A galaxy is a cluster of stars, stellar remnants, interstellar gas, and dust that is gravitationally bound together. The term comes from the Greek word “galaxias,” which means “stars”. Our galaxy, the Milky Way, is one example of a galaxy.
Hubble’s Classification of Galaxies
The classification of galaxies is based on morphology. This means that the differences in their appearance are based on how they appear visually. This system can be very useful for understanding the different properties of different galaxies. However, it is important to note that it is still a work in progress.
The Hubble classification scheme is based on the appearance of a galaxy in a two-dimensional image. As a result, its relationship with the physical properties of galaxies is only indirect. It has a number of shortcomings. One major problem is that it lacks some information, such as the shape of an individual galaxy.
Hubble’s classification of galaxies includes four main categories. The first category is elliptical. These have a smooth varying brightness, decreasing steadily from the center. Elliptical galaxies are mostly devoid of gas and contain old stars. The second type is lenticular. Ellipticals are further classified into different sub-categories.
The Hubble classification of galaxies provides a simplified way to describe their evolution. Hubble divided galaxies into elliptical and spiral types, and gave each type a corresponding number. In other words, “E0” means an almost-round galaxy, and “E7” indicates a galaxy with extreme ellipticality.
Elliptical galaxies
Elliptical galaxies are one of the four major classes of galaxies. They are smooth and featureless, and are approximately elliptical in shape. Edwin Hubble described the elliptical galaxy in his 1936 work, The Realm of the Nebulae. Hubble also described the spiral and lenticular galaxies. Elliptical galaxies are among the most common types of galaxies, although some are not elliptical.
The shape of elliptical galaxies is a consequence of the way in which they were formed. Because of their violent past, elliptical galaxies lack the gas necessary for star formation. The motion of stars in these galaxies reveals that galactic collisions have destroyed much of the gas in the galaxy. This process then triggers starbursts, which further deplete the gas reserves in the galaxy. Supernovae, on the other hand, blow large clouds of gas into deep space.
Elliptical galaxies are much older than their spiral counterparts. Their red color is caused by the fact that most of their gas has been exhausted, and star formation is not very common. Elliptical galaxies are generally quite dim compared to spiral galaxies.
Spiral galaxies
Spiral galaxies are objects that are surrounded by an extensive halo of dark matter. They are categorized based on the tightness of their spiral arms and the overall size of the central bulge. These differences can also be based on the relative amount of gas and dust in the halo. Type A spiral galaxies have about 2% of their mass made up of gas and dust. Type Sa spiral galaxies contain a smaller percentage of their mass as stars.
Spiral galaxies are dynamic entities, and they are hotbeds of star formation. Their disks are populated with young stars, while their diffuse halos are filled with older stars. The spiral arms are also where new stars are formed, which is why these galaxies are brighter than the disk around them.
The spiral arms of a galaxy are the result of the gravitational attraction of stars and gas. As a galaxy rotates, these waves cause a temporary pileup of stars and gas. The rotating galactic disk then spins these regions into spiral arms.
Bared Spiral galaxies
Bared spiral galaxies are similar to regular spiral galaxies, but feature a big bar of stars in the center. This bar can affect the motion of stars and the arms of the spiral. According to a 2008 study, the presence of a bar in the center of a spiral galaxy indicates its full maturity. Thus, bar spiral galaxies are often older than their arm counterparts.
Barred spiral galaxies are characterized by enhanced CO emission along an optical bar. However, many galaxies with barred morphology do not exhibit an optical bar. In contrast, the nearby spiral galaxy IC342 has a molecular bar but no apparent optical bar. CO emission imaging studies of barred spiral galaxies have also revealed the presence of spiral arms that trace the positions of young stars.
Barred spirals are classified into three subtypes: SBa, SBb, and SBc. The first two have tightly wound arms and the second has an elongated bar. The third category, SBm, describes irregular barred spirals, which are believed to have originated as full barred spirals.
Lenticular galaxies
A lenticular galaxy is a type of intermediate galaxy that contains a large disc but no spiral arms. This type of galaxy is the most common type of galaxy in our solar system. Lenticular galaxies have large-scale discs and are generally brighter than other types of galaxies.
Lenticular galaxies are also found in clusters. They are believed to have developed through ram pressure stripping of gas from spiral galaxies. However, the presence of a nearby companion could also influence the morphology of lenticular galaxies. Moreover, the absence of gas in some field lenticulars may be a result of internal processes.
Lenticular galaxies are quite similar to spiral galaxies in terms of luminosity distribution. However, they are dominated by bulge-dominated early-type galaxies and disk-dominated late-type galaxies. Lenticulars should be flatter than their counterparts in clusters, which suggests that internal factors may have played a role in their transformation.
Lenticular galaxies are brighter than their counterparts in other spiral classes. They have larger bulge-to-disk ratios than spiral galaxies and are more likely to form globular clusters than other types. This means that they may have evolved through mergers.
Variations in galaxy shapes
Scientists have used an artificial intelligence technique to identify variations in galaxy shapes. The technique is capable of removing the noise that clouds our astronomical knowledge. The astronomers developed the method after extensive training and testing on huge datasets derived from supercomputer simulations. They then applied the method to real data from Japan’s Subaru Telescope. The technique was successful in detecting variations in galaxy shapes that corresponded to the accepted models of the Universe.
The classification system divides galaxy shapes into four basic classes. These include spiral galaxies, barred spiral galaxies, elliptical galaxies, and irregular galaxies. The differences between the shapes of the different classes are related to the symmetry of the perturbation and the relative importance of the standing wave pattern.
Hubble first classified galaxies according to their shapes in 1936 in his book “The Realm of the Nebulae”. It has been useful for astronomers ever since. Elliptical galaxies look egg-shaped and are relatively featureless. They show a decrease in brightness as one moves away from the center. Usually, the long axis is four times as long as the short axis.
