Rhubarb Supernova

16-17 July 2021, 23:45 – 01:15 (BST)

Seeing: Average

Transparency: Average/poor

Conditions: After a run of cloudy nights, a very mild, cloudless evening with a light breeze blowing from the north. (And a near first-quarter moon sinking in the west.) I spent a lot of time carefully aligning and calibrating the astrophotography rig, only to find out later (for reasons I haven’t yet figured out) that it had stopped tracking after a few minutes, leaving me with over an hour’s worth of star trails. Oddly, it started tracking again (or attempted to, given that the autoguider had long since lost its target star) on the last few frames. I suspect a loose or snagged cable is the culprit, but I haven’t been able to replicate the fault in daylight. (This is the experience I expected to have when I first attempted this type of astrophotography back in 2018, not three years in. Oh well…)

While that was in progress (or not), I revisited some summer favourites with the XT10 at 133x: M57, M56, Albireo and M27. The dumbbell’s central star was visible again (with averted vision), but not as easily as it was at 171x.

Then, with my eyes sufficiently dark-adapted, it was onto some special extragalactic targets: 

BL Lacertae, Active Galactic Nucleus / Blazar

133x. Not the most visually exciting object (it looks like a faint, inconspicuous star in a field full of stars), but at approximately 900 million light years this is the second-farthest thing I’ve seen (after quasar 3C 273). I had to consult the MSA (chart 1123) to track it down, and even then I had to make a rough sketch of the field stars to be sure I had seen it.

BL Lac usually fluctuates between magnitudes 14 and 17, which would normally put it out of range of my scope, but it’s been exhibiting a lot of activity over the last few months and is now brighter than it’s ever been, currently pushing 13th magnitude.

SN 2021rhu, Supernova in galaxy NGC 7814, Pegasus

At long last, a supernova bright enough for my scope, and in a place where I could see it. That said, I had to wait for Pegasus to clear the neighbour’s fence before I could get the XT10 on it, but even though it was still low in the sky, the supernova was immediately obvious at both 133x and 171x, shining relatively brightly against the faint haze of NGC 7814 at approximately magnitude 12.5. It appeared as an intense star-like point slightly east of the galaxy’s centre (I wasn’t able to see the galaxy’s dark lane). Quite an impressive sight considering it’s 40 million light years away (but bad news for anyone unfortunate enough to be within a few light-years of the explosion).

Apparently SN 2021rhu (with that name I can’t help but think of it as the “rhubarb supernova”) is a type 1a supernova. This is caused by a white dwarf accreting matter from a nearby companion star until electron degeneracy pressure can no longer support its mass – resulting in a rapid collapse, a runaway nuclear reaction and a spectacular explosion that rips apart the entire star.

Also: I was treated to three very bright passes of the ISS over the course of the evening. One pass was so bright I could see a halo around it.

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I went out again the following night (17 July – 18 July) for another attempted imaging run which resulted in 8 minutes of perfect data followed by another hour or so of trailed images. [Insert expletives here.]

Fortunately I was able to salvage something from the session by sticking the camera on the XT10 and grabbing some data on Jupiter while a Ganymede shadow transit was in progress (below).

Detail-wise, there wasn’t much to choose between the image and the eyepiece view (at 240x): the brown barges above the NEB showed up well, but I didn’t spot the string of white ovals in the southern hemisphere (which are just about visible in the full-size image).

I also had a look at Saturn as it emerged from behind the neighbour’s tree. Several moons were visible and lots of fine detail was briefly visible in the planet and the rings, although the seeing quickly deteriorated.

I also had another look at SN 2021rhu (at 240x). The supernova was still quite prominent despite my eyes not being properly dark-adapted. It’ll be interesting to see if it’s still visible in August once the moon has got out of the way.

Nature note

The mosquitoes were out in force on both nights; in all the years I’ve been doing astronomy I’ve never been bitten quite like that before.