A galaxy is a vast system of stars, gas, dust, stellar remnants, and dark matter held together by gravity. Galaxies contain everything from young, newly formed stars to ancient stellar populations, along with planetary systems, nebulae, and other astronomical objects. Some also contain supermassive black holes at their centers.
Galaxies vary enormously in size, mass, shape, and appearance. Some contain only millions of stars, while the largest contain trillions. Their diameters range from a few thousand light-years to hundreds of thousands of light-years or more. A light-year is the distance light travels in one year, approximately 5.88 trillion miles.
The Milky Way, our home galaxy, contains hundreds of billions of stars, including the Sun. It is a barred spiral galaxy, meaning its stars form a flattened disk with spiral arms and a central bar-shaped structure. The solar system lies within one of its spiral arms, approximately 26,000 light-years from the galactic center.
Galaxies are not simply collections of stars scattered through space. They are dynamic systems that develop and change over billions of years. Gravity brings matter together, gas cools and forms stars, stars evolve and release material, and interactions between galaxies can reshape their structures.
Understanding galaxies helps astronomers investigate the history of the universe, the formation of stars and planetary systems, and the distribution of matter on the largest scales.
The main types of galaxies
Astronomers classify galaxies primarily according to their visible shapes and structural features. The traditional classification system, often called the Hubble sequence, organizes galaxies into several broad categories: spiral, barred spiral, elliptical, and irregular.
This classification describes their appearance rather than providing a complete explanation of how they formed. Galaxies can also change their shapes over time, particularly through gravitational interactions and mergers.
Spiral galaxies
Spiral galaxies have flattened, rotating disks containing stars, gas, and dust. Their most recognizable features are curved arms that extend outward from a central region.
These spiral arms are often bright because they contain concentrations of young, hot stars. They also contain clouds of gas and dust where new stars can form. Older stars are generally more widely distributed throughout the disk and central regions.
A spiral galaxy typically includes three major structural components: a central bulge, a disk, and a surrounding halo.
The central bulge is a relatively dense concentration of stars near the galaxy’s center. It often contains older stars and may surround a supermassive black hole.
The disk contains most of the galaxy’s spiral structure. Stars, gas, and dust orbit the center, generally following approximately circular paths. The disk is where much of the galaxy’s ongoing star formation takes place.
The surrounding halo extends far beyond the visible disk. It contains older stars, globular clusters, and large quantities of dark matter. Globular clusters are tightly packed, approximately spherical groups of stars, many of which are billions of years old.
The spiral arms are not necessarily permanent collections of the same stars. They are better understood as patterns of enhanced density that move through the disk. Stars and gas can enter and leave these patterns as they orbit the galaxy.
The Milky Way and the nearby Andromeda Galaxy are prominent examples of spiral galaxies.
Barred spiral galaxies
Barred spiral galaxies are a major subtype of spiral galaxies. They have an elongated, bar-shaped concentration of stars extending through the central region, with spiral arms emerging from the ends of the bar or nearby regions.
The bar is produced by the collective gravitational behavior of stars moving within the galactic disk. Rather than being a rigid structure, it is a dynamic arrangement maintained by the orbits of stars.
Bars can influence how gas moves through a galaxy. Gravitational forces associated with the bar can transport gas inward toward the central regions, potentially supplying material for star formation and feeding the central black hole.
The Milky Way is classified as a barred spiral galaxy. Its central bar is surrounded by a larger disk containing spiral arms, gas clouds, and billions of stars.
Elliptical galaxies
Elliptical galaxies have rounded or elongated shapes rather than prominent spiral arms. Their appearance ranges from nearly spherical to strongly flattened.
Unlike spiral galaxies, elliptical galaxies generally contain relatively little cold gas and dust available for forming new stars. As a result, many are dominated by older stellar populations and have comparatively low rates of ongoing star formation.
Their stars move along a variety of orbital paths rather than following the relatively organized circular orbits common in spiral disks. The combined gravitational influence of the galaxy holds these stars together.
Elliptical galaxies vary greatly in size. Small elliptical galaxies may contain relatively few stars, while giant elliptical galaxies can contain enormous stellar populations and occupy some of the largest galactic structures in the universe.
Many massive elliptical galaxies are found near the centers of galaxy clusters, where repeated gravitational interactions and mergers may have contributed to their development.
Their older stellar populations often give them a yellowish or reddish appearance because older stars tend to be cooler and emit a larger proportion of their visible light at longer wavelengths than young, massive blue stars.
However, not every elliptical galaxy is completely inactive. Some retain gas, acquire new material, or experience episodes of star formation.
Irregular galaxies
Irregular galaxies lack the well-defined spiral arms or smooth elliptical shapes characteristic of the other major categories.
Their appearances can be distorted, fragmented, or uneven. Some contain large concentrations of gas and dust, while others show complex arrangements of stars and star-forming regions.
Irregular shapes can develop through gravitational interactions with neighboring galaxies. A close encounter can stretch, compress, and redistribute a galaxy’s material. Smaller galaxies may also be distorted or gradually disrupted by larger companions.
Some irregular galaxies have not experienced major interactions and may simply have relatively disorganized structures.
The Large and Small Magellanic Clouds are well-known examples of nearby galaxies with irregular or irregular-like structures. They are gravitationally associated with the Milky Way and contain active regions of star formation.
Irregular galaxies are particularly useful for studying how gravity, gas dynamics, and star formation influence galactic evolution.
The structure of a galaxy
Although galaxies differ in appearance, their structures are governed by the same fundamental physical force: gravity.
Gravity attracts matter and influences how stars, gas, dust, and dark matter move. The resulting distribution of these components determines much of a galaxy’s shape, internal motion, and long-term development.
A galaxy’s visible stars represent only part of its total mass. Much of its mass is believed to exist in the form of dark matter, which does not emit, absorb, or reflect light in the ordinary way.
The galactic core and central black hole
Many galaxies contain a dense central region called the galactic nucleus. This region can include closely packed stars, gas, dust, and a supermassive black hole.
A supermassive black hole is an object with a mass millions or billions of times greater than the Sun’s. Its gravity is extremely strong near its event horizon, the boundary beyond which nothing can escape.
The Milky Way contains a supermassive black hole called Sagittarius A*, with a mass approximately four million times that of the Sun.
Not every galaxy has a visibly active central black hole. A black hole becomes especially luminous when gas and dust fall toward it. Before crossing the event horizon, some of this material forms an accretion disk, a rapidly moving structure heated by friction and other physical processes.
The heated material can release enormous amounts of radiation. Galaxies with exceptionally energetic central regions are known as active galaxies, and their central energy sources are called active galactic nuclei.
Some active galactic nuclei produce powerful jets of particles moving outward at speeds approaching the speed of light.
The black hole itself is not necessarily the brightest part of the system. Much of the radiation comes from the extremely hot material surrounding it.
Stars, gas, and dust
Stars make galaxies visible, but the material between stars is equally important to their evolution.
Interstellar gas consists primarily of hydrogen and helium, with smaller quantities of heavier elements. It exists in different physical states, including hot ionized gas, warm atomic gas, and cold molecular clouds.
Molecular clouds are especially important because they provide the conditions under which stars can form. These clouds contain molecules, including molecular hydrogen, and can become dense enough for gravity to overcome internal pressure and begin collapsing parts of the cloud.
Dust consists of microscopic solid particles made from elements and compounds such as carbon and silicates. Although it represents only a small fraction of the total mass of interstellar material, dust has important effects on how galaxies look and evolve.
Dust absorbs and scatters visible light, sometimes obscuring distant stars and star-forming regions. It also absorbs energy and re-emits it at infrared wavelengths.
Observing galaxies in different wavelengths allows astronomers to study components that visible-light observations alone might miss.
For example, radio observations can reveal cold hydrogen gas, infrared observations can penetrate some dusty regions and detect cool material, and ultraviolet observations can identify populations of hot, young stars.
The galactic halo and dark matter
A galaxy’s halo extends beyond its bright central regions and visible disk. It contains stars, globular clusters, hot gas in many massive galaxies, and a much larger distribution of dark matter.
Dark matter is called dark because it does not interact with electromagnetic radiation in ways that allow ordinary telescopes to detect it directly. Its existence is inferred primarily through its gravitational effects.
One important line of evidence comes from the rotation of spiral galaxies.
Stars and gas far from the centers of many spiral galaxies orbit at speeds that remain relatively high rather than decreasing as much as expected if most of the mass were concentrated in the visible central regions.
This observation suggests that galaxies contain substantial amounts of additional unseen mass distributed throughout their halos.
Other evidence comes from gravitational lensing, the bending of light by gravity. When light from a distant object passes near a massive galaxy or galaxy cluster, the gravitational field can bend its path. By studying this distortion, astronomers can estimate how mass is distributed, including mass that does not emit light.
The leading explanation is that galaxies are embedded in extended dark matter halos. These halos play an important role in how galaxies form, grow, and maintain their structures.
Although dark matter’s gravitational influence is well established, its underlying physical nature remains unknown.
How galaxies form
Galaxies formed through a long process involving the expansion of the universe, gravitational attraction, the growth of matter concentrations, and the cooling and collapse of gas.
The modern scientific explanation is based on the standard cosmological model, which describes a universe that began in a hot, dense state and has expanded over approximately 13.8 billion years.
In the early universe, matter was distributed with extremely small variations in density. Some regions contained slightly more matter than others.
Over time, gravity amplified these differences. Regions with greater density attracted additional matter, becoming increasingly concentrated.
Dark matter played a major role in this process because it could begin forming gravitational structures without being slowed by the pressure and radiation interactions that affected ordinary matter in the early universe.
As the universe expanded and cooled, ordinary matter became able to collect within these gravitational structures.
Gas fell into growing concentrations of matter, where it could cool and become dense enough to form stars. These developing systems eventually became the galaxies observed today.
Galaxy formation was not a single event that occurred everywhere at the same time. It was a gradual process involving many overlapping stages of gravitational growth, gas accumulation, star formation, and interaction.
The role of gravity in galaxy formation
Gravity is the primary force responsible for gathering matter into galaxies.
In the early universe, small differences in matter density created gravitational attractions that gradually drew material together. Dark matter accumulated into increasingly large structures called dark matter halos.
A dark matter halo is a gravitationally bound concentration of dark matter that provides the environment in which a galaxy can develop.
Ordinary matter, including hydrogen and helium, was attracted to these halos. As gas moved inward, it gained gravitational energy, much of which was converted into heat.
The gas could then lose energy through radiation. Once sufficiently cooled, it could settle into denser structures and eventually form stars.
The exact process depends on the mass of the halo, the amount of gas available, the surrounding environment, and the energy released by stars and black holes.
This is why galaxies can develop very different structures even though they are governed by the same basic physical laws.
How stars form inside galaxies
Stars form within dense regions of cold gas and dust, particularly molecular clouds.
A cloud can begin collapsing when gravity becomes strong enough to overcome the pressure and other forces supporting it. This can happen naturally when a region becomes sufficiently dense or when external events, such as shock waves from nearby stellar explosions, compress the gas.
As the cloud contracts, its central regions become denser and hotter. The collapsing material often develops a rotating structure, and the densest central region becomes a protostar.
A protostar is an early stage in the development of a star, before sustained hydrogen fusion begins in its core.
As the protostar continues to contract, its central temperature and pressure rise. When conditions become sufficiently extreme, hydrogen nuclei begin fusing into helium through nuclear fusion.
This process releases energy that helps support the star against further gravitational contraction.
The resulting star becomes a major component of the galaxy’s evolving stellar population.
Stars do not form at a constant rate in every galaxy. Galaxies rich in cold gas may produce stars actively, while galaxies that have exhausted, heated, or lost much of their star-forming gas may experience reduced star formation.
How galaxies grow through mergers and interactions
Galaxies do not evolve in isolation. They often interact gravitationally with nearby galaxies, and some eventually merge.
A galaxy merger occurs when two or more galaxies become gravitationally bound and combine into a larger system.
During an interaction, gravity can distort galactic disks, stretch stars into long streams, and pull gas toward central regions. These changes can trigger increased star formation by compressing gas clouds.
A merger does not necessarily mean that individual stars collide. The distances between stars are so enormous compared with their sizes that direct stellar collisions are relatively uncommon.
Instead, the most important effects involve changes in the overall gravitational structure and interactions between gas clouds.
When gas clouds collide and compress, they can form new stars. Meanwhile, the orbits of existing stars can be rearranged, potentially transforming a disk-shaped galaxy into a more rounded structure.
The Milky Way is expected to interact and eventually merge with the Andromeda Galaxy. Their future encounter is an example of how galaxies continue to evolve long after their initial formation.
Galaxy mergers are not always destructive. They can produce larger galaxies, redistribute matter, trigger star formation, and alter the surrounding environment.
The influence of supermassive black holes
Supermassive black holes can influence galaxy evolution through a process known as feedback.
When gas falls toward a central black hole, the surrounding accretion disk can release enormous amounts of energy. Some of this energy heats nearby gas or drives powerful outflows and jets.
These processes can affect how much gas remains available for star formation.
For example, energy released by an active galactic nucleus can heat gas and make it more difficult for that gas to cool and collapse into new stars. Outflows can also carry gas away from central regions.
This process is called active galactic nucleus feedback.
Stars themselves also influence their host galaxies. Massive stars release energy through radiation and stellar winds, while supernovae—the powerful explosions marking the deaths of certain stars—can heat and disperse surrounding gas.
Together, these processes regulate the supply of cold gas and influence how quickly galaxies form new stars.
The relationship between central black holes and their host galaxies is an important subject of modern astronomy. Observations reveal connections between the properties of massive black holes and the central regions of their galaxies, but the precise physical processes responsible for these relationships are still being investigated.
How galaxies change over time
Galaxies are not permanent, unchanging structures. Their appearance and physical properties evolve over billions of years.
One important factor is the availability of cold gas. Galaxies with abundant cold gas can continue forming stars, while those with limited supplies may gradually become less active.
As stars age, they produce heavier elements through nuclear reactions. These elements are released into surrounding space through stellar winds, planetary nebulae, and supernova explosions.
Astronomers call elements heavier than helium metals, even though many are familiar elements such as carbon, oxygen, silicon, and iron.
This enrichment changes the composition of gas from which future generations of stars form. It also provides the raw materials needed to build rocky planets and many of the chemical compounds found in planetary systems.
The environment surrounding a galaxy can also influence its development. Galaxies in dense clusters may experience interactions, gas stripping, and other processes that reduce their ability to form stars.
Over time, these different influences can produce galaxies with very different histories, even when their initial conditions were broadly similar.
How astronomers study galaxies
Astronomers cannot usually travel to other galaxies or examine them directly. Instead, they study the light and other forms of radiation that reach Earth from distant astronomical objects.
Different wavelengths reveal different physical processes and components.
Visible-light telescopes show the distribution of stars and the overall shapes of galaxies. Ultraviolet observations are especially useful for identifying hot, young stars. Infrared observations reveal cool material and allow astronomers to study regions obscured by dust.
Radio telescopes detect emissions from cold hydrogen gas and other sources. These observations help scientists map the distribution and motion of gas within galaxies.
X-ray telescopes reveal extremely hot gas, energetic stellar remnants, and activity around black holes.
By combining observations across the electromagnetic spectrum, astronomers can build a more complete picture of a galaxy’s structure and evolution.
Measuring the distances and ages of galaxies
Determining how far away a galaxy is provides essential information about its size, luminosity, and history.
Astronomers use several methods to estimate galactic distances. Nearby galaxies can sometimes be measured using individual stars with known relationships between their brightness and intrinsic luminosity.
For more distant galaxies, astronomers use methods involving supernovae, other distance indicators, and the relationship between cosmic expansion and redshift.
Redshift occurs when light from a distant object is shifted toward longer wavelengths. For distant galaxies, this is primarily associated with the expansion of space.
By measuring redshift and applying an appropriate cosmological model, astronomers can estimate a galaxy’s distance and the time its light has been traveling.
Because light takes time to travel, observing distant galaxies also means observing them as they were in the past.
For example, light from a galaxy billions of light-years away has traveled for billions of years before reaching our telescopes. Astronomers therefore see that galaxy at an earlier stage of its development rather than as it exists today.
This allows scientists to compare galaxies from different cosmic eras and investigate how their structures and properties have changed over time.
What galaxy observations reveal about the universe
Galaxies provide some of the most important evidence for understanding the history and large-scale structure of the universe.
Their distribution is not random. Galaxies are arranged in groups, clusters, and enormous interconnected structures called filaments. Between these structures are vast regions with relatively few galaxies, known as cosmic voids.
Together, these features form the cosmic web, the large-scale pattern of matter in the universe.
The cosmic web developed through gravitational growth from small density variations in the early universe. Studying its structure helps astronomers investigate how matter has evolved and how dark matter influences cosmic development.
Galaxies also provide evidence about the universe’s expansion. Measuring their distances and redshifts helps scientists understand how the expansion rate has changed over cosmic history.
Observations of distant galaxies are particularly valuable because they allow astronomers to investigate earlier stages of cosmic evolution.
The oldest and most distant galaxies detected by modern telescopes help scientists test theories about how quickly the first stars and galaxies formed, how early structures grew, and how the universe became enriched with heavier elements.
Some details of the earliest stages of galaxy formation remain uncertain. Scientists continue to investigate how the first galaxies accumulated gas, how quickly they formed stars, and how radiation from early stars and black holes influenced their surroundings.
Why understanding galaxies matters
Galaxies connect several major areas of astronomy and physics. Their structures reveal how gravity organizes matter, their stars preserve evidence of chemical evolution, and their central black holes provide opportunities to study extreme gravitational environments.
They also help scientists understand the origins of planetary systems. The elements needed to build rocky planets and many of the chemical compounds associated with life were produced through earlier generations of stars and distributed into space by stellar processes.
The Sun formed from gas and dust that had already been enriched by previous generations of stars. In this sense, the solar system is part of a much longer history of galactic evolution.
Studying galaxies also helps scientists test fundamental ideas about dark matter, cosmic expansion, and the formation of large-scale structures.
Although astronomers have developed a detailed framework for understanding how galaxies form and evolve, important questions remain. The nature of dark matter is still unknown, the precise mechanisms governing galaxy growth are complex, and the earliest stages of galactic evolution are still being explored.
What is clear is that galaxies are not isolated collections of stars frozen in time. They are evolving systems shaped by gravity, gas dynamics, stellar evolution, black holes, and interactions with their surroundings. Their histories preserve a record of how matter has organized itself across the universe over billions of years.
