When a star runs out of the nuclear fuel burning at its core, it collapses under its own weight. This death is so violent and sudden that it sends out a shock wave that hits the star's outer layers and causes them to explode in a supernova — the largest type of explosion humans have ever seen.
Somewhere between 10,000 and 20,000 years ago, ancient humans probably saw one such explosion glowing in the skies. It came from a dying star about 20 times the mass of our sun. The supernova was about 2,500 light-years away, but it glowed so bright that it was visible from Earth.
"This was before written language, but the universe kept a beautiful record of the blast," science writer Corey Powell recently wrote on Twitter.
That's because the explosion's blast wave is still expanding through space, at a speed of about 220 miles (350 kilometers) per second. The edges of the blast wave push material from the supernova through low-density matter in space, heating and compressing tendrils of dus and gas.
So the supernova explosion lives on as a web of colorful, glowing ribbons in the sky. This is called the Cygnus Loop because it's located in the constellation Cygnus from our vantage point on Earth.
NASA's Hubble space telescope snapped a striking photo of these supernova remnants, which was released last month. It captures a small section of the blast wave in unprecedented detail.
The European Space Agency, which co-manages Hubble with NASA, published the image below last month.
The entire blast wave spans an area of the sky six times larger than the full moon.
The image above, captured by NASA's Galaxy Evolution Explorer telescope before it shut down in 2013, shows the full Cygnus Loop.
Hubble first snapped a photo of the Cygnus Loop in 1991, though at that time the space telescope's primary mirror had an abnormality that made its images a little blurry. Hubble had launched just one year earlier; it would be another two years before astronauts visited and fixed the telescope.
Hubble was the first space telescope designed for in-orbit maintenance. Astronauts have visited it four more times to replace parts and upgrade its hardware.
As a result, the telescope takes much better pictures now, and we can see the Cygnus Loop in better detail than ever before.
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