Astronomy
The Lyman-Continuum Emission from Star-Forming Galaxies
Quick fact
Lyman-continuum photons have wavelengths shorter than 91.2 nanometers—so energetic they can ionize hydrogen. These photons are produced by the hottest, most massive stars, which live only a few million years, and they are thought to have reionized the universe within the first billion years after the Big Bang.
Why this is interesting
You've seen pictures of glowing galaxies, but did you know some of their light is so energetic it can strip electrons from atoms? This extreme ultraviolet light, called Lyman-continuum emission, may hold the key to how the universe became transparent.
Read the full explanation
Understanding The Lyman-Continuum Emission from Star-Forming Galaxies
Imagine a very hot, massive star—tens of times the mass of our Sun. Its surface temperature exceeds 30,000 K, causing it to emit a flood of ultraviolet light. Some of that light is so energetic that its wavelength is shorter than 91.2 nanometers—this is the Lyman continuum. When such a photon hits a neutral hydrogen atom, it has enough energy to knock the electron free, ionizing the atom. Around the star, this creates a bubble of ionized gas known as an H II region. In a star-forming galaxy, many such massive stars exist, each creating its own ionization bubble. If these photons escape the galaxy's gas and dust, they can travel into intergalactic space and ionize the diffuse gas between galaxies.
A deeper explanation
The key to Lyman-continuum emission lies in the physics of atomic hydrogen. The ionization energy of hydrogen is 13.6 electronvolts, corresponding to a photon wavelength of 91.2 nm—the Lyman limit. Massive O-type and B-type stars, with surface temperatures above ~30,000 K, produce a significant number of photons above this energy. The total production of LyC photons scales steeply with stellar mass, making star-forming galaxies powerful factories of ionizing radiation. However, the escape of these photons from the galaxy is not trivial. Neutral gas and dust in the interstellar medium can absorb LyC photons before they leave. The escape fraction—the proportion of LyC photons that actually exit the galaxy—is a crucial parameter. In the early universe, the first galaxies likely had high escape fractions, allowing their LyC emission to ionize the intergalactic medium, leading to the epoch of reionization. Today, astronomers study local star-forming galaxies to measure escape fractions and understand the conditions that allow LyC leakage, providing insight into how the universe became transparent.