The galaxy is often imagined by us as traditionally shown in encyclopedias and documentaries - a huge spiral of blue smoke, in which bunches of stars hide, in the middle of which the core shines brightly. However, such a "star island" is just one kind of correct structure. After all, there are also wrong galaxies, devoid of pronounced nuclei and sleeves - they huddle in space like an egg broken in weightlessness.
So, what does it take to call an object a galaxy?
First, it is the presence of stars and star clusters in it - they make up the lion's share of the visible matter of the galaxy. But only visible: most of the mass of any galaxy is made up of layers of gas and dust, molecular clouds and dark matter. Second, all this wealth must be linked in the gravitational system and rotate around the common center of mass. Usually, it is a galactic center, which we will talk about further - but its absence is not an obstacle.
In addition to internal gravitational interaction, galaxies interact with each other. Smaller "star islands" revolve around large ones - and they build connections with other giants, including the large-scale structure of the universe. But unlike planets and their satellites, galaxies are famous for their "predatory" manners. Our Milky Way is close to absorbing its satellites, the Big and Small Magellanic Clouds, in a couple of billion years' time and then being "eaten" by the Andromeda galaxy.
A visible characteristic of the galaxy is its size - as well as its stellar content and range. However, there is no precision here. There are galaxies that can hold hundreds of millions of stars within a hundred or two light-years radius. But there are others that have thousands of stars scattered over the same hundred light-years. Therefore, the only clear criterion here is the gravity separation from the nearby "islands" and the presence of its own center of mass. Thus, in the Universe at the same time, there are galaxies with several thousand luminaries, and with hundreds of trillions of stars.
As you can see, there are no clear limits to what can be considered a galaxy. Therefore, they are so diverse, often unimaginable. These are super-bright powerful quasars, and the Great Attractor, and huge star fields stretching over millions of light-years. But even the most ordinary galaxies have something to surprise.
Spiral galaxies
The classic spiral galaxy is an elliptical galaxy with spiral sleeves retreating from the center-badge. It also actively rotates (as indicated by the spiral shape) and has pronounced gas and dust components. Sleeves of spiral galaxies differ greatly in composition from the center: they are rich in free visible matter, which is why stars are actively formed. Still, the prevailing number of spiral galaxies has a bar-jumper. Galaxies of this class are the most common in the Universe: they account for 55% of the total number of "star islands".
As a rule, these galaxies have few sleeves, and the spiral is twisted only by a few turns. The exact reason why galaxies do not spin "tighter" is not known. Yes, the stars move around the center of the galaxy very quickly, accelerating closer to the center, and freely migrate from one part of the helix to another. But this is not enough to "freeze" galactic sleeves in space.
One of the most probable theories is that the helix is formed under the influence of density waves. They compress the clouds of gas and dust that enter the sleeves, "fixing" them and activating the starvation. There are mostly massive and bright blue stars that have existed for only a few million years, and therefore hardly change their position. All this contributes to the stability of the sleeves.
Incorrect galaxies
Wrong galaxies are the rarest of the "star islands". They look like torn clouds, and they repeat their structure. They are full of gases, dust, and stars, but there are no main structural elements - spirals, badges, etc. Some of them resemble elliptical or spiral galaxies. Many wrong galaxies have become like this because of the gravitational influence from the side that distorted their shape. But there are "star islands" that have taken on this form by themselves.
A special kind of irregular galaxies are dwarf ones - they are full of gas for star formation, they have few metals (in astronomy - elements other than hydrogen and helium), and they are very compact in size. These three factors create ideal conditions for the formation of very large, bright and short-lived stars. Scientists believe that this is what the first galaxies of our universe looked like. With the help of the James Webb telescope, capable of penetrating 100 million years after the Big Bang, scientists expect to see more wrong galaxies filled with super-giant stars.