Germany compared the Sun to hundreds of similar stars and found it fluctuated up to five times weaker. 

Using data from NASAs now retired Kepler Space Telescope, they selected stars with a similar surface temperature, age and rotation to the Sun - in the Milky Way galaxy.

Researchers say it's not clear whether the Sun has been 'going through a quiet period' for 9,000 years or is actually less bright than other similar stars.