Feedback Effects: Radiation and Winds
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 06
Radiation, winds & the next stars
How the first luminous sources changed the gas around them—and helped decide where another generation of stars could form.
The first stars changed the conditions for the next ones.
Imagine a young group of stars inside a gas cloud. Their light travels outward, the gas begins to respond, and some of the material that might have formed new stars moves away.
Elsewhere, a dense pocket can be squeezed by the expanding gas. Whether it survives and collapses depends on how quickly it cools and how strongly gravity holds it together.
This interaction is called feedback: stars and accreting black holes change the material that supplies their own growth and the formation of future stars.[1], [4], [7]
What makes this a feedback process?
Gravity draws gas into dark matter halos. Gas that loses enough energy can become denser and form stars. Some gas may also reach a black hole and fuel an accretion flow.
These luminous sources return energy and momentum to their surroundings. The response changes how much gas remains available, where it collects, and when it can form another generation.[1], [6], [7]
A starburst is an episode of unusually intense star formation. Many young stars concentrated in one region can act together, producing radiation and, later, overlapping explosions.
Their influence depends on the setting. A small early halo holds gas less tightly than a much larger system, while a dense cloud can shield its interior from some incoming radiation.[2], [3]
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Gas gathers
Inflow and cooling supply material for gravitational collapse.
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Sources become active
Young stars and accretion flows illuminate and disturb nearby gas.
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The supply changes
Gas can disperse, remain heated, collect elsewhere, or return later.
How does radiation change a cloud?
Radiative feedback includes several different physical processes. Keeping them separate helps explain why more light does not always produce the same response.
| Mechanism | What happens | Why it matters |
|---|---|---|
| Photoionization heatingHydrogen threshold: 13.6 eV | A photon removes an electron from an atom. Energy above the ionization threshold can heat the gas. | Higher thermal pressure can drive expansion and hinder collapse.[1] |
| Molecule destructionLyman–Werner band: 11.2– |
Ultraviolet radiation can break apart molecular hydrogen, H2. | Pristine minihalos can lose an important route for cooling their gas.[3] |
| Radiation pressureDirect momentum transfer | Absorbed or scattered photons give momentum to matter. | Radiation can push gas and, where present, dust; the strength depends on what absorbs or scatters the light.[2] |
An electron volt, or eV, is a unit of energy. The 13.6 eV threshold applies to ground-state hydrogen.
Ionized bubbles and evaporating clouds
An H II region is a region of ionized hydrogen. Gas there often reaches temperatures around 10,000 K, although the actual temperature depends on the radiation and cooling. Its thermal pressure can drive an expanding boundary into surrounding neutral material.
When radiation heats a cloud’s exposed gas enough for it to flow away, astronomers call the process photoevaporation. This can remove part of a small halo’s gas supply.[1]
Does destroying H₂ always stop star formation?
No. H₂ is especially important for cooling pristine minihalos. Dense molecular columns can absorb incoming Lyman–Werner radiation and protect gas farther inside—a process called self-shielding.
Molecules can also form again. Larger, hotter halos may cool through atomic hydrogen, and enriched gas has additional cooling routes. The result depends on the cloud and the radiation it receives.[3], [4]
What do stellar winds and supernovae do?
A stellar wind is material flowing away from an individual star. A galactic outflow is gas moving outward on a larger scale, potentially driven by several sources and mechanisms together.
Winds meet surrounding gas
Collisions between a fast wind and ambient material create shocks that heat gas and transfer momentum. Multiple stellar sources can help excavate cavities.
But pristine Population III stars lacked the heavy-element absorption lines that help drive strong winds in many modern massive stars. Their corresponding winds are expected to be weak or absent under many conditions; other mass-loss mechanisms remain possible.[5]
Explosions expand into the cloud
A successful supernova sends ejecta and a shock into its surroundings. It can heat gas, accelerate it outward, and transport newly available elements.
Earlier radiation may already have thinned the local gas. This changes how the explosion expands and how quickly its energy is lost through radiation.[2], [6]
Not every massive star produces a successful explosion. The different outcomes, and the elements they release, are explored in Primordial Supernovae: Element Synthesis.
How can a black hole influence distant gas?
Gas falling toward a black hole can release energy before crossing the event horizon. An actively accreting massive black hole may power an active galactic nucleus, or AGN. A black hole with little fuel can be comparatively quiet.
Radiation from the accretion flow can heat and ionize surrounding gas. Fast winds can also collide with ambient material, generating shocks and hot bubbles that drive larger outflows. How effectively this moves galactic gas depends on cooling, mixing, and the surrounding material.[15]
X-rays have competing effects
X-rays from accreting remnants, including black holes fed by stellar companions, can travel through gas that would absorb softer radiation more readily.
They heat the gas, potentially making collapse harder. But the free electrons created by ionization can also assist reactions that build H₂, helping some gas cool. Models must follow both effects to determine the outcome.[7]
When does feedback suppress or encourage star formation?
A cloud responds according to its density, temperature, shielding, and gravitational binding. The important comparison is between how quickly it can cool and collapse and how quickly it is heated, stripped, or disrupted.[4]
Less gas ready to collapse
Heating increases pressure. Radiation can remove molecules needed for cooling, while outflows move material away from a star-forming region.
Star formation can slow or pause until gas cools, returns, or is replaced by fresh inflow.
Some gas becomes easier to collapse
An expanding region or shock can compress a neighboring cloud. If that cloud sheds heat, remains bound, and collapses before disruption, new star formation may follow.
Compression alone is insufficient: a crushed cloud can also be heated or dispersed.
“Positive” and “negative” describe the effect on star formation. They do not assign value to the outcome.[4]
Can a region recover after its ionizing source fades?
Yes. As gas recombines, remaining free electrons can help molecular hydrogen form. Under suitable conditions, molecular cooling assists renewed collapse in a former H II region.
Recovery still requires gas to remain nearby or collect again. Its chemistry, density, and later irradiation all matter.[3]
Where does the displaced gas go?
Leaving a star-forming cloud, leaving a galaxy, and escaping its entire halo are different outcomes. Some material travels outward but remains gravitationally bound. It can cool and return in a galactic fountain.
Other material reaches the gas surrounding the galaxy or escapes farther into intergalactic space. At the same time, fresh inflow may continue supplying the system. These exchanges form part of the baryon cycle—the movement of ordinary matter between galaxies and their surroundings.[8]
Outflows also carry heavy elements. Enrichment changes the cooling options of later clouds, but its distribution is uneven. Where the material eventually collects matters for the next generation.
Reionization changes the supply beyond individual galaxies
As ionizing radiation spread between early galaxies, it transformed much of the intergalactic hydrogen from neutral to ionized. This was an extended, patchy transition. Quasar absorption measurements indicate that reionization-related variations persisted to about 1.1 billion years after the Big Bang.[9]
Heating the surrounding gas can make it harder for small halos to acquire new material. Feedback can also open paths through which more ionizing photons escape their hosts. The balance between photon production, escape, and absorption determines how feedback affects reionization.[7], [14]
How do astronomers observe feedback?
Astronomers study both the sources and the gas around them. A spectrum separates light by wavelength, revealing emission and absorption features. Doppler shifts and line shapes provide clues to motion along our line of sight.
Resolved emission and line profiles
JWST can measure infrared spectra of distant galaxies, including light originally emitted at visible wavelengths. Broad or shifted components can indicate fast-moving gas.
Researchers also examine spatial structure and competing explanations such as rotation, mergers, and turbulence. A broad line alone does not establish an escaping galactic wind.[10]
Several gas phases
Ionized, neutral, and molecular gas respond differently and leave different spectral signatures. Absorption against a background light source provides another view of material along the sightline.
The gas visible in one line is only part of the outflow. Estimates of its mass and escape prospects depend on density, geometry, and the host’s gravitational pull.[10], [11]
Outflows can move material without shutting a galaxy down
A 2026 GA-NIFS study used JWST/NIRSpec to examine massive, luminous star-forming systems at redshifts around 3–9. Several contained gas with emission-line profiles consistent with ionized outflows.
Under the researchers’ modeling assumptions, the measured gas could not escape its hosts. The inferred outflows mainly redistributed dust and metals, generally removing too little ionized gas to strongly suppress star formation.
The result concerns a selected sample and the observed ionized component. Detecting an outflow does not by itself establish long-term quenching, a sustained suppression of star formation.[11]
Listening for the response of intergalactic hydrogen
The redshifted 21-centimeter signal from neutral hydrogen offers a complementary probe of early heating and ionization. The SKA-Low science program aims to study this gas across cosmic dawn and reionization, connecting the luminous sources with changes in their wider surroundings.[12]
What are the main uncertainties?
Simulations combine gravity, gas motion, chemistry, radiation, and stellar evolution. They cannot resolve every process at every scale, so researchers use approximations and test how their results change with resolution and physical assumptions.
Different projects make different choices. FIRE-2 investigates stellar feedback in galaxy formation; CROC couples gas dynamics and star formation to an approximate treatment of radiative transfer for reionization. These are complementary approaches with distinct strengths and limits.[6], [13]
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What did the first sources emit?
The masses, companions, and lifetimes of early stars affect the radiation and explosions supplied to their surroundings.
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How much reaches dense gas?
Shielding, open channels, and the arrangement of clouds determine where energy and momentum are deposited.
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How much gas returns?
Cooling, halo growth, and continuing inflow determine whether an interruption becomes a lasting change.
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Which observations distinguish the models?
Galaxy spectra, gas motions, and reionization measurements must be explained together.
A galaxy’s next stars depend on what happens to its gas.
Radiation changes temperature and chemistry. Winds and explosions redistribute matter. Accreting black holes can influence both their own fuel and the surrounding galaxy. The resulting growth can be interrupted, redirected, or renewed.
Meanwhile, gravity continues bringing systems together. The next article follows merging and hierarchical growth, as small galaxies combine and inherit the gas, stars, and chemical history of their predecessors.
Sources and further reading
Observational examples reviewed in September 2026. Simulation results and inferred outflow properties depend on the physical assumptions and systems studied.
- Whalen, Abel & Norman (2004) — Radiation Hydrodynamical Evolution of Primordial H II RegionsPhotoionization, heating, and gas evacuation around the first luminous sources.
- Wise et al. (2012) — The Birth of a Galaxy. II. The Role of Radiation PressureRadiation momentum and its interaction with other stellar feedback mechanisms.
- Johnson, Greif & Bromm (2007) — Local radiative feedback in the formation of the first protogalaxiesMolecular destruction, shielding, and renewed molecule formation in relic ionized regions.
- Ahn & Shapiro (2007) — Does Radiative Feedback by the First Stars Promote or Prevent Second Generation Star Formation?How gas density, cooling, and radiation determine whether collapse advances or stalls.
- Krtička & Kubát (2006) — The winds of the hot massive first starsWhy pristine hydrogen–helium stars cannot simply be assigned modern metal-driven winds.
- Hopkins et al. (2018) — FIRE-2 Simulations: Physics versus Numerics in Galaxy FormationStellar feedback, numerical resolution, and the methods used to model galaxy formation.
- Jeon et al. (2014) — Radiative Feedback from high mass X-ray binaries on the formation of the first galaxies and early reionizationCompeting effects of X-ray heating and ionization on early gas and black-hole growth.
- Anglés-Alcázar et al. (2017) — The Cosmic Baryon Cycle and Galaxy Mass Assembly in the FIRE SimulationsGas inflow, outflow, recycling, and exchange between galaxies.
- Bosman et al. (2022) — Hydrogen reionisation ends by z = 5.3: Lyman-alpha optical depth measured by the XQR-30 sampleQuasar absorption measurements supporting a late end to hydrogen reionization.
- Carniani et al. — JADES: The incidence rate and properties of galactic outflows in low-mass galaxies across 3 < z < 9JWST spectroscopy of outflows and the observational limits on their detection.
- Rodríguez Del Pino et al. (2026) — GA-NIFS: high prevalence of dusty and metal-enriched outflows in massive and luminous star-forming galaxies at z ~ 3–9A sample in which inferred ionized outflows mainly redistribute material around their hosts.
- SKA Observatory — Epoch of Reionization science programPlanned use of redshifted 21-centimeter hydrogen signals to study early heating and ionization.
- Gnedin (2014) — Cosmic Reionization On Computers I. Design and Calibration of SimulationsCoupled radiative transfer, gas dynamics, and star formation with finite resolution and calibrated parameters.
- Trebitsch et al. (2017) — Fluctuating feedback-regulated escape fraction of ionizing radiation in low-mass, high-redshift galaxiesHow feedback clears paths that allow stellar ionizing radiation to escape.
- Faucher-Giguère & Quataert (2012) — The physics of galactic winds driven by active galactic nucleiAccretion-powered winds, shocks, and the transfer of energy to surrounding galactic gas.
All articles in this chapter
- Gravitational Clumping and Density Fluctuations
- Population III Stars: The Universe’s First Generation
- Early Mini-Halos and Protogalaxies
- Supermassive Black Hole Seeds
- Primordial Supernovae: Element Synthesis
- Feedback Effects: Radiation and Winds — you are here
- Merging and Hierarchical Growth
- Galaxy Clusters and the Cosmic Web
- Active Galactic Nuclei in the Young Universe
- Observing the First Billion Years