APOD: A Double Star Cluster (2013 Jan 01)

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APOD: A Double Star Cluster (2013 Jan 01)

Post by APOD Robot » Tue Jan 01, 2013 5:06 am

Image A Double Star Cluster

Explanation: Few star clusters are seen to be so close to each other. Some 7,000 light-years away, though, this pair of open or galactic star clusters is an easy binocular target, a lovely starfield in the northern constellation Perseus. Also visible to the unaided eye from dark sky areas, it was cataloged in 130 BC by Greek astronomer Hipparchus. Now known as h and chi Persei, or NGC 869 (above right) and NGC 884, the clusters themselves are separated by only a few hundred light-years and contain stars much younger and hotter than the Sun. In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.

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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by owlice » Tue Jan 01, 2013 5:49 am

What a lovely sparkly start to the new year!
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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by ta152h0 » Tue Jan 01, 2013 6:03 am

Maravilhoso ! in the language I spoke once daily. Reminds me of a professor I had in Phil 201 who would ask, if he was here today, " do you mean a double star cluster or a double star cluster " ? starting the year in style
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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Ann » Tue Jan 01, 2013 9:26 am

It's a lovely picture indeed! The diffraction spikes around the brighter stars, which contrasts with the "modesty" of the multitude of smaller star, combined with the strong (and real) color contrast between the blue and the red stars, make this picture look like a celestial treasure trove full of greater and lesser precious stones. It is lovely indeed.

Note how you can tell at a glance that the clusters are young. They are relatively compact, especially NGC 869 (which is the cluster on the lower left, by the way), and they contain several bright stars that stand out because of their brightness. Also, most of their brightest stars are blue. There is a great color contrast between the blue and the red stars, which clearly suggests that these stars are very massive. Massive stars start out very blue and then turn very red when they become red giants. Even though the stars in the famous outline of Orion don't form a "cluster", we can see, nevertheless, some similarities between Orion and the Double Cluster: the obvious brightness of the brightest stars, the preponderance of blue stars, the great color contrast between red and blue stars.

Because there are some very red stars in or near the Double Cluster, we can say that these clusters are old enough that some of their most massive stars have evolved into red giants. But the clusters are young enough that they still retain a lot of blue giants. We are almost certainly talking about an age of less than ten million years.

It is interesting to compare the appearance of clusters of different ages. Here is a huge picture (5.0 MB!) of the Hyades and the Pleiades. You can see that the stars of the Hyades are neither very blue nor very red, which clearly suggests that there are no supergiants here. At the same time there are indeed small color contrasts between the redder and the bluer stars, which suggests that the brightest stars of the Hyades are just a bit more massive than the Sun, and they are close to the turning point when they exhaust the hydrogen in their cores and turn into (modest) red giants. Note that apart from bright star Aldebaran, which doesn't belong to the cluster, the brightest stars of the Hyades don't seem to be exceptionally bright. The cluster doesn't even stand out all that well against the background. All of this suggests that the Hyades is several hundred million years old. According to Wikipedia, the age of the Hyades is about 625 million years.

Compare the Hyades with the Pleiades, which is so much bluer than the Hyades. All the brightest stars are blue, and the cluster stands out very clearly against the background. On the other hand, the blue color of the nebulosity around the cluster tells us that the stars in the Pleiades are not hot enough to ionize the gas and make it glow red. Also note that there is no "Barnard's Loop" around the Pleiades. All this suggests that although the stars are blue and the cluster is young, the combined ultraviolet light from the cluster isn't enough to ionize the gas in the vicinity. The age of the Pleiades cluster is estimated to be between 75 and 150 million years.

Note, by the way, the small and relatively rich cluster to the upper left of the Hyades, NGC 1647. The overall color of this cluster appears to be slightly blue, bluer than the Hyades. And indeed it is. According to my software, the brightest stars of this cluster are of spectral class late B and A. I know nothing about NGC 1647, but it might actually be about the same age as the Pleiades. This paper says that its age is about 190 million years, older than the Pleiades but considerably younger than the Hyades.

Finally, let's take a look at rich cluster M11. Note that apart from one yellowish star, there appear to be no excessively bright stars in the rich cluster. This suggests that the really massive stars that must have been here when the cluster was young have already exploded as supernovae, or at least sloughed off their outer envelopes and turned into tiny white dwarfs. Note, too, that there appears to be a preponderance of blue stars in the cluster, but these blue stars don't stand out because of their brightness. There are several yellow and reddish stars here, too. We can clearly tell from the visual appearance of this cluster that it is definitely older than the Pleiades. Because of the large number of blue stars we may guess that it is younger than the Hyades. The age of M11 is estimated to 220 million years.

Ann
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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Boomer12k » Tue Jan 01, 2013 8:17 pm

Thank you Ann.

I like the depth and the contrast between the clusters and starry background.

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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Marco Angelini » Wed Jan 02, 2013 4:28 pm

First of all, I would like to thank everybody, personally and on behalf of my friends of ASTROBRALLO, for the nice comments about our picture of the Double Cluster and the explanations about it.

Despite the open clusters are probably not the main target for the amateur astronomers, I love them. First of all because they are subjects the photos of which are not so far from their visual aspect through a pair of binoculars or a telescope (of course with a lower perception of the colours in the visual observation). Then because for those subjects the stars, often neglected in the pictures of nebulae or galaxies, are finally the leading actresses of the scene.
The Double Cluster in Perseo is one of the finest example among the several beautiful star fields offered by the arms of the Milky Way.

Ann,
Very interesting the physical comparisons with the other open clusters and even with the Orion outline which less immediate. I have a curiosity: even there are red giant stars in or near both clusters, the main red giants stars seem to lie in the NGC 884's side and probably they belong to this cluster. If this is true, can we say that the NGC 884 is a little bit older than its twin NGC 869?

Thanks and I wish you all a shining (like the Double Cluster) and peaceful 2013!

Marco

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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Anthony Barreiro » Wed Jan 02, 2013 7:54 pm

Thank you to Marco and your colleagues at Astrobrallo for this lovely photo, and to Ann for your latest stellar evolution tutorial! I've been looking at the double cluster, Hyades, Pleiades, NGC 1647, and Orion through my small telescope and binoculars during several recent clear nights. Like Marco said, they're all beautiful through the eyepiece of even a modest instrument. Last night I was studying sigma Orionis -- check out that baby for some hot OB action! Looking closely at the pictures on apod and reading Ann's explanations of the stars' colors adds immensely to my understanding and appreciation. Thanks all, and happy new year.
May all beings be happy, peaceful, and free.

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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Ann » Wed Jan 02, 2013 9:49 pm

Marco Angelini wrote:
Ann,
Very interesting the physical comparisons with the other open clusters and even with the Orion outline which less immediate. I have a curiosity: even there are red giant stars in or near both clusters, the main red giants stars seem to lie in the NGC 884's side and probably they belong to this cluster. If this is true, can we say that the NGC 884 is a little bit older than its twin NGC 869?
Very good question, Marco! I would have agreed with you, except that several sources have claimed, over the years, that NGC 884 is younger than NGC 869! Actually, there is some foundation for that claim. Look at the cluster centers. The center of NGC 869 (at lower left) contains some bluish stars that are clearly evolved: that is, they have exhausted the hydrogen in their cores and expanded and brightened to become blue supergiants. My software classifies the two brightest stars in NGC 869 as supergiants of spectral classes B2Ia and B3Ia. This means that they are intrinsically slightly cooler and yellower than Alnitak and Alnilam in Orion's Belt, but they are hotter and bluer than Rigel. The fact that Alnilam, Alnitak and Rigel have turned into supergiants means that they have started to "age".

The core of NGC 884 contains fainter stars. These stars could be less massive than the brightest stars of NGC 869, but they could also be equally massive but younger than the brightest stars of NGC 869 and therefore unevolved and consequently less bright. The stars in the center of NGC 884 could still be fusing hydrogen to helium in their cores, just like the Sun.

If NGC 884 is indeed younger than NGC 869, then the red stars that seem to surround NGC 884 (and in one case, even "penetrate it"), must be older than the stars in the core of NGC 884. For all I know, that may be the case! When it comes to Orion, I think many astronomers suspect that reddish Betelgeuse is older than the blue supergiants in the constellation of the Hunter.

On the other hand, I have come to have great respect for the "individuality" of stars. Maybe Betelgeuse is not older than than the blue supergiants of Orion. Maybe it has just aged faster, for one reason or another! Human beings don't all age equally fast, and I suspect that stars don't, either.

And as for the red stars in or near the Double Cluster: maybe some of them are "prematurely gray" by having turned red too soon!

Ann
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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Marco Angelini » Wed Jan 02, 2013 11:47 pm

Thank you to you to, Anthony for your positive comments!

Ann,

Thank you for your very interesting and clear explanation about the age of the Double Cluster. Even if the foundation of the most accredited claim seems to be consistent and supported by theory and facts, your second idea of "individuality" of stars (and its likeness with the human beings) is absolutely fascinating...is there any reference or bibliography related to this topic?

Marco

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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Ann » Thu Jan 03, 2013 6:56 am

Marco Angelini wrote: Ann,

Thank you for your very interesting and clear explanation about the age of the Double Cluster. Even if the foundation of the most accredited claim seems to be consistent and supported by theory and facts, your second idea of "individuality" of stars (and its likeness with the human beings) is absolutely fascinating...is there any reference or bibliography related to this topic?
One of the most recent results from the Hubble Space Telescope says that globular clusters age at different rates.

One of my own favorite odd stars, HIP 43465, is almost certainly a member of cluster M67. According to Wikipedia, M67 is 4 billion years old, which is very old for an open cluster. At this age, all stars of spectral class B ought to have burnt out and disappeared. Yet HIP 43465 is of spectral class B8. The star certainly appears to be a member of the cluster, since its proper motion appears to be perfectly in line with the proper motion of the other stars in the cluster. Its parallax is uncertain, but given the difficulties of measuring parallaxes of individual stars inside clusters - at least clusters as distant and relatively compact as M67 - its parallax too, seems fine. And its brightness is very similar to the brightness of many other stars in the cluster. But its color and temperature are very strange.

Follow this link, and you'll come to an "illustration" showing two open clusters and some "color-magnitude diagrams". The open cluster on the upper right is the Pleiades, of course, and the open cluster on the lower left is M67. Note the exceptionally blue star in M67 at 8 o'clock. This star is just as blue as one of the "sisters" in the Pleiades, and it's the same typical spectral class, too! It's like having "one Pleiad" on the run from her own perhaps 100-million-year-old cluster, taking refuge in a cluster that is about 40 times older!

I've spent a considerable time going through star catalogs, comparing the "expected brightness" of blue stars as judged by their spectral classes with the likely brightness of them based on their parallaxes as measured by the Hipparcos catalog. Very often the stars were not what astronomers had expected them to be. That's why I believe that stars have more surprises up their "sleeves" than we usually give them credit for.

Ann
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Re: APOD: A Double Star Cluster (2013 Jan 01)

Post by Anthony Barreiro » Thu Jan 03, 2013 9:23 pm

Ann wrote:... .

Follow this link, and you'll come to an "illustration" showing two open clusters and some "color-magnitude diagrams". The open cluster on the upper right is the Pleiades, of course, and the open cluster on the lower left is M67. Note the exceptionally blue star in M67 at 8 o'clock. This star is just as blue as one of the "sisters" in the Pleiades, and it's the same typical spectral class, too! It's like having "one Pleiad" on the run from her own perhaps 100-million-year-old cluster, taking refuge in a cluster that is about 40 times older!

... .

Ann
Aha! So that's where the missing Pleiad has fled! There have been many speculations, none conclusive, about why cultures around the world have called the Pleiades some variation of "the seven sisters" when most modern naked-eye observers see at most six stars. One can only surmise how her sisters tormented this poor star to cause her to take such a desperate journey! I hope she is being treated well by her older and wiser adoptive family.
May all beings be happy, peaceful, and free.