Starship Asterisk*

by **owlice** » Sun Jan 26, 2014 7:51 pm

Anthony, you might be interested in the Astrobites site, which is written by grad students for undergrads; see here: http://astrobites.com/

by **Anthony Barreiro** » Sun Jan 26, 2014 7:32 pm

geckzilla wrote:It's also difficult to say with certainty which stars aren't actually part of any given cluster. A couple of chance alignments could easily throw you off. Must be why at least one of those papers I noticed they focused on studying the very centers of the clusters rather than the extended regions around them.

Yes, I did understand that much. This whole discussion has been very helpful to me. I'm not at a level where I can digest much from astrophysics journal articles, but if somebody explains it slowly and doesn't use too many big words, I can understand a bit more over time ... .

by **geckzilla** » Sun Jan 26, 2014 7:13 pm

It's also difficult to say with certainty which stars aren't actually part of any given cluster. A couple of chance alignments could easily throw you off. Must be why at least one of those papers I noticed they focused on studying the very centers of the clusters rather than the extended regions around them.

by **Anthony Barreiro** » Sun Jan 26, 2014 4:59 pm

Ann wrote:
My ramblings were motivated by my own confusion at the similarity yet difference of the two members of the Double Clusters. Why does one of them have red giants but not the other?

On the other hand, one of the papers that Geckzilla provided links to claimed that the halo population of the Double Cluster probably belongs to both clusters, in a way that is slightly similar to a planet orbiting the center of mass of a binary star. (Or rather, imagine a whole retinue of planets orbiting a binary star!) And five out of six red supergiants in the Double Cluster appear (by visual inspection) to belong to the halo population, where they might be orbiting both of the clusters. So when it comes to why one of the clusters in the Double Cluster has red giants but the other has none, it could be that the question is moot.

I can't resist showing you this infrared picture of the Double Cluster. The large yellow-green stars are the red supergiants, whereas the small blue-white stars are the non-red stars. The core of NGC 869 is obvious, but the core of NGC 884 is almost lost in the glare of one of the red supergiants.

Ann

Thanks Ann. I read that paper, or as much of it as I could understand, but didn't make the connection to the possibility that if the red supergiants are orbiting the common center of mass of both clusters, it's just a coincidence that most of the red supergiants we see from our perspective are lined up in front of NGC 884, rather than NGC 869. The answer to my puzzlement seems to be: the universe is big, and I am not at the center of the universe!

by **Ann** » Sat Jan 25, 2014 8:24 pm

Nitpicker wrote:Scientifically speaking, if you know something, despite all evidence suggesting something else, it is likely that you don't really know that something.

Statistically speaking, there are enough possibilities in the universe for me to be not puzzled by the lack of red giants in some clusters (and the existence of red giants in others ).

Well and nitpickingly spoken, Nitpicker.

My ramblings were motivated by my own confusion at the similarity yet difference of the two members of the Double Clusters. Why does one of them have red giants but not the other?

On the other hand, one of the papers that Geckzilla provided links to claimed that the halo population of the Double Cluster probably belongs to both clusters, in a way that is slightly similar to a planet orbiting the center of mass of a binary star. (Or rather, imagine a whole retinue of planets orbiting a binary star!) And five out of six red supergiants in the Double Cluster appear (by visual inspection) to belong to the halo population, where they might be orbiting both of the clusters. So when it comes to why one of the clusters in the Double Cluster has red giants but the other has none, it could be that the question is moot.

I can't resist showing you this infrared picture of the Double Cluster. The large yellow-green stars are the red supergiants, whereas the small blue-white stars are the non-red stars. The core of NGC 869 is obvious, but the core of NGC 884 is almost lost in the glare of one of the red supergiants.

Ann

by **rstevenson** » Sat Jan 25, 2014 4:27 pm

That's a quite good description of science, Anthony. That's what scientists do, they learn to ask more intelligent questions, over and over again. You're a (budding) scientist!

Rob

by **Anthony Barreiro** » Sat Jan 25, 2014 3:36 pm

Ann wrote: I love the blue brilliance of the Pleiades, of course, but the cluster's lack of red giants is a mystery to me.

Thanks Ann, Judy, and the Nitpicker for your help.

As an interested amateur with no formal education in astrophysics, I love the ongoing dynamic tension between my growing understanding of stars and their utter complexity and bottomless mystery. All I have learned to do is to ask (hopefully) more intelligent questions.

by **Nitpicker** » Sat Jan 25, 2014 10:37 am

Scientifically speaking, if you know something, despite all evidence suggesting something else, it is likely that you don't really know that something.

Statistically speaking, there are enough possibilities in the universe for me to be not puzzled by the lack of red giants in some clusters (and the existence of red giants in others ).

by **Ann** » Sat Jan 25, 2014 10:20 am

Nitpicker wrote:
Ann wrote:Why do some young clusters contain bright red giant stars and others do not? That is the million dollar question.
A million dollars? I like those odds. I shall take a punt ...

A red giant is in a late phase of stellar evolution. I expect there is a range of life expectancy across the stars of different masses which eventually become red giants (0.3 - 8 solar masses). So, perhaps the red giants that we see in some younger clusters are just (lower mass?) short-lived examples of red giants.

Why doesn't the Pleiades contain any red giants?

http://en.wikipedia.org/wiki/Pleiades wrote:
The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years.

A hundred million years is certainly long enough for some members of the cluster to have evolved into red giants. But perhaps the Pleiades lacked stars that were massive enough to have evolved into red giants in that time? Well, in my amateur opinion, the Pleiades contains a sufficient number of B-type stars that it is likely that there should have been one or two members that were massive enough to have used up their core hydrogen and turned into red giants after 10⁸ years. After all, it is rare to see a young cluster that contains a number of late B-type stars but not a single early B-type star - and an early B-type star ought to have turned into a red giant after a hundred million years.

I love the blue brilliance of the Pleiades, of course, but the cluster's lack of red giants is a mystery to me.

Ann

by **Nitpicker** » Sat Jan 25, 2014 9:49 am

Ann wrote:Why do some young clusters contain bright red giant stars and others do not? That is the million dollar question.

A million dollars? I like those odds. I shall take a punt ...

A red giant is in a late phase of stellar evolution. I expect there is a range of life expectancy across the stars of different masses which eventually become red giants (0.3 - 8 solar masses). So, perhaps the red giants that we see in some younger clusters are just (lower mass?) short-lived examples of red giants.

Edit: it may be higher mass stars which live shorter lives, burning their fuel at a disproportionately high rate.

by **Ann** » Sat Jan 25, 2014 7:55 am

I also found your references interesting, geckzilla.

As a color commentator, the question that interests me most is this one: Why do most young clusters sport at least one red giant, while a few of them do not? As for clusters with red giants, check out M6, NGC 3293 and the Jewel Box Cluster, NGC 4755. Examples of clusters with no red giants are the Pleiades, IC 2602 (also known as the Southern Pleiades) and possibly M36, although there are yellow and red stars close to the latter. This widefield picture shows two obvious clusters, M38 at top left (at about 11 o'clock) and M36 at bottom left, at about 7 o'clock. It is obvious that both clusters are young, but it is also obvious that M38 contains several red giants, whereas the core of M36 contains none. But there are several red stars at the outskirts of M36, and there are some bright red stars some distance away from M36 which may well be associated with the cluster.

Please note that the red stars are often at the outskirts of a young cluster. That is the case of M6, NGC 3293 and M36. In constellation Orion, red supergiant Betelgeuse is also "at the outskirts " of the action. Red supergiant Antares is more "centrally placed" at Upper Scorpius, though perhaps slightly at the outskirts after all. (Antares and Betelgeuse are not members of clusters, of course, but of associations.)

There are no red giants in or around the Lagoon Nebula or in or around its "neighbour", the Trifid Nebula. Open cluster M21, not far from the Trifid Nebula, is an example of a young cluster with no red giants. This picture of M21 shows some not so bright but very red objects in M21. Clearly they are nowhere near as bright as the brightest blue stars in the cluster. Is it possible that they are pre-main sequence stars, which are brighter and redder than the main sequence stars they will evolve into? Are they, for some reason, just dust-reddened? Could some of them be background objects? What are they?

Why do some young clusters contain bright red giant stars and others do not? That is the million dollar question.

Ann

by **geckzilla** » Sat Jan 25, 2014 6:34 am

I meant in order to try to understand them. I'm terrible at reading those papers too but there are plenty of parts that are intelligible. "Oh no, they're talking about flux again! [skip, skip, skip]" (I've tried figuring out what flux means a couple of times now but haven't quite gotten it.)

by **Anthony Barreiro** » Sat Jan 25, 2014 6:13 am

geckzilla wrote:
Anthony Barreiro wrote:Thanks Ann. If these very bright stars are in spectral classes B and A, they're not going to look red. I'm still trying to understand why 884 has bright red stars and 869 does not. Those red stars are visible in a 200 mm telescope. I had always assumed that 884 was older than 869. Now I'm perplexed.
I recommend reading papers instead of looking at pictures, however pretty they may be.

Physical properties of the B and Be star populations of h and χ Persei.

The stellar population of h and χ Persei: cluster properties, membership, and the intrinsic colors and temperatures of stars.

The star formation history and mass function of the double cluster h and χ Persei.

Thanks for the references. Currie et al. looks interesting, and Slesnick, Hillenbrand, and Massey was already referenced by Bruce. I don't know enough about the methodology of photometry and spectroscopy to read these papers critically, but they at least concur that NGC 884 and NGC 869 are the same age.

I find your advice puzzling, though. If I hadn't seen those red stars through a telescope and in pictures, I wouldn't have had occasion to look into these papers.

by **geckzilla** » Sat Jan 25, 2014 2:42 am

Anthony Barreiro wrote:Thanks Ann. If these very bright stars are in spectral classes B and A, they're not going to look red. I'm still trying to understand why 884 has bright red stars and 869 does not. Those red stars are visible in a 200 mm telescope. I had always assumed that 884 was older than 869. Now I'm perplexed.

I recommend reading papers instead of looking at pictures, however pretty they may be.

Physical properties of the B and Be star populations of h and χ Persei.

The stellar population of h and χ Persei: cluster properties, membership, and the intrinsic colors and temperatures of stars.

The star formation history and mass function of the double cluster h and χ Persei.

by **Anthony Barreiro** » Sat Jan 25, 2014 2:19 am

Ann wrote:
Anthony Barreiro wrote:If NGC 869 and NGC 884 are the same age, why would 884 have some very prominent bright red giant stars while 869 has none? My first guess is that 884 must have started out with bigger stars that have already evolved into giants, while all of 869's stars were born small enough to still be on the main sequence.
Actually, the brightest stars in NGC 869 are giants or supergiants. My software classifies the two brightest stars in the core of NGC 869, V520 Per and HD 14143, as supergiants of spectral classes B3Ia and B2Ia. The core of NGC 869 contains brighter stars than the core of NGC 884, and it is possible that the stars in the core of NGC 884 are mostly unevolved (but I would guess that the brightest of them have begun to evolve, even though they have not yet become supergiants).

The red stars between the two clusters and at the outskirts of NGC 884 have fainter V magnitudes than the brightest blue stars in the clusters. The brightest star belonging to either of the two clusters appears to be HD 14433, a star of spectral class A1Ia and a V magnitude of 6.40. In Fabian Neyer's picture, this star is located to the upper right of NGC 884's "double core".

Rob Gendler has created a great picture of the Double Cluster, where you can begin to appreciate the relative brightness of the different stars. Note that it is a big picture of more than 800 KB.

Ann

Thanks Ann. If these very bright stars are in spectral classes B and A, they're not going to look red. I'm still trying to understand why 884 has bright red stars and 869 does not. Those red stars are visible in a 200 mm telescope. I had always assumed that 884 was older than 869. Now I'm perplexed.

by **Ann** » Fri Jan 24, 2014 5:35 am

Anthony Barreiro wrote:If NGC 869 and NGC 884 are the same age, why would 884 have some very prominent bright red giant stars while 869 has none? My first guess is that 884 must have started out with bigger stars that have already evolved into giants, while all of 869's stars were born small enough to still be on the main sequence.

Actually, the brightest stars in NGC 869 are giants or supergiants. My software classifies the two brightest stars in the core of NGC 869, V520 Per and HD 14143, as supergiants of spectral classes B3Ia and B2Ia. The core of NGC 869 contains brighter stars than the core of NGC 884, and it is possible that the stars in the core of NGC 884 are mostly unevolved (but I would guess that the brightest of them have begun to evolve, even though they have not yet become supergiants).

The red stars between the two clusters and at the outskirts of NGC 884 have fainter V magnitudes than the brightest blue stars in the clusters. The brightest star belonging to either of the two clusters appears to be HD 14433, a star of spectral class A1Ia and a V magnitude of 6.40. In Fabian Neyer's picture, this star is located to the upper right of NGC 884's "double core".

Rob Gendler has created a great picture of the Double Cluster, where you can begin to appreciate the relative brightness of the different stars. Note that it is a big picture of more than 800 KB.

Ann

by **Boomer12k** » Fri Jan 24, 2014 12:06 am

Awesome wide shot...love the Double Cluster, fascinating to see the clouds of Hydrogen...

by **Chris Peterson** » Thu Jan 23, 2014 10:54 pm

Anthony Barreiro wrote:If NGC 869 and NGC 884 are the same age, why would 884 have some very prominent bright red giant stars while 869 has none? My first guess is that 884 must have started out with bigger stars that have already evolved into giants, while all of 869's stars were born small enough to still be on the main sequence.

Born at the same time doesn't mean born under identical conditions. So a slightly different evolutionary history isn't unreasonable.

by **Anthony Barreiro** » Thu Jan 23, 2014 10:51 pm

If NGC 869 and NGC 884 are the same age, why would 884 have some very prominent bright red giant stars while 869 has none? My first guess is that 884 must have started out with bigger stars that have already evolved into giants, while all of 869's stars were born small enough to still be on the main sequence.

by **Ron-Astro Pharmacist** » Thu Jan 23, 2014 9:59 pm

The first question that came to mind when seeing this APOD was what could have initiated such intense star formation in these and other local open clusters. It was interesting to read that it could have been a 70 million year old supernova that initiated star formation in the vicinity of the dust clouds that became the Alpha Persei and Pleiades clusters. Also that it could be continuing outward in a ring of expansion.(Feb 2014 Sky and Telescope - Observing the Milky Way, Part III)

by **starsurfer** » Thu Jan 23, 2014 8:44 pm

Ann wrote:You may have been right to be shocked at my ignorance, starsurfer. I may have seen Fabian Neyer's image before, but I'm afraid I don't remember it.

Of course I, the complete amateur, can't know if the red strands of nebulosity have anything to do with the Double Cluster. I stated my impression, and that means nothing more than that: my impression.

Ann

I wasn't shocked at your ignorance, I was shocked by the fact that it was todays APOD after such a long time. I always take great pleasure in reading your comments and you seem to have a most innate ability of expressing emotion in your comments. As I might have said previously, I would love to meet you and give you a hug preferably whilst looking at the night sky!

The nebulosity is uncatalogued and has never been studied, it would be pretty cool if it was the remnant of the original emission nebula.

by **BDanielMayfield** » Thu Jan 23, 2014 8:40 pm

Thanks Chris, and I found an answer to my own question, demonstrating your point. Here’s the abstract of a 2002 study of these clusters:

The Star Formation History and Mass Function of the Double Cluster h and Chi Persei
Authors: Catherine L. Slesnick, Lynne A. Hillenbrand (Caltech), Philip Massey (Lowell Observatory)
(Submitted on 9 May 2002)

Abstract: The h and Chi Per "double cluster" is examined using wide-field (0.98 deg x 0.98 deg) CCD UBV imaging supplemented by optical spectra of several hundred of the brightest stars. Restricting our analysis to near the cluster nuclei, we find identical reddenings (E(B-V)=0.56+/-0.01), distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the two clusters. In addition, we find an IMF slope for each of the cluster nuclei that is quite normal for high-mass stars, Gamma=-1.3+/-0.2, indistinguishable from a Salpeter value. We derive masses of 3700 M_Sun (h) and 2800 M_Sun (Chi) integrating the PDMF from 1 to 120 M_Sun. There is evidence of mild mass segregation within the cluster cores. Our data are consistent with the stars having formed at a single epoch; claims to the contrary are very likely due to the inclusion of the substantial population of early-type stars located at similar distances in the Perseus spiral arm, in addition to contamination by G and K giants at various distances. We discuss the uniqueness of the double cluster, citing other examples of such structures in the literature, but concluding that the nearly identical nature of the two cluster cores is unusual. We fail to settle the long-standing controversy regarding whether or not the double cluster is the core of the Per OB1 association, and argue that this may be unanswerable with current techniques. We also emphasize the need for further work on the pre-main sequence population of this nearby and highly interesting region.

So the age disagreement was due to young interlopers from outside the cluster confusing the age estimates.

The area I highlighted in blue made me want to ask, how long does it take for a star to reach it’s main sequence lifetime if 11-13 million year old stars are still called “pre-main sequence” stars?

Bruce

by **Chris Peterson** » Thu Jan 23, 2014 8:03 pm

BDanielMayfield wrote:Also there seems to be some divergence of opinion as to the ages of these open star clusters. The description says they “are both 13 million years young based on the ages of their individual stars, evidence that they were likely a product of the same star-forming region.”

But the link in the description under “NGC 869 and NGC 884” says that They are both quite young: h is listed at 5.6, chi at 3.2 million years (Sky Catalog 2000)”
Is Sky Catalog 2000 out of date as to these clusters ages?

I'm not sure I'd generally consider Sky Catalogue 2000.0 (which I assume is what the SEDS site means by "Sky Catalog 2000") to be a particularly reliable source for this kind of information. In the absence of newer information, I'd go with a primary source making a good case for the 13 million year age and near simultaneous formation of both clusters.

by **BDanielMayfield** » Thu Jan 23, 2014 7:44 pm

Comments, even when they are wrong, lead to improved understanding after ideas in error are corrected. But it’s more pleasant for everyone when errors are pointed out in a polite manner. If people feel like they can’t express an idea without being jumped on if it’s wrong then commenting is suppressed.

And, how do we know that Ann’s suggestion about the red gas in today’s APOD being related to the double clusters isn’t at least partially correct? The description says that the clusters are 7,000 lys away. How far away is the red gas?

Also there seems to be some divergence of opinion as to the ages of these open star clusters. The description says they “are both 13 million years young based on the ages of their individual stars, evidence that they were likely a product of the same star-forming region.”

But the link in the description under “NGC 869 and NGC 884” says that They are both quite young: h is listed at 5.6, chi at 3.2 million years (Sky Catalog 2000)”
Is Sky Catalog 2000 out of date as to these clusters ages?

Bruce

by **Anthony Barreiro** » Thu Jan 23, 2014 7:01 pm

APOD Robot wrote:... Not seen in binoculars though, and not often depicted in telescopic images of the region are faint clouds of reddish ionized hydrogen gas found throughout this remarkable cosmic skyscape. A color composite, the image includes narrowband data to enhance emission from the hydrogen clouds. ...

A link to further information about these emission nebulae would have been helpful. Are they listed in any catalogues? Have their distances and proper motions been measured? That would help us understand whether they might be the birth nebulae of the star clusters in this image, otherwise unrelated interstellar clouds that the clusters happen to be moving through (like the reflection nebulae around the Pleiades), or clouds that simply happen to lie along the same line of sight as seen from Earth.

It is a remarkably beautiful and interesting image. I had never seen an image of the double cluster like this before. Rather than beating myself up for being ignorant, I'm appreciating an unexpected discovery.

Starship Asterisk*

APOD: Double Cluster in Perseus (2014 Jan 23)

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