Nova Persei 1901, or GK Persei for short, is a nova in the constellation of Perseus that lies at a distance of roughly 1400 light years and spans roughly 1 lightyear in diameter.  A nova is an astronomical event characterised by a sudden spike in brightness of a white dwarf star in a binary system. This increase in brightness is triggered when the white dwarf, drawing material from its donor star, undergoes a thermonuclear explosion on its surface, unleashing a burst of energy and temporarily increasing its luminosity. This thermonuclear explosion takes place when the white dwarf accumulates enough material from its donor star so that the ever-increasing pressure and heat trigger an explosion. The so-called donor star can be a main sequence star or even a subgiant - for GK Persei the latter is the case. As details would go beyond the scope of this text, I recommend reading [url=https://www.aavso.org/vsots_gkper]this[/url] for anyone interested.  As the name suggests, the nova was discovered in 1901 when it was pretty bright at magnitude 2.7. The nova attained a maximum brightness of mag. 0.2 (even brighter than Betelgeuse!) and then faded over the years, reaching a magnitude of roughly only 12 over the next 3 years. Nowadays the brightness sits at an average of mag 13-14. However this nova is not in a steady-state and undergoes frequent change in brightness - years after the discovery it was observed that the nova shows spikes in magnitude (in the order of 2-3 mag increase) that repeat regularly about every 3 years for a period of about 2 months which is due to outbursts in the system. More about those outbursts later in the text. For the Deep Sky Collective (DSC) “GK Persei '' was an interesting target as it images only very rarely due to it being a pretty hard target as it’s faint and small. With the small FOV group of the DSC we started gathering data in November and imaged for a total of 3 months. As the weather this season hasn’t been very kind, not everyone was able to contribute, however with  a total of 14 people working on GK Persei - 1 Editor, 2 stackers, 1 additional MSGR providers, myself coordinating it and only 9 photographers- we still managed to get a total of 265h of integration - the longest ever on GK Persei.  As was the case for Cas A, we didn’t really know what to expect in the different channels. As we knew the Nova emits in Ha and Oiii getting data here was a no-brainer. We also decided to give Sii a go but after a few subs and no evidence of signal we gave up on that - note that this does not mean there is no Sii at all and if you plan on going after this target we recommend double checking for yourself as our test wasn’t super in-depth. Broadband was another surprise for us as it showed some interesting structures and we were able to distinguish between certain parts of the Nova with these observations - more on that later.  Before getting into any details in the picture, I want to thank everybody who contributed to this project. As always, it was a pleasure working together with everybody!  Tim Schaeffer - [url=https://www.astrobin.com/users/xSky_Watcher/]@Tim Schaeffer[/url]  Justin P. - [url=https://www.astrobin.com/users/lokisastro/]@lokisastro[/url]  Steeve Body - [url]https://steevebody.com/[/url] Carl Björk - [url=https://www.astrobin.com/users/Palmito/]@Palmito[/url]  BTB Astroteam Brentenriegel - [url=https://www.astrobin.com/users/franziii/]@Franz Gruber[/url]  Bogdan Borz - [url=https://www.astrobin.com/users/Bobinius/]@Bogdan Borz[/url]  Nicola Beltraminelli - [url=https://www.astrobin.com/users/NBeltraminelli/]@Nicola Beltraminelli[/url]  Sebastian Donoso - [url=https://www.astrobin.com/users/georgian82/]@georgian82[/url]  Antoine and Dalia Grelin - www.galactic-hunter.com Mike Hamende - [url=https://www.astrobin.com/users/tosjduenfs/]@Mike Hamende[/url]  Jason Jacks - [url=https://www.astrobin.com/users/jmdl101/]@jmdl101[/url]  Paul Kent - [url=https://www.astrobin.com/users/narrowbandpaul/]@Paul Kent[/url]  Nicolas Puig - [url=https://www.astrobin.com/users/orion13/]@Nicolas PUIG[/url]  David Wood - [url=https://www.astrobin.com/users/david_wood/]@David Wood (SDAA TARO)[/url]   Special thanks to:  [list] [*]Justin, who stacked all of our files and ran a few tests to get the best result. Thanks for his efforts to the project and patience with stacking.  [*]Carl, who gave the project members access to his NAS to upload their data and make the data transmission significantly easier. Also thanks to him for stacking Ha, as we had some problems with the initial stack and Justin wasn’t available.  [*]Steeve, our editor who produced this wonderful and time-consuming edit you see here. [*]​​​Paul, Nicola, and Antoine and Dalia, who provided the team with MSGR references which is always a crucial step when it comes to collaborations.  [/list] [size=200]Interesting features in our data[/size]  [i]- Nova and Planetary nebula surrounding GK Persei[/i] As said in the beginning of the text, GK Persei is a nova, meaning that a main sequence star is being consumed by a white dwarf. The thermonuclear explosion (process briefly explained at the beginning of the text) not only results in an increase in brightness, but also in the ejection of the outer layers of the accumulated material into space. This expelled material forms a shell around the nova, which can expand and cool over time, creating the observed nova remnants, which in this case is the round structure around the central star (see picture).  What’s interesting though is that white dwarfs, remnants from the death of rather low to mid-mass stars, can be associated with planetary nebulae. A planetary nebula is what is left of that low to mid-mass star after its core collapsed on itself - if the end-of-life star is not above a threshold of 8 solar masses (i.e mass of our sun) its collapse does not produce enough energy to compress the star’s core into a neutron star or black hole, but this collapse is halted by “[url=http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/whdwar.html#c3]electron degeneracy pressure[/url]”. With this, a white dwarf (WD for short) is formed, just like the one in the GK Persei system.  During this formation of the WD, the outer layers of the progenitor star are gradually expelled into outer space where they are then ionised resulting in emissions in various wavelengths (such as Ha, Oiii). The main ionisation processes are ionisation from the UV emissions from the newly formed WD and ionisation from interaction with interstellar matter - as the resulting planetary emits light on its own it is called an emission nebula.  Note that these nebulae might not always be visible in Ha/Oiii due to their sheer age or just insufficient ionisation. In the case of GK Persei, this Planetary nebula (PN) is visible, giving us a cool region that hosts both a PN and a nova. This PN can be seen as the bigger shell around the nova. Note that this PN is a bipolar (or two-lobed) PN, meaning it has outflows in two opposite directions. This specific PN strongly reminded me of the “[url=https://esahubble.org/images/opo9738f/]Hubble 5[/url]” nebula, just bigger. With this project the DSC is happy to present the first clear pic of this planetary nebula from Amateurs! Revealing it takes a lot of integration time due to it being extremely faint, so it was the perfect target for our collaboration project. Note that these observations are the first that allow us to make clear measurements on this PN - We’ve found the width of the PN to be 880 arcsec at its maximum and 500 arcsec at its minimum. Previously there existed no clear observations due to its sheer faintness and numbers were mostly speculative, so we’re happy to have added those measurements.  Below you find a zoomed-in image of GK Persei and its associated PN - both highlighted so it’s easier to make out all of the features that were just described. We’ve also added a picture for this [url=https://arxiv.org/pdf/2201.10332v1.pdf]research paper[/url]  that was our initial reference for starting this project - the paper talks about cold molecular gas mapping  around GK Persei. [b][img]https://lh7-us.googleusercontent.com/ffMrkqtkCZifYJh2ApwzN4H144zZr_OghnbGlYfDr-OU7QprMCi8PsMQgnK_D-mTCPirOJdV2Xz-JTuKeV_MPbhw4ahwtNHqAI5XTEQqE_AHrQ8GC2YTEq6fBzCZ5e4bOdiN9o3VIY9gWUOg-0SRaK8[/img][/b][size=50]© Left image: DSC / Right image: Kamin ́ski et al.: Cool molecular gas in novae[/size] - Broadband structures around GK Persei As can be seen in our picture of GK Persei, surrounding the Nova and planetary nebula is a big cloud of grey matter - this is what we call integrated flux nebula, better known as IFN. Note that this IFN is located in our own galaxy.  Whether or not this IFN is associated directly to GK Persei or is just back/foreground dust is unknown as of now (according to my research). While it certainly looks like it could be part of the stream, we shouldn’t be too quick to make judgments purely based on an image. In fact, when looking at this from a wider FOV it seems like a random patch of IFN.  Note that this Broadband signal has been mistakenly picked up as Oiii signal in previous photographs - with our image we made sure to continuum subtract to properly isolate Ha and Oiii which nicely showcases both the Broadband signal and the Narrowband Planetary nebula signal. Below is a LRGB only picture. [img]https://cdn.astrobin.com/ckeditor-files/112994/2024/5e5c3d8f-453c-44b5-bf97-74281b93eea6.jpeg[/img] [size=50]©Image: DSC[/size] - Details in GK Persei As GK Persei itself is really small at 120 arcsec, getting any meaningful details in it is really hard - with our setups however, we were able to resolve GK Persei pretty well and are super happy with what we have.  When looking at professional observations of GK Persei the exact details become even clearer. The shell surrounding the central stars consists of little bubbles of matter, expelled during the thermonuclear explosions. Together these bubbles form a dense pack of matter in a pretty circular shape. Of course that’s only how we see it and in reality this is a sphere of matter around the central stars - GK Persei has seen several in-depth measurements of these bubbles which enabled professional researchers to construct [url=https://miguelsantander.files.wordpress.com/2012/11/gk-per-expansion-and-orbit.gif]3D maps of the Nova[/url] showing the expansion in full detail. In the link above, red is matter expanding away from earth and blue towards earth - representing the red- and blue-shift respectively. For a more in detail explanation of these measurements, I recommend reading [url=https://arxiv.org/pdf/1210.5884.pdf]this paper[/url].  Note that by comparing our image of GK Persei to older ones from big scopes we can even make out the expansion of GK over the years - in the GIF below, we compare our image to one from the Caelum Observatory, [url=https://commons.wikimedia.org/wiki/File:Nova_Persei_1901.jpg]see image[/url]. The GIF below showcases the expansion from 2012 to 2024. [img]https://lh7-us.googleusercontent.com/CBoqlqQmyNVz8L6Ycqcc9KljR88a_WRBU7qj0u4g73EhSkLk3I1HIFrxEtTWlGrzlYj2NSYaFiq-GQApDjeN_2jHXOeAc2JTAd2wNhR4LSrs0AmJD8ZgmJQgoVjd2Cj-77Pkr0tplR9UQeW0Rtk8pI4[/img] ©Image: DSC / [url=http://www.caelumobservatory.com/gallery/gkper.shtml]Caelum Observatory[/url] For any further questions about the project, feel free to leave a comment! For interest in participation in our new sister group, the Deep Sky Companions (DSC2), feel free to reach out to us!  If you want to see our image in greater detail, feel free to go to the DSC’s website, where the image is uploaded in full resolution ( 9240 x 6184 px), enabling you to explore the picture by yourself and being able to zoom in on every tiny detail! Link to our website:  [b][url=https://deepskycollective.com/m51]https://deepskycollective.com/gkpersei[/url][/b] Integration overview  For this project 9 photographers worked together and racked up a total of 265h50m.  A per-channel overview is given below.  13h00 L / 14h20 R / 16h15 G / 15h45 B / 91h55 Oiii / 114h35 Ha Total: 265h50 We hope that you enjoy this image!  Text written by [url=https://www.astrobin.com/users/xSky_Watcher/]@Tim Schaeffer[/url] , organiser and co-ordinator of the project and proofread by the team.