Cassiopeia A is the young remnant of a massive star that exploded unnoticed, and the brightest radio source in the sky beyond the Solar System.
At a glance
| type | supernova remnant |
|---|---|
| distance from the Sun | 11,100 light-years (3.4 kpc) |
| its light left it | 11,100 years ago |
| size in the piece | about 16 light-years acrossmodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Cassiopeia A
Cassiopeia A is the expanding debris of a core-collapse supernova, the death of a massive star. It is among the youngest remnants in the Milky Way, yet its explosion seems to have gone essentially unrecorded, probably because dust dimmed it. It was first found as an intense radio source, and at radio wavelengths it outshines everything in the sky beyond the Sun.
Its shell of shocked gas glows in X-rays, where Chandra’s first images revealed a point at its centre: the neutron star left by the collapse. Light from the original explosion, scattered off distant dust clouds and arriving late as light echoes, has been caught and studied, showing the star was stripped of most of its hydrogen before it died.
Because it is young and bright, its knots of oxygen, silicon and iron can be mapped in three dimensions, showing how the explosion turned the star inside out.
What you see in the piece
It is drawn as a hollow shell of shocked gas in Cassiopeia, faint in visible light at its distance and behind the dust; it stands out in the radio band, where supernova shells shine.
Measured or modelled
Whether the explosion was glimpsed by any observer at the time is uncertain. The shell’s drawn extent is modelled from its size on the sky and distance.
Questions
How far away is Cassiopeia A?
Cassiopeia A is about 11,100 light-years (3.4 kpc) from the Sun.
Why was the Cassiopeia A supernova not seen?
It is thought dust along the line of sight dimmed the explosion so much that it was not bright enough to be noticed.