A galaxy is a group of astronomical objects that are bound gravitationally.
Think of planets and their natural satellites, comets and asteroids, stars and stellar remnants (such as neutron stars or white dwarfs), the interstellar gasses between them, cosmic dust, and cosmic rays, dark matter, etc. All these items are held together by the force of gravity that 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 thanks to 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 definitely don’t look the same. Astronomers have recognized several types of galaxies according to their visual appearance. This galaxy morphological classification system, known as the Hubble sequence, or Hubble Tuning Fork, was invented by American astronomer Edwin Hubble in 1926, and it’s a significant part of the study of galaxy evolution.
The scheme divides galaxies into categories based on their shape. It is roughly divided into elliptical galaxies and spiral galaxies. Hubble gave the elliptical galaxies numbers from zero to seven, with E0 galaxies having an almost round shape and E7 very stretched out and elliptical.
The spiral galaxies were given letters from "a" to "c," with "Sa" galaxies appearing more tightly wound and "Sc" galaxies more loosely wound. The spiral galaxies were further sub-divided into normal spirals and barred spirals (which have a B in their designation), with barred spirals containing a bar of stars running through the central bulge.
Lenticular galaxies, designated S0, represent a transition between ellipticals and spirals.
Hubble also found that some galaxies did not fit into this classification system - they had odd shapes, were very small or very large, etc. These are termed irregular galaxies.
The Hubble system was later extended by Gérard de Vaucouleurs, who argued that rings and lenses are also important structural components of spiral galaxies. De Vaucouleurs' system keeps Hubble's basic division of galaxies 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 as they don’t have much gas and cosmic dust to form new stars. Consequently, elliptical galaxies are mostly made of old stars with low mass, and they 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, giving off more red light. But they are kind of brighter at the center — where star density is greater and where there is most likely a supermassive black hole. Presumably, this black hole supplies elliptical galaxies with the force of gravity necessary to keep the system together.
Elliptical galaxies account for around one-third of all known galaxies and between 10-27% of galaxies in the Virgo Supercluster, a mass concentration of galaxies that encompass the Virgo Cluster and the Local Group, two galaxy groups 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 sizes:
- Giant elliptical galaxies, can contain up to a trillion stars and span two million light-years across, meaning that you would need to travel for a million years at the speed of light to traverse them from end to end. Astronomers believe that giant elliptical galaxies are formed by the merger or collision with other elliptical galaxies.
According to a study by astrophysicist Daniel P. Whitmire, giant elliptical galaxies were once more compact. At this stage, they might have emitted lethal doses of radiation to young planets within them. Therefore, he theorizes that giant elliptical galaxies are not likely to harbor potentially habitable planets.
- Dwarf elliptical galaxies are much smaller than typical elliptical galaxies. They generally contain very little gas and have scant 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 around 10,000 light-years wide and is a satellite galaxy orbiting at a distance of roughly 50,000 light-years from the center of the Milky Way (around 70,000 light-years from Earth).
Spiral galaxies are thought to be the most recurrent in our universe. Around 60% of all galaxies are thought to be spiral galaxies.
As their name indicates, these galaxies are spiral-shaped. They consist of a flat, rotating disk of stars, cosmic dust, and interstellar gas, which spins around a central bulge made up of older, dimmer stars. The bulge is believed to contain a supermassive black hole.
The disk of stars orbiting the bulge separates into arms that circle the galaxy. These spiral arms contain a wealth of gas and dust and younger stars that shine brightly before their often rapid demise.
The bulge is surrounded by a galactic halo made of older, dimmer stars that are spread through several globular clusters (spherical groups of stars).
It is not fully understood what process creates and maintains the spiral arms. These galaxies rotate differentially—everything orbits 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 disk to wind up into a spiral form. If this were the only process involved in creating the spiral, we would likely see galaxies with a large number of tightly wrapped spiral arms. But most spiral galaxies have between two and four main arms.
Researchers believe the spiral form is also affected by density waves, which travel through the disk and cause stars and gas to "pile up" at the crest.
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 of fragments of the main arms. The Sun is located in one of these branches off the Sagittarius arm, called the Orion Spur.
Barred Spiral Galaxy
Barred spiral galaxies are spiral galaxies in which the arms do not stretch all the way to the center but connect with the ends of a bar-shaped center made of bright, young stars. According to a 2008 study by NASA, bars form when stellar orbits in a spiral galaxy divert from their path after a process of destabilization that is usually linked to the galaxy’s age and evolution.
The affected stars in the spirals begin to describe a more elongated orbit that “stretches out” the center of the galaxy, so it ends up looking like an extended bar. This bar structure channels interstellar gas inflows towards the center of the spiral galaxy, which fuels star formation.
Approximately 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 both elliptical and spiral galaxies.
They are called “lenticular” because they are in the shape of a lens. They can be compared to spiral galaxies in that they have a galactic bulge and a flat disk surrounding them. However, they do not have spiral arms or clearly-defined spiral arms. Therefore, they don't appear spiral-shaped.
The formation of lenticular galaxies is not clearly understood. One theory is that lenticular galaxies used to be spiral galaxies that have “grown old” and consumed most of their gas and cosmic dust. In fact, lenticular galaxies do not produce an important number of new stars because they have run out of matter to do so. As a result, they are made of mostly old stars, like elliptical galaxies. Another prominent theory is that lenticular galaxies are formed when two spiral galaxies collide.
Irregular galaxies are called this because they do not have a distinct regular shape, and therefore, they do not neatly fit 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 probably affected by its gravitational force, which distorted them into their current irregular shape.
Many irregular galaxies appear to be older than spirals but younger than ellipticals, leading some astronomers to hypothesize that irregular galaxies may be in an ‘in-between’ stage.
Irregular galaxies can also be classified as Irregular I (Irr I), which feature some structure but not enough to be classified as another type of galaxy, and Irregular II (Irr II), which does not have any kind of recognizable structure at all. There are also dIrr (dwarf irregular) galaxies.
Irregular galaxies are most frequently small, and they can contain lots of gas and cosmic dust, as well as both old and young stars.
Peculiar galaxies are those which do not fit in any other category of the Hubble classification scheme as 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 are constantly interacting with each other. This is why many peculiar galaxies can also be called interacting galaxies. This is also why they have extremely unusual shapes, an elevated rate of star formation, and more than one active central nucleus.
Perhaps some of the most famous peculiar galaxies are the Antennae Galaxies, which are interacting with each other in the constellation Corvus and are expected to fully collide (and become one) in about 400 million years.