OR: Lake Sonoma - October 23 2025 - Part 1
Muriel Dulieu Holzer
On Thursday 10/23/2025 I went to Lake Sonoma for what would be the last observing session of this moon cycle. The drive through the vineyards at twilight was beautiful as always with streaks of red in the sky. Arriving there, I found Steve Gottlieb already set up with his 14.5” Starmaster. There were high thin clouds extending in stripes in the sky. Thankfully they partially cleared up later in the evening. I wanted to see the comets again first. And for the rest of the evening, I was prepared with my observing lists based on the Orion Deepmap 600 list of objects. For each RA number from 17 and above, I had made a list with all the objects listed with that RA number. I would be using my 10” Orion Dobsonian and my Seestar S50.
I started by looking at the comet C/2025 A6 Lemmon. In case, like me, you wonder what the name means:
C stands for the comet type. C indicates a non-periodic, long period (> 200 years, maybe a one time visitor) comet.
2025 is the year of discovery.
A6 is the discovery sequence: A indicates the half month in which it was discovered, and the number shows the order of discovery. A6 means it was the 6th comet discovered between January 1-15, 2025.
Lemmon refers to the discoverer’s name. In this case the discoverer is the Mount Lemmon survey, an automated sky survey telescope located near Tucson Arizona. When it detects a new comet, the discoverer's name becomes “Lemmon” in honor of that facility.
The comet Lemmon was visible to the naked eye, conveniently placed at the vertex of a right angle between Alphecca (Alpha Coronae Borealis) and Izar (Epsilon Bootis). It seemed brighter than before with a longer tail spanning at least twice the field of view of my 8x42 binoculars. I only saw one tail but thanks to Moshen’s beautiful picture (see TAC post “Comet C/2025-A6 (Lemmon) from Lake Sonoma” from 10/25/2025), I learned that most comets have two tails:
the dust tail made of dust particles reflecting sunlight is the one I saw with my binoculars, while
the bluish ion tail made of ionized molecules from the comet’s sublimating ice only showed up on Moshen’s long exposure photograph.
Retrospectively, I just realized that my Seestar picture shows the ion tail as well, it is inside the dust tail close to the coma (see below).
The comet had a small bright nucleus which is its solid body likely formed of volatile ices, dust and rock, organic compounds (molecules that contain carbon bonded with other elements) and dark carbon materials (carbon rich substances that absorb most of the sunlight that hits them instead of reflecting it).
I was wondering if the slight green hue that showed up on my Seestar picture of the comet’s coma was related to its origin from the Oort cloud. Comets from the Oort cloud formed in the outer solar nebula, beyond the giant planets, where it is extremely cold. Therefore they contain more volatile ices (like CO and CH_4). In comparison, comets from the Kuiper Belt formed closer, in a somewhat warmer region, where many of the most volatile materials were lost or never condensed, and are richer in water (H_2O) and carbon dioxide(CO_2) as a result.
How does that relate to the green hue in the coma of the Oort cloud comets? The volatile ices, more abundant in the Oort comets, are often richer in volatile carbon compounds that include the parent molecules of C_2 and CN which, when they are excited by sunlight, produce the green glow one can see in the coma.
The other comet I looked at was C/2025 R2 (SWAN). This is also a non periodic comet likely from the Oort cloud. It was discovered by the SWAN instrument which stands for Solar Wind ANisotropies. It is one of the instruments aboard the SOHO (Solar and Heliospheric Observatory) spacecraft, a joint mission between NASA and ESA launched in 1995. Its purpose is to observe how hydrogen in the solar system interacts with the solar wind. What makes SWAN particularly powerful at detecting comets is that SWAN looks at ultraviolet light which is emitted by hydrogen atoms. When comets get close to the sun, sunlight breaks apart water vapor, and hydrogen gas is released. While SWAN monitors the entire sky continuously, when it sees a new bright localized hydrogen source that moves night after night, it knows that it is the signature of a new comet.
The comet SWAN was located on a straight line between Theta Aquilae and Alpha Capricorni, a double star. It appeared greenish as well on the Seestar. Its coma looked more or less round in my 10”. I could somewhat see a short dust tail. But I could not see any ion tail on the Seestar even though it has one. Maybe next year the tails will be more visible when it gets closer and brighter.
M56 is a globular cluster between Albireo in Cygnus and the parallelogram of Lyra. In my 10”, it appeared as a fuzzy patch with crooked arms and a dozen stars distinctly visible. The stars looked a lot brighter in the 14.5”, I suspect the quality of the mirror to be the cause.
I was interested to learn that M56 is likely an immigrant cluster, meaning that it may not have formed in the Milky Way’s original system. M56’s orbit swings far above and below the Milky Way’s disk and passes through the plane of the galaxy only occasionally. This kind of orbit is typical of clusters that were captured from smaller galaxies that merged with ours. Moreover the stars in M56 have very low metallicity, only about 1/50th the Sun’s, and their element ratios resemble those of stars in the population of clusters thought to have originated from the Gaia-Enceladus event, the ancient dwarf galaxy that merged with the Milky Way 8 to 11 billion years ago.
NGC 6503 is a field dwarf galaxy in Draco located about 20 million light-years away. The “field” refers to the fact that it does not belong to a larger group or cluster of galaxies and hence is gravitationally alone. This galaxy sits at the boundary of our local group and right on the edge of a cosmic void, a huge empty region of space called the Local Void. Because of its location, NGC 6503 helps astronomers study how galaxies form and evolve in isolation, without recent collisions or mergers, and without the gravitational interference of neighbors. It is used to test models of dark matter halos and star formation in low-density cosmic environments.
Looking at NGC 6503 in my 10”, I could see a diagonal elongated glow. In reality, it is a SA(s)cd galaxy, an unbarred spiral galaxy with loosely wound arms that start directly from the center, with a small bulge.
NGC 6543, the Cat’s Eye Nebula, is a gorgeous planetary nebula located in Draco. The Hubble telescope images reveal at least 11 concentric dust shells around the inner portion of the nebula and two wobbling jets shooting in opposite directions. Its complex shape may be due to a binary star system in its middle, amongst other things.
I remember looking at it previously at Lake Sonoma in Steve’s 14.5” scope with a night-vision device and being delighted to see multiple concentric layers and the result of the bipolar jets shooting out from the core in opposite directions. What a view.
In my 10” Dob it looks more like a bright fuzzy bloated star, maybe slightly blue. I took a look at it again with a filter, and I can see a larger outer glow in the shape of an eye.
I took a picture with the Seestar hoping to see it better but it appears similarly small, round, fuzzy and bright with no details.
Richard Navarrete
--
Observing Sites, Observing Reports, About TAC linked at top of:
https://groups.google.com/forum/#!forum/sf-bay-tac
Subscribers post to the mailing list at:
sf-ba...@googlegroups.com,
---
You received this message because you are subscribed to the Google Groups "The Astronomy Connection (TAC)" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sf-bay-tac+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/sf-bay-tac/9ca003ad-40b4-4cc3-8468-3b9a19c7444bn%40googlegroups.com.
Alex
So sad I'm practically immobile for the 6th week (badly sprained ankle).
