Nova discovered in Cassiopeia
On March 18, 2021, the Japanese amateur astronomer Yuji Nakamura discovered a nova in the constellation Cassiopeia. A faint object had appeared in a set of 15s exposures taken with a 135mm lens, on a location where no star had been seen four days earlier. The brightness was originally magnitude 9.6, but it rapidly increased to about 8.0. The nova is now officially classified as V1405 Cas.
Discovery of V1405 Cas by Yuji Nakamura. Two images of the same area of the sky, four days apart. The faint object in the green circle (left) was not visible four days before (right).
Novae are relatively rare but can be spectacular events. For example, in 2013 a nova in the constellation Delphinus reached peak brightness of magnitude 4.5, which made it well visible with the naked eye. Novae also change very quickly. Large brightness swings can take place within a matter of hours or days. There are very few cosmic events outside of our solar system that move so fast and can be observed by amateur astronomers.
V1405 Cas is a classical nova. Classical novae are typically seen in double-star systems. One of the companion stars is a compact white dwarf at the end of its life-cycle. The other is either a regular star the size of our sun, or a red giant. Hydrogen is pulled from the larger star and forms an accretion disk around the dwarf. The hydrogen is pulled in from the disk and compacted to high density as it experiences the high gravitational force of the dwarf. As it reaches the dwarf’s surface, temperature can rise to 10 million Kelvin. This can cause nuclear fusion on the dwarf’s surface. This is essentially a big hydrogen bomb going off. The explosion propels hydrogen gas from the system at incredible speeds. In the case of V1405 Cas, scientists have measured that hydrogen gas is ejected at 1,600 km/sec. Novae are distinctly different from supernovae. The latter involves an exploding star leaving a very different star behind if any. In the case of a nova, both stars typically survive and can go through a similar event again.
At the point of writing, the nova has still maintained it’s brightness of around magnitude 8. The development of its brightness can be followed through the website of the American Association of Variable Star Observers (AVVSO). Under the heading ‘Pick a star’, fill in the name ‘V1405 cas’ and select ‘Plot a Light Curve’.
More research on the nature of this nova is ongoing. One question astronomers would like to answer is which double-star system is the source of this nova. Currently the most likely candidate is a double-star system known as CzeV3217.
Observation
Luckily there was a clear night on March 29, so the nova could be photographed. The best time to photograph is just after sunset. The telescope setup that was used was actually meant for a very different object this evening, so was not the ideal tool for the job, but worked out well.
The setup consisted of the Takahashi FSQ-106 with 645RD 0.72x reducer, coupled to the ZWO ASI6200MM camera. The massive field of view of this combination of more than 5 degrees was overkill for this object, so the final image was heavily cropped in.
The following frames were collected:
Luminance: 20 x 30s @ gain 100/21
Red: 10 x 60s @ gain 100/21
Green: 10 x 60s @ gain 100/21
Blue: 10 x 60s @ gain 100/21
This resulted in an LRGB image with a total of 40 mins exposure. Individual exposures were shorter than usual, as the focal ratio of the combination was f/3.8, much faster than usual.
As far as processing goes, things were kept simple. Stacks for each filter were subjected to DBE, noise reduction with MMT and deconvolution (Lum only). The R, G and B channels were combined and manually stretched. Also the luminance was stretched manually and added to the RGB non-linear image. Final tweaks were made using CurvesTransformation.
Nova V1405 Cas as captured at th AstroWorldCreations observatory. An LRGB image of In total 40 mins exposure
At an earlier similar event with AT2019abn, there were images available of the same galaxy from a year before. That allowed a before/after comparison. Looking back at earlier images of the Bubble Nebula, it turned out that the site of the nova was not in the field of view of those images. So in this case only the ‘after’ can be photographed. But perhaps the nova will fade away after some time. If so, it would allow a comparison at a later date.
This image has been published on Astrobin.