What is a galaxy? What Types of Galaxy Are There? Where Do They Come From?

A look at what galaxies are, the types of galaxies that we can see, the "tuning fork" and galaxy formation - where do galaxies come from?

Galaxies are vast collections of stars, gas, and dust that are held together by gravity. They come in a variety of shapes and sizes, and each one has its unique characteristics. Billions of galaxies populate our universe, and everyone is an enormous group of stars that exist in combination in space. We are living in one of these galaxies known as the Milky Way, named after the path of milky light that its stars make across the Earth's sky.

At the start of the twentieth century, we believed that the Universe consisted of just the Milky Way galaxy. It wasn't until 1924 that Edwin Hubble discovered that some of the nebulae previously observed were distant galaxies. Since then, we have discovered hundreds of thousands of galaxies. We estimate there are between 100-125 billion galaxies in the visible Universe. A single telescope image can come with dozens of them and it's simple to assume that the universe is brimming over with them, In fact, however, the galaxies are separated by nonvenomous expanses of empty space. Multiply the diameter of a galaxy by about 20 and that is the distance between it and another one.

What is a Galaxy?

​Galaxies are vast collections of stars, gas, dust and invisible "dark matter" and they are held together by gravity. Recent research suggests that at the very centre of many galaxies, there exists a black hole. Galaxies come in a variety of shapes and sizes, from spiral galaxies with distinct arms to irregular galaxies with no discernible structure.

By any standard, galaxies are massive. Even dwarf galaxies are measured in thousands of light years (1 light year = 9.46 million, million kilometres). By comparison, the Milky Way is about 100,000 light-years across, and the biggest galaxies are over a million light-years across. The smallest galaxies contain a few million stars, whereas the Milky Way contains 500 billion and the biggest contains a trillion stars. each star follows its path around the galaxy's centre. The sun goes around the centre of our galaxy roughly once in 250 million years; that's one galactic year.

What is the Milky Way?

Stars surround Earth whichever way we look out. These stars all belong to our galaxy, the Milky Way Galaxy. The Milky Way is a spiral galaxy. The spiral arms have an abundance of bright young stars and gas clouds and are the major location for star formation. The entire Milky Way galaxy is situated within a spherical halo consisting of globular clusters which contain some of the oldest stars in the galaxy.

The Milky Way Galaxy is an enormous selection of stars, gas, and dust, which in combination make up a disc-shaped body that measures about 100,000 light-years across and about 4,000 light-years thick.

We know there are hundreds of billions of stars in the Galaxy however, we don't know precisely how many. The number of stars has been estimated using a function for the Galaxy's mass. This is regarded as about 1,000 billion solar masses. This figure not only covers the visible stars, gas, and dust but also additional material that hasn't been discovered yet. This "dark matter" is thought to be up to 90% of the Milky Way's overall mass, and we don't know what it is made from, we only know it is there.

The overall mass and our knowledge of the part of the Galaxy we are living in are then used to estimate the number of stars in the Milky Way galaxy. Most astronomers agree between 200 - 500 billion.

Galaxy Formation - Where Do Galaxies Come From?

​Galaxies are formed through a process of gravitational attraction between gas and dust particles. As these particles come together, they form clouds that eventually collapse under their own gravity, forming stars. Over time, these stars come together to form galaxies. The evolution of galaxies is influenced by a variety of factors, including the amount of gas and dust present, the presence of supermassive black holes, and interactions with other galaxies. By studying the formation and evolution of galaxies, astronomers can gain a better understanding of the history and structure of the universe.

Many astronomers currently think that galaxies build up from smaller ones through a series of collisions in a process known as hierarchical galaxy formation. Small gas-rich galaxies in the young universe crashed together and merged to make bigger ones in a galaxy formation process. Copious stars were produced using up gas. Once this gas ran out, elliptical galaxies emerged. Central black holes also consumed gas, limiting the growth of galactic bulges. Later collisions added stars and occasionally gas, meaning discs could grow further or be disrupted. And so the variety of galaxies we see today can be explained. This process accounts for why distant galaxies are bluer and more irregular and why ellipticals are seen in the most clustered regions. The web-like disruption of galaxies - with filaments linking groups and clusters is due to gravity acting on the massive dark matter haloes. Outstanding puzzles include how the first stars ad central black holes formed.

What are the Types of Galaxy?

Galaxies come in four main types and these are classified according to the galaxies' shape and structure. The four are - spiral, barred spiral, elliptical and irregular. This last type has no defined shape or structure so each irregular has an individual appearance.

Galaxy Classification "Tuning Fork" Diagram

Edwin Hubble introduced a classification scheme for galaxies that are referred to today as the "Hubble Tuning Fork Diagram."

This scheme provides for subcategories of both elliptical and spiral galaxies and introduces two new primary types of galaxies, lenticular and irregular. Hubble realised that elliptical galaxies could be classified by how round or flat they look. He classified spiral galaxies by how large and bright their central regions are and how tightly their arms are wound. Hubble also noticed that some spiral galaxies have a bright line, or bar, running through their central regions, and called these barred spiral galaxies. A transition type between the elliptical and spiral galaxies, with a central bulge and a disk but no spiral arms, are known as lenticular galaxies. The final classification, irregular galaxies, are neither spiral nor elliptical and can have any number of shapes.

Spiral Galaxies

Spiral galaxies such as the Milky Way and the Andromeda Galaxy are named for the arcs of bright stars that corkscrew into their centres. They are classified according to how tightly wound the arms are - from type Sa to Sc in Hubble's Sequence, os Sba to Sbc if there is a central bar. M74 pictured is type Sa. The spiral is a density wave embedded in a flattened disc of stars and gas that is arranged around a central bulge. Bright stars form where gas clouds are compressed. The disc is full of young stars and gas and tends to be blue; the bulge appears redder. Discs form when a cloud of gas collapses under its gravity, spinning faster as it shrinks vertically. Spirals are common across space apart from in the centres of galaxy clusters, where discs are easily destroyed by collisions

Barred Spiral Galaxies

Barred spirals have a bar-shaped hub of stars in the centre. The arms spiral out from each end of the bar. The Milky Way is thought to be a barred spiral, although it is difficult to tell from our limited viewpoint. The spirals and barred spirals are further sub-classified according to the size of their central bulge and the tightness of their arms (see the tuning fork below).

Elliptical Galaxies

Shaped like rugby balls, elliptical galaxies are much like the bulges of spirals but lack a disc. They contain little gas and a few stars are being formed within them. Old, red stars are the norm, travelling about the centres on an included elliptical orbit. Elliptical galaxies are often found in groups in the centres of galaxy clusters. At the heart of many elliptical galaxies lie a black hole, the mass of which typically scales with the galaxy's size. Ellipticals are thought to be the result of many collisions between galaxies - resulting in the likes of NGC 1132 pictured. In each smash-up, stars are created until the available gas is used up. Those stars then age and fade. Star formation may also be curtailed by the central black hole consuming gas.

Irregular Galaxies

Irregular galaxies do not fall into any of the other main classification categories. They have no distinctive shape. This may be because they have been distorted in a collision or they may have formed that way. Some dwarf galaxies condensed haphazardly from gas clouds and haven't settled into an organised state. About a quarter of galaxies are irregular, and they were more common still in the young Universe. Examples include the Large and Small Magellanic Clouds, two dwarf galaxies near the Milky Way that can easily be seen with the naked eye in the southern night sky. The former, with its prominent central bar, is shown in the picture.

Lenticular Galaxies

Lenticular galaxies are lens-shaped, their classification falling between spirals and ellipticals. Many are similar to spiral galaxies, containing relatively large discs and bulges but lacking spiral arms. These may be faded spirals in which star formation has ceased.

Others are likely to be the results of galactic collisions, which could have ripped off a part of a larger disc, or shut down star formation after a vigorous burst. Examples of lenticular galaxies include the stunning NGC 5866 in Draco, M84 and M86 two bright galaxies in Virgo.

Unusual Shaped Galaxies

The Antennae galaxies are a merging pair of spiral galaxies undergoing a gentle collision that started a few hundred million years ago. Long, antenna-like trails of stars pulled out from the centre of each galaxy give the pair its name. Billions of stars are being formed due to the disruption. A similar merger could engulf the milky way if it eventually collides with the Andromeda galaxy. The cartwheel galaxy formed when two galaxies collided 200 million years ago. The crash flung out a vast ring of gas, 150,000 light-years across which glows with new stars. In similar collisions the gas ring can be knocked 90 degrees, producing a polar ring galaxy such as NGC 5128 in Centaurus.

Structure of Galaxies

​Galaxies are vast collections of stars, gas, and dust that are held together by gravity. The structure of a galaxy can be divided into several components, including the central bulge, the disk, and the halo. The central bulge is a dense region at the centre of the galaxy that contains a high concentration of stars. The disk is a flattened region that surrounds the central bulge and contains the spiral arms. The halo is a spherical region that surrounds the disk and contains a lower concentration of stars. Understanding the structure of galaxies is crucial to understanding how they form and evolve.

The Central Bulge

The galactic bulge, often known as just the bulge, is a cluster of stars that is part of a bigger stellar formation. In most spiral galaxies, the central cluster of stars is referred to as the bulge. Historically, it was believed that bulges were elliptical galaxies with a disc of stars surrounding them, but high-resolution photographs taken by the Hubble Space Telescope have shown that many bulges sit at the centre of a spiral galaxy. It is presently believed that there are at least two different kinds of bulges: ones that resemble spiral galaxies and ones that resemble elliptical galaxies.

The Galactic Disc

The majority of a galaxy's stars make up the stellar (or galactic) discs. The disk is composed of stars which form the stellar component, and cold gas and dust which form the gaseous component. The majority of the stars in the stellar populations of disc orbit the galactic centre in roughly circular paths, showing very little random motion. Because the motion of the disc material is primarily in its plane (with very little vertical motion), discs can be quite thin. The disc of the Milky Way, for instance, is about 1 kLy thick, while discs in other galaxies might have different thicknesses.

Stellar Halo

Star fields and globular clusters make up the almost spherical population of the stellar halo. The stellar halo surrounds most giant elliptical galaxies in addition to the majority of disc galaxies. Since the stellar halo only makes up a small portion of a galaxy's stellar mass (around 1%), it has a far lower luminosity than the rest of the galaxy.

In spiral galaxies, where the spherical shape of the halo contrasts with the flat disc, the separation between the halo and the main body of the galaxy is most obvious. There is no clear separation between the other parts of the galaxy and the halo in an elliptical galaxy.

Dark Matter Haloes

Galaxies are much more massive than they look. Around 90 per cent of their mass is not luminous stars and gas but an unknown "dark matter". Its arranged in a spherical halo, which governs the motions of the stars within. This invisible cocoon depicted in purple in the image of galaxy cluster Abell 1689 explains why the outskirts of spiral galaxies spin faster than if they were influenced by the number of stars and gas alone. Dark matter also governs how galaxies clump together under gravity to form filaments and clusters. Astronomers are still trying to discern what dark matter is. It must be exotic as it does not absorb or emit light; it may be in the form of subatomic particles. Physicists are looking for candidates through varied experiments such as catching neutrinos in Antarctic ice.

Supermassive Black Holes

Supermassive black holes with millions or billions of times the mass of our sun lurk in the hearts of elliptical galaxies and the bulges of spirals. Their mass scales with the size of the bulge, implying that the black hole governs how big the bulge can grow. By guzzling gas or expelling winds during active phases supermassive black holes may limit how many stars can be formed before the fuel is exhausted.

Intersecting Galaxies and Galaxy Mergers

Although galaxies are usually far apart, sometimes they collide - with spectacular effect. A small galaxy can punch a hole as it passes through a larger one, as happened with the cartwheel galaxy in Sculptor. Long tails of stars and gas are often thrust out when one galaxy grazes another giving each a tadpole-like look. Powerful collisions can rip the hearts of galaxies apart, while the resulting shockwaves can trigger the births of millions of stars.

When a galaxy is captured by another's gravity, they eventually merge, altering the character of the whole. Strands of stars from cannibalised dwarf galaxies can still be seen within the Milky Way. It's thought that collisions are key to how galaxies grow, and what they look like.

Distant Galaxies

With the keen eyesight of the Hubble Space Telescope, astronomers can see across 90 per cent of the Universe. One of the deepest images taken by astronomers so far is the Hubble Ultra Deep Field which in 2004 revealed thousands of distant galaxies - most no more than flecks of light. Because light travels at a fixed speed of no more than 300 million m/s it takes a long time to traverse these vast distances. So when we view the distant Universe we are also peering back in time. Distant galaxies tend to be bluer than ones nearby, suggesting that more stars were being formed in the past. Many of these distant galaxies are irregular in shape as well. There were also more active galaxies in the young Universe.

Our Galactic Neighbourhood of Galaxies

The Milky Way is one of over 40 galaxies in a cluster known as the Local Group. The closest member is the Canis Major Dwarf is colliding with our galaxy and is about 42,000 light years from the galactic centre. The farthest galaxy is about three million light-years away. The two largest galaxies are the Andromeda galaxy and the Milky Way. The large and small Magellanic clouds are two irregular galaxies in the group.

The Future of Galaxy Exploration

As technology continues to advance, so does our ability to explore and study galaxies. With the launch of new telescopes and spacecraft, such as the James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope, we will be able to observe and analyze galaxies in even greater detail. This will allow us to better understand the formation and evolution of galaxies, as well as the role they play in the larger structure of the universe. The future of galaxy exploration is exciting and full of possibilities.