Spiral Density Waves and the formation of Spiral Arms
A density wave is analogous to a traffic jam on a highway: Cars moving into the jam slow and bunch up and them speed
up and spread out when the leave the jam. In a spiral density wave, the GMC’s slow down and bunch up as they enter
the jam and collapse to form stars. The first stars to form are the very luminous O, B and A stars that are many
thousands of times more luminous than the Sun and they (these new stars) define the limits of the spiral arms. The
other new stars (cooler F,G, K and M) keep moving out of
the density wave.
In the Milky Way the orbital speed of stars in the vicinity
of the Sun is 220 km/s while the speed of the spiral arms
is only 125 km/s. So the GMC’s enter the spiral density
wave from behind and exit the front of the density was
new stars.
You can see the relationship between the GMC’s, the
new O,B and A stars and the density wave in the image of the Whirlpool Galaxy below.
Implication: How do the number of stars between the spiral arms compare to the number of stars in the spiral arm?
Implication: A wave needs a source of excitation to exist. For example, there are no waves on the surface of a pond until
a stone is thrown into the pond. What is the excitation for these density waves?