Complex sentence: Historically this star was used as a luminosity standard to compare with other stars to check for variability, but it was later discovered that Bellatrix itself is an eruptive variable, changing in luminosity by a few percent over time.
Split into simpler sentences: Historically this star was used as a luminosity standard to compare with other stars to check for variability. But it was later discovered that Bellatrix itself is an eruptive variable, changing in luminosity by a few percent over time.