APOD: Salt, Pepper, and Ice (2018 Sep 18)

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APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by APOD Robot » Tue Sep 18, 2018 4:06 am

Image Salt, Pepper, and Ice

Explanation: There's a "camera" comet now moving across the sky. Just a bit too dim to see with the unaided eye, Comet 21P / Giacobini-Zinner has developed a long tail that makes it a good sight for binoculars and sensitive cameras. The movement of the Comet 21P on the sky was captured last week in the featured time-lapse video compressing 90 minutes into about 2.5 seconds. What might seem odd is that the 21P's tail is not following the comet's movement. This is because comet tails always point away from the Sun, and the comet was not moving toward the Sun during the period photographed. Visible far in the background on the upper left is the Salt & Pepper star cluster, M37, while the bright red star V440 Auriga is visible just about the frame's center. This 2-km ball of dust-shedding ice passed its nearest to the Sun and Earth only last week and is now fading as it crosses into southern skies. Comet 21P should remain visible, however, and photogenic to stabilized cameras, for another month or so.

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by neufer » Tue Sep 18, 2018 4:47 am

https://www.britannica.com/science/comet-astronomy/Nature-of-comets#ref1223611 wrote:


<<In 1951 German astronomer Ludwig Biermann studied the tails of comets and showed that the ion tails flowed away from the Sun at speeds in excess of 400 km per second. He suggested that the phenomenon had to be associated with some sort of “corpuscular radiation” flowing outward from the Sun. In fact, he had suggested the existence of the solar wind, which was not directly detected for another 8 years. The outflowing dust and gas in the coma interacts with the solar wind and sunlight. The molecules and free radicals are ionized by charge exchange with the solar wind. Once ionized, they are caught up in the Sun’s magnetic field and flow away at high velocity in the solar wind. The process forms long, narrow, straight trails that glow blue in colour because of the presence of CO+ molecules. However, the major ion in cometary ion tails is H2O+, which does not glow at visible wavelengths. Those tails point almost exactly away from the Sun because the solar wind velocity is typically about 400 km per second, much larger than the orbital velocities of almost all comets. The ion or plasma tails are known as Type I tails.

The fine dust suffers a different fate as it is blown away from the Sun by radiation pressure on the tiny grains. That forms a broad, curved, sometimes yellow-coloured tail following the comet in its orbit and pointed generally away from the Sun, which is known as a Type II tail. The grains are blown into a larger orbit than the comet nucleus, and that results in their slowing because of the laws of planetary motion, causing them to lag behind the nucleus. The dust follows the comet around its orbit but eventually disperses into the zodiacal dust cloud.>>
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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Boomer12k » Tue Sep 18, 2018 8:19 am

WOW... super neat...

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by heehaw » Tue Sep 18, 2018 9:36 am

Moves like a crab!

DanP

Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by DanP » Tue Sep 18, 2018 12:57 pm

Very nice!

The apparent motion of the comet on the sky is a combination of the respective motions of the comet and the Earth around the Sun.
As their velocity vectors are not not necessarily aligned the direction of the apparent motion may completely differ from the tail direction.
As seen from the comet the tail direction is mostly directed opposite to the Sun because solar radiation and solar wind push the ions and dust grains.

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Ann » Tue Sep 18, 2018 3:23 pm

M37. Photo: Bob Franke.
Scrooge McDuck. Image: Disney (Carl Barks?)


















The salt-n-pepper cluster of M37 is the Scrooge McDuck of open clusters in Auriga: Rich and old!

M37 is not terrifically old by any means, not even as open clusters go, but it is old enough that it is sporting a good many bits of ground pepper, the many red giants of the cluster. Clusters, unlike people, don't grow grayer or whiter as they age, but redder (or at least yellower). The white grains of salt are of course main sequence stars, mostly of spectral class A.

And if M37 doesn't have an entire cosmic vault full of stars, it nevertheless has a lot of them. According to Wikipedia, it contains over 500 identified stars (and almost certainly many unidentified ones). The mass of M37 is about 1,500 M, about twice that of the Pleiades, and its estimated age is 346.7 to 550 million years. The Pleiades is generally assumed to be about a hundred million years.

The more massive an open cluster is, the longer it can be expected to survive. (Although admittedly, the nature of its environment is very important too.)

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by MarkBour » Tue Sep 18, 2018 4:05 pm

Ann wrote:
Tue Sep 18, 2018 3:23 pm
... [M37's] estimated age is 346.7 to 550 million years. The Pleiades is generally assumed to be about a hundred million years. ...
Ann
When you say that the open cluster M37 is estimated at 346.7 to 550 million years old, it would seem to imply that none of the stars in the cluster are much older than that. Is that correct, or can a 500-million-year-old cluster contain stars that are billions of years old? I am wondering, since I assume that such clusters form together, not that they have stars that formed apart and somehow came together later.
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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by BDanielMayfield » Tue Sep 18, 2018 9:31 pm

MarkBour wrote:
Tue Sep 18, 2018 4:05 pm
Ann wrote:
Tue Sep 18, 2018 3:23 pm
... [M37's] estimated age is 346.7 to 550 million years. The Pleiades is generally assumed to be about a hundred million years. ...
Ann
When you say that the open cluster M37 is estimated at 346.7 to 550 million years old, it would seem to imply that none of the stars in the cluster are much older than that. Is that correct, or can a 500-million-year-old cluster contain stars that are billions of years old? I am wondering, since I assume that such clusters form together, not that they have stars that formed apart and somehow came together later.
Stars of open clusters do form together, but not exactly at the same time. For example we can currently observe star forming regions where we have both young stars already brightly shining together with dense pockets of dust in which new stars are forming, So the stars of an open star cluster won't all have exactly the same age, but close, at least astronomically speaking.

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Astronymus » Tue Sep 18, 2018 9:31 pm

There is a small object moving in the right upper corner. Is it an asteroid? Can I name it? :mrgreen:

Image
» Oh, yeah. Oooh, ahhh, that's how it always starts. But then later there's running and... and screaming. «

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Ann » Tue Sep 18, 2018 9:42 pm

MarkBour wrote:
Tue Sep 18, 2018 4:05 pm
Ann wrote:
Tue Sep 18, 2018 3:23 pm
... [M37's] estimated age is 346.7 to 550 million years. The Pleiades is generally assumed to be about a hundred million years. ...
Ann
When you say that the open cluster M37 is estimated at 346.7 to 550 million years old, it would seem to imply that none of the stars in the cluster are much older than that. Is that correct, or can a 500-million-year-old cluster contain stars that are billions of years old? I am wondering, since I assume that such clusters form together, not that they have stars that formed apart and somehow came together later.
Stars that belong to the same cluster are indeed (mostly) the same age. It is impossible for a billion-year-old star to be a true member of a 500 million-year-old cluster!

So while there are certainly many Sun-like stars in M37 in terms of mass, there are no Sun-like stars in M37 in terms of age.

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by BDanielMayfield » Wed Sep 19, 2018 2:08 pm

Astronymus wrote:
Tue Sep 18, 2018 9:31 pm
There is a small object moving in the right upper corner. Is it an asteroid? Can I name it? :mrgreen:

Image
I couldn't find what you've found Astronymus, but in looking for it I noticed what looks like a meteor about 1/3rd of the way into the video cutting across part of the comet's tail.

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Astronymus » Wed Sep 19, 2018 2:52 pm

BDanielMayfield wrote:
Wed Sep 19, 2018 2:08 pm
Astronymus wrote:
Tue Sep 18, 2018 9:31 pm
There is a small object moving in the right upper corner. Is it an asteroid? Can I name it? :mrgreen:

Image
I couldn't find what you've found Astronymus, but in looking for it I noticed what looks like a meteor about 1/3rd of the way into the video cutting across part of the comet's tail.

Bruce
It's in the upper right corner. Best is to watch the video on Youtube on fullscreen.

My guess is that that streak you mention is a satellite
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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by neufer » Wed Sep 19, 2018 3:43 pm

Astronymus wrote:
Tue Sep 18, 2018 9:31 pm

There is a small object moving in the right upper corner. Is it an asteroid? Can I name it? :mrgreen:
Image
Given the negligible vertical motion I would guess that it is an asteroid.

(However, since the Earth's motion is makes the comet move slightly to the right;
it is a little surprising that the asteroid moves to the left.)
Art Neuendorffer

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Re: APOD: Salt, Pepper, and Ice (2018 Sep 18)

Post by Mebbee » Wed Sep 19, 2018 6:02 pm

neufer wrote:
Wed Sep 19, 2018 3:43 pm
Astronymus wrote:
Tue Sep 18, 2018 9:31 pm

There is a small object moving in the right upper corner. Is it an asteroid? Can I name it? :mrgreen:
Image
Given the negligible vertical motion I would guess that it is an asteroid.

(However, since the Earth's motion is makes the comet move slightly to the right;
it is a little surprising that the asteroid moves to the left.)
Mebbee it's an anti-asteroid