Somewhere in the distant universe, there are galaxies that appear to have used up their brightest years long before Earth existed. Their stars still shine, their gravity still holds enormous collections of matter together, and their histories remain written across billions of years of cosmic evolution. Yet the activity that once filled them with new stars has slowed dramatically or stopped altogether.
Astronomers sometimes call these systems dead galaxies. The name sounds dramatic, but it describes something specific: galaxies in which star formation has largely ceased. They are not literally lifeless, and many contain billions of stars that will continue shining for an extraordinarily long time.
The mystery is how they reached this state.
Stars form from cold, dense clouds of gas, principally molecular hydrogen. A galaxy that continues receiving fresh gas can keep producing new generations of stars. So why do some galaxies lose that ability? Why do others shut down rapidly after a spectacular burst of star formation, while neighbouring galaxies continue creating stars for billions of years?
The answer is not one universal cosmic switch. Astronomers have identified several processes that can deprive galaxies of their star-forming fuel, heat that fuel until it cannot collapse into stars, or prevent fresh material from arriving.
Supermassive black holes, exploding stars, interactions with other galaxies and the enormous halos of dark matter surrounding galaxies can all play a role.
Observations of ancient galaxies are now revealing that some systems stopped forming stars surprisingly early in the universe’s history. Understanding why is one of the most important problems in modern galaxy evolution—and it may explain how the universe acquired the enormous variety of galaxies we see today.
The first misconception to clear up is that a galaxy stops forming stars because all its existing stars have burned out.
That is not how galactic ageing works.
A galaxy can contain billions or even trillions of stars and still have almost no ongoing star formation. The key ingredient is not the number of stars already present. It is the supply of cold gas available to create new ones.
Stars form when sufficiently dense regions of gas collapse under gravity. As the material contracts, it can fragment into smaller clouds and eventually produce stars. The most massive stars burn through their nuclear fuel relatively quickly, while smaller stars can shine for billions or trillions of years.
If a galaxy stops producing new stars, its stellar population gradually changes. The short-lived, massive, blue stars disappear first. Older, longer-lived stars become increasingly dominant, and the galaxy’s overall light can become redder.
This is why astronomers often distinguish between star-forming galaxies, which frequently appear blue because of their young stellar populations, and quiescent galaxies, which tend to look redder.
Colour alone is not enough to establish that a galaxy has stopped forming stars. Dust can redden the light of a galaxy that is still actively forming stars, so astronomers combine observations across different wavelengths with measurements of stellar populations, gas and other indicators.
The important distinction is that a galaxy can remain full of stars while losing the ability to make new ones.
The process that suppresses star formation is known as galaxy quenching. It can occur over different timescales, and different galaxies may reach the same quiet state through very different evolutionary histories.
A useful starting point is the recent review A Review of Galaxy Quenching — Part I: Defining the Problem and Observational Results, which surveys the observational evidence for how galaxies stop forming stars.
Imagine a galaxy as a vast city whose factories manufacture stars. The factories require raw materials, and the main supply is cold gas.
At first, many galaxies have access to substantial reservoirs of gas. Material falls into their gravitational wells from the surrounding cosmic environment, while mergers and interactions can bring in additional supplies. Gas already inside a galaxy can also cool and collect into star-forming clouds.
But that supply is not guaranteed to last forever.
A galaxy may consume its cold gas through star formation. Some gas is returned to the surrounding environment when stars shed their outer layers or explode as supernovae, but other processes can heat or expel gas, making it unavailable for further star formation.
If fresh gas stops arriving, the galaxy can gradually exhaust the material that remains available.
Astronomers call one version of this process starvation or strangulation. The galaxy is not necessarily stripped of every gas cloud in a sudden event. Instead, its access to new fuel is cut off, and star formation dwindles as its existing reservoir is depleted or disrupted.
This process can be particularly important for smaller galaxies orbiting within larger groups or clusters. Once they enter the environment of a much more massive system, their ability to draw in fresh material may change dramatically.
The timescale varies. Some galaxies decline gradually over billions of years; others lose their star-forming capacity much more rapidly.
The distinction matters because observations show that galaxies do not all follow the same path from vigorous star formation to quiescence.
3. Supermassive black holes can interfere with an entire galaxy’s future
At the centre of many massive galaxies sits a supermassive black hole, containing millions or billions of times the mass of the Sun. Despite their enormous gravitational influence, black holes do not simply suck up all the gas in a galaxy. Their importance comes partly from what happens when material falls toward them.
Gas spiralling toward a black hole can release tremendous amounts of energy. The resulting active galactic nucleus, or AGN, may shine across the electromagnetic spectrum and launch powerful winds or jets of particles.
Those outflows can affect gas far beyond the black hole’s immediate surroundings.
If energy from the active nucleus heats the galaxy’s gas or drives cold material outward, clouds that might otherwise collapse to form stars can be disrupted. In massive galaxies, the black hole may also help keep the surrounding hot gas from cooling enough to replenish the cold reservoir.
This process is known as AGN feedback.
Astronomers have strong reasons to believe that black-hole feedback is an important ingredient in galaxy evolution. Without some mechanism to suppress star formation in massive galaxies, many theoretical models produce galaxies with properties that do not match observations.
Yet the details remain challenging. Scientists must determine how much energy escapes the immediate surroundings of the black hole, how it couples to gas, and whether it mainly expels cold material, heats gas that remains behind, or prevents fresh gas from cooling.
NASA’s research on supermassive black holes and galaxy evolution discusses these open questions.
4. Some galaxies became quiescent astonishingly early
For a long time, astronomers expected the earliest galaxies to be predominantly active, assembling their stars while gas was abundant. The discovery of massive galaxies with little ongoing star formation at surprisingly early cosmic times has complicated that picture.
In March 2024, astronomers reported the discovery of an exceptionally early quiescent galaxy using the James Webb Space Telescope. The galaxy was seen as it existed roughly 700 million years after the Big Bang, when the universe was only a small fraction of its present age.
Its star formation appeared to have declined dramatically after a relatively brief period of intense activity.
The discovery raised an immediate question: how could a galaxy build a substantial population of stars and then stop producing new ones so early?
The Reuters report on the early dead galaxy observed with Webb describes the finding and the uncertainty surrounding its shutdown.
These possibilities are not mutually exclusive. A burst of star formation can itself generate supernovae and stellar winds that reshape the remaining gas. An active black hole may contribute to the process, while the galaxy’s broader environment determines whether it can replenish its supply.
There is also an important observational limitation: astronomers see distant galaxies as they were when their light began travelling toward us. A galaxy observed in a quiescent state at an early epoch may not have remained permanently inactive. It could, under the right conditions, acquire new gas and begin forming stars again.
The discovery therefore does not mean that every early galaxy shut down for the same reason. It demonstrates that some galaxies reached a quiet phase much earlier than many simple pictures of galaxy evolution would suggest.
5. Exploding stars can make it harder for new stars to form
Black holes are not the only sources of energy capable of disrupting a galaxy’s gas.
Massive stars end their lives in powerful supernova explosions. These events inject energy and momentum into the surrounding interstellar medium, heating gas and driving expanding shells and outflows.
When many massive stars form within a short period, their combined stellar winds and supernovae can have a substantial effect on the host galaxy.
This is known as stellar feedback.
In some galaxies, feedback can push gas out of the regions where stars form. In others, it can stir and heat the gas, reducing the amount that can collapse into new stars. The effectiveness of this process depends on the galaxy’s gravitational pull, its gas distribution and how efficiently the energy couples to the surrounding material.
For a small galaxy, even a relatively modest outflow may remove a significant fraction of its available gas. A much more massive galaxy can hold onto gas more effectively, although its star formation may still be influenced by heating and other feedback processes.
Stellar feedback can therefore contribute to quenching, especially in lower-mass systems, but it does not explain every massive, long-quenched galaxy on its own.
In some cases, the galaxy may need additional help to remain quiet after the initial outflow. Hot gas can cool, and fresh material may eventually fall back. This is one reason astronomers distinguish between the process that initially suppresses star formation and the mechanism that keeps it suppressed over long periods.
A galaxy can be shut down by one combination of events and kept quiet by another.
6. Galaxy clusters can strip away the fuel supply
A galaxy does not evolve in isolation. Many live within groups or clusters containing hundreds or thousands of galaxies, along with enormous quantities of hot gas and dark matter.
When a smaller galaxy falls into such an environment, it encounters the hot gas filling the space between cluster members. As the galaxy moves through this medium, the surrounding gas can exert pressure on its own interstellar gas.
If that pressure becomes strong enough, it can strip gas out of the galaxy. The process is called ram-pressure stripping.
Imagine holding a loosely packed cloud of gas inside a galaxy while the galaxy races through a dense atmosphere. The surrounding medium can push on the cloud and remove it, particularly from the outer regions where gravity holds it less tightly.
Once stripped, that gas is no longer readily available to form stars in the galaxy.
The effect is especially important for satellite galaxies moving through dense cluster environments, where the surrounding gas can be hot and the relative velocities high. Gravitational interactions with the larger system and with other galaxies can also disturb or remove gas.
These environmental processes help explain why galaxies in dense regions of the universe are, on average, more likely to be quiescent than comparable galaxies in less crowded environments.
The details vary, however. Some satellite galaxies retain cold gas for a time after entering a cluster, and some continue forming stars before their supplies finally run low.
Astronomers therefore study both the galaxy’s internal physics and its location within the cosmic web. Where a galaxy lives can be just as important as what is happening inside it.
7. The hot halo problem: When gas is present but cannot make stars
One of the most puzzling features of massive quiescent galaxies is that they do not always lack gas altogether.
Some are surrounded by large reservoirs of extremely hot gas. The material may contain far more mass than the cold gas needed to build new stars, yet it does not readily condense into star-forming clouds.
Why not?
Gas must lose energy to cool. In a massive galaxy’s gravitational halo, infalling material can be heated to very high temperatures by shocks and compression. If it remains hot, it cannot easily settle into the dense, cold clouds required for star formation.
This offers a possible route to long-term quenching: instead of removing every last trace of gas, a galaxy can be prevented from cooling and replenishing its cold supply.
The difficulty is that hot gas radiates energy away. If nothing counteracts that cooling, some of it should eventually cool and flow inward.
This is where black-hole feedback may become important again. Jets from an active galactic nucleus can inflate enormous cavities in the surrounding gas, stir the atmosphere and transfer energy into it. If that heating offsets enough of the energy lost through radiation, the gas may remain too hot to fuel widespread star formation.
The process is sometimes called maintenance-mode feedback: the black hole helps maintain a galaxy’s quiet state after its star formation has already declined.
The balance is not always stable. In some environments, cooling gas can still reach the centre and trigger new star formation. Astronomers have observed systems in which black-hole activity and star formation appear to regulate one another rather than following a simple one-way shutdown.
NASA’s account of the Phoenix Cluster describes a striking example in which a central black hole’s feedback is not preventing all the surrounding gas from cooling, allowing an unusually intense episode of star formation.
8. Did galaxies simply use up their gas too quickly?
In some cases, the answer may be partly yes.
A galaxy that undergoes an intense burst of star formation can convert a large amount of cold gas into stars over a relatively short period. If it cannot replenish that gas quickly enough, the burst will eventually fade.
Astronomers call galaxies that have recently undergone a rapid decline in star formation post-starburst galaxies. Their light can reveal evidence of a strong episode of star formation in the recent past, followed by a much quieter period.
The timing is important. Massive, short-lived stars disappear relatively quickly, but certain types of stars leave distinctive spectral signatures that can persist after a burst has ended. By studying these signatures, astronomers can estimate when star formation declined and how rapidly the change occurred.
Yet gas consumption alone may not explain why the shutdown persists. If a galaxy remains surrounded by material that can cool and fall inward, it may eventually start forming stars again.
Recent research has found that many post-starburst galaxies have depleted molecular gas reservoirs, although some retain substantial quantities of gas despite low star formation. That variation suggests that the shutdown can involve more than one mechanism.
A 2026 report on the EMBERS I study describes observations of post-starburst galaxies using atomic hydrogen and molecular-gas tracers. The findings point to a reduced cold-gas supply in many systems while highlighting exceptions that require additional explanation.
This distinction is essential. A galaxy can stop forming stars because its fuel has been consumed, because the fuel has been removed, because it cannot cool, or because the remaining gas is not forming stars efficiently. These outcomes can look similar from a distance, but they tell different stories about the galaxy’s evolution.
9. How astronomers reconstruct the shutdown
A galaxy’s present appearance is only one piece of evidence. To understand why star formation stopped billions of years ago, astronomers combine several kinds of observation.
Telescopes such as the James Webb Space Telescope can observe distant galaxies whose light has been stretched by cosmic expansion. Their spectra and brightness help astronomers estimate stellar populations and investigate how early galaxies evolved.
Radio and submillimetre measurements
Instruments such as ALMA can investigate cold molecular gas and dust, helping researchers determine whether a quiet galaxy has lost its star-forming material or still contains a substantial reservoir.
Stellar spectra
The light from a galaxy contains clues about the ages and composition of its stars. Spectral features can reveal whether a galaxy formed most of its stars long ago or experienced a more recent burst.
Researchers then compare those observations with computer simulations of galaxy evolution. The goal is not merely to reproduce the number of red galaxies in the universe, but to explain their gas content, stellar ages, environments and histories.
A useful resource is the NASA Hubble material on exhausted gas reservoirs and early quenching, which discusses observations of massive galaxies that had already become quiescent in the early universe.
10. Could a galaxy start forming stars again?
The word dead can make a galaxy’s future sound settled. In reality, quiescence is not always permanent.
A galaxy that has stopped forming stars might later acquire fresh gas through interactions or accretion. A merger could deliver additional material, and gas that was previously too hot to form stars might eventually cool if the conditions change.
Such events can lead to renewed star formation, sometimes called rejuvenation.
But restarting is not guaranteed. A massive galaxy may continue to host a hot gas halo, while energy from its central black hole prevents that gas from cooling efficiently. A satellite galaxy in a dense cluster may have lost much of its gas and be unable to replenish it.
The galaxy’s environment, mass, gas reservoir and history all influence what happens next.
Nor does renewed star formation necessarily restore the galaxy to its earlier state. The new episode may be modest compared with the intense activity that built most of its stars, or it may transform the galaxy’s appearance more substantially.
That is why astronomers treat quenching as an evolutionary process rather than a single irreversible event.
A quiet galaxy is not necessarily a galaxy whose story has ended. It is one whose star-forming activity has changed—and whose next chapter depends on what happens to its remaining gas.
The real mystery is why the shutdown lasts so long
The universe contains galaxies that once built stars at extraordinary rates but now produce very few. Some reached this state billions of years ago, when the cosmos itself was young. Their existence tells us that building a galaxy and keeping it active are two very different challenges.
The evidence points to a combination of causes. Some galaxies lose their cold gas to powerful outflows. Others become starved of fresh material. In massive systems, hot gas halos and supermassive black holes can make it difficult for new fuel to reach the star-forming regions. In galaxy clusters, the surrounding environment can strip gas away.
No single explanation accounts for every galaxy.
That is what makes the problem so interesting. Astronomers are not simply trying to explain why a galaxy becomes red or why its youngest stars disappear. They are trying to reconstruct a complicated sequence of events from light that has travelled across cosmic time.
The James Webb Space Telescope, radio observatories and large galaxy surveys are giving researchers increasingly detailed evidence about these histories. Each observation helps narrow the possibilities, but important questions remain about how the different mechanisms interact and which ones dominate in different environments.
A galaxy that stopped forming stars billions of years ago is not a failed galaxy. It is evidence that the universe has many ways of shaping the lives of its largest structures.
And somewhere in the distant light reaching Earth today, astronomers may be seeing another galaxy at the moment its brilliant era of star formation comes to an end.
