This month sees the climax of the passage of comet C/2017 T2 PanSTARRS, as it reaches its brightest predicted magnitude while passing close to the galaxy pair known as M81 and M82 (see below). We first reported on this comet in the middle of 2019, when there was conjecture that it could become bright enough to be seen with the naked eye.
Sadly, this turned out to be wishful thinking, but on the plus side the comet has been a steady, albeit dimmer, performer during its time in our skies. It’s also well-placed for UK viewing.
Read more about comets:
- A guide to comets
- The story of comet Hale-Bopp
- How Comet Interceptor will encounter a pristine comet in space
It begins the month at an estimated mag. +11.6 in Camelopardalis. Not the easiest constellation to identify nor navigate, the comet passes close to mag. +4.6 TYC 4530-2010-1 on the night of 4/5 May, heading east towards the northern border of Camelopardalis with Ursa Major.
It crosses on the night of 17/18 May, a time when it’ll be close to the 11th magnitude galaxy IC 529. It continues southeast, passing close to mag. +5.2, 27 Ursae Majoris on the nights of 19/20 and 20/21 May, as it approaches M81 and M82.
The comet’s closest approach to the galaxy pair occurs on the nights of 22/23 and 23/24 May. It’s expected to be mag. +11.5. The galaxies are listed at mag. +7.0 for M81 and +8.6 for M82.
Tracking southeast, it passes close to IC 2574, a 10th magnitude dwarf galaxy. During May, it should be visible in a small scope. It appears small if you plan to image it.
The comet’s brightness is estimated to stay above mag. +12 until July 2020.
Follow the trail of C/2017 T2 PanSTARRS
Take our deep-sky tour to follow the comet past galaxies great and small
Comet C/2017 T2 PanSTARRS passes close to M81 and M82, and you can locate them by extending a diagonal across the pan of the Saucepan from Phecda (Gamma (γ) Ursae Majoris) through Dubhe (Alpha (a) Ursae Majoris) for the same distance again. M81 and M82 are 2˚ to the north of the point you reach.
Bode’s Galaxy, M81, is a bright spiral galaxy. It shines at mag. +7.8 and appears as a 20×11 arcminute glowing oval. The core is bright, showing good condensation, but the outer spiral arms need larger apertures to see convincingly as they are thin and spindly. M81 is located 12 million lightyears away.
Despite being dimmer than M81, 9th magnitude M82 is easy to find because it lies 37 arcminutes to the north of M81. M82 is known as the Cigar Galaxy because of its distinctive shape. Being so close to M81, it’s easy to get both objects into the same low-power field of view. When you do, the contrast in shape is quite stunning. M82 is an irregular galaxy with high star-formation activity and a prototype for a class of starburst galaxies.
Smaller apertures reveal a view which looks like a regular sideways-on spiral. With larger apertures, the core begins to show a mottled appearance with bright knots separated by darker dust lanes. M82 is a member of the M81 group and shares its neighbour’s distance of 12 million lightyears.
Our next target is NGC 3077, a disturbed elliptical galaxy which is also a member of the M81 group. Again, this galaxy is easy to locate as it’s in the same field as M81 and M82, marking the eastern vertex of a right-angled triangle formed with M81 and M82, M81 sitting at the right angle.
NGC 3077 appears as a circular glow through a 150mm scope, much smaller than M81 or M82, with an apparent diameter around 1.5 arcminutes. A similar, albeit slightly larger appearance greets a 250mm scope. Even larger apertures will begin to show a weak, larger glow about 4 arcminutes across which surrounds the 2-arcminute central condensation and wide core.
We move slightly further from the M81 and M82 region for our next target, the dwarf spiral galaxy IC 2574. An outlying member of the M81 and M82 group, IC 2574 will also get a close pass by C/2017 T2 PanSTARRS this month, the comet lying 20 arcminutes from the galaxy’s centre at 01:00 BST (00:00 UT) on 27 May.
Also known as Coddington’s Nebula, IC 2574 shines at mag. +10.6 but has a low surface brightness, thanks to its 12×6 arcminute apparent size. A study of the galaxy reveals that 90% of its mass is tied up in dark matter. Locate Coddington’s Nebula 2.9˚ east and half a degree south of M82.
NGC 2976 is another unusual looking galaxy, located 1.3˚ to the south of M81. Although it’s classed as an unbarred spiral galaxy, mag. +10.8 NGC 2976’s spiral arms are difficult, if not impossible to see. A small scope will show it as a weak glow 3 arcminutes across, but having low surface brightness and no evidence of any definite core.
A 300mm scope at 200x shows a glowing ellipse with a mottled, uneven texture. Averted vision will show several brighter patches within the galaxy’s irregular border. These are evident on the northwest side. This object is another member of the M81 group and is 11.6 million lightyears away. It was discovered by William Herschel in 1801.
Next is NGC 2985, the brightest member of the NGC 2985 group. It shines at mag. +10.4 and is located 3.2˚ north and 28 arcminutes west of M81. A 150mm scope reveals a fuzzy glow about 1 arcminute across. Its nucleus appears stellar through small instruments. A mag. +12.5 star sits 1 arcminute from the nucleus on the eastern side.
This is a marker because with a 250mm instrument the galaxy’s outer halo, formed from a complex wrapping of tightly wound spiral arms, extends beyond this star to present a halo 2 arcminutes across
How to photograph C/2017 T2 PanSTARRS
As it passes M81 and M82 the comet is expected to be shining with an integrated magnitude of +11.6. With a small apparent size, the comet should be on a par with the galaxies, making for an interesting and balanced photograph as long as the clouds stay out of the way.
Comets are notoriously awkward objects to image against starry backgrounds.
While we go to great lengths to remove the effects of Earth’s rotation using equatorial tracking or autoguiding mounts, comets mess this up completely by having their own relative motion against the stars.
Tracking the stars keeps them crisp and sharp, as long as your polar alignment is accurate, but a comet will often blur.
Just to complicate matters, a comet’s rate of motion varies depending on how close they are to Earth and the inclination of their orbit to the line of sight of the observer.
In the case of C/2017 T2 PanSTARRS, when close to M81 and M82 its motion across the sky is in the order of 50 arcminutes a day.
Converting this to a minute rate, 50 arcminutes a day 24 hours is equivalent to 2 arcseconds every minute, which is not too bad.
With a required field size of at least 2˚ to capture the comet and galaxies comfortably at closest approach, this rate of movement is equivalent to 1/3,600th of a 2˚ field every minute.
Wider fields will be more tolerant of the comet’s motion.
Exposures in the order of 30, 60, 90 and 120 seconds should work fine for single shots. If you intend to stack images, the situation is a bit different as stacking accumulates timed motion.
Stack sixty 60-second exposures for example and you’re integrating a scene which covers an hour’s worth of cometary movement.
Using the rate above this equates to 120 arcseconds or 2 arcminutes, roughly 1/15th the Moon’s apparent diameter. As comets go, this isn’t excessive.
It’s helped by the fact that T2 PanSTARRS is now moving away from Earth, its closest approach having occurred in late December 2019.
Various techniques exist for processing comets with deep-sky backgrounds. One popular method is to process for the background stars and galaxies, getting your processing software to remove any moving objects between frames – i.e. the comet.
Then process for the comet itself, attempting to remove background stars.
Finally, both sharp images can be combined to give a view showing the comet in sharp relief against its backdrop.
But the slower motion of T2 PanSTARRS can work against you as there’s not enough motion to reveal it as a convincing moving entity between frames.
- Recommended equipment: Camera with a lens of focal length 600mm (APS-C) or 1,000mm (full frame)
Your image scale will be determined by the lens or scope attached to your camera’s front. A lens of 600mm focal length attached to a non-full frame DSLR (eg, APS-C) will deliver a 2˚ (long frame dimension) field. A full frame camera with a 1,000mm lens achieves the same field: 2˚ is the smallest workable size for close approach.
An aligned tracking mount is needed. The M81/M82 field is located near the northern celestial pole. The apparent movement of stars due to Earth’s rotation is reduced compared to what you’d see if the field were closer to the celestial equator. Maximum exposures depend on the setup, but a minimum of 30-60 seconds is achievable.
Set your camera’s ISO to a mid-low value, say ISO 800-1600. For lens-based setups, fully open the lens (lowest f-number). For 30” exposures, use the camera’s 30” setting. For longer values, set the camera to bulb mode and use a remote shutter release to control shutter opening. Use a stopwatch to control exposure lengths.
Accurate focusing is important, so take your time. Pre-focus on a star such as Dubhe (Alpha (a) Ursae Majoris) using your camera’s Live View facility. Set the Live View display to maximum magnification. Approach focus, achieve it and wind through it. Repeat until you recognise what focus looks like, then wind to it.
The night of 23/24 May presents the closest pass with the comet passing M81 by 1.5˚. Align the camera frame so the mid-point of the line joining the comet to M81 is in the centre; having both objects on the frame diagonal works well. It may also be worth noting that there are several other faint galaxies in the vicinity (see above).
The comet’s slow speed may allow several minutes of images to be stacked without blurring. DeepSkyStacker can stack and average images, allowing flats and darks to be used. Darks are equivalent exposures with the lens cap fitted. Flats are an image of an evenly illuminated white light source, exposing to 50-70% saturation.
Pete Lawrence is an experienced astronomer and astrophotographer. These guides originally appeared in the May 2020 issue of BBC Sky at Night Magazine.