The Sun is not a perfectly steady lightbulb. Its brightness quivers slightly: it dims a bit, then brightens. For Earth's climate, even slow variations stretched over thousands and millions of years matter. They could have left their mark in the geological record of the last 540 million years.

Astronomer Geller measured the brightness of 170 stars in the open cluster M67. These stars are nearly the same age as the Sun and similar in size and temperature. Their spread on the color–luminosity diagram looks like a smeared band rather than a thin line. Part of that smearing is just measurement noise, like graininess in an old photograph. But when the noise is subtracted, a small intrinsic spread remains: the stars do indeed "breathe" in brightness.

The new analysis shows that the intrinsic brightness variations of such stars do not exceed 5–9 percent. This is an upper limit; the real changes are likely even smaller. For comparison, pure random noise gives about 10 percent. So the Sun probably did not suddenly dim or flare in the past. But even gentle variations could have nudged the climate into warmer or colder epochs.