220P/McNaughtThe comet that was supposed to be boring.

Before the outburst, late May 2026

mag 18

Fourteen hours later, 31 May

mag 11

Reported peak, June

mag 8.2

Each step down the scale is roughly two and a half times brighter. It covered ten of them in under a day.

This comet was supposed to be boring.

220P/McNaught, found by Robert McNaught in 2004, loops the Sun every five and a half years. Small, faint, forgettable. Then in June it exploded, brightening by a factor of thousands in under fourteen hours. Then in August it did it again.

The green glow is real. It's carbon gas, C₂, fluorescing in sunlight at 516 nanometres. The fan above the core is dust being pushed off the nucleus.

Here's the problem. Over three hours the comet drifted across the sky against the stars. Stack for sharp stars and the comet smears into a streak. Stack for a sharp comet and every star becomes a trail.

So you do both, then blend them. Ninety frames, three hours, one comet that refuses to die.

---

The long version

Robert McNaught discovered this Jupiter-family comet from Siding Spring on 20 May 2004. On a 5.5-year orbit, it's the kind of object that usually passes unremarked.

Then, in a window of under fourteen hours spanning 30–31 May 2026, it brightened by roughly seven magnitudes, with some estimates putting the peak near magnitude 8.2. Perihelion followed on 14 June. A second outburst in August brightened it again by a factor of several hundred, and it was still holding near magnitude 9 at the end of that month. It makes its closest approach to Earth, 1.043 AU, on 12 October 2026.

The coma's green cast is diatomic carbon (C₂) fluorescing at 516 nm, a common signature in an active comet's inner coma. A short dust fan extends from the condensed nucleus.

On the processing: over the 3.6-hour run the comet moved 137 pixels against the star field, measured at 22.8″/h against an ephemeris prediction of 23″/h. A single stack can only keep one of the two sharp. The final image is a composite, one stack registered on the stars and one registered on the comet's own motion, combined so both are sharp. The smeared comet in the star-aligned stack was modelled from the comet's measured velocity and subtracted rather than masked, which leaves the star field completely intact.

Acquisition

Date	17 September 2026, 01:23–05:00 EEST
Integration	89 × 120 s at ISO 800, 2 h 58 m total
Tracking	Sidereal, dithered every 2 frames, no meridian flip
Calibration	100 bias, 30 flats, matched darks

Equipment

Optics	GSO RC8 carbon, 203 mm, with TS 2″ Photoline 0.8× reducer, 1270 mm at f/6.3
Camera	Canon EOS M50, no filter
Mount	HEQ5 Pro on PLL C82 pier, EQMOD
Guiding	ASI220MM Mini on TS 50 mm, pulse-guided, 4.13″/px, guide FWHM 3.16″
Control	N.I.N.A. 3.2.0.9001, PHD2 2.6.14
Image scale	0.604″/px, plate-solved at 1269.5 mm
Processing	Siril 1.4.4, dual registration (global star alignment and comet/asteroid registration), Winsorized sigma stacking, SPCC against Gaia DR3, background extraction; composite assembled with a motion-blur model of the comet's measured velocity

NGC 7331 + Stephan's QuintetA single field, two very different distances

NGC 7331, and NGC 7320 in the Quintet

40 Mly

Stephan's Quintet, the four interacting members

290 Mly

Deer Lick Group

300 Mly

Everything in the frame looks like one neighbourhood. Only two of these objects actually are.

The big spiral is NGC 7331, about 40 million light years away in Pegasus. It's often called a twin of our own galaxy, same size, same shape. If you could fly out and look back at the Milky Way, this is roughly what you'd see. Herschel found it in 1784. Messier missed it completely.

Those four little smudges crowding its edge? Not companions. They're background galaxies eight times further out. Astronomers call them the Deer Lick Group, the fleas on the deer.

Top right, that tight knot of five is Stephan's Quintet, the first compact galaxy group ever discovered, in 1877. Four of them are genuinely tearing each other apart 290 million light years away. The fifth is a fraud. It's only 40 million light years off and just happens to sit in the same line of sight.

Three hours, two targets, one 8-inch telescope from a backyard north of Bucharest.

---

The long version

NGC 7331 is one of the brightest galaxies William Herschel catalogued, in 1784, and one of the more conspicuous objects Messier never recorded. It's an unbarred spiral in Pegasus at roughly 40 million light years, frequently described as a structural analogue of the Milky Way. Its central bulge rotates counter to the disk, a sign of a disturbed history.

The four small galaxies apparently clustered along its eastern flank, NGC 7335, 7336, 7337 and 7340, are not associated with it at all. They lie some 300 million light years beyond, and the chance alignment earned the whole arrangement the name Deer Lick Group.

About half a degree to the northwest sits Stephan's Quintet, found by Édouard Stephan at Marseille in 1877 and the first compact galaxy group ever identified. NGC 7317, 7318A, 7318B and 7319 are a genuinely interacting system at roughly 290 million light years, with NGC 7318B ploughing into the group at around 800 km/s. NGC 7320 is a foreground interloper at about 40 million light years, a discordant redshift that fuelled decades of argument before it was settled as a line-of-sight coincidence.

Framing both in one field needed a camera rotation of 90°. They sit about 28 arcminutes apart, against a field of 1°00′ × 40′.

Acquisition

Date	16 September 2026, 21:15–23:50 EEST
Integration	54 × 120 s at ISO 800, 1 h 48 m total
Calibration	100 bias, 30 flats, matched darks

Equipment

Optics	GSO RC8 carbon, 203 mm, with TS 2″ Photoline 0.8× reducer, 1270 mm at f/6.3
Camera	Canon EOS M50, no filter
Mount	HEQ5 Pro on PLL C82 pier, EQMOD
Guiding	ASI220MM Mini on TS 50 mm, pulse-guided, 4.13″/px
Control	N.I.N.A. 3.2.0.9001, PHD2 2.6.14
Image scale	0.604″/px, plate-solved at 1269.5 mm
Processing	Siril 1.4.4, calibration, registration, Winsorized sigma stacking, RBF background extraction, SPCC against Gaia DR3, generalised hyperbolic stretch