The question is very interesting and very relevant to the studies we do. Understanding why galaxies are shaped the way they are and how their structure is related to their formation processes and their evolution is a fundamental part of my research. It is important that you know that the fact that our galaxy is spiral is not a rarity, there are many that have a very similar shape. In fact, most galaxies are spiral. Properties such as its mass, its environment, or the age of the universe influence whether that number is higher or lower. We classify galaxies into two types: elliptical and spiral. When the universe was younger, the fraction of spiral galaxies was much higher than now that the universe is older, since the galaxies are becoming elliptical by internal processes that we call secular. Also, in general, the smaller the galaxies, the more likely they are to be spiral rather than elliptical.
To understand why they have this shape, you have to know that a galaxy is made up of stars, clouds of gas and dust, and dark matter. We can consider this composition as a fluid, that is, the galaxies behave like a fluid. If we have some particles with an initial rotation speed and a well of gravitational potential, fluid physics causes a rotating disc to be generated in the same way that when we open a faucet the water falls down the drain making circles or when we remove the foam from a cup of coffee often spiral structures are formed). The action of gravity and the conservation of angular momentum cause all matter to flatten and form a very finite disk that retains that rotation. When galaxies evolve and get bigger and bigger, that rotation speed is lost due to frictional movements. Then random speeds start to dominate and galaxies become more elliptical, distorting that orderly motion. Collisions between galaxies, which are relatively frequent, also destroy the spiral shape of galaxies. Elliptical galaxies are usually old and have stopped forming stars, and spiral galaxies also usually still have many young stars and even continue to form new stars, as in the Milky Way.
The smaller the galaxies, the more likely they are to be spiral rather than elliptical.
A spiral galaxy is very flattened, with a privileged dimension that is that of the disk, where most of the stars are contained. In that disk there may or may not be spiral arms. The spiral arms are areas in which the density of stars is greater and that works as disturbance waves, the stars that come from behind collide and accumulate and that makes the spiral arms get bigger and bigger (it is precisely in the arms where more stars are formed). Spiral galaxies may have a central bulge, which is more spherical in shape and almost always contains an older stellar population; In addition, they can also have a bar, which is when the spiral arms do not fully reach the center of the galaxy, but there is a structure between them and the center in the shape of a bar. There are many studies on why the bars arise and what implications they have on the evolution of galaxies, but it is still an open research front.
Also, spiral galaxies have a halo that is like a sphere in which there are widely scattered stars with more random speeds than the rest of the galaxy in the disk. And then there is, of course, the dark matter component. All galaxies are formed in halos of dark matter, which is something that we have not been able to observe, although we have been able to detect it. The way dark matter was detected was because the rotation speed we see in the outermost part of galaxies is higher than it would be if dark matter didn’t exist. This fact was discovered by an American astrophysicist named Vera Rubin.
The Milky Way has a bulge, a halo and dark matter and it is very possible that it has a bar, although we are not totally sure. And it has five spiral arms, one of which, well outside, is our solar system.
Helena Dominguez Sanchez She is a doctor in astrophysics and a researcher at the Institute of Space Sciences (ICE-CSIC, Barcelona).
Question sent via email by Noah Martinez Garcia
we answer is a weekly scientific consultancy, sponsored by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco ‘For Women in Science’ program, which answers readers’ questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists), who answer these questions. Send your questions to us at [email protected] or via Twitter #nosotrasrespondemos.
Coordination and drafting: victory bull