Nova Cassiopeiae 2021, also known V1405 Cassiopeiae, was a nova in the constellationCassiopeia. It reached a peak brightness of magnitude 5.449 on May 9, 2021, making it visible to the naked eye.[4] It was discovered by Japanese amateur astronomer Yuji Nakamura of Kameyama, Japan, at 10:10 UT on March 18, 2021. The nova was first seen by Nakamura in four 15 second CCD exposures with a 135mm F/4 lens, when it was at magnitude 9.3. Nothing was seen brighter than magnitude 13.0 with the same equipment in exposures taken at 10:12 UT on March 14, 2021.[4][5][6] For the first seven months after discovery, the nova's brightness stayed at a rough plateau, fading and rebrightening at least eight times; it is considered a very slow nova.[7] After the seven month long series of peaks, Nova Cassiopeiae began a linear decline in brightness.[4] This nova has been detected throughout the electromagnetic spectrum, from radio to gamma rays.[8][9]
All novae are binary stars, consisting of a white dwarf orbiting a "donor star" from which the white dwarf accretes material. Spectra taken of Nova Cassiopeiae around maximum brightness showed that the nova was an FE II type novae.[10] The ejecta from FE II novae is believed to come from a large circumbinary envelope of gas (which was lost from the donor star), rather than the white dwarf.[11]TESS observations revealed an orbital period of 4.52138±0.00012 hours for the binary system.[12]
