Image source [1]: NGC 1300, an example of a barred spiral galaxy.
Hello friends, today we will talk about a new galaxy theme where I will give a simple explanation about galaxies so that we all learn from our universe. A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
Let's say that a galaxy is a very large set of stars that are held together by their self-gravitation, that is, by the gravitation created by the set of all of them. Our sun (a typical star) is in our galaxy which we call the Milky Way (a typical galaxy), which also contains the order of another 10 ^ 11 stars. The stars allowed us an easy classification. Giving the value of the mass, all the properties were practically fixed, if they were of the main sequence. And those that were not, belonged to well-differentiated types. Galaxies, on the other hand, do not allow themselves to be classified. Each galaxy seems to belong to a type formed by it alone. The fact that galaxies are angularly extensive contributes to this. However, we will see how galaxies are intrinsically more complex. The first classification is due to Hubble. According to this, the galaxies are of five types: Spiral (60%), elliptical (13%), lenticular (22%), irregular (3%) and peculiar (1%). As seen in the image 2.
Image source [2]: Types of galaxies according to the Hubble classification scheme: an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.
Where E = is an abbreviation of elliptical, S = are the spiral arms and a, ab, b, bc, c are the separation of their arms from the body, example: Sa arms very little open.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
Image source [3]: A spiral home to exploding stars.
An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure, and their stars are in some random orbits around the center.
Image source [4]: The giant elliptical galaxy ESO 325-G004.
A lenticular galaxy is a type of galaxy intermediate between an elliptic and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disc galaxies (like spiral galaxies) that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation.
Image source [5]: NGC 4866 is a lenticular galaxy located in the constellation of Virgo.
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.
Image source [6]: NGC 1427A, an example of an irregular galaxy. It is an Irr-I category galaxy about 52 Mly distant.
A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar.
Image source [7]: ESO 162-17 is an example of a peculiar galaxy 40 million light-years away in the constellation Carina.
Active galaxies
In Astrophysics, it is synonymous with temporal variability at scales much lower than the life of an astronomer. Normally, the universe, while being the result of an explosion, seems immutable. If we look at M31 after 10 years, we see exactly the same as today. We could not say the same if we observed in 10 ^ 10 years. But an active galaxy, on the other hand, has appreciable temporal variations, so its activity is striking and prevents us from the existence of phenomena extraordinarily energetic. The entire galaxy does not vary, but only its nucleus, a central point, not resolved angularly or with the best interferometric techniques.
Image source [8]: A jet of particles is being emitted from the core of the elliptical radio galaxy M87.