Crab Nebula

The Crab Nebula (M1) is the expanding wreckage of a supernova explosion observed and recorded by Chinese, Japanese, and Arab astronomers in 1054 AD. For roughly three weeks, the "guest star" was bright enough to be seen in daylight; it gradually faded over the following 22 months.

Now, almost a thousand years later, the nebula is a chaotic filigree of glowing gas expanding outward at 1,500 km/s. The visible structure is about 11 light-years across and still growing measurably from year to year — astronomers can directly photograph the expansion by comparing images taken decades apart.

At the heart of the nebula lies the Crab Pulsar — a neutron star about 20 km in diameter spinning 30 times per second. The pulsar is the crushed remnant of the original star's core: an entire stellar mass squeezed into a sphere the size of a city, where a teaspoon of material would weigh a billion tons. The pulsar emits beams of radio waves, light, and X-rays that sweep across Earth like a lighthouse, allowing it to be detected at every wavelength.

The Crab Nebula is too faint for naked-eye observation but is one of the most-studied objects in the sky across radio, optical, X-ray, and gamma-ray astronomy. It's sometimes called the "standard candle" of X-ray astronomy because its emission is so well-characterized.