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The Milky Way Dr Bryce 29:50 Class notices Homework: We are moving towards the end of semester, it is vital that you maximise your grade by completing all your homework CSP observing exercise Exam behaviour The Milky Way galaxy appears in our sky as a faint band of light “All sky view”

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The Milky Way

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the milky way

The Milky Way

Dr Bryce


class notices
Class notices
  • Homework: We are moving towards the end of semester, it is vital that you maximise your grade by completing all your homework
  • CSP observing exercise
  • Exam behaviour
“All sky view”
  • The Milky Way in Visible light

Dusty gas clouds obscure our view because they absorb visible light

This is the interstellar medium that makes new star systems

interstellar medium
Interstellar Medium
  • Can both absorb and emit light
  • Most of the interstellar medium is gas and it is easiest to observe when it forms an emission cloud/nebula
  • Good examples of this include the Orion Nebula
  • Because the gas is predominantly hydrogen we see lines associated with atomic or ionized hydrogen

We see our galaxy edge-on

Primary features: disk, bulge, halo, globular clusters

globular clusters
Globular clusters
  • We know from our H-R diagrams that globular clusters are old
  • One way to map the Milky Way is to consider the distribution of globular clusters
Our interpretation of the Milky Way
  • Disk is thin and wide
  • Note spiral arms and bar

Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:

1.0 x 1011MSun

Sun is about 8kpc from the galactic centre

orbital velocity law
Orbital Velocity Law
  • The orbital speed (v) and distance from the galactic centre (d) of an object on a circular orbit around the galaxy tells us the mass (M) within that orbit

Star-gas-star cycle

Recycles gas from old stars into new star systems

hii regions
HII regions
  • “H two”
  • Strong emission lines
  • A central hot star emits UV photons which ionize the hydrogen
  • When an electron is recaptured by a proton the HII line is emitted
hii regions20
HII regions
  • Require a hot star to have formed in a molecular cloud
  • The hotter the star the larger the HII region can be
  • HII regions tend to be red – see the Rosette Nebula
The Milky Way at X-ray Wavelengths
  • X-ray emission is produced by hot gas bubbles and X-ray binaries

Supernova remnant cools and begins to emit visible light as it expands

New elements made by supernova mix into interstellar medium


Radio emission in supernova remnants is from particles accelerated to near light speed

Cosmic rays probably come from supernovae


Multiple supernovae create huge hot bubbles that can blow out of disk

Gas clouds cooling in the halo can rain back down on disk


Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons

Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules


Molecular clouds in Orion

  • Composition:
  • Mostly H2
  • About 28% He
  • About 1% CO
  • Many other
  • molecules
gas recycling
Gas recycling
  • Stars make new elements by fusion
  • Dying stars expel gas and new elements, producing hot bubbles (~106 K)
  • Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K)
  • Further cooling permits molecules to form, making molecular clouds (~30 K)
  • Gravity forms new stars (and planets) in molecular clouds

Gas Cools

interstellar gas temperature
Interstellar gas temperature
  • Molecular clouds are dense and at low temperatures (~10K)
  • Interstellar gas is much less dense and much warmer (~10,000K)
  • We also see very hot (~1 million K) gas from Supernova shock waves, it is these regions that are responsible for the X-ray bubbles
The Milky Way at 21cm wavelength
  • Neutral hydrogen in confined to the plane of the Milky Way
21cm line
21cm line
  • Associated with the lowest energy level of Hydrogen
  • Doesn’t involve the hydrogen atom interacting with another photon so we can “see” this line anywhere in space
dark nebula
Dark Nebula
  • Associated with interstellar dust
  • Dust particles block the photons from the stars behind them
  • Dust will re-emit in the infra-red
the development of our model
The development of our Model
  • Galileo first observed that the Milky Way is made up of stars and many astronomers have tried to map it
  • For example Herschel used star counts, see below
early models
Early models
  • Were incorrect as they didn’t include the effects of interstellar dust which will dim starlight (this effect is called extinction) and interstellar reddening
  • It is for these reasons that we actually find it easier to study other galaxies rather than the galaxy in which we live

We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light


Halo: No ionization nebulae, no blue stars

 no star formation

Disk: Ionization nebulae, blue stars  star formation


Halo Stars:

0.02-0.2% heavy elements (O, Fe, …),

only old stars

Halo stars formed first, then stopped

Disk Stars:

2% heavy elements,

stars of all ages

Disk stars formed later, kept forming

spiral structure
Spiral Structure
  • We can easily observe spiral arms in other galaxies but within the Milky Way our view is hindered by the effects of interstellar gas and dust

Spiral arms are waves of star formation

  • Gas clouds get squeezed as they move into spiral arms
  • Squeezing of clouds triggers star formation
  • Young stars flow out of spiral arms
cloud collisions
Collisions cause the flattening of the disk

Upwards or downwards motions tend to be cancelled out

Cloud collisions
  • Possible models for rotation
  • Wheel or Merry-go-round
  • Planetary or Keplerian
  • Milky Way doesn’t rotate like either of these models
milky way s rotation curve
Milky Way’s rotation Curve
  • Is “flat”
  • This means that the distribution of mass in the Milky Way continues outwards past the luminous material (stars)
  • The dark matter could be brown dwarfs, white dwarfs, Jupiters, Black holes or elementary particles, they are not emitting light but they are exerting gravitational influence

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H


Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

gravitational microlensing
Gravitational microlensing
  • A dark object in the galactic halo (MACHO) could act as a lens because of the curvature of spacetime around it.
  • Black holes would be the strongest type of microlens

Infrared light from center

Radio emission from center


Swirling gas near center

Orbiting star near center


Stars appear to be orbiting something massive but invisible … a black hole?

Orbits of stars indicate a mass of about 4 million MSun


X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in