Scientists working at the Solar Dynamics Observatory have spotted a massive solar filament that stretches across a good portion of the sun's visible surface. It's about 533,000 miles long (857,780 km) – equivalent to 67 Earths lined up in a row.
NASA says this solar filament — one of the longest ever recorded — is an enormous line of colder material that's hovering in the Sun's atmosphere, the corona. According to APOD, the scar is actually hot gas held aloft by the Sun's magnetic field; if viewed from the side it would appear as a raised prominence:
(Image credit: jp-Brahic; Earth shown for scale)
Filaments like this one can last for days before disappearing.
They're also known to erupt out into space, ejecting solar material that either rains back down or escapes out into space (a "moving cloud" known as a coronal mass ejection).
Check out another picture of the solar filament here.