Milky Way - Theories

Andrew Conway

Main observations to explain

  1. the rotation curve: speed vs distance from galactic centre
  2. spiral structure
  3. stellar populations: galactic halo, disk and centre

Note that only broad features of these observations are established with certainty. E.g. the galaxy has a spiral structure, but there isn't agreement on the structure of spiral arms.

GAIA - upcoming observations

GAIA - diagram

GAIA info

Source: ESA Copyright: ESA

The Milky Way - rotation

Milky Way rotation

Red - theory; Blue - observed; Yellow - the Sun

Source: Brews ohare CC-BY SA 3.0

Explaining the rotation curve

We need to explain the rotation curve, specifically these two features:

  1. Higher than expected speeds near the galactic centre.
  2. Flat curve beyond the Sun's orbit.

Keplerian orbits

Kepler's third law states that the cube of the radius r is proportional to the square of the period T and the mass M about which the body is orbitting:

r3 = constant × M T2

where M is the mass.

Note: we're considering a circular orbit for simplicity, but the argument holds for an elliptical orbit too.

Keplerian rotation curve

The circumference of a circle is 2π times r, so the speed in an orbit is given by dividing it by the period:

v= 2πr / T

Then, using Kepler's third law, we can substitute for T to give:

v2 = constant × M / r

This tells us two important things:

  1. The greater the mass M, the greater the speed.
  2. The greater the radius r, the smaller the speed.

Mass distribution

Galactic centre

Spiral arms - simplistic

Wave motion

Wave - animation

wave animation

Source: Penn State University CC BY-NC 3.0

M51 - photograph

M51 photograph

Source: NASA/ESA Public Domain

Spiral arms as density waves

See animations here:

Spiral arms and star formation

M81 - spiral galaxy

M81 photograph

Source: NASA/ESA Public Domain

M81 - caption

M81 may be undergoing a surge of star formation along the spiral arms due to a close encounter it may have had with its nearby spiral galaxy NGC 3077 and a nearby starburst galaxy (M82) about 300 million years ago.

M81 is one of the brightest galaxies that can be seen from the Earth. It is high in the northern sky in the circumpolar constellation Ursa Major, the Great Bear. At an apparent magnitude of 6.8 it is just at the limit of naked-eye visibility. The galaxy's angular size is about the same as that of the Full Moon.

This image combines data from the Hubble Space Telescope, the Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX) missions. The GALEX ultraviolet data were from the far-UV portion of the spectrum (135 to 175 nanometers). The Spitzer infrared data were taken with the IRAC 4 detector (8 microns). The Hubble data were taken at the blue portion of the spectrum.

(Edited from

Populations I, II and III

Stars are placed into one of three possible populations:

Formation of the Milky Way