Supernova
Cassiopeia A
The brightest radio source in the sky; remnant of a supernova around 1680
The remnant of a supernova that exploded about 340 years ago — one of the last observed in the Milky Way, though dust may have hidden the explosion. The neutron star at its centre was discovered in Chandra's first-light image; JWST revealed the remnant's "Green Monster" structure.
| Type | Supernova |
|---|---|
| Constellation | Cassiopeia |
| Distance | 3,400 pc (11,000 light-years) |
| Explosion | ~1680 |
| Expansion speed | 4,000–6,000 km/s |
| Centre | Neutron star |
| Right ascension (J2000) | 23h 23m 24.0s |
| Declination (J2000) | +58° 48′ 54″ |
In the sky
Visible from the whole Northern Hemisphere and south to latitude 31° S. North of latitude 31° N it never sets. It stands highest at midnight in September, the best time to observe it.
Through a telescope from Earth
Light's journey
The light you see tonight set out 11 thousand years ago. At that time the first farming villages were being founded; there was no writing yet. At the speed of Voyager 1, the fastest craft leaving the Solar System (16.9 km per second), the trip would take 197 million years.