White dwarfs are supposed to be boring when it comes to colour: dense stellar embers that cool for billions of years, drifting from blue-white through white to orange and eventually red. A set of observations reported by Heise suggests they can be stranger than that, including what is described as the first green star and the reddest white dwarf yet found.
A star's colour is normally a proxy for temperature. Hotter objects radiate more of their energy at short wavelengths and look blue; cooler ones look red. But the perceived colour of a white dwarf is also written by its atmosphere — hydrogen-dominated, helium-dominated, or polluted with heavier elements pulled in from a shredded planet — and by extreme surface gravity, which broadens absorption lines in ways that change which wavelengths escape.
That is how a green-looking star can exist. No simple temperature model produces green: our eye integrates a broad spectrum, and the Sun's own peak output lies in the green yet still appears white to us. Colours as unusual as green, or an extreme red, point to unusual atmospheric physics rather than a plain thermometer reading — and they are useful precisely because they stress models of how these remnant atmospheres evolve.
For astronomers the interest is in the outliers. A white dwarf redder than cooling theory comfortably predicts constrains those theories: on opacity, on atmospheric composition, on how quickly heat escapes.
The sample specifics, temperatures and spectral analysis belong in the original report and the underlying paper; the headline is that the colour palette of stellar remnants is wider than the textbooks imply.
