Pretty much just the title. Why does gravity shape some galaxies in a spiral? Why not a square or a another shape?
Thanks!
In: Physics
A galaxy is a blob of millions of stars. This blob is spinning as a way of counteracting its gravitational attraction to itself. The inside has to keep moving faster than the outside. By spinning at different speeds, the blob gets twisted into a spiral.
The spirals in galaxies are regions of great star and dust density. Often, they are waves – a little bit like sound waves – of density that travel in circles around the galactic plane, supporting star formation and aggregation and making themselves visible. Sometimes they are temporary structures of density that have for some reason been sheared into a pattern by external forces, but these are more rare because they do not last as long.
So, their existence is basically a matter of wave dynamics. What processes can create such waves, and what kinds of waves can survive for (from what we can observe of galaxies of different ages) billions of years?
Answering in reverse order, a popular model for how spiral or bar waves can survive because they can take advantage of the orbital motion of stars. Stars close to the center of the galaxy can exchange energy with stars and dust farther away, a bit like the Earth exchanges rotational energy with the moon because of tidal forces. This slows down matter near the center of the galaxy and speeds it up on the outside. Since this process happens more along the arms, they can sustain themselves over time.
This model is popular, but there is still debate around it. Sadly there is even more debate about how these waves form. They can be formed by interaction with nearby galaxies or clusters, but this doesn’t explain all kinds of arms, and we can see arms in galaxies that don’t appear to have had recent interactions with nearby galaxies. So the question of formation is much more open.
Latest Answers