Immense islands of stars, gas, dust, and dark matter — the building blocks that make up the large-scale universe. From the quiet spiral to the most luminous objects in the cosmos.
A galaxy is a gravitationally bound system of stars, gas, dust, and — by far the dominant mass component — dark matter. The scales involved are almost impossible to grasp: our own galaxy, the Milky Way, has a diameter of roughly 100,000 light-years and contains an estimated 100 to 400 billion stars. Across the entire observable universe, current estimates — based on deep images from the Hubble and James Webb Space Telescopes — range from several hundred billion to around two trillion galaxies, depending on the counting method, from gigantic elliptical giants with several trillion stars down to dwarf galaxies with only a few million. Galaxies rarely occur in isolation: they gather into groups, clusters, and superclusters along a filamentary cosmic web — the largest known structure in the universe.
The estimated mass distribution of the Milky Way shows a pattern typical of most galaxies: the luminous, directly visible material — stars, gas, and dust — accounts for only a small fraction of the total mass. The rest is supplied by an invisible component whose existence can only be inferred from its gravitational effects: from the rotation curves of galaxies, which spin faster than their visible mass alone would allow. Without this additional gravitational "glue," many galaxies simply could not hold together.
In 1926, Edwin Hubble proposed a classification scheme that — with later extensions — remains the standard of galaxy morphology to this day: the "tuning-fork diagram." It sorts galaxies purely by their visual appearance, not by their age or physical evolution. Hubble himself initially suspected an evolutionary sequence running from left (elliptical) to right (spiral) — an assumption now considered disproven. The classification nevertheless remains useful because it correlates with real physical quantities: gas content, star-formation rate, and bulge-to-disk ratio.
The abstract classification becomes tangible through concrete examples. Four galaxies illustrate the range of what falls under the term "galaxy" — from our own cosmic home to one of the most massive known supermassive black holes.
Our own galaxy — viewed from the inside, which makes it harder to map than distant galaxies. A central bar channels gas toward the core, from which spiral arms such as the Perseus and Sagittarius Arms extend. The Sun sits in a smaller spur, the Orion Arm.
The nearest large spiral galaxy, visible to the naked eye as a faint smudge in the constellation Andromeda. Larger in diameter than the Milky Way, though with a comparable total mass. It is approaching us, blueshifted, at around 110 km/s — the beginning of a future collision.
A gigantic elliptical galaxy at the center of the Virgo Cluster. Its supermassive black hole was the first ever imaged directly, in 2019 — the famous "ring of fire" captured by the Event Horizon Telescope. A relativistic jet of matter shoots thousands of light-years out into space.
A satellite galaxy of the Milky Way, visible to the naked eye in the southern sky. It has no ordered spiral or elliptical structure but is rich in gas and star formation — it hosts the Tarantula Nebula, one of the most active star-forming regions in our local surroundings.
The Milky Way consists of three major structural components. The central bulge is a dense, roughly spherical concentration of mostly old stars. It is surrounded by the flat, rotating disk, where the spiral arms lie — this is where practically all active star formation takes place, driven by gas and dust. The outer frame is formed by the halo: a spherical region sparsely populated with old stars and globular clusters, which also holds the reservoir of dark matter and extends far beyond the visible disk.
The Sun orbits the galactic center at a distance of roughly 27,000 light-years, embedded in the Orion Arm — a relatively small spur between the more massive Perseus and Sagittarius Arms. At an orbital speed of around 220–240 km/s, it takes about 225 to 250 million years to complete one full orbit, known as a galactic year. Since its formation, the Sun has therefore completed only about 20 such orbits.
At the center of the Milky Way sits a supermassive black hole, Sagittarius A*, with roughly 4.3 million solar masses. Its existence and mass were not observed directly but reconstructed over decades from the orbits of individual stars, which circle the invisible point in extremely tight, fast ellipses. Reinhard Genzel and Andrea Ghez received the 2020 Nobel Prize in Physics for this work. In 2022, the Event Horizon Telescope also achieved the first direct image of Sagittarius A*'s shadow.
Galaxies are not isolated, static objects — on cosmic timescales, they collide, merge, and deform one another. When two galaxies approach each other, tidal forces come into play: the nearer edge of a galaxy is pulled more strongly than the farther one, tearing out long, extended "tidal tails" of stars and gas. When interstellar gas is compressed and densified in the process, it can trigger enormous bursts of star formation — so-called starbursts, in which a galaxy forms thousands of new stars within just a few million years. The stars themselves rarely collide directly — the space between them is vast even within a single galaxy; what is mainly affected is the gas, dust, and large-scale structure.
Nearly every massive galaxy harbors a supermassive black hole at its center — most of them, like Sagittarius A*, are largely inactive today because almost no material is flowing into them. If, however, enough gas falls toward the black hole, an accretion disk forms: material heats up through friction to millions of degrees and radiates enormous amounts of energy — far more efficiently than any known nuclear fusion. Part of this energy is additionally hurled out into space via relativistic jets along the rotation axis, driven by the strong magnetic field around the black hole. Such centers are called active galactic nuclei (AGN).
Quasars are the most luminous form of active galactic nuclei: they can radiate more energy than all the stars of their host galaxy combined, outshining even the entire galaxy — even though the central radiation source is only a few light-hours across. Quasars occur predominantly in the early universe, when plenty of gas was still available to feed the black holes, and today serve astronomers as extremely distant "beacons" for mapping the structure of the young cosmos.
In 2024, the quasar J0529-4351 was confirmed as the brightest object known in the universe to date. Its central black hole is devouring material at a rate equivalent to one solar mass per day — making its accretion disk brighter than 500 trillion Suns. Despite decades of sky surveys, the object went unnoticed for a long time because it looked deceptively like an ordinary star.
A few orders of magnitude and observations that show just how extraordinary our picture of the galactic universe really is.
Relative to the cosmic microwave background — the closest thing to a "rest frame" for the universe — the Milky Way moves at roughly 600 to 630 km/s. The cause is the combined gravitational pull of distant mass concentrations, including the so-called "Great Attractor" in the constellation Centaurus. Even within a seemingly motionless universe, our cosmic home is anything but still.
When the Hubble Space Telescope spent ten days in 1995 photographing a tiny, seemingly empty patch of sky — smaller than a grain of sand held at arm's length — the resulting "Hubble Deep Field" revealed more than 3,000 galaxies. The James Webb Space Telescope has since pushed that depth significantly further, revealing galaxies from just a few hundred million years after the Big Bang.
Dwarf galaxies — small, faint, often without a distinctive structure — are by far the most common galaxy type in the universe. Around the Milky Way alone, several dozen such companion galaxies are now known, many of them discovered only through increasingly sensitive sky surveys. The striking spirals and ellipticals featured in this overview are exceptions, not the rule.
Massive galaxy clusters curve spacetime so strongly that they measurably deflect the light of more distant background objects — visible as distorted arcs or multiply imaged galaxies. This gravitational lensing effect provides some of the strongest evidence for dark matter, whose distribution can even be mapped this way. More on this on the Dark Matter & Energy page.