A supernova remnant is made of material ejected from an eploding star. A star that is roughly three times more massive than the sun or larger is doomed to end this way. When it does, it ejects most of its material in a rapidly expanding shock wave. This then seeds the surrounding area with heavier elements that go on to be part of new stars, solar systems, planets, trees, and cats. These remnants often contain copious amounts of ionized hydrogen and oxygen, which makes them particularly photogenic to astrophotographers using specialized filters.