Interstellar Medium and the Milky Way

Spiral Arms

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The stars and gas and dust clouds in the disk congregate in a spiral pattern. There are four parts to the spiral pattern in our Galaxy called spiral arms. There are many stars that are also in-between the spiral arms, but they tend to be the dimmer stars (G, K, M-type stars). Long-lived stars will move in and out of the spiral arms as they orbit the galaxy. Star formation occurs in the spiral arms because the gas clouds are compressed in the arms to form stars. The very luminous, short-lived O and B-stars and H II regions around them enhance the spiral outline. They outline the spiral pattern the same way Christmas lights around the edges of a house will outline the borders of the house at night. The O and B-type stars live for only a few million years, not long enough to move outside of a spiral arm. That is why they are found exclusively in the spiral arms.

differential rotation can make spiral pattern

Differential rotation provides an easy way to produce a spiral pattern in the disk. Differential rotation is the difference in the angular speeds of different parts of the galactic disk so stars closer to the center complete a greater fraction of their orbit in a given time. But differential rotation is too efficient in making the spiral arms. After only 500 million years, the arms should be so wound up that the structure disappears. Also, the spiral pattern should occupy only a small part of the disk. The observations of other galaxies contradicts this: the spiral arms in spiral galaxies rarely have more than two turns. Galaxies are billions of years old so the spiral pattern must be a long-lasting feature. What maintains the spiral pattern?

differential rotation winds spiral arms too tightly

Density Wave Theory

One popular theory says that the spiral structure is a wave that moves through the disk causing the stars and gas to clump up along the wave---a density wave. The spiral arms are where the stars pile up as they orbit the center. They are like the entrance ramps onto a busy highway during rush hour: when a car comes to the point where many other cars are merging, it slows down and cars jam up. But eventually, the car moves past the ramp and speeds up. A person in a helicopter above the traffic giving reports for the local radio station sees a traffic jam at the entrance ramp even though individual cars are moving through it. Another clump moving down the highway may be seen as cars pile up behind a slow-moving truck.

In a galaxy the spiral region of greater gravity concentrates the stars and gas. The spiral regions rotate about as half as fast as the stars move. Stars behind the region of greater gravity are pulled forward into the region and speed up. Stars leaving the region of greater gravity are pulled backward and slow down. Gas entering spiral wave is compressed. On the downstream side of wave, there should be lots of H II regions (star formation regions). This is seen in some galaxies with prominent two-armed spiral patterns. But there are some unanswered questions. What forms the spiral wave in the first place? What maintains the wave?

Self-propagating Star Formation

Another popular theory uses the shock waves from supernova explosions to shape the spiral pattern. When a supernova shock wave reaches a gas cloud, it compresses the cloud to stimulate the formation of stars. Some of them will be massive enough to produce their own supernova explosions to keep the cycle going. Coupled with the differential rotation of the disk, the shock waves will keep the spiral arms visible.

Computer simulations of galaxy disks with a series of supernova explosions do produce spiral arms but they are ragged and not as symmetrical and full as seen in so-called ``grand-design'' spirals that have two arms. There are spiral galaxies with numerous, ragged spiral arms in their disks (called ``flocculent'' spirals), so perhaps the self-propagating star formation mechanism is responsible for the flocculent spirals.

A ``grand-design''spiral seen face-on.

A flocculent spiral with ragged spiral arms.

Transient Spirals

In this scenario the spiral arms come and go. This behavior is seen in computer simulations of galactic disks. It is possible that all three theories may be correct. Some galaxies (particularly the grand-design spirals) use the density wave mechanism and others use the self-propagating star formation or transient spiral mechanisms. Our galaxy may be an example of a spiral that uses more than one. The spiral density waves could establish the overall pattern in the disk and the supernova explosions could modify the design somewhat.

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last updated: June 27, 2022

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Author of original content: Nick Strobel