---
title: "Starlight and intergalactic hydrogen"
description: "Follow escaped starlight through ionization and recombination, then read neutral-hydrogen maps and HERA upper limits."
canonical_url: "https://sandboxphysics.com/stories/reionization/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/reionization/"
last_modified: "2026-11-12"
---

# Starlight and intergalactic hydrogen

Sandbox Physics · 2026-11-12

Begin with ultraviolet light leaving a galaxy. Count ionizations and recombinations, follow an expanding volume, then read the radio signal left by neutral hydrogen. Work the examples as each new quantity appears.

1. L1 · Concept

   ## [How starlight changes intergalactic hydrogen](https://sandboxphysics.com/stories/reionization/starlight-and-hydrogen/)

   Ultraviolet photons escaping young galaxies ionize intergalactic hydrogen and must also replace ions lost through recombination.

2. L2 · Chapter

   ## [What does half ionized mean?](https://sandboxphysics.com/stories/reionization/ionized-volume-and-local-fraction/)

   A half-ionized parcel and a volume split into neutral and fully ionized regions can share an average but recombine at different rates.

3. L2 · Chapter

   ## [Supplying photons to an expanding volume](https://sandboxphysics.com/stories/reionization/photons-in-an-expanding-box/)

   Escaped photons grow the ionized volume while recombination consumes part of the supply; a comoving box keeps the hydrogen count fixed.

4. L3 · Chapter

   ## [Where the photons go when ionized bubbles meet](https://sandboxphysics.com/stories/reionization/when-ionized-bubbles-overlap/)

   Overlapping regions contain one copy of the gas. Follow excess supply into neutral volume, then distinguish volume conservation from bubble shape.

5. L3 · Chapter

   ## [Reading ionized bubbles through neutral hydrogen](https://sandboxphysics.com/stories/reionization/reading-a-neutral-hydrogen-map/)

   Derive the ideal high-spin-temperature 21 cm signal, then use simple waves to distinguish mean brightness, spatial power and Fourier phase.

6. L3 · Method

   ## [Recompute a fixed-source ionization history](https://sandboxphysics.com/stories/reionization/recompute-a-fixed-source-history/)

   Read the original cell field, check its photon balance, and use independent quadrature and explicit Fourier projections to examine the same history.

7. L4 · Paper guide

   ## [Reading HERA's 21 cm upper limits](https://sandboxphysics.com/stories/reionization/reading-hera-upper-limits/)

   Read the markers, errors and windows in HERA's power spectrum, recompute two published table rows, and connect an upper limit to early-universe physics.