Spiral galaxies & Density Wave Theory

Spiral galaxies are the most common type of galaxy, accounting for approximately 60% of all galaxies in the universe. (Other galaxies include elliptical galaxies and elliptical galaxies). Spiral galaxies usually have two, three, or four spiral arms that extend from the center to the edge of the galaxy. The spiral structure was discovered very early, but scientists have been unable to explain its stability for a long time.

What principle of spiral structure is maintained? And how can it last for billions of years?

Proposition 1: The galaxy is a rigid body, and all matter / stars travel at the same velocity (like a sphere). This implies that the orbital velocity of the star is proportional to the orbital radius in all parts of the galaxy <---> further away from the center, the star moving at a faster velocity.

This is NOT EXACTLY different from the actual observation (to a certain distance, the velocity of the star's orbit does not increase anymore).



The image of the M81 spiral galaxy combines data from the Hubble, Spitzer and GALEX space telescopes.

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Hypothesis 2: Stars with different orbital radii move at different angular velocities (the term in the industry is differential rotation). This hypothesis is based on factual observation, but not sufficient.

Because of the different angular velocities, the spiral arms of the galaxy will disintegrate in a very short time (~ a few million years). Meanwhile, the spiral structure must be sustainable billions of years.



Explain the spiral of spiral galaxies.


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Hypothesis 3: As hypothesis 2, adding a star density distribution in a galaxy is NOT a problem. This causes the orbits and angular velocities of stars to change over time, as they are strongly influenced by the gravity of other stars in the vicinity.

This is the density wave theory. This hypothesis is good and complex, take a look at the illustrated video