Nova Cas 2021, a ‘new star’ – a classical nova – discovered on 18 March 2021 in the northern constellation of Cassiopeia, has suddenly brightened in the past few days by around two magnitudes. It now shines at around magnitude +5.2, possibly within naked-eye range from a dark-sky UK location.
Nova Cas 2021, now formally designated as V1405 Cas by the International Astronomical Union (IAU),was discovered by Yuji Nakamura of Japan on 18 March, when it shone at around magnitude +9.6. It then proceeded to brighten rapidly, hitting around magnitude +7.8 within 24 hours and peaking on 20 March at +7.5. Within hours of the discovery being announced, professional astronomers from the National Astronomical Observatory of Japan obtained a spectrum of the nova with the 3.8-m Seimei Telescope at Okayama Observatory, which revealed the telltale features of a classical nova.
The most concise description of a nova I have read is from a 1992 paper penned by Robert E. Williams, currently Astronomer Emeritus at the Space Telescope Science Institute (STScI) and a former Director of the Institute and of the Cerro Tololo Inter–American Observatory in La Serena, Chile. He writes ’The outbursts of novae are caused by nuclear reactions on the surface of a degenerate white dwarf which has accreted gas from the Roche-lobe overflow of a secondary companion in a close binary system’.
Since V1405 Cas’ discovery, the nova has been under constant watch by both amateur and professional astronomers. A light-curve produced from observations submitted to the American Association of Variable Star Observers (AAVSO: www.aavso.org) shows that the nova slowly declined in brightness until around 8 April, sinking to around magnitude +8.5. (UVBRI photometry reveals it has remained around a half to three-quarters of a magnitude brighter through R and I filters, respectively). Since mid-April, V1405 Cas has been slowly brightening; as of 6 May it shone visually at close to magnitude +7.
In the past few days it has brightened rapidly, the latest visual observation, from 10 May (~09h UT), had the nova at magnitude +5.2. This makes it the brightest nova seen from UK shores since V339 Delphini, which peaked at magnitude +4.3 in August 2013, and V5668 Sagittarii, which peaked at magnitude +4.0 onInterestingly, visual brightness estimates and those from the VRI bands (UBVRI photometry) appear to have converged during this latest brightening episode.
The Astronomer’s Telegram (www.astronomerstelegram.org) number 14614, dated 8 May, reported on spectral observations made by astronomers that are part of the ANS (Asiago Novae and Symbiotic stars Collaboration: a growing Italian network of small- and medium-size telescopes performing spectroscopy [low- and medium-resolution single dispersion, and Echelle high-resolution modes) and UBVRC IC CCD photometry of symbiotic stars and novae). It reported that a spectrum obtained on 8 May showed V1405 Cas looking like a ‘textbook example’ for a FeII nova at around maximum brightness.
Novae can be classified by their spectrum and by looking at the visual light-curve for their behaviour, notably the time taken to rise to maximum and subsequently fall to minimum. Let’s turn again to the aforementioned Robert Williams for an explanation of the former: ‘Spectra of post-outburst novae display either He+N or FeII lines as the most prominent non-Balmer lines at maximum light. Spectral diagnostics indicate physical conditions for ‘He/N’ spectra that are consistent with their origin in the white dwarf (WD) ejecta, whereas ‘FeII’ spectra point to their formation in a large circumbinary envelope of gas whose origin is the secondary star.’
The General Catalogue of Variable Stars (GCVS) recognises three types of novae based on the light-curve, fast novae (Na), slow novae (Nb) and very-slow novae (Nc). The AAVSO website (www.aavso.org/vsots_novae) has a detailed description of each class.
V1405 Cas is easily visible through a pair of 10×50 binoculars or a small telescope at the precise position RA 23h 24m 47.73s, Dec +61° 11’ 14.8” (J2000.0). It’s handily placed in Cassiopeia, lying just 2.5 arcminutes north-east of the magnitude +8.9 star HIP 115566 (SAO 20610), which itself is located a 24-arcminute hop to the south of the bright open star cluster Messier 52 (NGC 7654).
Cassiopeia’s location at this time of the year is not ideal, but you should have little trouble in locating its very familiar ‘W’ shaped asterism of bright stars. By about 11pm BST, the end of nautical twilight from London, Messier 52, found six degrees north-west of magnitude +2.3 Caph (beta [β] Cassiopeiae, the most westerly lying star in the ‘W’), lies 25 degrees up in north-north-east. It climbs to an altitude of 45 degrees above the north-eastern horizon by about 3.15am BST.
Astronomically dark skies are just about hanging on for the majority of England, Northern Ireland and Wales (though by mid- to late-May they’ll be gone). From 52 degrees north, the sky is astronomically dark from about 11.35pm to 1.20am BST; from Birmingham V1405 Cas lies around 30 degrees up at midnight. The Moon is out of the way presently (new on 11 May), so, if you can find an observing site free from major light pollution there’s a good chance, given a transparent night and a dark-adapted eye, you may be able to spot V1405 Cas without optical aid.
Novae are among the most fascinating and unpredictable objects to observe. In this millennium, only around a dozen or so have been bright enough to see with the naked eye, with the majority of those located too far south to view from the UK. There’s no telling how long this nova will remain as bright and behave more long-term, so take any chance you get to observe it.