Galaxies
Gravitationally bound systems of stars, gas, dust, and dark matter.
Overview
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter. They range from faint dwarfs containing a few million stars to giant ellipticals with trillions, and they are the basic structural units of the visible universe.
- That galaxies exist outside the Milky Way was only established in 1923–24.
- Rotation curves show galaxies contain far more mass than their visible matter accounts for.
- Most large galaxies host a supermassive black hole at their centre.
- Galaxy morphology is strongly shaped by mergers and interactions, not just by initial conditions.
Morphology: the Hubble sequence and its limits
Before the mid-1920s it was an open question whether the spiral 'nebulae' were clouds inside the Milky Way or separate stellar systems. In 1923–24 Edwin Hubble identified Cepheid variable stars in the Andromeda nebula and applied Leavitt's period–luminosity relation to them, showing it lies far beyond our own Galaxy and settling the question. Everything below rests on that result.
Hubble's classification, still the standard vocabulary, sorts galaxies into ellipticals (E0 to E7 by apparent flattening), spirals (Sa to Sc by how tightly wound the arms are and how large the bulge is), barred spirals (SBa to SBc), lenticulars (S0, disc-shaped but with little star formation), and irregulars.
The scheme is descriptive, not evolutionary — the 'early' and 'late' labels sometimes attached to it reflect a discarded idea that galaxies evolve along the sequence. Morphology correlates strongly with other properties: ellipticals are typically old, red, gas-poor and found in dense environments, while spirals are actively forming stars and contain substantial cold gas.
The Milky Way from inside
Our own Galaxy is a barred spiral roughly 100,000 light-years across, with the Sun about halfway out from the centre. Determining its structure is unusually hard because we are embedded in the disc, looking through its dust — the existence of the central bar was not firmly established until infrared surveys penetrated that obscuration.
At the centre lies Sagittarius A*, a compact radio source identified as a supermassive black hole of roughly four million solar masses through decades of tracking individual stellar orbits around it, and imaged at horizon scale by the Event Horizon Telescope in 2022.
The dark matter evidence
Measured rotation curves of spiral galaxies stay approximately flat far beyond the visible disc, rather than declining as the enclosed visible mass would predict. Vera Rubin's systematic measurements in the 1970s established this as a general property, not a peculiarity of a few systems.
The same conclusion follows from independent lines of evidence: gravitational lensing of background objects, the velocity dispersion of galaxies within clusters, and the pattern of fluctuations in the cosmic microwave background. All indicate substantially more gravitating mass than the light traces. The nature of that mass is unknown, and its identification remains one of the major open problems in physics.
Galaxies interact
- Mergers between comparable galaxies disrupt discs and can produce ellipticals; the resulting tidal tails are directly observable.
- Star formation is often triggered by interaction, as gas is compressed and driven inward.
- The Milky Way is currently absorbing smaller satellite galaxies, and stellar streams in the halo are the debris.
- The Milky Way and the Andromeda Galaxy are approaching one another and are expected to interact in a few billion years.
- Environment matters: galaxies in dense clusters are more often gas-poor ellipticals, having been stripped of star-forming material.
Active galaxies
In some galaxies the central black hole is actively accreting, producing a luminous nucleus that can outshine all the stars combined. These active galactic nuclei appear under many observational labels — Seyfert galaxies, radio galaxies, blazars, quasars — and the prevailing unified model attributes much of that variety to viewing angle relative to an obscuring torus and to jet orientation, rather than to fundamentally different objects.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- How many galaxies are there?
- Estimates from deep survey fields extrapolated across the sky run into the hundreds of billions of observable galaxies, and some analyses push higher once very faint dwarfs are included. Any specific number is a model-dependent extrapolation from a small surveyed area, not a count, and it should be read that way.
- What is dark matter and how do we know it exists?
- It is an inferred component of mass that interacts gravitationally but does not emit or absorb light detectably. The evidence is that galaxies rotate too fast at their edges for their visible mass, that clusters bind galaxies moving faster than visible mass allows, that gravitational lensing reveals more mass than light traces, and that the cosmic microwave background fluctuation pattern requires it. What it is made of is unknown.
- Is the Milky Way going to collide with Andromeda?
- The two galaxies are approaching each other and an interaction is expected in a few billion years. 'Collision' overstates it at the stellar level: galaxies are mostly empty space, and individual star collisions would be extraordinarily unlikely. What would change is the structure of both galaxies, as gravity reshapes their discs.
- Does every galaxy have a supermassive black hole?
- Most large galaxies appear to, and there is a well-established correlation between black hole mass and the properties of the host bulge, suggesting the two grow together. Some small and dwarf galaxies show no evidence for one, and whether they genuinely lack them or simply host black holes too small to detect is an active question.