Starship Asterisk*

Comet NEOWISE and Nebulae

Explanation: Would you brave wild animals to photograph this sky? One astrophotographer did -- and we all get to reap the rewards. First, thousands of stars were visible with many of the brightest impressively blue. Next, several red-glowing nebulae were discernible, including the California Nebula on the far right, and, above it, the Heart and Soul nebulae. But the real reason to brave the local wildlife was Comet NEOWISE, visible on the left. In the featured long-duration composite taken last week, Comet NEOWISE's blue-glowing ion tail points straight up, away from the rising Sun, while the Sun-reflecting dust tail trails off toward the right. The picture combines three exposures taken consecutively over 10 minutes from the same location near Miedzygórze, Poland. A moonlit dirt road shows the path ahead, while the Śnieznik Mountains is visible on the horizon. Comet C/2020 F3 (NEOWISE) passes its closest to the Earth next week, after which the 5-km wide, evaporating, icy dirtball will fade as it glides back to the outer Solar System.

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Ah, lovely! Note the Double Cluster at upper right, to the right of the red Heart and Soul nebulas. The cluster to the upper right of the Double Cluster looks bluer than the cluster to the left, and that's correct too, because the right one does not contain any red giants.

Note Alpha Persei Moving Cluster below the Double Cluster, and the California Nebula at bottom right.

Ann

Beautiful image, the foreground really makes it. It resembles an impressionist work.
The only thing I don’t care for is the exaggerated color on the nebula, it looks unnatural and is distracting. Actually I think cutting off the right third would make a stronger composition.
Sorry just the illustrator in me getting nit picky. Beauty Of course is in the eye of the beholder and opions are like... This is one of the best, out of many, images of the comet I’ve seen!

Eric

Kudos to Jarek Oszywa: great photo!
Some clouds over to the left also!

For the past few days, I have seen some good pictures of the comet. These are all Earth based pictures. I wonder if we would be able to see some closeup pictures of the comet body in space.

spaceaman wrote: ↑Mon Jul 20, 2020 1:43 pm For the past few days, I have seen some good pictures of the comet. These are all Earth based pictures. I wonder if we would be able to see some closeup pictures of the comet body in space.

For the most part, there would be little advantage to images made in space. With something like the Hubble Telescope, we could get higher resolution, but that's not necessarily useful for an active comet, where the coma obscures the nucleus and where the structure in the coma and tail is large enough to be fully resolved from the ground.

APOD Robot wrote: ↑Mon Jul 20, 2020 4:09 am
Explanation: Would you brave wild animals to photograph this sky? One astrophotographer did. In the featured long-duration composite taken last week, Comet NEOWISE's blue-glowing ion tail points straight up, away from the rising Sun, while the Sun-reflecting dust tail trails off toward the right. The picture combines three exposures taken consecutively over 10 minutes from the same location near Miedzygórze, Poland. A moonlit dirt road shows the path ahead, while the Śnieznik Mountains is visible on the horizon.

https://www.rollingstone.com/politics/p ... d-1030897/

I enjoyed looking at Dr. Nemiroff's "Lecture #11 on Comets and Meteors (http://asterisk.apod.com/viewtopic.php?f=24&t=18013)

That lecture did not give much detail about the cause for the shape of a comet's tail. Tentatively, here's my view of it:
A bright comet with two bright tails is a chance to watch a gravitational density-separation process in action. Considering an outbound comet:

• The heat causes sublimation and out-gassing, which also kicks lots of dust and small particles off of the main nucleus.
• At the outset, all of these are travelling in the same orbit, and could stay roughly together, as a nucleus with an atmosphere, or exosphere, anyway. But the solar wind and solar radiation both press on this collection. Apparently, the gas molecules are apt to get ionized, which I guess increases the forces they experience from the solar wind. No doubt, the ions can return to the ground state, but they still experience force.
• So, the gas molecules, being the lightest, are getting a much greater force per unit mass than the rest. And they are thus being accelerated to a higher orbit. Indeed, coming off from the path of the comet, they look almost like a straight line outward.
• Next, the dust and small particles are getting much less force per unit mass than the gas molecules. (Their surface for impact of the wind and radiation is much smaller than it would be if they had been dissolved into a gas of those same molecules, and they have heavier elements, so they are accelerated far less than the gas.) Nevertheless, the small particles are getting a lot more pressure per unit mass than the nucleus is. So, they spread, each at their own rate, but all generally being pushed into higher orbits than the nucleus. The spread of this tail is not only from the variances in the dust, but also they are spread throughout the differing times of release.
• The nucleus itself is also being pushed by these forces, but since it is a large dense body, the push it receives is very small. By comparison to the other parts, it is just travelling in an orbit, and stays below all of the rest.

An inbound comet would be undergoing very similar effects. I find it more complex to think about what the tail would look like right after perihelion. Not at all sure how the skewed ""higher" orbits would look as all of this rounds perihelion in comparison to the material coming out on the outward journey.

Chris mentioned doing some very detailed simulations of this. I wonder if his models painted a different picture.

neufer wrote: ↑Mon Jul 20, 2020 1:57 pm

Would you brave Wallace animals to be interviewed on Fox News?

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APOD Robot wrote: ↑Mon Jul 20, 2020 4:09 am
Explanation: Would you brave wild animals to photograph this sky? One astrophotographer did. In the featured long-duration composite taken last week, Comet NEOWISE's blue-glowing ion tail points straight up, away from the rising Sun, while the Sun-reflecting dust tail trails off toward the right. The picture combines three exposures taken consecutively over 10 minutes from the same location near Miedzygórze, Poland. A moonlit dirt road shows the path ahead, while the Śnieznik Mountains is visible on the horizon.

Art Neuendorffer (MIT class of 1967)