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Published 3rd June 2010 by

Billions of galaxies populate our universe, each one a vast group of stars that exist together in space. We live in one of these galaxies called the Milky Way, named after the path of milky light that it stars make across the Earth's sky.
Astronomical Objects Series
  1. Guide to the Constellations and Mythology
  2. What are Asteroids, Meteors and Comets?
  3. Binary Stars and Double Stars
  4. Variable Stars
  5. Supernova and Supernovae
  6. Types of Nebula and Nebulae
  7. What Is a Black Hole? Black Holes Explained - From Birth to Death
  8. Quasars
  9. Pulsars - The Universe's Gift to Physics
  10. What is inside a Neutron Star?
  11. Gamma Ray Bursts
  12. Kuiper Belt
  13. What is an Exoplanet?
  14. Galaxy Types and Galaxy Formation
  15. The Messier Catalogue
  16. The Caldwell Catalogue
  17. 25 Stunning Sights Every Astronomer Should See

It was once thought to be the only galaxy in the universe, but astronomers now estimate that the universe contains about 125 billion galaxies.

Whichever direction we look we find galaxies. A single telescope image can include dozens of them and it's easy to think that the universe is brimming over with them, In reality, however, the galaxies are separated by vast expanses of empty space. Multiply the diameter of a galaxy by about 20 and that's the distance between it and another one.

What is a Galaxy?

By any standard, galaxies are large. Even the 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 largest 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 largest contain a million stars. each star follows it's own path around the galaxies centre. The sun goes around the centre of our galaxy roughly once in 250 million years; that's one galactic year.

The galaxies are held together by gravity and also contain gas, dust and large amounts of invisible "dark matter". Recent research suggests that at the very centre of many galaxies there exists a black hole.

Galaxy Formation

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 an merged to make bigger ones. 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.

Types of Galaxy

Galaxies come in four main types and these are classified according to a 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.

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 loud of gas collapses under its own gravity, spinning faster as it shrinks vertically. Spirals are common across space apart from in the centres of galaxy clusters, were discs are easily destroyed by collisions

M51 Whirlpool Galaxy

M51 Whirlpool Galaxy


Barred Spirals

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).

NGC1300 Barred Spiral Galaxy

NGC1300 Barred Spiral Galaxy


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 galaxies size. Ellipticals are through 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.

Elliptical Galaxy NGC1316 Cerdit: NASA/Hubble

Elliptical Galaxy NGC1316 Cerdit: NASA/Hubble


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 in a haphazard manner 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.

M82 Cigar Galaxy Irregular Galaxy

M82 Cigar Galaxy Irregular Galaxy


Lenticular Galaxies

Lenticular galaxies are lens-shaped, their classification falling between spirals and ellipticals. Many are similar to spiral galaxies, containing relatively disc and large bulge but lacking the 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.

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.

Intersecting Galaxies

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. For instance, the disc of the larger galaxy in Arp 273 pictured) has been twisted into a rose shape by its companion.

Distant Galaxies

With the keen eyesight of the Hubble Space Telescope, astronomers can see across 90 percent 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 Univers.

Galaxy Classification "Tuning Fork"

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

Hubble Tuning Fork Diagram

Hubble Tuning Fork Diagram

Windows to the Universe

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.

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 in 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.

Our Galactic Neighbours

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.

Photos of Galaxies

For some stunning images of galaxies taken by the Hubble Space Telescope take a peek at HubbleSite.

Tutorial Series

This post is part of the series Astronomical Objects. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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