Supernova 2025rbs is among the currently observable extragalactic supernovae discovered through automated sky surveys. For the timely documentation and analysis of such transient events*, the website rochesterastronomy.org/snimages, curated by David W. Bishop, has established itself as a central and reliable source. Updated daily, it provides concise access to current supernova discoveries worldwide, including discovery data, magnitude estimates, classification details, and links to imaging and spectra. For the planning and execution of astrophotographic observations—such as those presented here for SN 2025rbs—the site serves as a valuable and practice-oriented resource.

_{* …Transient events are short-lived astronomical phenomena that appear and evolve over timescales ranging from seconds to months, before fading away or becoming indistinguishable from their surroundings.}

The entry for 2025rbs:

Discovered on July 14th by the Gravitational-wave Optical Transient Observer (GOTO), which is an international network of robotic, wide‑field optical telescopes (40cm „of the shelf“-astrograpphs iwth f/2.7) initially designed to detect the electromagnetic counterparts of gravitational-wave events like neutron star mergers. Operating from two sites—La Palma (Canary Islands) and Siding Spring (Australia)—each node comprises dual mounts equipped with eight 40 cm telescopes, delivering a combined field-of-view of ~88 square degrees and enabling rapid sky coverage within minutes of a gravitational-wave alert. Fully operational, GOTO conducts an all‑sky survey every 2–3 days and has already discovered thousands of transient sources.

A Supernova of Type Ia

A Type Ia supernova is a thermonuclear explosion of a white dwarf star in a binary system. This occurs when the white dwarf accretes enough mass (typically from a companion star) to approach the Chandrasekhar limit of approximately 1.4 solar masses. The Chandrasekhar limit defines the maximum mass a white dwarf can sustain before gravitational collapse. Once the this limit is exceeded, the electron degeneracy pressure („Elektronenentartungsdruck“) can no longer support the star, triggering a runaway thermonuclear explosion that disrupts the entire white dwarf (there is no remnant like SNR at type II supernovae). This mechanism results in a highly consistent peak luminosity (distortion happens always around the same mass), making Type Ia supernovae valuable standard candles („Standardkerzen“) in cosmology. As one key characteristics of Type Ia supernovae no hydrogen lines can be found in their spectra (distinguishing them from Type II supernovae).

September 07, 2024 – NGC 7331 and Stephan`s Quintet

The host of 2025rbs is not an unknown galaxy for astrophotographers, as it is prominently located next to the so-called ‘Stephen’s Quintet’ a compact group of five galaxies notable for its dramatic tidal interactions, including active collisions and gas shocks. I had already imaged this region of the sky in early September 2024 using my ASI 294MC Pro color camera. The results can be found in my earlier post (NGC 7331 and Stephan`s Quintet)

During the image processing of the image, which was exposed for 5 hours (60×300″) at the time, the focus was not on the resolution of the galaxy nucleus of NGC 7331, which is why this image is not the first choice to serve as a reference for the detection of a supernova directly in the core area of the galaxy.

July 21, 2025 – Sequence Planning

The goal of the actual sequence is to capture the supernova (approx. 14 mag) with sufficient signal-to-noise ratio, avoid overexposure of the bright galactic core, and obtain a dynamically balanced dataset suitable for HDR image processing in PixInsight.

As an initial step, I will conduct a series of test exposures at 30 s, 60 s, and 300 s in Luminance to assess the signal-to-noise ratio and the behavior of the bright galactic core across different integration times. The outcome of this evaluation will inform the final design of the imaging sequence in NINA, with the aim of optimizing dynamic range and preserving detail in both the supernova and the central galaxy regions.

According to the NOVA DSO Tracker, the suitable observable period in this night is about 3.5 hours, between 11:15 pm and 02:55 am with an object`s altitude of 40° to 75° above the horizon, and azimuth between 75° and 150°.

Meteoblue predicted that there might will be a chance to clear skies from 11 pm onwards.. Actually (its 7.45 pm now) we have 15 km/h of wind and rain with 1 mm/hr 🙁 … 30 min later: a rainbow is spending some hope that the prediction maybe will come reality 🙂

July 21, 2025 – Session Notes

The sky cleared up right at the planned start of the preparation for the imaging session around 10pm. The first single frame of 60s (gain 100, -10°C) already shows the SN very prominently:

In the end, I was able to expose 31 frames of 60s each (at gain 200, -10°C) before PHD2 suddenly failed to produce a reasonable guiding (… it’s now finally the time to take a closer look at the basic settings of PHD2 (which were always already set and ready in the „good old“ ASIAIR workflow)).

Within the calibrated, registered and integrated capture the Supernova can be recognized very prominently. The key here is to use a very cautious stretching (Histogramm Transformation and Generalized Hyperbolic Stretch). I would have estimated the brightness being about about 13.5 to 14.0 mag (a possible comparison star is marked accordingly in the inverted image together with its Simbad dataset):

In an image processing without consideration of the SN, just focussing on the resolution of the spiral arms of the galaxy, the SN would almost completely disappear in the central area of NGC 7331, as the following image shows.

„Astrophotography reminds us once more: only with patience and precision do the faintest structures unveil themselves – they are waiting for those who look with understanding!“