In our universe there are systems which are made up of large amounts of gas, dust, billions of stars and planets orbiting around some stars, like our Solar System. All these elements are held together by the force of gravity and we call them galaxies.
Galaxies are specially interesting because of the motions they have and the different shapes they acquire. Although the similarities among them were useful for astronomers such as Edwin Hubble, who in 1926, was able to classify them according to their appearance. (If you wish, before going further, you can learn more about the classification of galaxies in the activity Properties of galaxies by clicking on this link: https://voyages.sdss.org/expeditions/expedition-to-galaxies/galaxies-3/).
Hubble´s original classification is still used today, although it has been modified with subsequent discoveries because galaxies with similar shapes may have differences in their properties and behavior. Hubble recognized two main types of galaxies; the irregular ones wich do not have a defined shape and the regular ones wich are also divided into elliptical and spirals. Elliptical galaxies seem to have oval or spherical shapes and spiral galaxies can be of two types; barred wich have a central bar and two large arms, or normal spirals wich have a large number of short arms. Hubble arranged the types of galaxies in a sequence, connecting the elliptical galaxies with the spiral ones in a progression that can be considered as an evolutionary sequence. You can observe this sequence in figure 1.
Most of the galaxies in our universe have been classified into two main categories: spirals which seem to have flat shapes like the M51 galaxy, and ellipticals like M105 which have spherical or elliptical shapes.
Figure 2. Spiral galaxy M51. The Whirpool galaxy. Credits: NASA, ESA, S. Beckwith (STScI) y elHubble Heritage Team (STScI/AURA). Messier-51
Most of the galaxies in our universe have been classified into two main categories: spirals which seem to have flat shapes like the M51 galaxy, and ellipticals like M105 which have spherical or elliptical shapes. Distinguishing between one and other may seem simple if we look carefully at their shapes. But can we rely only on what we can see to classify them? Our eyes may deceive us sometimes, if that is the case we will need to use other galaxy characteristics in order to be certain that we have classify them correctly.
Ever since the galaxies existence was discovered, astronomers have dedicated themselves to study every aspect of them and although there is still much to be discovered yet, for now we are going to focus only on how the objects inside of them move, mainly the gas and stars. As a teaser, we can tell you that now we know that in general spiral galaxies move in an orderly manner which is very different to the way elliptical galaxies move. We can make use of the characteristics of these motions and use them to identify to which category they belong to. Would you like to know how?