Nova Cas 2021 increases in brightness

The nova in Cassiopeia that appeared on 18 March has provided another surprise – an outburst of over five times its brightness in a matter of a few days, to become bright enough to be visible with the naked eye.

Reports show that the nova has been slowly increasing in brightness since mid April. As of 6 May, the object was at a visual magnitude of about 7, having been discovered at a magnitude of 7.8.

However, on the evening of 7 May, visual observers noted magnitudes brighter than magnitude 6. SPA member Bob Steele reports:  ‘I made two estimates this evening at 2150 UT and 2208 UT with 15×63 binocs in good conditions. Both produced a visual magnitude of 6.0. Last night it appeared to me to be mag 7.0. ‘

These magnitudes are confirmed by reports sent to the American Association of Variable Star Observers. Very few novae display a secondary maximum, and this is usually fainter than the original peak. So observers are urged to keep watch on this star. Our previous news story gives more information about the location of the nova.

Update 8 May. The latest visual estimates, and the photo below, put the nova at about magnitude 5.5. This makes it the brightest nova visible from the UK since Nova Sagittarii 2016 No 4, which peaked at magnitude 5.4, and the brightest nova well placed for viewing since Nova Delphini 2013. at mag 4.3.

Update 11 May. The rise in brightness appears to be levelling off at about magnitude 5.3, according to estimates supplies to the American Association of Variable Star Observers.

Update 26 May. After a surge in brightness to magnitude 5.3 lasting a few days, the nova has now settled back to about magnitude 7.5. The object’s spectrum has the characteristics of a nova, and it is categorised as a slow nova, which may take several months to decline in brightness.

NCas2021widefinder
A wide-field view of the region of the nova in Cassiopeia, which is marked with a cross.
NCas-changes
Increase in brightness of Nova Cas (circled) between 20 March and 8 May 2021 at 23h. Photo: Robin Scagell