When you think of the Milky Way, you probably picture that familiar spiral shape we all have come to know and love. That spiral structure is very common for galaxies, Over three-quarters of the one's we have observed, are in this pinwheel shape. Our own galaxy is actually in a subgroup called barred spiral galaxies. But how exactly spiral galaxies get their shape is still up for debate.
Most galaxies spin around a central point, it's usually a supermassive black hole. It is tempting to think that those spiral arms are practically solid structures, with the same stars all sticking together as they revolve around the centre of the galaxy. If that were true, that would mean that the stars on the tip of the arms have to move faster than the stars near the centre to keep up.
This doesn't jive well with the laws of physics. In reality, objects move faster when they are closer to the point they are orbiting around, otherwise, they would lose the battle with gravity and fall in. This is Kepler's third law and that is why Earth's orbit takes a year while Mercury's is just 88 days.
This means though that if the arms were made up of the same stars then as the galaxy rotates, the arms would get stretched out and wound up like they were in a cosmic taffy machine. Our Sun takes around 250 million years to do a lap of the galaxy, which sounds like a long time but on an astronomical timescale, it's pretty quick.
At those speeds, spiral galaxies should wind their arms into oblivion before any appreciable length of time and we shouldn't be observing as many of these galaxies as we do. Astronomers call this THE WIND-UP PROBLEM and they have come up with two explanations for the arms.
One is that the arms are not material, but are actually the product of a density wave. As stars travel around the galaxy, they will come to areas where they will bunch up for a while and then areas were they are more spread out. This means that even though the stars are moving inside the arms, the arms themselves look pretty stationary. You can think of it as a traffic jam from a distance the jam appears to stay in one place on the highway, but the cars that make up that jam are actually flowing through it.
What would cause these density waves in the first place is still hotly debated. Astronomers have found that spiral arms could be caused by other nearby galaxies or driven by the large bar shape at the centre of some galaxies. But bars and companion galaxies are not universal, so explaining galaxies that have nither with the density wave model is tricky.
The other popular explanation is Stochastic Self-Propagating Star Formation, but its friends call it SSPSF. The basic idea behind it is Star formation in a galaxy, causes a shock wave, that kicks off more star formation. The rotation of the galaxy stretches the bright new stars into the familiar spiral arms and they burn out and die as the chain reaction propagates and arms move on.It is possible SSPSF and the density wave model actually work together and stars are formed in the denser areas, making the arms look brighter with their hot new stars.
So those are the most accepted explanations of spiral information, but confirming them can only be answered after hundreds of millions of years of observation. So if you are looking for a career with job security, might I suggest Astronomy?
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