A galaxy’s colour is a mixture of many birthdays
A galaxy contains stars born at many different times. A small recent generation can contribute a conspicuous amount of blue light because its most massive young stars are so luminous. Those stars also use up their lives quickly.
If new stars keep arriving, they keep refreshing that young population. If star formation declines, the bright blue contribution gradually falls and older populations become more prominent in the combined optical light. This is why the recent supply of stellar births matters even when much of the stellar mass was assembled long ago.
Two galaxies with the same visible diameter can have different gas reserves, stellar masses and birth histories. Their similar size gives us a starting comparison; their fuel and light explain the colour difference.
Reading: Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1 · Original fixed-slice and static-restoring-force materials
The environment can remove fuel or stop the refill
Cold, dense gas supplies new stars. A galaxy moving through hot cluster gas experiences a relative wind. Where its momentum transfer overcomes the galaxy’s resistance for long enough, the wind can move disk gas away from the region where stars were forming.
A second route acts on the larger reservoir around the galaxy. If fresh gas no longer replenishes the disk, existing cold gas can continue forming stars for a while and gradually run down. This is often called starvation: the refill is cut off while the remaining supply is consumed.
The two routes can overlap. Their timing depends on the gas distribution and on the environment encountered along the path. A strong pressure peak, a long exposure and a depleted outer reservoir therefore lead to different histories of the same basic fuel problem.
Reading: Köppen, Jáchym, Taylor & Palouš, Ram Pressure Stripping Made Easy, arXiv:1806.05887v1 (2018) · Dou, Yu & Hou, Pre-processing and protection during cluster infall, arXiv:2605.23314v1 (2026)
Give the population time to change its light
The stellar light responds after the change in fuel. Previously formed stars remain present, and different masses evolve on different schedules. There is no single instant when every galaxy of a given size must turn red.
In a controlled light calculation, compare a history that keeps forming stars with one that is deliberately stopped after 650 million years. At later ages the stopped history lacks the new blue contribution. The plotted samples show how that specified history affects global colour; the shutdown time was an input.
Reading: Bruzual & Charlot, Stellar population synthesis at the resolution of 2003, astro-ph/0309134v1 · Original fixed-slice and static-restoring-force materials
Dust can redden an actively forming galaxy too
Light crossing dust loses a larger fraction of its blue component in the optical example used here. The same underlying stars can then look redder even while new stars are forming. A red image can therefore motivate a question about both stellar ages and the light’s path.
More than one band, spectral features or measurements at other wavelengths help compare these explanations. The dust screen in our light exercise is separately adjustable, so you can hold the formation history fixed and see how much that one change affects the colour.
Reading: Calzetti et al., The Dust Content and Opacity of Actively Star-Forming Galaxies, astro-ph/9911459v1 / ApJ533 (2000) · Original fixed-slice and static-restoring-force materials
Follow the two links in the calculation separately
The gas-sheet model follows a prescribed wind pulse into gas motion and the fuel counted inside a disk volume. Its consumption rule continues outside the disk, so the all-gas star-formation rate is identical for every wind path. Its wind calculation therefore supplies the disk-fuel link.
The separate population exercise takes specified birth histories and calculates their global light. Putting a shutdown into that exercise demonstrates the history-to-colour link. Predicting a real galaxy’s global colour first requires a law for how the displaced gas continues forming stars. If the light escapes freely and all stars are included, their populations can be added directly. Colours measured only in a disk or tail region also require the stars’ later positions.
Some galaxies have already spent time in a smaller group before arriving at their present cluster. Their fuel and stellar ages can carry that earlier environmental history. The final reading guide explains how observations test this pre-processing, alongside the cluster passage.
Reading: Original fixed-slice and static-restoring-force materials · Dou, Yu & Hou, Pre-processing and protection during cluster infall, arXiv:2605.23314v1 (2026)