Starship Asterisk*

Kemble's Cascade of Stars

Explanation: This line of stars is real. A little too faint to see with the unaided eye, Kemble's Cascade of stars inspires awe when seen with binoculars. Like the Big Dipper though, Kemble's Cascade is an asterism, not a constellation. The asterism is visible in the northern sky toward the long-necked constellation of the Giraffe (Camelopardalis). This string of about 20 unrelated stars, each of similar brightness, spans over five times the angular width of the full moon. Stretching diagonally from the upper left to the lower right, Kemble's Cascade was popularized last century by astronomy enthusiast Lucian Kemble. The bright object near the top left of the image is the relatively compact Jolly Roger open cluster of stars, officially designated as NGC 1502.

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APOD Robot wrote:

This line of stars is real. (...) This string of about 20 unrelated stars, each of similar brightness...

Yeah, well. This Coathanger of stars is also real:


And the Coathanger stars are also of a similar brightness, and they are also unrelated. They go by the fancy name of Brocchi's Cluster, but they are not a cluster, because they are unrelated. That's what makes them (and Kemble's Cascade) asterisms.

Anyway, back to Kemble's Cascade. I have looked up the spectral classes, distances and luminosities of three of the stars. The apparently brightest of the stars of Kemble's Cascade is a star of spectral class B9IV. "B9" means that it is slightly hotter and bluer than Vega, and "IV" means that it is very slightly evolved (that is, it has almost used up the hydrogen in its core and is about to evolve into a giant). It is located at a distance of 385 light-years, and its luminosity in yellow-green light (that is, its V luminosity, which is how the luminosity of stars is traditionally measured) is 106 solar luminosities (or 106_☉).

The reddest star that I have looked up is HD 24065, a star of spectral type K5. Stars of spectral class K5 or M are interesting, because they are either very faint (like 61 Cygni) or exceedingly faint (like Proxima Centauri), or else they are very respectably bright (like Aldebaran) or else really truly bright (like HD 137071), or super-bright like Betelgeuse and Antares.

My point is that there seems to be no "middling stars" with "middling luminosities" of about 20-30_☉ or 1_☉, like Sirius or Pollux or the Sun, among the K5 or M-type stars.

The bright late K- or M-type stars are near the end of their lives, and their inflated red brilliance is actually their last hurrah as normally functioning stars (which in the case of stars can last for millions of years, but you know what I mean). So HD 24065 is a very respectable K5-type star of almost 400 solar luminosities, and the next stage of its existence will be the white dwarf stage.

As for HD 25443, near the top of Kemble's Cascade as seen in the APOD, it is the intrinsically hottest, brightest and most distant of all the stars in this asterism. It is a star of spectral class B0III at a distance of some 3,700 light-years, with a luminosity of some 2,000_☉. (Actually it is brighter than that, because the light from this star is very obviously reddened and dimmed by dust.)

It seems obvious to me that HD 25443 is an outlier of NGC 1502, or at least it is clearly related to NGC 1502. We have good reasons to believe that the actual distance between HD 25443 and NGC 1502 is considerably less than a hundred light-years, even though the Gaia parallax suggests otherwise.

(Or maybe Gaia doesn't even suggest that there is such a difference in distance between HD 25443 and NGC 1502. Gaia's measurements come with a certain uncertainty, which I didn't consider or show in my annotation of today's APOD.)

And finally, today's APOD doesn't show us the complete Kemble's Cascade. The end opposite NGC 1502 has actually been cut off in the APOD. Here you can see the complete Kemble's Cascade (and the orientation is different, of course):


Ann

According to Google:

A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline

whereas

An asterism [is] a pattern of stars that is not a constellation.

So... basically an asterism is a pattern that isn't a pattern ...???

I love how this is carefully referred to as an asterism (a pattern of stars) which they say is different from that of a constellation. Do people think the constellations are actually big dippers, goats, twins, bulls etc?

All I could think of was AWL! Just beautiful! Kitty said awl!

As I understand it, Asterisms are familiar patterns of stars that have not been “officially” recognized as constellations. Some are actually found within bigger constellations such as Orion’s Belt. Other examples include the big and little dippers.

Astronut

gmPhil wrote: ↑Tue Jan 03, 2023 11:11 am According to Google:

A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline

whereas

An asterism [is] a pattern of stars that is not a constellation.

So... basically an asterism is a pattern that isn't a pattern ...???

I don't think that's a good description of current usage. A modern constellation is a rectilinear area of the sky with its boundaries defined by the IAU, and which typically encloses a grouping of stars with some sort of historical association with a named pattern of some sort (which is all a constellation used to be). Any recognized pattern of stars is an asterism, whether it is associated with the constellation it lies within or not.

This is constellation Crux (the lighter whitish area at center).


This is Crux (or the Southern Cross) as we are used to thinking of it. We may possibly call it an asterism, although it is a bit big and sparse for an asterism. The three blue stars are at similar distances (some 300 light-years), but while the proper motion of the two middle blue stars is similar, the "bottom" star, Acrux, moves faster to the south and slower to the west than the two other blue stars. And the top red star, Gacrux, is half as far away as the blue stars.


This is cluster NGC 4755. It is located quite close to Beta Crux in constellation Crux. It is a proper cluster, because all the stars in it were born at the same time. They are all at (more or less) the same distance and move in (more or less) the same direction through space.


The bright blue stars that seem to trace the upper outline of the Milky Way form the Sco-Cen association. These stars are related, and they were born from the same quite large and multi-generational region of star formation. But they are too spread out, and they are at too dissimilar distances and too dissimilar ages to be a cluster.

Ann

Chris Peterson wrote: ↑Tue Jan 03, 2023 2:46 pm

gmPhil wrote: ↑Tue Jan 03, 2023 11:11 am According to Google:

A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline

whereas

An asterism [is] a pattern of stars that is not a constellation.

So... basically an asterism is a pattern that isn't a pattern ...???

I don't think that's a good description of current usage. A modern constellation is a rectilinear area of the sky with its boundaries defined by the IAU, and which typically encloses a grouping of stars with some sort of historical association with a named pattern of some sort (which is all a constellation used to be). Any recognized pattern of stars is an asterism, whether it is associated with the constellation it lies within or not.

So is every constellation also an asterism? It would make definitional sense that the set of constellations is a subset of the set of asterism. In that even the full set of stars in the defined rectilinear area of a constellation is just a "recognized pattern" of stars.

johnnydeep wrote: ↑Tue Jan 03, 2023 3:49 pm

Chris Peterson wrote: ↑Tue Jan 03, 2023 2:46 pm

gmPhil wrote: ↑Tue Jan 03, 2023 11:11 am According to Google:


whereas


So... basically an asterism is a pattern that isn't a pattern ...???

I don't think that's a good description of current usage. A modern constellation is a rectilinear area of the sky with its boundaries defined by the IAU, and which typically encloses a grouping of stars with some sort of historical association with a named pattern of some sort (which is all a constellation used to be). Any recognized pattern of stars is an asterism, whether it is associated with the constellation it lies within or not.

So is every constellation also an asterism? It would make definitional sense that the set of constellations is a subset of the set of asterism. In that even the full set of stars in the defined rectilinear area of a constellation is just a "recognized pattern" of stars.

I think every constellation is associated with a classical constellation, which was associated with an asterism. Some of those asterisms, however, are all but impossible to really notice. And we've changed the common asterisms associated with some of the constellations. Like a big dipper instead of a bear, or a teapot instead of an archer.

Chris Peterson wrote: ↑Tue Jan 03, 2023 3:56 pm

johnnydeep wrote: ↑Tue Jan 03, 2023 3:49 pm

Chris Peterson wrote: ↑Tue Jan 03, 2023 2:46 pm

I don't think that's a good description of current usage. A modern constellation is a rectilinear area of the sky with its boundaries defined by the IAU, and which typically encloses a grouping of stars with some sort of historical association with a named pattern of some sort (which is all a constellation used to be). Any recognized pattern of stars is an asterism, whether it is associated with the constellation it lies within or not.

So is every constellation also an asterism? It would make definitional sense that the set of constellations is a subset of the set of asterism. In that even the full set of stars in the defined rectilinear area of a constellation is just a "recognized pattern" of stars.

I think every constellation is associated with a classical constellation, which was associated with an asterism. Some of those asterisms, however, are all but impossible to really notice. And we've changed the common asterisms associated with some of the constellations. Like a big dipper instead of a bear, or a teapot instead of an archer.

Yes, there are the "common" asterisms. I'm just advocating for the idea that even the set of all stars in a constellation delineated area of the sky should also be considered to be an asterism.

johnnydeep wrote: ↑Tue Jan 03, 2023 4:26 pm

Chris Peterson wrote: ↑Tue Jan 03, 2023 3:56 pm

johnnydeep wrote: ↑Tue Jan 03, 2023 3:49 pm

So is every constellation also an asterism? It would make definitional sense that the set of constellations is a subset of the set of asterism. In that even the full set of stars in the defined rectilinear area of a constellation is just a "recognized pattern" of stars.

I think every constellation is associated with a classical constellation, which was associated with an asterism. Some of those asterisms, however, are all but impossible to really notice. And we've changed the common asterisms associated with some of the constellations. Like a big dipper instead of a bear, or a teapot instead of an archer.

Yes, there are the "common" asterisms. I'm just advocating for the idea that even the set of all stars in a constellation delineated area of the sky should also be considered to be an asterism.

I think that "asterism" very much carries with it the sense of a clearly discernible pattern. The set of all stars is the constellation, not the asterism. Most constellations contain more than one asterism- one or more that are directly associated with the traditional constellation (which may vary with culture), and any number that are not.

Chris Peterson wrote: ↑Tue Jan 03, 2023 4:48 pm

johnnydeep wrote: ↑Tue Jan 03, 2023 4:26 pm

Chris Peterson wrote: ↑Tue Jan 03, 2023 3:56 pm

I think every constellation is associated with a classical constellation, which was associated with an asterism. Some of those asterisms, however, are all but impossible to really notice. And we've changed the common asterisms associated with some of the constellations. Like a big dipper instead of a bear, or a teapot instead of an archer.

Yes, there are the "common" asterisms. I'm just advocating for the idea that even the set of all stars in a constellation delineated area of the sky should also be considered to be an asterism.

I think that "asterism" very much carries with it the sense of a clearly discernible pattern. The set of all stars is the constellation, not the asterism. Most constellations contain more than one asterism- one or more that are directly associated with the traditional constellation (which may vary with culture), and any number that are not.

Ok.

The five inner stars of the Big Dipper form "sort of" a cluster, because these stars are extremely similar when it comes to spectral classes, luminosities and motion through space. But they are a bit spread out to count as a cluster. The outermost stars, Alkaid and Dubhe, are unrelated.

Note that the relative positions of the five innermost stars of the Big Dipper will be relatively unchanged in 50,000 years, but Alkaid and Dubhe will have moved considerably.

Ann

I'm trying to think of a humorous phrasing for the abbreviation IAU which matches the notion of "evil industrial bulldozers of the romantic and historical terms of astronomy". Let's see. The IAU: The Insensitive Astronomical Undoers. Probably someone has done this better.

MarkBour wrote: ↑Sat Jan 07, 2023 7:52 am I'm trying to think of a humorous phrasing for the abbreviation IAU which matches the notion of "evil industrial bulldozers of the romantic and historical terms of astronomy". Let's see. The IAU: The Insensitive Astronomical Undoers. Probably someone has done this better.

I'm not at all sure what this has to do with this image, nor why you feel it necessary to malign the IAU. Perhaps you don't think there is a need for standardization. If this was meant as a joke, I don't understand it.

bystander wrote: ↑Sat Jan 07, 2023 5:50 pm

MarkBour wrote: ↑Sat Jan 07, 2023 7:52 am I'm trying to think of a humorous phrasing for the abbreviation IAU which matches the notion of "evil industrial bulldozers of the romantic and historical terms of astronomy". Let's see. The IAU: The Insensitive Astronomical Undoers. Probably someone has done this better.

I'm not at all sure what this has to do with this image, nor why you feel it necessary to malign the IAU. Perhaps you don't think there is a need for standardization. If this was meant as a joke, I don't understand it.

I wasn't sure how to take it, either. The old names are rightfully kept. But all manner of new names, for small bodies, for craters, for geological structures, are drawn from a wide range of cultures. The IAU is very inclusive in that respect.

Chris and bystander, please accept my apology. It was meant as a joke. Or, more accurately, a sarcastic jab, which may be undeserved.

The following notes attempt to explain my remark.
- - - - - - - - - -
If the IAU intends that part of their mission is not just to improve communication among astronomers, but also to influence the rest of society, then sometimes they will become associated with unwelcome intrusions on established language. Whether this is just a public relations issue, or whether they are potentially misguided at times in such efforts, I am not qualified to say. (Of course I hold opinions on thousands of topics on which I am not qualified to say!)

I find looking at language and study of the ancients a very pleasant journey. So, when it comes to "planetes asteres" (πλάνητες ἀστέρες) or "cōnstellātiō", I may be obnoxious. I enjoy imagining how these words connect ancient and modern people, common folk like myself, who look up into the night sky in wonder. I like to think about this across ages of time and vast distances on our world and incredibly larger expanses of spacetime out in other systems around other stars. It's a bunch of romantic notions in my mind, I guess.

I would like a couple of solid words that can be used by average parents to share the awe of the night sky with their children. If the IAU hopes to redefine "constellation" to stand for "a region of the sky, bordered by arcs of right ascension and declination", or "planet" to involve clearing of regions and hydrodynamic equilibrium, then these efforts come into conflict with my romantic notions.

So, I find some results of the IAU annoying. And they would probably find me annoying. I realize that annoying is not always bad.
(Using the word in a mild sense -- annoying := "troublesome, vexation, causing irritation, late 14c.")

MarkBour wrote: ↑Sun Jan 08, 2023 12:30 am If the IAU hopes to redefine "constellation" to stand for "a region of the sky, bordered by arcs of right ascension and declination", or "planet" to involve clearing of regions and hydrodynamic equilibrium, then these efforts come into conflict with my romantic notions.

Well, they defined "constellation" that way about a century ago. And it does make sense... before then, there was no clear way to identify where many astronomical objects were in the sky. Tiling the sky makes good sense, and the existing constellations made for a good naming system. That did not limit "constellation" to not still have its classical meaning, however. There is nothing inaccurate about referring to the asterism of Orion as the constellation of Orion.

Chris Peterson wrote: ↑Sun Jan 08, 2023 3:04 am

MarkBour wrote: ↑Sun Jan 08, 2023 12:30 am If the IAU hopes to redefine "constellation" to stand for "a region of the sky, bordered by arcs of right ascension and declination", or "planet" to involve clearing of regions and hydrodynamic equilibrium, then these efforts come into conflict with my romantic notions.

Well, they defined "constellation" that way about a century ago. And it does make sense... before then, there was no clear way to identify where many astronomical objects were in the sky. Tiling the sky makes good sense, and the existing constellations made for a good naming system. That did not limit "constellation" to not still have its classical meaning, however. There is nothing inaccurate about referring to the asterism of Orion as the constellation of Orion.

Back in 1930, I see. I'm surprised, I thought it was a much more recent action.
And, if they aren't telling me to throw out my old usage, then that's fair enough.

MarkBour wrote: ↑Sun Jan 08, 2023 9:46 am

Chris Peterson wrote: ↑Sun Jan 08, 2023 3:04 am

MarkBour wrote: ↑Sun Jan 08, 2023 12:30 am If the IAU hopes to redefine "constellation" to stand for "a region of the sky, bordered by arcs of right ascension and declination", or "planet" to involve clearing of regions and hydrodynamic equilibrium, then these efforts come into conflict with my romantic notions.

Well, they defined "constellation" that way about a century ago. And it does make sense... before then, there was no clear way to identify where many astronomical objects were in the sky. Tiling the sky makes good sense, and the existing constellations made for a good naming system. That did not limit "constellation" to not still have its classical meaning, however. There is nothing inaccurate about referring to the asterism of Orion as the constellation of Orion.

Back in 1930, I see. I'm surprised, I thought it was a much more recent action.
And, if they aren't telling me to throw out my old usage, then that's fair enough.

How old is your usage? There has never been any agreement on which stars belonged in constellation asterisms. Our current way of depicting constellations with stick figures or lines is quite new... started in the 1950s. And different artists and catalog creators drew (and still draw) different patterns. For most of the time that people have been creating star charts, the constellations were represented by drawings of what they represented lying over the star region, with little or no attempt to connect individual stars to the representation. There are only a few cases where an individual star was commonly claimed to be important to the constellation (Regulus as the heart of Leo, Antares as the heart or head of Scorpius, Aldebaran as the eye of Taurus).

So the actual asterism representing any constellation has never been formally defined, or universally recognized.

Chris Peterson wrote: ↑Sun Jan 08, 2023 2:49 pm

MarkBour wrote: ↑Sun Jan 08, 2023 9:46 am

Chris Peterson wrote: ↑Sun Jan 08, 2023 3:04 am
Well, they defined "constellation" that way about a century ago. And it does make sense... before then, there was no clear way to identify where many astronomical objects were in the sky. Tiling the sky makes good sense, and the existing constellations made for a good naming system. That did not limit "constellation" to not still have its classical meaning, however. There is nothing inaccurate about referring to the asterism of Orion as the constellation of Orion.

Back in 1930, I see. I'm surprised, I thought it was a much more recent action.
And, if they aren't telling me to throw out my old usage, then that's fair enough.

How old is your usage? There has never been any agreement on which stars belonged in constellation asterisms. Our current way of depicting constellations with stick figures or lines is quite new... started in the 1950s. And different artists and catalog creators drew (and still draw) different patterns. For most of the time that people have been creating star charts, the constellations were represented by drawings of what they represented lying over the star region, with little or no attempt to connect individual stars to the representation. There are only a few cases where an individual star was commonly claimed to be important to the constellation (Regulus as the heart of Leo, Antares as the heart or head of Scorpius, Aldebaran as the eye of Taurus).

So the actual asterism representing any constellation has never been formally defined, or universally recognized.

Point taken. And I'm sure there's much handiness that has been gained with the 1930 IAU convention. I was guilty of "We mock that which we don't understand." in taking a jab at the IAU.

Of course they could have named their handy sectors "constellareas", or some such term. I'm talking not about specific constellations, just the term itself, which is evidently derived from latin and some sources indicate it had been in use as far back as the 14th century.