Some stars, known as red giants, are incredibly large, with sizes up to 1,500 times bigger than our sun. These massive stars are in the final stages of their life cycle, having exhausted their fuel and expanded to enormous sizes.
Imagine a star so big that it would engulf the entire solar system, including all the planets. That's what a red giant star would look like. As they expand, they can sometimes be seen from Earth, but only briefly. These brief appearances are known as 'red giant flares'.