What are the different types of galaxies near the earth?
Answers
spiral, barred spiral, elliptical, and irregular
Elliptical galaxies have a spherical or oval shape with no internal structure and nearly uniform brightness. The stars in their midst come and go in all directions in a haphazard way.
If they were not moving, they would eventually fall towards the center of the galaxy.
It would collapse under its own gravity, but because of their movement, the stars are subjected to a centrifugal force that prevents them from fall towards the center
The elliptical galaxies are mainly composed of old and red stars and are more or less devoid of young and massive stars.
They contain only a very small amount of gas and dust and the interstellar medium is virtually non-existent. These two facts are related since gas is the necessary ingredient for star formation.
If it is absent, no new star can be formed and all the stars present were formed at a time when interstellar gas was still available. They are necessarily stars with a very long life, therefore not very massive and red.
Spiral galaxies are more complex.
They consist essentially of two elements: a spherical nucleus surrounded by a disk of matter in which a spiral structure appears.
There is a great diversity of form, from a huge nucleus surrounded by small spiral arms to a tiny nucleus with very long arms.
This class of galaxies is further subdivided into two groups: the normal spirals, in which the arms develop directly from the nucleus, and the barred spirals which have a large central bar whose extremities are the starting point of the arms (c. is the case of our Galaxy).
For spiral galaxies, it is not a random internal agitation that prevents gravitational collapse, but an overall rotation of the galaxy. Each star circles around the nucleus and it is this orbital movement that gives rise to a centrifugal force. The global rotation of the galaxy is also responsible for the flattening of the whole and the formation of the disc.
The spirals have stars of all ages and masses, and a large amount of gas and dust. Here too, the two facts are linked because a rich interstellar medium means that there is still enough material to form many stars, hence the presence of young and massive stars.
However, this is only true in the spiral arms and for this reason the arms appear brighter and stand out from the rest to give the galaxy its characteristic appearance.
Between spirals and ellipticals there is an intermediate case, that of lenticular galaxies. Like the spirals, they have a bulky nucleus and a disc, but, like the ellipticals, they lack helical arms and have a relatively poor interstellar medium.
Finally, there is a last category, that of irregular galaxies, which contains all the galaxies that do not fit into the three preceding groups. These galaxies have a mostly misshapen appearance and are very rich in gas and dust. They can be classified in two groups.
First galaxies have an irregular appearance but the distribution of matter is actually very regular, such as the Magellanic Clouds. These are now considered spirals that have failed to complete their training.
The second type is that of truly irregular galaxies, as much from the visual point of view as from that of the distribution of matter. This irregularity can have various origins such as strong activity in the nucleus or a collision with another galaxy.
