Starship Asterisk*

Two Comets and a Star Cluster

Explanation: Two unusual spots are on the move near the famous Pleiades star cluster. Shifting only a small amount per night, these spots are actually comets in our nearby Solar System that by chance wandered into the field of the light-years distant stars. On the far left is comet C/2017 O1 ASAS-SN, a multi-kilometer block of evaporating ice sporting a bright coma of surrounding gas dominated by green-glowing carbon. Comet ASAS-SN1 shows a slight tail to its lower right. Near the frame center is comet C/2015 ER61 PanSTARRS, also a giant block of evaporating ice, but sporting a rather long tail to its right. On the upper right is the Pleiades, an open cluster dominated by bright blue stars illuminating nearby reflecting dust. This exposure, taken about two weeks ago, is so deep that the filamentary interstellar dust can be traced across the entire field. The Pleiades is visible to the unaided eye, but it should require binoculars to see the comets.

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Great picture! I love the colors. The Pleiades are very blue, comet ASAS-SN1 is very green, whereas comet PanSTARRS is colorless. I would guess that the green comet is moving towards the Sun, feeling a powerful headwind of ionized particles from the Sun, and developing a green coma in response to it. The colorless comet is likely leaving the Sun behind, and returning to its days of colorlessness.

Ann

"Straaanngers....in the niiiiight....exchanging glances....."

Blue Eyes....

:---[===] *

Fantastic image with the Pleiades rising out of the wider dust field. I had the pleasure of attending a Damian Peach talk last week on images of Jupiter and spoke to him briefly. He was part of the amateur planetary imaging boom in the early 2000s and has built his skills since then.

Beautiful photo of the Pleiades! Shows extremely clearly that the dust illuminated by the stars of the Pleiades is NOT any remnant of the formation of that cluster, but is merely the dusty neighborhood through which the Pleiades star cluster happens to be passing.

Beautiful wallpaper material for PC background!

Ann wrote:Great picture! :D I love the colors. The Pleiades are very blue, comet ASAS-SN1 is very green, whereas comet PanSTARRS is colorless. I would guess that the green comet is moving towards the Sun, feeling a powerful headwind of ionized particles from the Sun, and developing a green coma in response to it. The colorless comet is likely leaving the Sun behind, and returning to its days of colorlessness.

ASASSN1 is inbound, with perihelion on 14 October (it will also be only 0.7 AU from Earth). Its bright coma is the product of being pristine (with a 10,000 year period, it hasn't lost much material). The direction it is traveling doesn't matter.

C/2015 ER61 has no tail and very little outgassing because it is apparently a dormant comet, having lost most of its volatile material. It does have some activity, and flared last April. Its perihelion passage was on 10 May. Again, the direction of travel doesn't matter, only the volatility and actual distance from the Sun.

heehaw wrote:Beautiful photo of the Pleiades! Shows extremely clearly that the dust illuminated by the stars of the Pleiades is NOT any remnant of the formation of that cluster, but is merely the dusty neighborhood through which the Pleiades star cluster happens to be passing.

Indeed, a great demonstration of the dust and the Pleiades being together by happenstance. And it's a great comet image too!

I finally saw C/2017 O1 (ASASSN) on the morning of 29-September-2017, but it wasn't convincing in my 16x70 binoculars from a reasonably dark site, the NJ Pines where M31 (The Andromeda Galaxy) was easily seen with unaided eyes. I needed my 85 mm spotting scope at 60x to be certain that ASASSN was there. I didn't even try for fainter C/2015 ER61 (PANSTARRS) on 29-September, but I had seen it a couple of times back in April 2017 when it was in outburst using 10x50 binoculars, and even once with the 16x70s from my suburban front yard.

However, I am puzzled by Chris' remark that "C/2015 ER61 has no tail." In Peach's image, there's a nice tail, especially in proportion to the size of the coma.

Joe Stieber wrote:However, I am puzzled by Chris' remark that "C/2015 ER61 has no tail." In Peach's image, there's a nice tail, especially in proportion to the size of the coma.

I think he probably meant to write coma, in reference to the greenish halo that it lacks in the image.

Joe Stieber wrote:However, I am puzzled by Chris' remark that "C/2015 ER61 has no tail." In Peach's image, there's a nice tail, especially in proportion to the size of the coma.

Indeed. I meant no gas tail, and even the dust tail is quite small for a comet that size (presumably because the lack of volatiles means little dust is ejected).