord Rosse), using a
  72-inch telescope, classified the nebulae into
  featureless ellipticals and whirlpool-like spiral
  nebulae

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History of Galactic ( Extragalactic) Astronomy

• 1785 - Herschel attempted to determine the shape
  and size of Galaxy
• Assumptions
• All stars have same intrinsic brightness
• Star are arranged uniformly throughout the MW
• He could see to the edge of the MW

Herschel could not account well for the effects
of dust. More dust along the disk causes the
distribution of stars to drop-off artificially
objects more than a few kpc from the Sun are
obscured by dust.
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History of Galactic ( Extragalactic) Astronomy

• Kapteyn (early 1900s) used stellar parallax to
  estimate the true size of the Galaxy ? Kapteyn
  Universe
• 10kpc diameter and 2kpc thick with the Sun less
  than a kpc from the center (rather heliocentric)
• Tried to estimate scattering due to ISM gas but
  determined it to be insignificant (most
  obscuration is due to dust absorption which has a
  smaller wavelength dependence)
• Shapley (1919) observed that globular clusters
  are distributed asymmetrically in the sky and
  that if one assumes they are distributed about
  the center of the galaxy, this implies the Sun in
  not near the center of the Galaxy

• Estimated distances to globular clusters using
  variable stars and P-M relationship
• Concluded size to be 100kpc with Sun 15kpc from
  center
• Still wrongdidnt account for dust absorption
  which makes things look further away

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History of Galactic ( Extragalactic) Astronomy
Shapley realized that the globular clusters are
all orbiting the center of our Galaxy and map out
the true extent of the Galaxy.
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History of Galactic ( Extragalactic) Astronomy

• In 1920, the National Academy of Science hosted
  the Great Debate concerning the nature of the
  Spiral Nebulae were they island universes
  outside of the Milky Way?
• Shapley had MW size too big and therefore argued
  NO, they are part of the Milky Way
• Others at that time believed the Kapteyn model of
  a much smaller MW and argued YES, they are
  separate galaxies.

In 1922-1924 Edwin Hubble resolved the
controversy using the superior 100-inch telescope
at Mount Wilson. He observed Cepheid variables
in Andromeda and, using the P-M relation
(distance method), determined its distance to be
300kpc -- well outside of the MW (still off by a
factor of 2 due to poor Cepheid calibrations)
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Morphology of our Galaxy
Also in the early 1900s, the first kinematic
studies of the MW revealed the velocities of
those globular clusters were 250 km/s, much
higher than the mass of the smaller Kapteyn
galaxy model would require. So the galaxy must
contain more stars (and mass) than Kapteyn
originally thought in order to keep the star
clusters from flying off.

• First detailed kinematic model (Lindblad 1927)
  revealed
• A spherical component with random motions (250
  km/s) ? HALO
• A flattened component with rotational motion
  measured at 200 to 300 km/s near the Sun DISK
• A third component, also spherical, exists in the
  center of the galaxy BULGE
• Stars here also move on mostly random orbits

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Morphology of our Galaxy

• The three components of our galaxy (disk, halo
  and bulge) also differ in the mix of the types of
  stars they contain
• Population I Hot, blue stars and young open
  clusters accompanied by gas and dust are
  primarily found in the disk of the Milky Way
• Population II red stars and older globular
  clusters are found in the halo of the Milky Way

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Morphology of our Galaxy
Plotting stars on HR diagrams showed that the
populations differed in age and metallicity
(enrichment of elements heavier than
Helium) Pop I young and metal rich Pop II old
and metal poor
Disk mainly Pop I Halo mainly Pop II Bulge
mix of Pop I and II
Since most stars are smaller than the sun, the
Milky Way actually contains far more than 23
billion stars more like 200 billion
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Differential Rotation

• Everything in the Galaxy orbits around the
  Galactic center
• Material closer to the center travels on faster
  orbits (takes less time to make one full orbit)
• Similar to the way the planets orbit the Sun
• Orbital periods at different distances from the
  Galactic center can tell us the distribution of
  mass in the Galaxy
• Examining motions of stars in the disk are most
  helpful for mapping the distribution of mass

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How do we locate the center of the Milky Way?

• Cant see center directly with visible light
  because of obscuring clouds in the plane of the
  Galaxy

• Look above the plane of the galaxy

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M15
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M13
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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

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Globular cluster system
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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).

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Globular clusters in Sagittarius
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Spiral arms
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Spiral Structure

• Many external galaxies show spiral structure
• Hard to see the morphology of the MW (since we
  are in it!)
• Use other galaxies properties to determine the
  nature of the WM and determine we are in a spiral
  galaxy

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Spiral Structure
In other galaxies, HII regions and OB
associations trace out spiral arms.

• Using spectroscopic parallax, we can place the
  nearby O and B stars at their proper distances.
• They appear to delineate spiral arms.
• Since O and B stars are young objects, spiral
  arms are associated with star formation.
• Problem Cant see very far in the optical

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Tracing spiral arms
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Spiral arms can be traced from the positions of
clouds of atomic hydrogen
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21 map of spiral arms
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Spiral arms
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Mass of the Galaxy

• We can use Keplers Third Law to estimate the
  mass of the Milky Way inside the Suns orbit
• Suns distance from center of Milky Way
  8,500 pc 1.8 x 109 AU
• Period of Suns orbit around the center of the
  Milky Way 230 million years (2.3 x 108 yr)

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Simplified form of Keplers 3rd law using
convenient units
Where M in solar masses a in AU P in Earth years
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Mass of the Milky Way within the Suns orbit

• Where M in solar masses, a in AU, P in Earth
  years
• Mass within Suns orbit is 1011 M?
• Total mass of MW Galaxy is 1012 M?
• Total number of stars in MW Galaxy ? 2 x 1011

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Keplers 3rd Law applied to Binary Stars

• Where
• G is gravitational constant
• G 6.6710-11 m3/kg-s2 in SI units
• m1, m2 are masses (kg)
• P is binary period (sec)
• A is semi-major axis (m)

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Keplers 3rd Law applied to Galaxy
Where M(r) is mass inside r (kg)
Change from P to velocity v
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Rotation curve of Milky Way
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Understanding Rotation Curves
Since the Milky Way rotation curve shows no drop
in velocities beyond the visible edge of the disk
(around R15 kpc), this indicates the presence of
some additional, non-luminous material ? Dark
Matter (matter too dim or weakly interacting to
be detected by current technology)
Even though Dark Matter is detected through
measurements of the Galactic Disk, it is not
necessarily confined to the disk and is likely to
be distributed throughout the Galactic Halo. Most
galaxies appear to exist within these Dark Matter
Halos.
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What is the Dark Matter? Neutrinos low mass
particles that interact via gravity or weak
nuclear force. Most neutrinos, produced from
nuclear fusion, pass easily through the Sun.
Common particles but low mass prevents them from
contributing more than few percent of dark
matter. WIMPs - Weakly Interacting Massive
Particles exotic subatomic particles predicted
via supersymmetric extensions of the Standard
Model. Predicted rest mass of 200-500 times that
of proton. Being searched for in particle
accelerators and large detector experiments.
MACHOs - Massive Compact Halo Objects White
Dwarf Stars, Red Dwarfs (0.2 Msun), Brown Dwarfs
(lt0.08 Msun), Neutron Stars, Black Holes. Recall
that the more massive remnants result from
relatively rare high, mass stellar progenitors.
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The Galactic Nucleus

• Inner 500pc of Galaxy
• Extinction makes optical studies impossible -
  use radio or IR
• Stellar density is 107 stars per pc3 (compared to
  0.1 in the solar neighborhood)
• If the Sun were near the GC
• Nearest star would be 1000AU away
• A million stars brighter than Sirius in the night
  sky
• Total starlight more than 200 times brightness of
  the full Moon

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The Galactic CenterOptical vs Radio observations
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Supermassive Black Hole in the Galactic Center
Radio image (10 pc across) shows feature known as
SgrA West center of this is SgrA
Radio image (80 pc across) shows feature SgrA and
radio filaments
Investigate IR stellar motions in region about
1pc across (a few lightyears) to estimate BH mass
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• Measure proper motions of stars around Galactic
  Center
• Adaptive optics at large telescopes improved
  ground-based resolution to 0.5 in IR (stellar
  positions measured to 0.002)
• 90 stars identified and proper motions (largest
  at 1400 km/s!) centered about SgrA to within
  0.1
• Velocities consistent with Keplarian motion (all
  mass at center)
• M 2.6 /- 0.2 x 106 Msun

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Curvature of the paths near SgrA constrain the
volume of the mass to Schwarzchild radius (few
x 106 km) ? Supermassive Black Hole