APOD: Pleiades: The Seven Sisters Star Cluster (2021 Nov 24)

[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, taken from Florida, USA, 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]

Lovely APOD!!

However, trying to annotate it with text inserted into the images almost broke me... 🥺

Pleiades star names.

Pleiades foreground and background stars.

Pleiades distinctive features.

I know I misspelled "distinctive" in my labeled image, but I couldn't figure out a way to correct it without going back and re-doing the whole thing from scratch, and I didn't have the energy for it. Sorry!

Phew!

Ann

P.S. Oh, by the way. I checked a number of Gaia parallaxes for stars of the fifth and sixth magnitude. The Gaia parallaxes seem to converge at a distance of slightly below 440 light-years for the Pleiades, maybe 430 to 435 light-years.

[Ann]

I have to ask again, what's up with that too-bright but too-faint orange star, HD 23463, smack in the middle of the Pleiades? Its Gaia parallax, 7.5458 ± 0.0461, places it at a distance of some 432 light-years, which is "the perfect distance" to the Pleiades!!

So is this star a member of the Pleiades, then? But how can it be? It is way too bright to be a K-type main sequence star, and it is way too faint to be a massive Pleiades B-type star that has evolved into a red giant. Indeed, if this star was a K-type dwarf at the distance of the Pleiades, it would barely be visible in this image, and certainly not be surrounded by obvious diffraction spikes!

And if it was a recently evolved red giant, the product of a B-type main sequence star that had just run out of hydrogen in its core, we would expect it to be at least more or less as bright as the brightest blue stars in the cluster.

The Jewel Box star Cluster with red giant star DU Crucis. Image: ESO/Y. Beletsky.

---

Take a look at the image of NGC 4755 and its red giant star, DU Crucis. Yes, this orange star is indeed fainter than the blue giants in the cluster, some 1½ magnitudes fainter than the brightest blue giant of the cluster (at upper right). Yes, but the red star in the center of the Pleiades is more than 4½ magnitudes fainter than the luminary of the Pleiades, Alcyone!

The only explanation I can think of for the faintness of HD 23463 is that this red star is not a true member of the Pleiades after all. Instead, I think it is an old and very moderate red giant, perhaps similar to Pollux, but a little redder. This star has to be older than the Pleiades, because we expect the most massive stars to use up their core hydrogen and turn into red giants first, and massive blue stars should give rise to bright red giants. Indeed, it wouldn't surprise me if the red star of the Pleiades is perhaps an old Altair, a star with less than twice the mass of the Sun, turned red giant. Alcyone, by contrast, is believed to have a mass of 5.9-6.1 M_☉. And if Alcyone hasn't evolved into a red giant yet, we certainly don't expect an equally young but much more lightweight star to have used up its core hydrogen already.

So what is this old relatively lightweight star doing in the center of the Pleiades if it is not a member of the cluster? Well, perhaps it just blundered into the cluster as the Pleiades and the star were orbiting the center of the Milky Way at different speeds. HD 23463 does seem to orbit slower than the average proper motion of the Pleiades.

Ann

[XgeoX]

A little trivia…

The logo for Subaru cars is a highly stylized Pleiades cluster!





Eric

[orin stepanek]

One of the most beautiful naked eye sights in the night sky!

6 Kitty sisters; pretty also!

[E Fish]

A little trivia…

The logo for Subaru cars is a highly stylized Pleiades cluster!





Eric

Which is one of the reasons why I chose to buy a Subaru.

[Guest]

I have been puzzled for several years about why, when the image is of the Pleiades and the caption gives the standard clause about other names for the group ("Seven Sisters and M45"), it doesn't also call out that this cluster is known as Subaru. I have decided that the reason must be that to include an alternate naming from one non-English language but not others would be divisive.

[johnnydeep]

I have been puzzled for several years about why, when the image is of the Pleiades and the caption gives the standard clause about other names for the group ("Seven Sisters and M45"), it doesn't also call out that this cluster is known as Subaru. I have decided that the reason must be that to include an alternate naming from one non-English language but not others would be divisive.

I doubt that would be the reason. Probably just that not every fact about every APOD can possibly be included. But in this case, I think it's an unfortunate oversight since "Subaru" is also the name of a famous powerful telescope! True, it's operated by the National Astronomical Observatory of Japan, but it's located in Hawaii and enjoys multi-national fame. See https://subarutelescope.org/en/

[johnnydeep]

I have to ask again, what's up with that too-bright but too-faint orange star, HD 23463, smack in the middle of the Pleiades? Its Gaia parallax, 7.5458 ± 0.0461, places it at a distance of some 432 light-years, which is "the perfect distance" to the Pleiades!!

Pleiades What is up with that star.png

So is this star a member of the Pleiades, then? But how can it be? It is way too bright to be a K-type main sequence star, and it is way too faint to be a massive Pleiades B-type star that has evolved into a red giant. Indeed, if this star was a K-type dwarf at the distance of the Pleiades, it would barely be visible in this image, and certainly not be surrounded by obvious diffraction spikes!

And if it was a recently evolved red giant, the product of a B-type main sequence star that had just run out of hydrogen in its core, we would expect it to be at least more or less as bright as the brightest blue stars in the cluster.

The Jewel Box star Cluster with red giant star DU Crucis. Image: ESO/Y. Beletsky.

---

Take a look at the image of NGC 4755 and its red giant star, DU Crucis. Yes, this orange star is indeed fainter than the blue giants in the cluster, some 1½ magnitudes fainter than the brightest blue giant of the cluster (at upper right). Yes, but the red star in the center of the Pleiades is more than 4½ magnitudes fainter than the luminary of the Pleiades, Alcyone!

The only explanation I can think of for the faintness of HD 23463 is that this red star is not a true member of the Pleiades after all. Instead, I think it is an old and very moderate red giant, perhaps similar to Pollux, but a little redder. This star has to be older than the Pleiades, because we expect the most massive stars to use up their core hydrogen and turn into red giants first, and massive blue stars should give rise to bright red giants. Indeed, it wouldn't surprise me if the red star of the Pleiades is perhaps an old Altair, a star with less than twice the mass of the Sun, turned red giant. Alcyone, by contrast, is believed to have a mass of 5.9-6.1 M_☉. And if Alcyone hasn't evolved into a red giant yet, we certainly don't expect an equally young but much more lightweight star to have used up its core hydrogen already.

So what is this old relatively lightweight star doing in the center of the Pleiades if it is not a member of the cluster? Well, perhaps it just blundered into the cluster as the Pleiades and the star were orbiting the center of the Milky Way at different speeds. HD 23463 does seem to orbit slower than the average proper motion of the Pleiades.

Ann

How definitive is that distance figure for HD 23463? This page shows a table of data for it that puts it at 2700 ly - https://stargazerslounge.com/topic/3067 ... b=comments

There's no reference given for the 2700 ly figure (is it a screenshot of some Sky & Telescope app's database?), and perhaps GAIA is the undisputed standard these days?

PS - nice job on the annotated images in your prior post! Sorry the effort "broke you", but I trust future attempts will be more pleasant experiences. And yes, with Paint 3D, once you've hit enter on your text box, the text becomes dumb pixels that you can no longer edit. There are other tools that use objects and layers that allow unlimited editing, but Paint 3D is good for quick stuff.

[Fred the Cat]

What strikes me in the image are the tendrils or filaments apparent in the interstellar medium through which the cluster is passing. Perhaps just an illusion since I can find no references to an actual physical cause but beautiful none-the-less.

Just finding a dark sky location in Florida seemingly might be a difficult task; let alone creating such a dramatic image. Applause for the photographer.

[Ann]

I have to ask again, what's up with that too-bright but too-faint orange star, HD 23463, smack in the middle of the Pleiades? Its Gaia parallax, 7.5458 ± 0.0461, places it at a distance of some 432 light-years, which is "the perfect distance" to the Pleiades!!

Pleiades What is up with that star.png

So is this star a member of the Pleiades, then? But how can it be? It is way too bright to be a K-type main sequence star, and it is way too faint to be a massive Pleiades B-type star that has evolved into a red giant. Indeed, if this star was a K-type dwarf at the distance of the Pleiades, it would barely be visible in this image, and certainly not be surrounded by obvious diffraction spikes!

And if it was a recently evolved red giant, the product of a B-type main sequence star that had just run out of hydrogen in its core, we would expect it to be at least more or less as bright as the brightest blue stars in the cluster.

The Jewel Box star Cluster with red giant star DU Crucis. Image: ESO/Y. Beletsky.

---

Take a look at the image of NGC 4755 and its red giant star, DU Crucis. Yes, this orange star is indeed fainter than the blue giants in the cluster, some 1½ magnitudes fainter than the brightest blue giant of the cluster (at upper right). Yes, but the red star in the center of the Pleiades is more than 4½ magnitudes fainter than the luminary of the Pleiades, Alcyone!

The only explanation I can think of for the faintness of HD 23463 is that this red star is not a true member of the Pleiades after all. Instead, I think it is an old and very moderate red giant, perhaps similar to Pollux, but a little redder. This star has to be older than the Pleiades, because we expect the most massive stars to use up their core hydrogen and turn into red giants first, and massive blue stars should give rise to bright red giants. Indeed, it wouldn't surprise me if the red star of the Pleiades is perhaps an old Altair, a star with less than twice the mass of the Sun, turned red giant. Alcyone, by contrast, is believed to have a mass of 5.9-6.1 M_☉. And if Alcyone hasn't evolved into a red giant yet, we certainly don't expect an equally young but much more lightweight star to have used up its core hydrogen already.

So what is this old relatively lightweight star doing in the center of the Pleiades if it is not a member of the cluster? Well, perhaps it just blundered into the cluster as the Pleiades and the star were orbiting the center of the Milky Way at different speeds. HD 23463 does seem to orbit slower than the average proper motion of the Pleiades.

Ann

How definitive is that distance figure for HD 23463? This page shows a table of data for it that puts it at 2700 ly - https://stargazerslounge.com/topic/3067 ... b=comments

There's no reference given for the 2700 ly figure (is it a screenshot of some Sky & Telescope app's database?), and perhaps GAIA is the undisputed standard these days?

PS - nice job on the annotated images in your prior post! Sorry the effort "broke you", but I trust future attempts will be more pleasant experiences. And yes, with Paint 3D, once you've hit enter on your text box, the text becomes dumb pixels that you can no longer edit. There are other tools that use objects and layers that allow unlimited editing, but Paint 3D is good for quick stuff.

As you say, there is no reference for the 2700 ly figure, so I guess Gaia is a better source. That said, I have known about that orange star in the middle of the Pleiades ever since I bought a nice rather big poster of the Pleiades - HD 23463 stood out there like a sore thumb - and I have assumed that this star is a background star.

But if you check out this Simbad source for HD 23463 and compare it with the Simbad source for its apparent neighbor, HD 23479, you can see that the parallaxes of these two stars are very similar (within error bars), but their proper motions are quite different. And it is the white star, HD 23479, that shows the typical Pleiades proper motion. The orange star is much slower in its southward motion.

Ann

[johnnydeep]

I have to ask again, what's up with that too-bright but too-faint orange star, HD 23463, smack in the middle of the Pleiades? Its Gaia parallax, 7.5458 ± 0.0461, places it at a distance of some 432 light-years, which is "the perfect distance" to the Pleiades!!

Pleiades What is up with that star.png

So is this star a member of the Pleiades, then? But how can it be? It is way too bright to be a K-type main sequence star, and it is way too faint to be a massive Pleiades B-type star that has evolved into a red giant. Indeed, if this star was a K-type dwarf at the distance of the Pleiades, it would barely be visible in this image, and certainly not be surrounded by obvious diffraction spikes!

And if it was a recently evolved red giant, the product of a B-type main sequence star that had just run out of hydrogen in its core, we would expect it to be at least more or less as bright as the brightest blue stars in the cluster.

The Jewel Box star Cluster with red giant star DU Crucis. Image: ESO/Y. Beletsky.

---

Take a look at the image of NGC 4755 and its red giant star, DU Crucis. Yes, this orange star is indeed fainter than the blue giants in the cluster, some 1½ magnitudes fainter than the brightest blue giant of the cluster (at upper right). Yes, but the red star in the center of the Pleiades is more than 4½ magnitudes fainter than the luminary of the Pleiades, Alcyone!

The only explanation I can think of for the faintness of HD 23463 is that this red star is not a true member of the Pleiades after all. Instead, I think it is an old and very moderate red giant, perhaps similar to Pollux, but a little redder. This star has to be older than the Pleiades, because we expect the most massive stars to use up their core hydrogen and turn into red giants first, and massive blue stars should give rise to bright red giants. Indeed, it wouldn't surprise me if the red star of the Pleiades is perhaps an old Altair, a star with less than twice the mass of the Sun, turned red giant. Alcyone, by contrast, is believed to have a mass of 5.9-6.1 M_☉. And if Alcyone hasn't evolved into a red giant yet, we certainly don't expect an equally young but much more lightweight star to have used up its core hydrogen already.

So what is this old relatively lightweight star doing in the center of the Pleiades if it is not a member of the cluster? Well, perhaps it just blundered into the cluster as the Pleiades and the star were orbiting the center of the Milky Way at different speeds. HD 23463 does seem to orbit slower than the average proper motion of the Pleiades.

Ann

How definitive is that distance figure for HD 23463? This page shows a table of data for it that puts it at 2700 ly - https://stargazerslounge.com/topic/3067 ... b=comments

There's no reference given for the 2700 ly figure (is it a screenshot of some Sky & Telescope app's database?), and perhaps GAIA is the undisputed standard these days?

PS - nice job on the annotated images in your prior post! Sorry the effort "broke you", but I trust future attempts will be more pleasant experiences. And yes, with Paint 3D, once you've hit enter on your text box, the text becomes dumb pixels that you can no longer edit. There are other tools that use objects and layers that allow unlimited editing, but Paint 3D is good for quick stuff.

As you say, there is no reference for the 2700 ly figure, so I guess Gaia is a better source. That said, I have known about that orange star in the middle of the Pleiades ever since I bought a nice rather big poster of the Pleiades - HD 23463 stood out there like a sore thumb - and I have assumed that this star is a background star.

But if you check out this Simbad source for HD 23463 and compare it with the Simbad source for its apparent neighbor, HD 23479, you can see that the parallaxes of these two stars are very similar (within error bars), but their proper motions are quite different. And it is the white star, HD 23479, that shows the typical Pleiades proper motion. The orange star is much slower in its southward motion.

Ann

Ok, good enough for me! Then I guess that is just one unusual pseudo-Pleiades "sister"!

[Ann]

Ok, good enough for me! Then I guess that is just one unusual pseudo-Pleiades "sister"!

Yes, a pseudo-Pleiades "sister". Its color combined with its brightness is anomalous for a circa 100 million year old cluster whose most massive member is a B-type giant more than four magnitudes brighter than the oddball orange star.

Also the proper motion of HD 23463 is different than the average proper motion of the true members of the Pleiades, which is about 19 milliarcseconds/year in right ascension and -44 milliarcseconds/year in declination. It is the southward motion of HD 23463, -12 milliarcseconds/year, that is very much slower than the average proper motion of the Pleiades.

Ann