A galaxy is a group of gravitationally bound astronomical objects.
Think of planets and their natural satellites, comets and asteroids, stars and stellar remnants (such as neutron stars or white dwarfs), interstellar gases between them, cosmic dust and cosmic rays, dark matter, etc. All of these elements are held together. by the force of gravity which keeps them attracted to each other to form a system. This system is called a galaxy.
The universe is full of galaxies. Scientists have estimated different numbers of galaxies using data collected by telescopes and interplanetary space probes, such as NASA’s Hubble Telescope and NASA’s New Horizon spacecraft. In 2020, they calculated that there were about two trillion galaxies in the observable universe.
As you can imagine, not all of these galaxies have the same characteristics, and they certainly don’t look alike. Astronomers have recognized several types of galaxies based on their visual appearance. This system of morphological classification of galaxies, known as the Hubble Sequence, or Hubble Tuning Fork, was invented by American astronomer Edwin Hubble in 1926, and it is an important part of the study of the evolution of galaxies.
The diagram divides galaxies into categories based on their shape. It’s approximately divided into elliptical galaxies and spiral galaxies. Hubble gave elliptical galaxies numbers ranging from zero to seven, with E0 having an almost round shape and E7 very elongated and elliptical.
Spiral galaxies were given letters from “a” to “c”, with “Sa” galaxies appearing more tightly coiled and “Sc” galaxies appearing more loosely coiled. Spiral galaxies have been subdivided into normal spirals and barred spirals (which have a B in their designation), with barred spirals containing a bar of stars crossing the central bulge.
Lenticular galaxies, designated S0, represent a transition between ellipticals and spirals.
Hubble also discovered that some galaxies did not fit this classification system – they had odd shapes, were very small or very large, etc. These are called irregular galaxies.
The Hubble system was later extended by Gérard de Vaucouleurs, who argued that the rings and lentils are also important structural components of spiral galaxies. De Vaucouleurs’ system retains Hubble’s basic galaxy division but introduces a more elaborate classification system for spiral galaxies based on the presence and types of bars, rings, and spiral arms.
Elliptical galaxies are the most abundant. They have spherical or oval shapes. They are not very active because they don’t have much gas and cosmic dust to form new stars. Therefore, elliptical galaxies are mostly made up of low-mass old stars and are not as bright as other types of galaxies. They tend to contain less gas and dust than spiral galaxies, which means fewer stars are born and existing stars tend to be older and emit more red light. But they are a bit brighter in the center – where the density of stars is greater and where there is most likely a supermassive black hole. Presumably, this black hole provides the elliptical galaxies with the necessary gravity force to hold the system together.
Elliptical galaxies make up about a third of all known galaxies and between 10 and 27 percent of the galaxies in the Virgo Supercluster, a massive concentration of galaxies that encompass the Virgo Cluster and the Local Group, two groups of galaxies that contain the Milky Way Galaxy (our “home” galaxy) and the Andromeda Galaxy, one of our closest “neighbors”.
There are two subtypes of elliptical galaxies based on their size:
- Giant elliptical galaxies can contain up to a trillion stars and span two million light-years, meaning it would take a million years to travel at the speed of light to pass through them. from one end to the other. Astronomers believe that giant elliptical galaxies are formed by merging or colliding with other elliptical galaxies.
According to a study by astrophysicist Daniel P. Whitmire, the giant elliptical galaxies were once again compact. At this point, they could have emitted lethal doses of radiation onto the young planets within them. Therefore, he theorizes that giant elliptical galaxies are unlikely to host potentially habitable planets.
- Dwarf elliptical galaxies are much smaller than typical elliptical galaxies. They generally contain very little gas and have little evidence of recent star formation. However, dwarf elliptical galaxies are more common than giant elliptical galaxies.
One of the best-known dwarf elliptical galaxies is the Sagittarius Dwarf Elliptical Galaxy, which is about 10,000 light-years across and is a satellite galaxy orbiting at a distance of about 50,000 light-years from the center of the planet. Milky Way (about 70,000 light-years from Earth).
Spiral galaxies are considered the most recurrent in our universe. It is believed that around 60% of all galaxies are spiral galaxies.
As their name suggests, these galaxies are spiral-shaped. They consist of a flat, rotating disc of stars, cosmic dust and interstellar gas, which revolves around a central bulge of older, dimmer stars. The bulge is thought to contain a supermassive black hole.
The disk of stars orbiting the bulge separates into arms that encircle the galaxy. These spiral arms contain a wealth of gas and dust and younger stars that glow before their often rapid demise.
The bulge is surrounded by a galactic halo made up of older, dimmer stars that propagate through several globular clusters (spherical groups of stars).
It is not fully understood what process creates and maintains the spiral arms. These galaxies spin differently – everything spins at the same speed, so the time it takes to complete a full rotation increases with distance from the center. This differential rotation also causes any disturbance in the disc to spiral into a spiral shape. If this were the only process involved in the creation of the spiral, we would probably see galaxies with large numbers of tightly wrapped spiral arms. But most spiral galaxies have between two and four main arms.
The researchers believe the spiral shape is also affected by density waves, which pass through the disk and cause stars and gas to “build up” at the ridge.
Our galaxy, the Milky Way, has four spiral arms – two major arms called Scutum-Centaurus and Perseus and two minor arms named Norma and Sagittarius. It also has a number of branches made from fragments of the main arms. The Sun is located in one of these branches of the arm of Sagittarius, called Orion Spur.
Barred spiral galaxy
Barred spiral galaxies are spiral galaxies in which the arms do not extend to the center but connect at the ends of a bar-shaped center composed of bright young stars. According to a 2008 study by NASAbars form when the stellar orbits of a spiral galaxy deviate from their trajectory after a process of destabilization generally linked to the age and evolution of the galaxy.
The stars affected in the spirals begin to describe a more elongated orbit which “stretch” the center of the galaxy, so that it ends up looking like an extended bar. This bar structure channels interstellar gas flows towards the center of the spiral galaxy, which fuels star formation.
About half of the known spiral galaxies have bars. In fact, the Milky Way is officially classified as a barred spiral galaxy.
Lenticular galaxies often share characteristics with elliptical and spiral galaxies.
They are called “lenticular” because they have the shape of a lens. They can be compared to spiral galaxies in that they are surrounded by a galactic bulge and a flat disk. However, they do not have clearly defined spiral arms or spiral arms. Therefore, they do not appear in a spiral shape.
The formation of lenticular galaxies is not clearly understood. According to one theory, lenticular galaxies were once spiral galaxies that “aged” and consumed most of their gas and cosmic dust. In fact, lenticular galaxies do not produce significant numbers of new stars because they lack the material to do so. As a result, they are mostly composed of old stars, like elliptical galaxies. Another important theory is that lenticular galaxies form when two spiral galaxies collide.
Irregular galaxies are so called because they do not have a distinct regular shape, and therefore they have doesn’t fit neatly into any of the Hubble categories.
They lack spiral arms and a nuclear galactic bulge, and overall they tend to look very chaotic. Some astronomers believe that irregular galaxies were originally elliptical or spiral galaxies that suffered from structural alterations due to mergers and/or interactions with other galaxies.
This is likely the case with the Magellanic Clouds, two irregular dwarf galaxies that orbit the Milky Way and were likely affected by its gravitational pull, which warped them into their current irregular shape.
Many irregular galaxies appear to be older than spirals but younger than ellipticals, leading some astronomers to speculate that irregular galaxies may be at an “intermediate” stage.
Irregular galaxies can also be classified as Irregular I (Irr I), which show some structure but not enough to be classified as another type of galaxy, and Irregular II (Irr II), which have no sort of recognizable structure . There are also Irr (irregular dwarf) galaxies.
Irregular galaxies are mostly small and can contain a lot of gas and cosmic dust, as well as old and young stars.
Peculiar galaxies are those that do not fit into any other category of the Hubble classification scheme because they are unusual in shape, size and/or composition.
They are believed to be formed by the collision of two or more galaxies, whose gravitational forces constantly interact with each other. This is why many particular galaxies can also be called interacting galaxies. This is also why they have extremely unusual shapes, a high rate of star formation, and more than one active central core.
Perhaps some of the most famous particular galaxies are the Antennae galaxies, which interact with each other in the constellation Corvus and are expected to fully collide (and become one) in about 400 million years.