As part of our search for emission-line objects in 26 Galactic globular clusters, we found a small emission nebula in M 22 (NGC 6656). Its spectral lines and mass indicate that it is the remnant of a nova.
A catalogue of ancient Chinese observations contains a record of a 'guest star' observed in the year 48 BCE very close to the position of the globular cluster.
Together with a remnant age of roughly 2,000 years estimated from its brightness, this suggests that the underlying nova has been observed by humans about 2,000 years ago.
This would be one of the oldest confirmed extrasolar observations recorded by humans.
A nova is an explosion caused by accreted matter on the surface of a white dwarf in a binary system.
The explosion increases the brightness of the white dwarf by several orders of magnitude, causing previously invisible stars to become much brighter, sometimes even visible to the naked eye. The white dwarf loses parts of its mass in the explosing, this ejected mass is distributed into the space surrounding the binary system. The ejected mass is illuminated by the UV radiation from the white dwarf and re-emits the energy in visible wavelengths, which can be observed as emission lines.
While a handful of novae are observed in the Milky Way each year, only two novae have been observed in Galactic globular clusters in the last 150 years.