"and of the Roman emperors as low as Severus"
Bust of Septimius Severus (after 204AD) - Credit: Roby
The Roman Empire existed between 44BC and 1453AD. What is now known as the Ancient Roman Empire came to an end in 476, while the Eastern Roman Empire (or Byzantine Empire) lasted until 1453.
The first Roman emperor was Augustus, who ruled from 27BC to 14AD. Septimius Severus (often known simply as Severus), the first emperor of the Severan dynasty, ruled from 193 to 211.