Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, held together by gravity and spanning from tens of thousands to hundreds of thousands of light-years across. The word derives from the Greek galaxias, meaning "milky," a reference to the Milky Way, the galaxy containing Earth and the Solar System. Astronomers estimate that the observable universe contains at least two trillion galaxies, though this figure is subject to ongoing revision as telescope technology improves.

Classification and morphology

Galaxies are classified primarily by their visual morphology, a system formalised by Edwin Hubble in 1926 and known as the Hubble sequence or Hubble tuning-fork diagram.

Elliptical galaxies

Elliptical galaxies (designated E0 through E7) are smooth, featureless systems with an approximately ellipsoidal shape. They contain predominantly old, low-mass stars and very little cold gas or dust, resulting in low rates of ongoing star formation. Giant ellipticals rank among the most massive galaxies known.

Spiral galaxies

Spiral galaxies display a flattened disk with winding arms of younger stars, gas, and dust radiating from a central bulge. They are subdivided into ordinary spirals (Sa through Sd) and barred spirals (SBa through SBd), in which a linear bar of stars connects the nucleus to the arms. The Milky Way is classified as a barred spiral, most likely of type SBbc. Spiral galaxies are the most visually prominent class and account for a large fraction of galaxies in the present-day universe.

Lenticular galaxies

Lenticular galaxies (S0) are intermediate between ellipticals and spirals: they possess a disk and central bulge but lack prominent spiral arms and have largely exhausted their star-forming gas.

Irregular galaxies

Irregular galaxies lack a clear symmetric structure and are often the result of gravitational interactions or mergers. The Large Magellanic Cloud and the Small Magellanic Cloud, satellite galaxies of the Milky Way, are well-known examples.

Dwarf galaxies

Dwarf galaxies are the most numerous type in the universe. They contain from a few million to several billion stars and are commonly found orbiting larger host galaxies. Dwarf spheroidals, dwarf ellipticals, and dwarf irregulars are the principal subtypes.

Formation and evolution

The prevailing cosmological model holds that galaxies began forming a few hundred million years after the Big Bang, as small overdensities in the distribution of matter collapsed under gravity.

Early universe and the first galaxies

Observations by the James Webb Space Telescope, operational from 2022, have revealed galaxies existing within a few hundred million years of the Big Bang, some of which are unexpectedly massive and structured for their age. These findings have prompted discussion about refinements to standard models of galaxy formation, though they have not yet overturned the broader Lambda-CDM model.

Hierarchical assembly

The dominant theoretical framework — hierarchical structure formation — proposes that small structures formed first and subsequently merged to build progressively larger systems. Dark matter halos provided the gravitational scaffolding within which ordinary (baryonic) matter accumulated to form stars and eventually galaxies.

Galaxy mergers

Galaxies interact and merge over cosmological timescales. Mergers can trigger intense bursts of star formation (starbursts), feed supermassive black holes at galactic centres, and transform spiral galaxies into ellipticals. The Milky Way is expected to merge with the Andromeda Galaxy in approximately 4.5 billion years.

Quenching

Star formation in a galaxy can be suppressed — a process called quenching — by feedback from an active galactic nucleus, supernovae-driven winds, or the stripping of gas as a galaxy moves through a hot intergalactic medium. Quenched galaxies eventually become red and dead systems dominated by old stellar populations.

Active galactic nuclei and supermassive black holes

Most large galaxies harbour a supermassive black hole at their centre, with masses ranging from millions to tens of billions of solar masses. When surrounding material accretes onto the black hole, the nucleus becomes highly luminous, producing an active galactic nucleus (AGN). The most extreme AGN are observed as quasars, which can outshine an entire galaxy by factors of hundreds. There is strong observational evidence for a correlation between the mass of the central black hole and properties of the galactic bulge, suggesting co-evolution of black holes and their host galaxies.

Galaxy groups, clusters, and the large-scale structure

Galaxies are not distributed randomly in space; they cluster into hierarchical structures.

Galaxy groups

A galaxy group is a gravitationally bound collection of typically fewer than fifty galaxies spanning a few million light-years. The Milky Way belongs to the Local Group, which also includes the Andromeda Galaxy, the Triangulum Galaxy, and several dozen dwarf galaxies.

Galaxy clusters

Galaxy clusters contain hundreds to thousands of galaxies within a region of roughly 10 to 30 million light-years and are among the most massive gravitationally bound structures in the universe. They are permeated by hot intracluster gas visible in X-rays and contain large quantities of dark matter.

Superclusters, filaments, and voids

At the largest scales, galaxy clusters assemble into superclusters connected by filaments of galaxies and separated by vast, near-empty cosmic voids. This pattern — resembling a cosmic web — is a direct consequence of the initial density fluctuations seeded during cosmic inflation.

Observation and study

The systematic study of galaxies beyond the Milky Way began in earnest in the 1920s, when Edwin Hubble used the 100-inch Hooker Telescope at Mount Wilson Observatory, California, to resolve individual Cepheid variable stars in the Andromeda Nebula, establishing it as a separate galaxy far outside the Milky Way. This discovery, sometimes called the Great Debate resolution, fundamentally changed humanity's picture of the universe.

Modern galaxy surveys such as the Sloan Digital Sky Survey (SDSS) have catalogued hundreds of millions of galaxies and provided the statistical foundation for contemporary cosmology. Space observatories including the Hubble Space Telescope and the James Webb Space Telescope have extended observations to the earliest epochs of cosmic history.

Frequently asked questions

Is the Milky Way a typical galaxy?

The Milky Way is a large barred spiral galaxy and is more massive than the majority of galaxies in the universe, most of which are small dwarf systems. In absolute terms it sits near the upper end of the size distribution but is far from the most extreme examples.

Is dark matter present in all galaxies?

Observational evidence strongly indicates that dark matter constitutes the dominant mass component of virtually all galaxies, with the visible stellar and gaseous matter representing only a minority of the total mass. A small number of unusual galaxies have been reported with little detectable dark matter, though these cases remain subjects of active research and debate.

Are galaxies moving away from each other?

Yes. The expansion of the universe causes galaxies that are not gravitationally bound to one another to recede at velocities proportional to their distance, a relationship described by the Hubble–Lemaître law. Galaxies within the same bound group or cluster do not expand away from each other.

Is it possible to see other galaxies with the naked eye?

Several galaxies are visible to the unaided human eye under dark-sky conditions. The Andromeda Galaxy (approximately 2.5 million light-years away) is the most distant object routinely visible without optical aid, appearing as a faint smudge in the constellation Andromeda. The Large and Small Magellanic Clouds are also naked-eye objects from the Southern Hemisphere.

Is the number of galaxies in the universe known precisely?

No precise count exists. A widely cited 2016 study estimated at least two trillion galaxies in the observable universe, but the figure depends on assumptions about faint, unresolved dwarf galaxies and may be revised as deeper surveys are completed.