---
title: "How starlight changes intergalactic hydrogen"
description: "Ultraviolet photons escaping young galaxies ionize intergalactic hydrogen and must also replace ions lost through recombination."
canonical_url: "https://sandboxphysics.com/stories/reionization/starlight-and-hydrogen/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/reionization/starlight-and-hydrogen/"
last_modified: "2026-11-12"
---

[Starlight and intergalactic hydrogen](https://sandboxphysics.com/stories/reionization/)

# How starlight changes intergalactic hydrogen

Sandbox Physics · 2026-11-12

Ultraviolet light escaping a young galaxy can knock electrons out of hydrogen atoms. As the affected regions grow and meet, more of the gas between galaxies changes from neutral atoms into charged particles. This is the basic picture of cosmic reionization.

Imagine colouring the gas according to its state. Give one colour to regions where electrons mostly remain attached to protons, and another to regions where they have been separated. Patches around galaxies would appear as ionized bubbles. Gas still occupies those patches. A bubble has no material skin like a soap bubble; its boundary marks a change in the state of hydrogen.

## First the light must escape

A photon is a packet of light energy. If a hydrogen atom absorbs a sufficiently energetic photon, its electron can escape the proton. Ground-state hydrogen needs at least 13.6 electronvolts for this process. A single ordinary visible-light photon falls below that threshold. Young, hot stars produce many higher-energy ultraviolet photons, which makes them useful sources for ionizing the surrounding gas.

Some of those photons are absorbed by gas or dust before they leave their galaxy. The fraction that reaches intergalactic space is called the escape fraction. Two galaxies could produce the same number of ionizing photons but release quite different numbers into their surroundings.

The useful quantity for this question is therefore the number of **escaping ionizing photons**. Brightness in a visible-light photograph has not yet told us that number. We focus here on light supplied by stars; determining the contributions of different kinds of source to the real reionization history requires observations. [Madau, Haardt and Rees, 1999, §3.1](https://arxiv.org/abs/astro-ph/9809058v1)

## Hydrogen can need another photon

A free electron can meet a proton and bind to it again. This process is recombination. To maintain an ionized region, the source must keep supplying photons to ionize hydrogen that has recombined. One hydrogen nucleus can require more than one photon over its history.

Consider a simple counting example. Suppose each absorbed photon produces one hydrogen ionization, with other uses of photons left out. Start with one hundred neutral hydrogen nuclei. After sixty ionizations and twenty recombinations, forty nuclei remain ionized. Twenty of the ionization events have been undone. The twenty recombinations need not have involved twenty distinct nuclei: the same nucleus may have recombined more than once.

Density affects how rapidly this happens. In denser gas, electrons and protons encounter one another more often. At the same temperature, maintaining ionization can therefore cost more photons in a dense region. Bubble growth depends on both the light supplied and the repeated recombinations occurring in gas that has already been ionized. [Mellema et al., C2-Ray method, §2](https://arxiv.org/abs/astro-ph/0508416v2)

When two bubbles meet, their overlap contains only one copy of the gas. Additional photons can replace recombinations or reach hydrogen that is still neutral. Simply drawing two larger spheres and taking their union makes it easy to lose track of that extra supply. A calculation of merging bubbles must account for both the gas and the photons.

## Transparency depends on the light

As neutral hydrogen disappears, absorption of hydrogen-ionizing light decreases. This is one meaning of the Universe becoming more transparent. Other wavelengths interact differently. Even a small residual amount of neutral hydrogen can strongly affect a resonant spectral line, and free electrons scatter radiation. Reionization changes how the gas interacts with light across the spectrum.

Redshifted 21 cm radiation offers a way to study the neutral gas. The line connects two groups of atomic spin states, whose relative populations are described by a spin temperature. In a useful ideal limit, that temperature is far above the microwave background temperature. Neutral regions then have a stronger ideal signal, while ionized regions lack that neutral-hydrogen contribution. This is the starting point for viewing ionized patches through the gas around them. [Furlanetto, Oh and Briggs, 2006, §2.1](https://arxiv.org/abs/astro-ph/0608032v2)

Real radio data contain bright foreground emission and instrumental effects. HERA's first results, released in 2021, placed upper limits on the strength of fluctuations; they did not locate individual early bubbles. A statistical upper limit and a map of where the bubbles lie carry different kinds of information. [HERA first results, fixed 2021 version](https://arxiv.org/abs/2108.02263v1)

The physical story connects light escaping galaxies, hydrogen being ionized repeatedly, and the spatial pattern an observer hopes to distinguish. Counting how much gas is ionized and finding where that gas lies are useful next steps with different answers to work out.

Continue with [What does half ionized mean?](https://sandboxphysics.com/stories/reionization/ionized-volume-and-local-fraction/) and [Supplying photons to an expanding volume](https://sandboxphysics.com/stories/reionization/photons-in-an-expanding-box/).

## Sources

1. [Madau, Haardt and Rees, 1999, §3.1](https://arxiv.org/abs/astro-ph/9809058v1)
2. [Mellema et al., C2-Ray method, §2](https://arxiv.org/abs/astro-ph/0508416v2)
3. [Furlanetto, Oh and Briggs, 2006, §2.1](https://arxiv.org/abs/astro-ph/0608032v2)
4. [HERA first results, fixed 2021 version](https://arxiv.org/abs/2108.02263v1)

## Keep reading

Starlight and intergalactic hydrogen · Reading path

### Photons, ionized volume and a radio signal

1. [You are here **How starlight changes intergalactic hydrogen**](https://sandboxphysics.com/stories/reionization/starlight-and-hydrogen/)
2. [Article **What does half ionized mean?**](https://sandboxphysics.com/stories/reionization/ionized-volume-and-local-fraction/)
3. [Article **Supplying photons to an expanding volume**](https://sandboxphysics.com/stories/reionization/photons-in-an-expanding-box/)
4. [Article **Where the photons go when ionized bubbles meet**](https://sandboxphysics.com/stories/reionization/when-ionized-bubbles-overlap/)
5. [Article **Reading ionized bubbles through neutral hydrogen**](https://sandboxphysics.com/stories/reionization/reading-a-neutral-hydrogen-map/)
6. [Article **Recompute a fixed-source ionization history**](https://sandboxphysics.com/stories/reionization/recompute-a-fixed-source-history/)
7. [Article **Reading HERA's 21 cm upper limits**](https://sandboxphysics.com/stories/reionization/reading-hera-upper-limits/)