What is a galaxy?

What is a galaxy, and what are the different types of galaxy found in the Universe? Find out about galaxy formation and the history of their study.

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A galaxy is a concentrations of millions or billions of stars, together with gas clouds and pockets of dust, all bound by gravity and swathed in a cocoon of mysterious dark matter.

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Galaxies are among the largest and most beautiful objects in the night sky. These huge swirling masses are where almost every star in the Universe is born, lives and dies.

There are probably over 100 billion of galaxies in the Universe and there are at least a hundred billion stars in our own galaxy the Milky Way, ranging from tiny red dwarfs to blue supergiants.

Some of the largest nearby galaxies appear in the night sky as faint smudges of light, and it was only in the early 20th century that astronomer Edwin Hubble proved that they lie well beyond the Milky Way, thus essentially settling astronomy’s Great Debate.

See galaxies through your telescope with our guide to best galaxies to observe.

Left: elliptical galaxy NGC 4621. Right: Spiral galaxy NGC 1015. Credit: ESA/Hubble & NASA, P. Cote / ESA/Hubble & NASA, A. Riess (STScl/JHU)
Left: elliptical galaxy NGC 4621. Right: Spiral galaxy NGC 1015. Credit: ESA/Hubble & NASA, P. Cote / ESA/Hubble & NASA, A. Riess (STScl/JHU)

Hubble also established that galaxies vary in shape and size. Two-thirds have distinctive spiral patterns, while the rest range from bulbous ellipticals to irregular blobs.

They can be dwarves containing millions of stars or giants harbouring trillions.

Astronomers are still piecing together why this is the case, but galaxy collisions and mergers seem to be important in determining how a galaxy evolves.

Central supermassive black holes also seem to govern how gas is consumed and when stars are formed within these cosmic conurbations.

Galaxy formation

6 beautiful galaxy collisions seen by the Hubble Space Telescope
6 beautiful galaxy collisions seen by the Hubble Space Telescope

Understanding how galaxies form is inevitably tricky. Many astronomers think that galaxies form by being built 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. 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 distribution 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 and central black holes formed.

The discovery of galaxies

Harlow Shapely and Heber Curtis: the astronomers of the Great Debate. Credit: Smithsonian Institution Archives / National Institute of Standards and Technology
Harlow Shapely and Heber Curtis: the astronomers of the Great Debate. Credit: Smithsonian Institution Archives / National Institute of Standards and Technology

It has only been in the past 100 years or so that astronomers have understood what galaxies are, and come to realise their importance.

Before then, they were thought to be spiral-shaped nebulae on the outskirts of our own Galaxy, the Milky Way.

To many astronomers they were a nuisance, prone to getting in the way of observations and easy to confuse with comets.

But over the years, interest in these objects grew and so did the controversy about their size. Were they small but nearby, or colossally huge and far away?

On 26 April 1920 two astronomers, Harlow Shapley and Heber Curtis, argued on this topic at what would come to be known as the Great Debate.

Shapley maintained that the Universe consisted of only one galaxy, our own Milky Way, while Curtis said that we lived in just one of many.

In the years that followed, techniques enabling astronomers to measure distance in space enabled Edwin Hubble to measure the distance to the Andromeda Galaxy at 900,000 lightyears (though this number has since been revised up to 2.5 million lightyears).

This placed it far outside the limits of Shapley’s galaxy. This meant that galaxies were huge.

Types of galaxies

Generally speaking, there are three main types of galaxy: elliptical, spiral and irregular, although this is only part of the story. There are many more kinds of galaxy that don’t fit exactly into these tight definitions. Below we’ll take a look at some of the different types of galaxy that exist in the Universe.

Spiral galaxies

Messier 51, also known as the Whirlpool Galaxy, is one of the most famous examples of a beautiful spiral galaxy. Credit: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)
Messier 51, also known as the Whirlpool Galaxy, is one of the most famous examples of a beautiful spiral galaxy.
Credit: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)

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, or Sba to Sbc if there is a central bar. M74, pictured below , is type Sa.

Galaxy M74 Credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)
Galaxy M74 Credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)

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 own 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.

Elliptical galaxies

Elliptical galaxy NGC 1132. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: M. West (ESO, Chile)
Elliptical galaxy NGC 1132. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: M. West (ESO, Chile)

Shaped like rugby balls, elliptical galaxies are much like the bulges of spirals, but lack any disc. They contain little gas, and few stars are being formed within them.

Old, red stars are the norm, travelling about the centre on inclined elliptical orbits.

Elliptical galaxies are often found in groups in the centres of galaxy clusters.

At the heart of many elliptical galaxies lies 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, above. 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 Galaxy NGC 4485 Hubble Space Telescope, 16 May 2019
Irregular Galaxy NGC 4485, as seen by the Hubble Space Telescope, 16 May 2019.

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 ordered 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 be seen easily with the naked eye in the southern night sky.

Lenticular galaxies

Lenticular galaxy NGC 5866. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: W. Keel (University of Alabama, Tuscaloosa)
Lenticular galaxy NGC 5866. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: W. Keel (University of Alabama, Tuscaloosa)

Lenticular galaxies are lens shaped, their classification falling between spirals and ellipticals

Many are similar to spiral galaxies, containing a relatively small 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 result of galaxy collisions, which could have ripped off 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, below, and M84 and M86, two bright galaxies lying near the centre of the vast Virgo Cluster.

Interacting galaxies

The Cartwheel Galaxy, by the Chandra X-ray Observatory and Hubble Space Telescope. Credit: X-ray: NASA/CXC; Optical: NASA/STScI.
The Cartwheel Galaxy, by the Chandra X-ray Observatory and Hubble Space Telescope. Credit: X-ray: NASA/CXC; Optical: NASA/STScI.

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 (above).

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 birth 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.

Unusually-shaped galaxies

The Antennae Galaxies. These two galaxies started to interact millions of years ago and will eventually merge together into one. Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration
The Antennae Galaxies. These two galaxies started to interact millions of years ago and will eventually merge together into one. Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration

The Antennae Galaxies, seen above, 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 when it eventually collides with the Andromeda Galaxy in an event known as the Andromeda-Milky Way collision.

The Cartwheel Galaxy formed when two galaxies collided 200 million years ago. The crash flung out a vast ring of gas, 150,000 lightyears across, which glows with new stars.

In similar collisions, the gas ring can be knocked 90º, producing a ‘polar ring’ galaxy such as NGC 5128 in Centaurus.

Distant galaxies

Hubble Ultra Deep Field
The galaxy-studded vista of the Hubble Ultra Deep Field. Credit: NASA, ESA, H. Teplitz, M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

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 around 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.

Active galaxies

Barred spiral galaxy UGC 6093 is an active galaxy, meaning it has an active galactic nucleus. Material is dragged towards the central supermassive black hole, heating up and causing the galaxy's core to shine brightly. Credit: ESA/Hubble
Barred spiral galaxy UGC 6093 is an active galaxy, meaning it has an active galactic nucleus. Material is dragged towards the central supermassive black hole, heating up and causing the galaxy’s core to shine brightly. Credit: ESA/Hubble

In active galaxies, the light from the accretion of gas, which is spiralling into a central supermassive black hole, outshines everything around it.

Some active galaxies shine so brightly that they can be seen right across the Universe. Around 10% also emit vast jets of energetic particles, which are visible to radio telescopes.

Dark matter in galaxies

A dark matter halo mapped in galaxy Abell 1689. Credit: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University)
A dark matter halo mapped in galaxy Abell 1689. Credit: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d’Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University)

Galaxies are much more massive than they look. Around 90% of their mass is not in luminous stars and gas, but in unseen ‘dark matter’.

It’s 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 above, explains why the outskirts of spiral galaxies spin faster than if they were influenced by the quantity 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.

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What’s clear is that we may know a lot about galaxies, their history and evolution, and the role that dark matter plays in their composition, but we still have a lot to learn.

The Rose Galaxy by Mark Large, Colchester, UK. Equipment: Altair Astro 10
The Rose Galaxy by Mark Large, Colchester, UK. Equipment: Altair Astro 10″ RCT, Skywatcher AZ EQ6-GT, QSI 683wsg-8 fitted, Astrodon filters, SBGI ST-i.
Arp 286 by Ron Brecher, Ontario, Canada. Equipment: SBIG STL-11000M, Baader LRGB filters, 10
Arp 286 by Ron Brecher, Ontario, Canada. Equipment: SBIG STL-11000M, Baader LRGB filters, 10″ ASA astrograph, Paramount MX, QHY5 guide, 80mm refractor, FocusMax, TheSkyX.
Barnard's Galaxy and The Little Gem by Kevin R. Witman, Cochranville, Pennsylvania, USA. Equipment: Stellarvue 102ED refractor, iOptron iEQ45 GEM, Modified Canon XS.
Barnard’s Galaxy and The Little Gem by Kevin R. Witman, Cochranville, Pennsylvania, USA. Equipment: Stellarvue 102ED refractor, iOptron iEQ45 GEM, Modified Canon XS.
NGC 7479 by John Slinn, Loxwood, W. Sussex, UK. Equipment: Vixen VMC200L, modded Canon 450D, PhD, Losmandy G11.
NGC 7479 by John Slinn, Loxwood, W. Sussex, UK. Equipment: Vixen VMC200L, modded Canon 450D, PhD, Losmandy G11.
Obscured Galaxy IC342 by Terry Hancock, Michigan, USA. Equipment: Astro-Tech AT12RC, AP 2.7
Obscured Galaxy IC342 by Terry Hancock, Michigan, USA. Equipment: Astro-Tech AT12RC, AP 2.7″ Reducer, Paramount GT-1100S German Equatorial Mount, QHY16200A mono CCD, QHYOAG-M Off Axis Guider, Optolong filters.
IC 342 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
IC 342 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
IC 342 by Dan Crowson, Dardenne Prairie, Missouri,USA. Equipment: SBIG ST-8300M, Astro-Tech AT90DT
IC 342 by Dan Crowson, Dardenne Prairie, Missouri,USA. Equipment: SBIG ST-8300M, Astro-Tech AT90DT
IC 1613 by Dan Crowson, Animas, New Mexico, USA Equipment: SBIG STF-8300M, Astro-Tech AT12RCT.
IC 1613 by Dan Crowson, Animas, New Mexico, USA Equipment: SBIG STF-8300M, Astro-Tech AT12RCT.
NGC 4945 by Warren Keller, Star Shadows Remote Observatory, Mayhill, NM, USA. Equipment: RCOS 16
NGC 4945 by Warren Keller, Star Shadows Remote Observatory, Mayhill, NM, USA. Equipment: RCOS 16″, Apogee Alta U9-LRGB, PlaneWave Ascension 200HR mount, ACP, MaxIm DL, FocusMax.
NGC 4945 Galaxy by Rafael Compassi, Presidente Lucena, Brazil. Equipment: SW 8
NGC 4945 Galaxy by Rafael Compassi, Presidente Lucena, Brazil. Equipment: SW 8″, ASI1600mm-cool, ZWO EFW, Optolong LRGB filters.
Centaurus A in LRGB by Haim Huli, Namibia. Equipment: ASA Astrograph 12
Centaurus A in LRGB by Haim Huli, Namibia. Equipment: ASA Astrograph 12″, ASA DDM85, FLI MicroLine 8300
Centaurus A by Fernando Oliveira De Menezes, Sao Paulo, Brazil. Equipment: Esprit 150mm triplet, Qhy 16200
Centaurus A by Fernando Oliveira De Menezes, Sao Paulo, Brazil. Equipment: Esprit 150mm triplet, Qhy 16200
NGC 4725 and its Neighbours by Andre van der Hoeven, Steinborn/Neroth, Germany. Equipment: TEC-140, QSI 583ws, NEQ-6
NGC 4725 and its Neighbours by Andre van der Hoeven, Steinborn/Neroth, Germany. Equipment: TEC-140, QSI 583ws, NEQ-6
NGC 6951 Galaxy (aka NGC 6952) by Bob Franke, Chino Valley, Arizona USA. Equipment: 12.5
NGC 6951 Galaxy (aka NGC 6952) by Bob Franke, Chino Valley, Arizona USA. Equipment: 12.5″ RCOS Ritchey-Chrétien, SBIG STL-11000, AstroDon LRGB filters, Paramount ME mount
NGC 7497 in the Integrated Flux Nebula by Bob Franke, Chino Valley, Arizona USA. Equipment: Takahashi FSQ-106ED, Losmandy G11, SBIG STF-8300, Baader LRGB.
NGC 7497 in the Integrated Flux Nebula by Bob Franke, Chino Valley, Arizona USA. Equipment: Takahashi FSQ-106ED, Losmandy G11, SBIG STF-8300, Baader LRGB.
NGC7497 + MBM 50 (Integrated Flux Nebula) by Álvaro Ibáñez Pérez, Las Inviernas/Navas de Estena, Spain. Equipment: TS115 Triplet APO Refractor, TS Optics 0,79x, NEQ6 Pro II Tuning Belts, EQMOD, CCD Atik 460EX mono, Baader LRGB, IDAS LPS in colour, Lunático EZG-60, SXLodestar, RoboFocus, FocusMax
NGC7497 + MBM 50 (Integrated Flux Nebula) by Álvaro Ibáñez Pérez, Las Inviernas/Navas de Estena, Spain. Equipment: TS115 Triplet APO Refractor, TS Optics 0,79x, NEQ6 Pro II Tuning Belts, EQMOD, CCD Atik 460EX mono, Baader LRGB, IDAS LPS in colour, Lunático EZG-60, SXLodestar, RoboFocus, FocusMax
NGC 1532 - LRGB by Dan Crowson, Animas, New Mexico, USA. Equipment: ST-8300M, Astro-Tech AT90EDT.
NGC 1532 – LRGB by Dan Crowson, Animas, New Mexico, USA. Equipment: ST-8300M, Astro-Tech AT90EDT.
NGC 3621 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG ST-8300M, Astro-Tech AT90EDT.
NGC 3621 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG ST-8300M, Astro-Tech AT90EDT.
NGC 3621 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG ST-8300M, Astro-Tech AT90EDT.
NGC 3621 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG ST-8300M, Astro-Tech AT90EDT.
UGC 6253 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
UGC 6253 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 4731 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 4731 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 3166, NGC 3169 & NGC3165 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 3166, NGC 3169 & NGC3165 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 5905 and NGC 5908 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC 5905 and NGC 5908 by Dan Crowson, Animas, New Mexico, USA. Equipment: SBIG STF-8300M, Astro-Tech AT12RCT
NGC300 and Distant Galaxy Clusters by Christian van den Berge, Netherlands. Equipment: Nikon D600, APM 700/107, Riccardi reducer, Fornax 51, Guided with Lacerta MGEN
NGC300 and Distant Galaxy Clusters by Christian van den Berge, Netherlands. Equipment: Nikon D600, APM 700/107, Riccardi reducer, Fornax 51, Guided with Lacerta MGEN
NGC 7793 in Sculptor by Warren Keller, Star Shadows Remote Observatory, NM, USA. Equipment: SSRO- RCOS 16
NGC 7793 in Sculptor by Warren Keller, Star Shadows Remote Observatory, NM, USA. Equipment: SSRO- RCOS 16″, Alta U9, PlaneWave Ascension 200HR, ACP, MaxIm DL, FocusMax.
NGC 1365 by Warren Keller, Star Shadows Remote Observatory, NM, USA. Equipment: RCOS 16
NGC 1365 by Warren Keller, Star Shadows Remote Observatory, NM, USA. Equipment: RCOS 16″, Alta U9, PlaneWave Ascension 200HR, ACP, MaxIm DL, FocusMax.
NGC4038 aka The Antennae Galaxies by Haim Huli, Kibutz Ramat Hakovesh, Israel. Equipment: ASA 12
NGC4038 aka The Antennae Galaxies by Haim Huli, Kibutz Ramat Hakovesh, Israel. Equipment: ASA 12″, ASA DDM85, FLI 8300 Mono
NGC2841 by Bernard Miller, Phoenix, AZ, USA. Equipment: Planewave CDK-17, FLI PL16803, Paramount ME
NGC2841 by Bernard Miller, Phoenix, AZ, USA. Equipment: Planewave CDK-17, FLI PL16803, Paramount ME
NGC2683 by Bernard Miller, Phoenix, AZ, USA. Equipment: Planewave CDK-17, FLI PL16803, Paramount ME
NGC2683 by Bernard Miller, Phoenix, AZ, USA. Equipment: Planewave CDK-17, FLI PL16803, Paramount ME
Große Magellanscheint Wolke by Mario Richter, Astrofarm, Kiripotib, Namibia. Equipment: Canons 450d, Teleobjektiv, Vixen GPDX.
Große Magellanscheint Wolke by Mario Richter, Astrofarm, Kiripotib, Namibia. Equipment: Canons 450d, Teleobjektiv, Vixen GPDX.