Starship Asterisk*

Double Cluster in Perseus

Explanation: This lovely starfield spans some seven full moons (about 3.5 degrees) across the heroic northern constellation of Perseus. Just right of center it holds the famous pair of open or galactic star clusters, h and Chi Persei. Also cataloged as NGC 869 (right) and NGC 884, both clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters 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. Always a rewarding sight in binoculars, the Double Cluster is even visible to the unaided eye from dark locations. 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. Visible toward the upper left of the wide field of view is another, smaller open star cluster, NGC 957, also of similar age, distance, and possibly related to the more famous Double Cluster in Perseus.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

What a fascinating image! Indeed, this is the first time I have seen reddish clouds of hydrogen in a picture centered on the Double Cluster.

The glowing hydrogen doesn't form an obvious "cavity" around the Double cluster. Hot clusters and even hot individual stars typically blow away the remnants of the gas and dust that they were born from with their fierce stellar winds. Examples of how hot stars and clusters blow away their natal gas clouds can be seen here, here and here.

But even though the red clouds of hydrogen don't obviously center on the Double Cluster, there is a long, broad, thick, curving "wall" of gas that can seen in the upper part of the picture, which just might be hydrogen blown away and piled up by the super-powerful winds of these two mighty clusters.

In fact, the more I look at the red filaments of hydrogen, the more many of them appear to "curve" around the Double Cluster. And some of them appear to curve around the other cluster in the image, NGC 957!

Fabian Neyer, I think you have just identified the remains of the birth cloud of the Double Cluster (and NGC 957)!!!

Ann

Ann wrote:What a fascinating image! Indeed, this is the first time I have seen reddish clouds of hydrogen in a picture centered on the Double Cluster.

The glowing hydrogen doesn't form an obvious "cavity" around the Double cluster. Hot clusters and even hot individual stars typically blow away the remnants of the gas and dust that they were born from with their fierce stellar winds. Examples of how hot stars and clusters blow away their natal gas clouds can be seen here, here and here.

But even though the red clouds of hydrogen don't obviously center on the Double Cluster, there is a long, broad, thick, curving "wall" of gas that can seen in the upper part of the picture, which just might be hydrogen blown away and piled up by the super-powerful winds of these two mighty clusters.

In fact, the more I look at the red filaments of hydrogen, the more many of them appear to "curve" around the Double Cluster. And some of them appear to curve around the other cluster in the image, NGC 957!

Fabian Neyer, I think you have just identified the remains of the birth cloud of the Double Cluster (and NGC 957)!!!

Ann

I take it you haven't seen Fabian's image before?
I'm shocked to see this as APOD considering it was released in October 2011! I'm pretty sure the nebulosity is in the background and is probably unrelated to the Double Cluster.
I'm also surprised that they only had a single image, there is also an identical LRGB image and the two together would have made a very nice mouseover.

I don't know why you would be "shocked to see this as APOD" simply because the image was released in October 2011.

The nebulosity is in the skyscape; there's nothing in the APOD that suggests it is related to the Double Cluster.

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.

Emphasis mine.

APOD Robot wrote:
Explanation: This lovely starfield spans some seven full moons (about 3.5 degrees) across the heroic northern constellation of Perseus. Just right of center it holds the famous pair of open or galactic star clusters, h and Chi Perseii.

[list]I'm angry [/list]It's h and Chi Persei

http://asterisk.apod.com/viewtopic.php? ... ii#p112932

owlice wrote:I don't know why you would be "shocked to see this as APOD" simply because the image was released in October 2011.

The nebulosity is in the skyscape; there's nothing in the APOD that suggests it is related to the Double Cluster.

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.

Emphasis mine.

I'm shocked as it is an image I didn't expect to see as APOD! Also I never stated anything about the nebulosity being related to the Double Cluster but I did reply to Ann's statement.

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

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.

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

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.

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

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.

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)

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.

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.

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

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

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.

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.

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.

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.)

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

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.

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

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 ).