APOD: Saturn in the Milky Way (2017 Jun 10)

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APOD: Saturn in the Milky Way (2017 Jun 10)

Postby APOD Robot » Sat Jun 10, 2017 4:11 am

Image Saturn in the Milky Way

Explanation: Saturn is near opposition in planet Earth's sky. Rising at sunset and shining brightly throughout the night, it also lies near a line-of-sight to crowded starfields, nebulae, and obscuring dust clouds along the Milky Way. Whitish Saturn is up and left of center in this gorgeous central Milky Way skyscape, a two panel mosaic recorded earlier this month. You can find the bright planet above the bowl of the dusty Pipe nebula, and just beyond the end of a dark river to Antares, alpha star of the constellation Scorpius. For now the best views of the ringed giant planet are from the Saturn-orbiting Cassini spacecraft, though. Diving close, Cassini's Grand Finale orbit number 8 is in progress.

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James Brink

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby James Brink » Sat Jun 10, 2017 4:59 am

When I see star field photos such as this one on June 10 I often see and clearly see here lines and arcs of stars that do not seem random. I have not read about this phenomenon which must relate to star formation. What is known?

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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Chris Peterson » Sat Jun 10, 2017 5:49 am

James Brink wrote:When I see star field photos such as this one on June 10 I often see and clearly see here lines and arcs of stars that do not seem random. I have not read about this phenomenon which must relate to star formation. What is known?

Almost all the structures you see are, in fact random. You are experiencing the effect of your brain detecting patterns even when they don't really exist. There are random star field generators which produce the same visual artifacts.
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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Ann » Sat Jun 10, 2017 6:10 am

James Brink wrote:When I see star field photos such as this one on June 10 I often see and clearly see here lines and arcs of stars that do not seem random. I have not read about this phenomenon which must relate to star formation. What is known?


The thing to understand about the lines and arc patterns that stars seem to form is that all stars are in motion.

Click to play embedded YouTube video.
Click to play embedded YouTube video.


















The video at left is a simulation of the motion of stars inside a massive cluster, and the video at right shows the (idealized) motion of stars in a spiral galaxy.

Click to play embedded YouTube video.


Here is a video showing the motion of two million stars in the Milky Way, as measured by the European sonds Gaia and Hipparcos. Bearing in mind that there are about two hundred billion stars in the Milky Way, it is a tiny, tiny fraction of the total stellar population of our galaxy that has had its various motions measured by these two space sonds.

The thing to remember is that all stars are in motion, and that the stars themselves are incredibly tiny compared with the vast, vast distances separating them. This adds to the impression that the stars are stationary, because it takes very much longer than a human lifetime for even the fastest-moving stars to appreciably change their apparent positions in the sky.

The stars look "relatively bigger" than they are when we look at them, because the atmosphere of the Earth scatters and spreads their light. Photography often makes stars look bigger, too, due to (among other factors) pixel bleeding. And as human beings we are utterly unable to picture in our minds the incomprehensible distances separating the stars. Also the sky looks pretty much like a canopy to us, where everything appears to be at the same distance from us. Nothing could be further from the truth. Take a look at how the Big Dipper would look if we saw it from other locations than our own:

Click to play embedded YouTube video.


The bottom line is that the lines and arcs that we see in the sky are frozen moments in time, seen from the vantage point of the Earth. The stars themselves, constantly in motion, will destroy the lines and arcs that grace our present-day sky and create other apparent lines and arcs in the sky in the future.

Ann
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heehaw

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby heehaw » Sat Jun 10, 2017 9:09 am

"The stars themselves, constantly in motion, will destroy the lines and arcs that grace our present-day sky and create other apparent lines and arcs in the sky in the future."
Indeed, and somewhere in our galaxy (or another) now, or some time in the past, a spot likely exists from which you, for a while, could have seen your own name written on the sky in stars!

zelaza

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby zelaza » Sat Jun 10, 2017 11:32 am

I'm unable to determine which of the bright objects is Saturn. Can anyone help?

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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby neufer » Sat Jun 10, 2017 11:50 am

zelaza wrote:
I'm unable to determine which of the bright objects is Saturn. Can anyone help?

The bright white one without the nebulosity (that is not a pipe).

Tap on the "You can find" part of the text.
Art Neuendorffer

douglas

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby douglas » Sat Jun 10, 2017 2:05 pm

" .. Cassini's Grand Finale orbit number 8 is in progress."

Here's something to ponder: will they find in measuring Saturn's mass and shape of gravity field that they're related to the rings, as unexpected as Jupiter's gravity field measures? That's a fast rate of rotation for a planet of that volume ..

" .. When Cassini and previous spacecraft have measured Saturn’s mass, the Doppler shift they’ve observed was actually caused by the combined mass of the planet and its rings. So while the total mass of Saturn and its rings is known, scientists don’t know the mass of the planet alone or the rings alone. Scientists also can’t tell precisely how non-spherical Saturn is.

Despite having about 750 times the volume of Earth, Saturn rotates more than twice as fast as Earth, completing a full rotation in less than 11 hours. That spin affects Saturn’s shape, according to Luciano Iess, a radio scientist at Sapienza University of Rome. “Saturn is fatter at the equator,” he said. “We know that Saturn is oblate due to the fast rotation, but the rings do not allow a precise measurement of that oblateness.”

https://saturn.jpl.nasa.gov/news/2976/c ... nstrument/

Nice picture, Mr. Nouroozi.

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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Ann » Sat Jun 10, 2017 7:47 pm

zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?


You can check out this 600 KB annotated version of today's APOD.

The Milky Way with the Lagoon Nebula at 10 o'clock,
and Saturn to the right of it.
The fact that the annotated image is as big as 600 KB means that I'm not allowed to post it as an image (and not as a link either, if I don't give everyone fair warning). But I am allowed to show you a smaller version of the image, which unfortunately isn't annotated. So let me try to explain.

The Lagoon Nebula. Photo: ESO/S. Guisard.

















At right you can see a picture of the well-known Lagoon Nebula. It is an emission nebula, and therefore it glows pink from ionized hydrogen and oxygen.

Now look at the picture at left, a small version of today's APOD. Can you see the Lagoon nebula in this picture at about 10 o'clock? The nebula is very small compared with the large swathe of the Milky Way, but it is quite noticeable because of its very pink color. It is almost white in the middle.

Please note that the Lagoon Nebula sits in a very dark dust lane of the Milky Way. To the right of the Lagoon Nebula the darkest dust gives way for a large, round, lighter-colored orange-brown patch. And smack in the middle of that round orange-brown patch is a bright white "star". That is Saturn.

Note the very white color of Saturn in this picture. In reality, if you observe Saturn, it will probably look pale yellow to you. Nevertheless, Saturn's color is fairly unusual. The bright stars in the sky are usually either bluer or redder than Saturn, a fact which is very obvious in today's APOD. Saturn's color is mostly due to the color of its own cloud tops, but it is also due to reflected light from the Sun. And most bright stars in the sky are either bluer or redder than the Sun.

Ann
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douglas

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby douglas » Sun Jun 11, 2017 2:26 pm

zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?


Because Saturn is a solar system object and the photographer used a reflector to take the picture the nearest objects to Earth will have the most prominent diffraction spikes.

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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Chris Peterson » Sun Jun 11, 2017 3:04 pm

douglas wrote:
zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?

Because Saturn is a solar system object and the photographer used a reflector to take the picture the nearest objects to Earth will have the most prominent diffraction spikes.

The image was made with a refractor- a 50mm Canon lens. The diffraction spikes around the bright objects are caused by the blades of the iris diaphragm. Note that there are ten spikes, which means this lens has a 5-blade iris. Most Canon 50mm f/1.8 lenses have (or had) 6, 7, or 8 blades, but they've made two models (1987-1990, 1990-2015) with five blades, so this image was clearly made with one of those. (They are very popular lenses, because they only cost about $100 and are very sharp... as we see in this APOD.)

The prominence of the diffraction spikes is unrelated to the distance from Earth, but is dependent upon the brightness of the object. Point sources show more prominent spikes than extended sources like planets, but at this image scale Saturn is virtually a point source, so its size is having little effect on diffraction (although I note that despite being a magnitude brighter than Antares, its diffraction spikes are no more prominent, possibly just slightly less so).
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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Ann » Sun Jun 11, 2017 3:29 pm

douglas wrote:
zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?


Because Saturn is a solar system object and the photographer used a reflector to take the picture the nearest objects to Earth will have the most prominent diffraction spikes.


That's not true. The diffraction spikes are caused by a combination of the equipment being used for the picture and the brightness of the object being photographed. It has nothing to do with the distance to the object in question.

The Earth's closest neighbor in space is the Moon, which is in orbit around the Earth. (Actually the Earth and the Moon are in orbit around their common center of mass, which, or so I believe at least, is located inside the Earth.) The average distance between the Earth and the Moon is ~ 350,000 kilometers. (I'm too lazy to google.)

The overexposed Moon in the night sky with diffraction spikes.
Source: https://hiveminer.com/Tags/pollux/Timeline

The eclipsed Moon in the Milky Way. Photo: Babak Tafreshi.










At left you can see an overexposed Moon with diffraction spikes. The Moon has been overexposed in order to bring out details in the night sky.

At right you can see the darkened, eclipsed Moon against the backdrop of the Milky Way. Note that the Moon is located almost exactly where Saturn is in the APOD of June 10, 2017. Because the Moon is eclipsed, we can easily see fine details in the Milky Way, which would otherwise be blotted out by the brilliant light of the full Moon.

I don't know if the photo at right has been taken with the sort of equipment that produces diffraction spikes in the first place. Nevertheless, an eclipsed Moon is so dark that it will not produce diffraction spikes under any kind of "normal circumstances", but a normal un-eclipsed full Moon may easily do so, in spite of being a very extended object.

Ann
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douglas

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby douglas » Sun Jun 11, 2017 7:31 pm

Chris Peterson wrote:
douglas wrote:
zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?

Because Saturn is a solar system object and the photographer used a reflector to take the picture the nearest objects to Earth will have the most prominent diffraction spikes.

The image was made with a refractor- a 50mm Canon lens. The diffraction spikes around the bright objects are caused by the blades of the iris diaphragm. Note that there are ten spikes, which means this lens has a 5-blade iris. Most Canon 50mm f/1.8 lenses have (or had) 6, 7, or 8 blades, but they've made two models (1987-1990, 1990-2015) with five blades, so this image was clearly made with one of those. (They are very popular lenses, because they only cost about $100 and are very sharp... as we see in this APOD.)

The prominence of the diffraction spikes is unrelated to the distance from Earth, but is dependent upon the brightness of the object. Point sources show more prominent spikes than extended sources like planets, but at this image scale Saturn is virtually a point source, so its size is having little effect on diffraction (although I note that despite being a magnitude brighter than Antares, its diffraction spikes are no more prominent, possibly just slightly less so).


Thanks on that with the iris diaphragm. I was wondering why there were so many spikes as reflectors using that many vanes would be .. Large.

I should possibly have said because planets have greater apparent size than stars/point sources their diffraction spikes are more prominent. Or the spikes are more 'full bodied'??

I can not agree that Antares' spikes are as prominent as Saturn's, :ssmile: .

douglas

Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby douglas » Sun Jun 11, 2017 7:37 pm

Ann wrote:
douglas wrote:
zelaza wrote:I'm unable to determine which of the bright objects is Saturn. Can anyone help?


Because Saturn is a solar system object and the photographer used a reflector to take the picture the nearest objects to Earth will have the most prominent diffraction spikes.


That's not true. The diffraction spikes are caused by a combination of the equipment being used for the picture and the brightness of the object being photographed. It has nothing to do with the distance to the object in question.

The Earth's closest neighbor in space is the Moon, which is in orbit around the Earth. (Actually the Earth and the Moon are in orbit around their common center of mass, which, or so I believe at least, is located inside the Earth.)
Ann


Here 'ya go, Ann, the Moon through a Schiefspiegler: no diffraction spikes, anywhere! :)

https://www.youtube.com/watch?v=hVbT5I5pdCc

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Re: APOD: Saturn in the Milky Way (2017 Jun 10)

Postby Chris Peterson » Sun Jun 11, 2017 7:54 pm

douglas wrote:I should possibly have said because planets have greater apparent size than stars/point sources their diffraction spikes are more prominent. Or the spikes are more 'full bodied'??

Usually, a greater apparent sized object reduces the prominence of diffraction. A point source places all of the energy in narrow lines. A broader source distributes it across a larger area, which reduces the surface brightness. That's why we rarely see diffraction spikes around images of the Moon. The diffracted light is just too spread out to be apparent.
Chris

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