Illustration of the Universe's first generations of stars. Scientists estimate that the first stars began to shine when the Universe was just 180 million years old, a fraction of its 13.8-billion-year age. Astronomers have detected these stars indirectly for the first time, by observing the signals in hydrogen gas that these stars illuminated. These stars are theorized to have been very massive, burning through their hydrogen supplies in around a million years or so, and then turning into the red giants and supergiants seen in this illustration.