APOD: Pleiades: The Seven Sisters Star Cluster (2020 Sep 09)

[APOD Robot]

Pleiades: The Seven Sisters Star Cluster

Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as large and clear as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six of the sister stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

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[Ann]

It's so nice to see a beautiful RGB picture of the lovely blue Seven Sisters in the sky!

Well, the Sisters look like they usually do, immersed in their delicate blue nebulosity, so I'd like to call attention to the orange star at 9 o'clock in the image. Because it is in fact a mystery that the Seven Sisters are so blue, and that there is no red star among them. The Pleiades is a rich cluster, whose total mass according to Wikipedia is about 800 solar masses, and it contains over 1,000 statistically confirmed member stars.

Such a rich cluster ought to have produced at least one star more massive than Alcyone, whose mass according to Wikipedia is about 6 solar masses. A star of, say, 8 solar masses, ought to have evolved into a red giant by now, in view of the fact that the Pleiades is considered to be about 100 million years old.

Think of all the open clusters that are dominated by blue stars but contain at least one red giant: M6, M7, M11, M25, M37, M38, M41, M44, NGC 457, NGC 3293, NGC 3532, NGC 3766, The Jewel Box, The Double Cluster of Perseus... okay, I give up now, but you get my drift. Believe me, it's hard enough to find an open cluster that has shed its natal cloud and is dominated by blue stars, but still doesn't contain a red giant!

Animation of the proper motion of the Pleiades and stars in its vicinity in 400,000 years. Author: Alexander Meleg.

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Well, one such cluster is IC 2602, and it is also known as... (drum roll) the Southern Pleiades. I guess that is because IC 2602 and the Pleiades both lack red giants!

Or do they? What about that orange star at 9 o'clock in today's APOD, HD 23712? Could that star be the "missing" red Pleiad?

No. No, that star is not a member of the entourage of the Seven Sisters, because the proper motion of HD 23712 is extremely different from the proper motion of the Pleiades. The animation at right shows the fascinating proper motion of the stars of M45. As you can see, they carry no bright red star in tow as they tumble southeastwards in the sky.

Please, bystander, let me keep this animation in my post, even though its larger (but not that much larger ) than 400 KB!

Ann

[rstevenson]

The cluster is always described as blue, of course, but there’s clearly a reddish-brown aura about it. Is that not part of the cluster? If so, why the color difference; if not, what is illuminating it?

Rob

[Eclectic Man]

Superb image.

I do, however have a request. For images where there is no definite 'right way up', could you display them in landscape rather than portrait orientation, please? Most will be viewed on computer screens in landscape orientation, and it would be more impressive to view full screen.

Thanks.

[Ann]

The cluster is always described as blue, of course, but there’s clearly a reddish-brown aura about it. Is that not part of the cluster? If so, why the color difference; if not, what is illuminating it?

Rob

Dusty nebulas in Taurus. Photo: Scott Rosen.

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That is clearly the extended dust cloud that the Pleiades themselves have blundered into and lit up with their blue light. Look at the picture at right which shows some other dark and dusty nebulas i Taurus.

So why do the dusty nebulas away from the blue light of the Pleiades in today's APOD seem to glow dimly with a "brown light"?

Integrated Flux Nebula, including faint molecular cloud MBM 54, located 900 light-years from the Earth in the direction of Pegasus. Photo: Scott Rosen.

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If you ask me, the darkly glowing dust nebulas probably reflect the integrated light of the Milky Way itself. They just might be distantly related to the faint dusty "cirrus clouds" of the Milky Way, known as the "Integrated Flux Nebula", or IFN. The IFN is supposed to reflect the overall light of the Milky Way, as far as I have understood it.

NGC 205, small elliptical satellite galaxy of Andromeda. NGC 205 does have a small population of relatively young blue stars in its center, diluting its otherwise all-yellow light. The B-V index of NGC 205 is +0.85, possibly the same overall value as the Milky Way. Photo: dcrowson.

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In one of the many books, magazines, papers and other stuff on astronomy that I have read over the years, I know I have come across the claim that the integrated B-V color of the Milky Way might be around +0.85. If that is true, it wouldn't surprise me. A B-V index of +0.85 is fairly red, and the Milky Way might indeed be a fairly red galaxy. We know our galaxy is large, and I'd say larger than most, and it clearly has a huge population of old red and yellow stars. Its star formation is winding down, and according to Wikipedia, the Milky Way is believed to be in a transitional stage, "the green valley", between vigorously star forming "blue" galaxies and "red and dead" elliptical galaxies.

So, in short, the brownish nebulas surrounding the brilliantly blue clouds of the Pleiades might be a kind of very faint "reflection nebulas" reflecting the integrated color of the Milky Way itself. Of course, we must never forget that dust is a reddening agent that reddens and dims light that passes through it. But dust also reflects light, and not just the brilliant blue light of hot young stars.

Ann

[orin stepanek]

Like a beacon in the sky is the Pleiades!

[TheZuke!]

"and the clarity of the observer's eyesight."

Yes, in my case, the Pleiades is a blur on a line to the West of Aldebaran and Orion's Belt.

Subaru!

[rstevenson]

The cluster is always described as blue, of course, but there’s clearly a reddish-brown aura about it. Is that not part of the cluster? If so, why the color difference; if not, what is illuminating it?

Rob

That is clearly the extended dust cloud that the Pleiades themselves have blundered into and lit up with their blue light. Look at the picture at right which shows some other dark and dusty nebulas i Taurus.

So why do the dusty nebulas away from the blue light of the Pleiades in today's APOD seem to glow dimly with a "brown light"?

If you ask me, the darkly glowing dust nebulas probably reflect the integrated light of the Milky Way itself. They just might be distantly related to the faint dusty "cirrus clouds" of the Milky Way, known as the "Integrated Flux Nebula", or IFN. The IFN is supposed to reflect the overall light of the Milky Way, as far as I have understood it.

Ann

If I’m understanding that correctly, the blue dust is close to the blue stars of the cluster, while the brown dust is far enough away that it can be considered background (and maybe some foreground) dust that is just coincidentally in the same field of view?

Rob

[BDanielMayfield]

Please, bystander, let me keep this animation in my post, even though its larger (but not that much larger ) than 400 KB!

Ann

Thanks Ann, and bystander too, for sharing, and allowing that time lapse of the Pleiades in motion! It well shows how clusters will always be passing in front of background stars, will occasionally be intersected by non member stars, and will usually have one or more stars zipping across them in the foreground.

Wow, what a busy place our galaxy is, with stars hurtling through space in all directions! And yet, due to the vast distances between them, they almost never ever collide, or even for that matter pass close enough to each other to upset the planetary systems almost all of them possess.

We bemoan the fact that the vast distances between the stars and the light speed limit make interstellar travel so difficult, but otoh those vast distances also help make long lasting habitability on planets possible. What a train wreck the galaxy would be if the distances between stars was much smaller than it is.

Bruce

[orin stepanek]

I got pretty good vision; and to me the Pleiades look like 1 giant star; kinda tear drop shaped! There does seem to be one that twinkles once in awhile! However that is here in the city!

[Ann]

The cluster is always described as blue, of course, but there’s clearly a reddish-brown aura about it. Is that not part of the cluster? If so, why the color difference; if not, what is illuminating it?

Rob

That is clearly the extended dust cloud that the Pleiades themselves have blundered into and lit up with their blue light. Look at the picture at right which shows some other dark and dusty nebulas i Taurus.

So why do the dusty nebulas away from the blue light of the Pleiades in today's APOD seem to glow dimly with a "brown light"?

If you ask me, the darkly glowing dust nebulas probably reflect the integrated light of the Milky Way itself. They just might be distantly related to the faint dusty "cirrus clouds" of the Milky Way, known as the "Integrated Flux Nebula", or IFN. The IFN is supposed to reflect the overall light of the Milky Way, as far as I have understood it.

Ann

If I’m understanding that correctly, the blue dust is close to the blue stars of the cluster, while the brown dust is far enough away that it can be considered background (and maybe some foreground) dust that is just coincidentally in the same field of view?

Rob

The Cocoon Nebula, The inner part is pink from hydrogen alpha emission, but the edges of the nebula are bluish from reflection nebulosity. The ultraviolet photons 'run out' and stop ionizing the hydrogen before the blue photons run out and cause dust particles to reflect the blue light from the central star. The long dark tendril of dust looks very dark, but might have been shown to reflect some brownish light from the vicinity. © Copyright: Federico Pelliccia.

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Widefield view of Orion, Taurus and Perseus. Photo: Ray Emery.

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Yes, that's how I understand it. The total luminosity of the Pleiades is very faint compared with the total luminosity of the Milky Way.

The picture at left shows tries to "put the Pleiades into perspective" compared with the Milky Way (even though I strongly believe that the Pleiades have been "dimmed" in order to make other details in the sky more visible). The picture at right demonstrates that different nebulas have different sizes, because the "effects of certain photons" run out before the effects of other photons do. That is why the central pink Hα of the Cocoon nebula does not extend as far as the blue reflection nebula that forms a blue border around the pink center.

Similarly, the blue light of the Pleiades that illuminates nearby dust clouds extends only so far.

P.S. Once again I posted a picture that is too big (but only about 150% of the permitted size). Again, please, bystander, let me keep it.

Ann

[neufer]

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Click to play embedded YouTube video.

Animation of the proper motion of the Pleiades and stars in its vicinity in 400,000 years. Author: Alexander Meleg.

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[orin stepanek]

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Click to play embedded YouTube video.

Animation of the proper motion of the Pleiades and stars in its vicinity in 400,000 years. Author: Alexander Meleg.

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Thanks Art; I needed that Rawhide flashback! One of my favorites, back in the day!

[Chris Peterson]

Thanks Art; I needed that Rawhide flashback! One of my favorites, back in the day! 😋

Still livin' the day here in Colorado. Pushing cattle by day, pulling photons by night...

And in color!
_

[neufer]

Thanks Art; I needed that Rawhide flashback!

• Many people are saying that I have done more for
  [fill in the blank] Starship Asterisk* than anyone in history

[orin stepanek]

Thanks Art; I needed that Rawhide flashback! One of my favorites, back in the day!

Still livin' the day here in Colorado. Pushing cattle by day, pulling photons by night...

And in color!
_
20190805_102303p.jpg

+1

[orin stepanek]

Thanks Art; I needed that Rawhide flashback!

• Many people are saying that I have done more for
  [fill in the blank] Starship Asterisk* than anyone in history

You have a way about you!

[MarkBour]

"Animation of the proper motion of the Pleiades and stars in its vicinity in 400,000 years. Author: Alexander Meleg."

Ann

Loved this animation of Alexander Meleg, Ann; thanks for sharing it. I have some folks I'll want to share it with !