The Cygnus Loop supernova remnant in optical light and X-rays. The optical image (reddish) has been taken from the 2nd digital sky survey, the X-ray images (greenish/bluish) are from a mosaic of several ROSAT PSPC observations. The optical light comes from cooling gas, resulting in a filamentary structures, as the cooling gas is being crushed by more highly pressured gas. The rapid cooling only happens once the shock velocity is below about 200 km/s. A part of Cygnus Loop is called the Veil nebula, and is often photographed by (advanced) amateur astronomers. The Cygnus Loop is a relatively old supernova remnant, of about 5000 year old, and at a distance of about 2000 light year. This for astronomical standards is relatively nearby. As a result the Cygnus Loop is rather large on the sky, about 3 degrees. By comparison, the Moon is only 0.5 degrees. If only our eyes were very sensitive, or could see X-rays, the night sky would be sprinkled with beautiful large structures!
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